Review of Transport Aircraft Ditching Accidents

Review of Transport Aircraft Ditching Accidents Olaf Lindenau ([email protected]), Thomas Rung ([email protected]) Institute for Fluid Dynamics and Ship Theory (M8), Hamburg University of Technology

Abstract Aircraft ditching, i.e. the controlled emergency landing on water, is of relevance for the design and certification of transport aircraft. Ditching investigations are generally based on scaled model tests as well as numerical simulations. Supplementary, the analysis of ditching accidents is another valuable investigation method. Although water impact accidents of aircraft are rare and cases of passenger transport jet aircraft ditching are almost none existent, some documented examples exist, which provide insight into the impact scenario. The paper presents a compilation of all known planned ditching events of passenger transport jet aircraft based on available accident data and reports. Emphasis is given to two recent planned ditching events of a Boeing 767 (in 1996) and a Boeing 737 (in 2002), which are analyzed in detail from an engineering point of view. As a typical example of an unplanned ditching, the accident of a Boeing 707 (in 2000) is investigated. The focus of the three case studies is put on extracting the impact conditions in view of the related hydrodynamic loads and analyzing the respective damages to the aircraft structure. Based on the investigated scenarios, the paper concludes the key factors for a successful aircraft ditching as well as those factors contradicting it and outlines remaining uncertainties. Moreover, the examples included highlight the crucial aspects of modeling and simulating aircraft ditching. The conducted accident analysis thus enhances the knowledge about transport aircraft ditching and provides guidance to goaloriented ditching investigations. Keywords: aircraft ditching, unplanned ditching, water impact, impact damage, impact loads, accident investigation

Contents 1 Introduction

2

2 Evaluation of transport jet aircraft ditching events

4

3 Ditching accident case studies 3.1 Planned ditching of Boeing 767 in 1996 off Comoros Islands . . . . . . . . . . . . 3.1.1 History of flight and analysis of water impact . . . . . . . . . . . . . . . . 3.1.2 Summary from ditching point of view . . . . . . . . . . . . . . . . . . . . 3.2 Unplanned ditching of Boeing 707 in 2000 in Lake Victoria . . . . . . . . . . . . . 3.2.1 History of flight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Analysis of flight data recorder parameters . . . . . . . . . . . . . . . . . . 3.2.3 Damages to aircraft structure . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.4 Analysis of accident from ditching point of view . . . . . . . . . . . . . . . 3.2.5 Comparison of ditching tests for this aircraft type to the unplanned ditching 3.3 Planned ditching of Boeing 737 in 2002 in Bengawan Solo River . . . . . . . . . . 3.3.1 History of flight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Analysis of damages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Evacuation and injuries . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.4 Analysis of accident from ditching point of view . . . . . . . . . . . . . . .

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4 Assessment of planned and unplanned ditching events

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5 Conclusion

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Bibliography

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1

Introduction

Water impact accidents of aircraft can be divided into two groups: ditching accidents of the airborne aircraft and overrun accidents with prior ground contact during take-off or landing. Moreover, the definition excludes all non survivable accidents where the aircraft also impacts a body of water but without a chance of survival. Ditching is further classified in planned ditching and unplanned ditching. The general aviation notion of “ditching” refers to an emergency landing on water as a planned water contact. The crew, with the aircraft under control, deliberately lands on water according to recommended procedures given in the flight manual. The destruction of the aircraft is accepted, provided the crew and passengers can safely escape and be rescued. As opposed to this, unplanned ditchings refer to water impacts without appropriate preparation. Consequently, only some, if any at all, measures of the recommended ditching procedure can be followed. For ditching performance analysis of aircraft a ditching event can be subdivided into four major consecutive phases (Fig. 1): approach, impact, landing and floatation phase. Approach phase

• approach conditions • environmental conditions

Impact phase

• hydrodynamic impact pressures • local deformations

Landing phase

• motion after impact • planing condition • global dynamic response of structure • detachment of components

Floatation phase

• floatation position • water ingress • evacuation

Figure 1: Phases of aircraft ditching. The approach phase covers the approach conditions for the airborne aircraft close to the water surface, which are then the initial conditions for the impact scenario. The approach conditions consist of the aircraft’s state of motion such as flight path angle, pitch attitude and aircraft speed. Moreover, the aircraft’s mass properties regarding weight, center of gravity and inertia as well as the flap setting, landing gear position, and engine thrust are of relevance. The environmental conditions contain wind and sea state. The decisive aspect about of the approach phase is to reduce the energy which has to be absorbed and transformed during the impact phase to a minimum. The reduction of the kinetic energy at the end of the approach phase can be achieved by reducing the aircraft weight by possible fuel jettisoning and, if possible, by using a maximum-flap configuration to lower the speed. Above all, the forward speed should be minimized while keeping a low to moderate sink rate, as the velocity directly determines the hydrodynamic pressure. The impact phase yields local deformations along the fuselage due to the hydrodynamic pressure. The landing phase covers the motion of the aircraft including the accelerations acting on the passengers and structural components of the aircraft. Critical situation occur when the aircraft nose dives into the water or the aircraft skips on the water surface. Accordingly, loads and accelerations exceed tolerable and survivable thresholds. Aspects of interest are the global dynamic impact response of the structure, as well as the possible detachment of aircraft components such as engines and flaps. Once the airplane comes to rest, the floatation phase starts, which is governed by water ingress and likely ends with the sinking of the airplane. The evacuation of the passengers and crew of the aircraft takes place within the floatation phase. The ditching of aircraft can be investigated by ditching accident analysis, by ditching tests and numerical simulations. Each of these three methods individually contributes to the assessment of ditching of aircraft and also the methods themselves are linked to each other. The approach is depicted in Fig. 2 as the “triangle of aircraft ditching investigation methods”. Ditching accident analysis refers to the technical investigation of actual transport aircraft water-impact accidents comprising their causes, circumstances, and consequences. Physical ditching tests are possible using prepared full-scale aircraft or scaled models. Model-scale ditching tests are the traditional way of ditching performance analysis of aircraft designs. The level of complexity of numerical ditching simulations ranges from simple analytical water-impact formulas to numerical simulation methods taking into 6th International KRASH Users’ Seminar (IKUS6), 15-17 June 2009, Stuttgart

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Numerical Simulation of Ditching

Ditching of Aircraft Ditching Tests

Ditching Accident Analysis

Figure 2: The triangle of aircraft ditching investigation methods.

account the complete aircraft and the near field of the surrounding fluids. While the ditching accident analysis is confined to a few aircraft under special conditions, ditching tests and numerical simulations of ditching allow to investigate aircraft ditching performance during the design phase considering various parameter influences. While generally ditching accident analysis within the official accident investigation is aimed at determining the causes of the accident, this paper is devoted to the ditching itself. As the key issue in ditching of aircraft is the impact on water, this paper focuses on the hydrodynamics of ditching, but also includes aerodynamic and mechanic aspects. There are a number of constraints in the analysis of aircraft ditching accidents: • As Table 1 shows ditchings are very rare events and occur for different aircraft under different circumstances. Thus, no meaningful statistical evaluation is possible. Alternatively, the ditching accident analysis has to be conducted for individual accidents or by clusters of selected cases. • In hardly any ditching accident the flight data recorder data of the water impact are available. Besides that, reliable observations of the first ditching phases and their documentation are rare. Hence, the aircraft state during approach and the sequence of events during impact and landing needs to be estimated. This estimation of leads to uncertainties regarding the actual conditions in the first three ditching phases. • Usually accident investigators are examining the final or post-final situation at the crash site. Next to the documentation of the state of the aircraft after the crash, they also force the difficulty to determine the state of the aircraft before the crash. In some cases videos and photos of the accident itself or at least of some of the four ditching phases are available. In addition to measured weather data the visual material reveals the environmental conditions. • Not necessarily all material gathered during the official accident investigation is released to the public. Mind, that the focus of the accident investigation is usually put on the cause of the accident. Apart from this, it has to be noted that the accident investigation and the published report reflects a certain view of a group of people on an event at a point in time. This view is affected by scientific (e.g. analytical, technological, and recovery capabilities) as well as non-scientific factors (e.g. significance of event, experience of investigators, political climate, relationship with regulatory authorities). The present study - like others - is condemned to rely on spares publicly available data. This basically consists of the official accident report and additional visual material. • Unlike numerical simulations and ditching tests, the accident analysis offers no direct insight into the physics of ditching and thus makes it difficult to reconstruct a cause-and-effect chain. As these kind of data are only available for existing aircraft they can only be used for future aircraft within the design stage. Despite the above mentioned constraints the analysis of ditching accidents gives irrefutable evidence of what


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happens in aircraft ditching. Thus, it shows what should be considered in numerical and experimental ditching investigations in the design phase of an aircraft. Accident analysis also reveals how regulations as well as experimental and numerical investigation methods match the actual accident scenarios. Ditching tests and numerical simulations of ditching may serve to investigate the relation between the impact conditions and the outcome of an actual accident in terms of motions, loads, and damages. Thus, ditching tests and numerical simulations can enhance the understanding and facilitate the analysis of such accidents.

2

Evaluation of transport jet aircraft ditching events

Table 1 summarizes the short list of passenger transport jet aircraft ditching accidents. All listed events accidents classify as ditchings due to the fact that the flight crew was aware of the necessity to ditch the aircraft and there was some time to prepare the passengers and the aircraft. The list explicitly excludes propeller driven as well as military and general aviation aircraft ditching events in order to focus on the ditchings of large passenger transport jet aircraft. Table 1: Planned ditching events of passenger transport jet aircraft. Date A/C Type

Location

On board / Fatalities

21 Aug. 1963 TU-124

Neva River, St. Petersburg, Russian Federation

52 / 0

Fuel exhaustion on diverted flight with non retracted undercarriage.

2 May 1970 DC-9

Carribean Sea, NE of St. Croix, Virgin Islands (US)

63 / 23

Fuel exhaustion resulting from continued, unsuccessful landing attempts.

11 Sep. 1990 B-727

Atlantic Ocean, SE off Newfoundland, Canada

18 / 18

Contact lost following fuel problem distress call; wreckage not located.

23 Nov. 1996 B-767

Indian Ocean, off Grande Comore, Comoros

175 / 125

Fuel exhaustion due to interference by hijackers.

16 Jan. 2002 B-737

Bengawan Solo River, near Klaten on Java Island, Indonesia

60 / 1

Engine failure in thunderstorm and unsuccessful restart attempts.

15 Jan. 2009 A320

Hudson River, New York, United States

155 / 0

Dual engine failure caused by bird strike.

Description

The first event refers to the Tupolev TU-124 ditching in 1963. The accident has been reported by Flight International (1964) and data has been compiled and discussed by Townshend (1965). A number of favorable aspects lead to the successful outcome of the ditching: the pilot handling of the ditching, the environmental conditions regarding weather and visibility, and the immediate rescue by a tug boat towing the aircraft to shore. Furthermore, the light weight of the aircraft due to empty fuel tanks, as well as the little damage experienced by the aircraft structure at water impact which preserved the floatation capability of the aircraft, have contributed to the positive outcome. The McDonnell Douglas DC-9 ditching in 1970 was investigated by the National Transportation Safety Board (NTSB) (1971). Additionally, an extensive analysis of this accident was conducted by the NTSB (1972). Despite the great search and rescue efforts conducted and reported by the Joint Rescue Coordination Centre (RCC) Halifax (1990) in connection with the Boeing B-727 ditching in 1990, no parts of the aircraft could be located with reasonable certainty. The flight crew had made a distress call to two other aircraft stating that they were low on fuel and planned to ditch the aircraft on the ocean (Fink, 1990). Due to the availability and wide distribution of a video recording, the Boeing B-767 ditching in 1996 is well known. The ditching will be discussed as case study in section 3.1. The ditching of the Boeing B-737 in 2002 is the second most recent ditching of a jet transport aircraft. This case proofs that modern type aircraft can be successfully ditched and will be discussed in section 3.3. The ditching of the Airbus A320 in 2009 was at the time of writing under investigation by the National Transportation Safety Board (NTSB) (2009).


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Along with the development of modern transport aircraft a number of studies regarding water-impact accidents have been performed. These compile the data of a number of accidents in order to identify technical and procedural deficiencies. The studies aim to give recommendations for improvements and provide guidance to adapt regulations. The latest of such studies are a review of transport aircrew training programs related to ditching by Cosper and McLean (1998) and a two-part study on “Transport water impact and ditching performance” sponsored by the Federal Aviation Administration (FAA) in 1996. The motivation were the transport aircraft water impact accidents occurring in the period 1959 to 1994, and the rising number of overwater operations of transport aircraft and the associated potential risk for water-impact accidents. The first part of the FAA study was performed by Patel and Greenwood (1996) having three objectives: first to review and analyze worldwide transport accident data relative to water impacts and ditching performance, second to compare the results of the study with current FAA requirements to determine their adequacy and relevancy, and third to conduct a survey of major worldwide airports to determine their proximity to water. The second part of the FAA study was conducted by Tahliani and Muller (1996) and comprises three topics: accident data analysis, airport water rescue and emergency floatation equipment. The ditching accidents of the B-767 in 1996 and of the B-737 in 2002 are not included in the previously published accident studies. Thus, the present paper reviews these accidents. Additionally, the unplanned ditching of a Boeing B-707 ditching in 2000 is analyzed in section 3.2 in order to widen the basis for the present study and as a typical scenario for an unplanned ditching, i.e. descent into a body of water short of the runway during approach.

3

Ditching accident case studies

3.1

Planned ditching of Boeing 767 in 1996 off Comoros Islands

On 23 Nov. 1996 at 12.20 UTC (15.20 local time) the Boeing 767-200ER Ethiopian Airlines Flight No. 961 with registration ET-AIZ ditched off the island Grande Comore, Union of the Comoros, in the Indian Ocean. The information presented here is extracted from the aircraft accident report by the Ethiopian Civil Aviation Authority - Flight Safety Department (1998). The hijacking situation has been analyzed by the Federal Aviation Administration (FAA) (1996). The accident is widely known because of the extensive news coverage following the accident, which includes a short video taken by a tourist from the beach at the crash site of the water impact and displays the disintegration of the aircraft (e.g. CNN interactive, 1996a,b,c). The Boeing 767-200ER is a mid-size, wide-body twin-jet airliner in an extended-range variant. The fuselage with a length of 48.5m has a height of 5.41m and a width of 5.03m. The B-767 features a low-wing cantilever monoplane of 47.6 m span and a conventional tail unit with a single fin and rudder. The aircraft is powered by two under wing mounted turbofan engines. The B-767-200ER with registration ET-AIZ was configured with 190 seats in twin-aisle arrangement: 12 first-class, 24 business-class, and 154 economy-class seats. 3.1.1

History of flight and analysis of water impact

The Boeing 767 with 163 passengers and 12 crew members was hijacked en-route from Addis Ababa, Ethiopia to Abidjan, Ivory Coast. The hijackers demanded to be flown to Australia and ignored the necessity of refueling. The aircraft eventually ran out of fuel and the pilot ditched the aircraft 500m away from the shore in the northern part of Grande Comore. The aircraft broke into four parts. 119 passengers and 6 crew members were fatally injured. As cause of the accident “unlawful interference by hijackers which resulted in loss of engines power due to fuel exhaustion” is given in the accident report. Due to the total engine power loss the digital flight data recorder (DFDR) and the cockpit voice recorder (CVR) stopped recording about 25 minutes before the impact. Thus, no data of the ditching can be concluded from the DFDR. The final approach and the ditching of the aircraft was video recorded by a tourist (Fig. 3). According to the accident report an altimeter reading of 150ft and an airspeed of 200kts were obtained less than 2 minutes before impact. Pilot and first officer had been left alone by the hijackers to assume control by this time. Before that the hijackers had partly made improper control inputs. Further, the report reveals that the flight crew had the intention to turn the aircraft to the left in order to ditch parallel to the wave crests. Regarding the impact motion of the aircraft, the report states that “the aircraft brushed the water in a left-wing low attitude and then was held in a straight and level attitude. After hitting a reef with its belly, it broke into four sections.” The fuselage broke behind the cockpit section, in front of the wing box and near the middle of


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a)

b)

c)

Figure 3: Sequence of still images taken from tourist video of Boeing 767 ET-AIZ ditching in 1996 off Grande Comore: a) first water impact of the left wing, b) almost straight and leveled aircraft during the landing phase, c) disintegration due to impact with a reef.

the aft fuselage part, Fig. 4. The left engine was found still attached to the wing and the right engine separated from the wing. The center section came to rest upside down. The impact to the reef in shallow water is given as reason for the destruction of the aircraft.

Figure 4: Boeing 767 ET-AIZ ditching in 1996 off Grande Comore: fuselage breakage (bottom) and seating arrangement with coloring according to injuries (top). Graphics taken from accident report by the Ethiopian Civil Aviation Authority - Flight Safety Department (1998) and colored according to injuries. With respect to the configuration of the aircraft at impact, the accident report states, that “the landing gears were in a retracted position with the doors closed. The RAM air turbine was deployed. The wing flaps were up.” The weight of the aircraft at impact was the actual take-off weight of 124.419t reduced by the consumed fuel. The weather at the accident site at the time of the accident is estimated with NE wind of 3 Bft, 30◦ C air and in the order of 25◦ C water temperature. The RAM air turbine provided power only for the stand-by-instruments. Thus “there was no vertical speed information required to maintain a 200-300fpm rate of descent, in accordance with the ditching procedure.” Instructions on ditching were provided by the Flight Operation and Flight Crew Training manual. In case of ditching the crew is expected to complete the ditching preparation checklist in the Quick Reference Handbook. But the circumstances of this accident did not allow the crew to perform in accordance with the procedure. “In addition, the procedures do not provide for ditching with both engines out.” There was an early awareness of the possibility of having to ditch among the flight crew, the area control and the hijackers. The pilot already discussed with the area control and hijackers the need for refueling and the otherwise possible ditching about three hours before the accident. About half an hour before the impact the pilot made an announcement speaking of an expected “crash landing”. No additional pre-emergency briefing of the passengers was carried out. Only the announcement to sit down and fasten the seat belts could be given by the lead flight attendant over the public address system. The hijackers did not allow further messages. According to the accident report, instructions were only given in English to the passengers from over 30 different nations. Thus, communication and language difficulties might have affected the emergency preparation. The first officer and crew assisted the passengers with donning on and handling life vests and showed how to assume the brace position during impact. Also all loose items were stowed in their appropriate place. Whether the preparations for ditching had been fully and correctly carried out could not be determined by the accident investigators. 6th International KRASH Users’ Seminar (IKUS6), 15-17 June 2009, Stuttgart

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The disintegration of the fuselage into four parts resulted in a rapid water ingress, but also enabled a rapid escape through the openings of the broken fuselage. Almost impossible was the escape for the passengers in the upside down positioned center section. Here about 50 people were fatally trapped. Apart from this, no clear pattern of injury related to seating position can be concluded from the accident report, Fig. 4. Among the surviving passengers the injuries ranged from minor to very serious. The injuries were mainly caused from impact but some also from drowning. Immediate rescue and first aid was given by people from the beach and the nearby hotel, soon joined and organized by official forces. A critical review of the accident report reveals that, apart form the information that the flaps were in retracted position, no further investigation and discussion into the reason for this is given. The flap position directly correlates with the approach speed consequently determining the kinetic impact energy. The maximum lift coefficient based on the wing area for a B-767 is given by Brune and McMasters (1990) with CL max,flaps = 2.45 for full flaps configuration. The order of the maximum lift coefficient for the clean configuration, i.e. no flaps deployed, may be reasonably assumed as CL max,clean ≈ 1.2. Taking further the ratio of maximum lift coefficients as equal to the respective ratio for actual flyable lift coefficients, the ratio of necessary approach speeds to achieve the same lift may be calculated as: 2 Vflaps

V2 clean

=

CLclean 1.2 ≈ ≈ 0.5 CLflaps 2.45

(1)

The kinetic energy is also proportional to the velocity squared. Thus, the kinetic energy at impact would have been in the order of 50% lower for the aircraft in full-flaps configuration compared to the aircraft ditching in clean configuration. In full-flaps configuration the aircraft could have been flying at about 70% of the speed required to give the same lift in clean configuration. 3.1.2

Summary from ditching point of view

Based on the information provided by the accident report, this water impact accident has to be rated as planned ditching. The pilot and to some extend also the crew and passengers were prepared for the water impact. The accident may also be ranked as an intentionally forced unsurvivable water-impact accident as the hijackers made improper control inputs and disagreed with the intention to perform an emergency landing. Key factors regarding the circumstances of the ditching can be split into factors contributing to an at least partly “successful” ditching and contradictory factors. Key factors positively affecting the outcome of this planned ditching were: • The low weight due to empty fuel tanks reduced the impact energy. • The landing gear was retracted following general ditching procedures. • Wind and seaway were moderate and consequently had no impact on the ditching situation. • The daylight and the closeness of the crash site to an occupied beach eased evacuation, rescue, and first-aid given to survivors. • The environmental conditions regarding air and water temperature opposed no additional immediate threat to the survivors and rescuers. On the other hand the following key factors are responsible for the high fatality rate (71% of the persons on board were fataly injured) of this planned ditching: • As the flaps were not set, the airspeed at impact was comparatively high and increased the kinetic energy at impact. • Pilot and first officer only late (if at all) assumed complete control over the aircraft. Thus, they might not have been able to carry out a maneuver close to the ditching instructions. • There was no vertical speed indication for the pilots to carry out the ditching maneuver. • The roll angle to the left and the low pitch attitude were opposed to general ditching procedures and probably increased accelerations and loads at impact. • The impact of the aircraft not only with the water surface, but also with a reef increased loads and accelerations at impact.


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• Accelerations and disintegration of the aircraft at impact to the water surface and the reef caused immediate fatal injuries to some of the persons on board the aircraft. • The disintegration of the fuselage in four parts lead to immediate flooding of the aircraft. Especially in the center part of the fuselage being turned up-side-down passengers died from drowning. • The pilots were not able to inform and prepare the cabin crew and the passengers for the ditching situation. • The pilots had no guidance or training for an all-engine-out ditching. Summing up, the at least partly “successful” outcome of the ditching was basically positively effected by the conditions at the crash site, while the circumstances on board the aircraft, the ditching maneuver itself and the preparation of crew and passengers seriously interfered with the survival chance of the persons on board the aircraft in this planned ditching.

3.2

Unplanned ditching of Boeing 707 in 2000 in Lake Victoria

On 3 Feb. 2000 at 17.36 UTC (20.36 local time) the Boeing 707-351C with registration ST-APY operated by Trans Arabian Transport (TAAT) ditched unplanned in Lake Victoria about 4km short of runway 12 of Mwanza Airport, Tanzania. The aircraft was carrying a crew of five all rescued by fishermen. News of the accident and photos of the damaged aircraft are compiled by the Flight International (2000a,b) and Sparaco (2000). Much of the accident description presented here is based on the report by Nyamwihura (2000) and the AIB Bulletin No. 1/00 (2000). The here performed analysis of the flight data recorder (FDR) parameters is based on the flight recorder replay report by Evans (2000).

Figure 5: Boeing 707 ST-APY afloat more than 12 hours after unplanned ditching in Lake Victoria in 2000. Missing engines, flaps, and ailerons and damaged left wing tip (left photo). Damaged lower forward fuselage structure in area of torn away nose landing gear (right photo). Photos: Nyamwihura (2006) The Boeing 707-351C is based on the design of the first commercially successful four engine passenger jet airliner. The B-707-351C was designed as a convertible passenger/freighter version with a fuselage length of 46.6m, a diameter of 3.76m, and a span of 44.4m. The aircraft features a low wing design with conventional tail unit. The four turbofan engines were mounted below the wing. At the time of the ditching the aircraft operated as cargo aircraft. 3.2.1

History of flight

The aircraft was operating a charter flight from Khartoum, Sudan to Mwanza, Tanzania for the purpose of uplifting some cargo of fish fillet bound for Brussels. Mwanza airport, elevation 3763ft has one runway 12/301 which is 3300m long and 45m wide. The crew contacted Mwanza Tower at 16.58 and were advised that there was no power at Mwanza Airport but efforts were being made to use a standby generator. After the generator became on line and the runway was lighted, the aircraft commenced a visual approach to runway 12. At 17.28 the captain reported turning final runway 12 and a landing clearance was given. The 1 “runway 12” designates runway direction for take-offs and landings with approximate heading 120◦ and “runway 30” with heading 300◦ respectively


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aircraft made a normal visual approach to runway 12. When it came close to the point were it was expected the touch down it was observed to overshoot after which it made a left turn. The controller advised the commander to try runway 30 since the wind was calm and runway 30 had approach lights. However, the commander chose to use runway 12 again. The accident report only makes explicit reference to the existence of approach lights for runway 30. As the approach to runway 12 is over the lake and the landing threshold close to the lakeshore, there is probably no approach light system for this runway. The aircraft was then observed to proceed on a left base2 for runway 12, executed a left base turn3 for runway 12 but could not come out aligned properly and therefore proceeded to continue for a wider right base turn for runway 12. According to the controller, he cautioned the crew of the height, as the aircraft descended very low over the water level while it was still far from the threshold of runway 12. The aircraft was seen falling into the lake with a bang moments later. The commander later reported that whilst executing another approach and with the runway ahead and slightly to the left the aircraft suddenly hit something. There was a lot of noise and the aircraft came to a stand still. The crew shut down the engines and fired the fire bottles. Fishermen, who were in the lake, witnessed that the aircraft was making a turn and reported two loud bangs. They sailed in the direction of the plane, assisted by the SOS flash messages from crew torches. Whereas two of the crew members were found swimming to the shore, the remaining three were found standing on the aircraft wing. They all wore life jackets. Apart from fuel leakage into the lake, no other third party damages or injuries were reported. On the following day some 18 hours after the accident the aircraft had drifted further in the waters. It was floating freely with the tail section partly submerged, but the forward fuselage section was standing at an angle, clear of water. The aircraft was later towed to a position close to the shore where it was de-fueled. The investigation of the wreckage revealed that both pilots had set the correct QNH = 1015hPa reading on their pressure altimeters. However, the altimeter on the (left) Pilot seat stopped at 4100ft while that in the (right) First Officer seat stopped at the correct elevation of lake Victoria, 3720ft. Since the radio altimeter could not be read at the time of the accident, this disagreement in the altimeter readings could well be a factor in the accident. After the accident the captain reported that he took over the controls after the first officer had failed twice to attain the correct height and turning radius for proper landing. However, the captain handed over controls and “blacked out” at the time when the aircraft was descending and it was too late to initiate a climb. The captain himself admitted that this was a “human factors accident”. He attributed his dizziness and the subsequent blacking out to the fact that he had not eaten food since the evening of the previous day. 3.2.2

Analysis of flight data recorder parameters

The flight data recorder of the aircraft was recovered and analyzed at the Air Accident Investigation Branch, Aldershot, United Kingdom. The “Flight Recorder Replay Report” by Evans (2000) gives a summary of the conditions of the Flight Data Recorder (FDR) and the Cockpit Voice Recorder (CVR). Both tapes exhibited damages typical of that due to water immersion, especially because the water was allowed to dry out gradually. The parameters recorded by the FDR are: pressure altitude, magnetic heading, normal acceleration, indicated airspeed, pitch and roll attitude and a FDR sequence number which increments every 10s. All 25 hours of data from the FDR were recovered. The data quality is described as satisfactory with a few data losses throughout the flight. However, due to the tape damage in the range of the final few seconds of the recording, there were many more relevant data errors. The data of the final 20min from the FDR given in tabular form in the appendix of the report were digitized for further analysis here. From the magnetic heading and the indicated airspeed an approximate track of the aircraft is derived by simple time integration of the data. Obviously faulty data were replaced by linearly interpolated values. No corrections were made to the indicated airspeed, used to derive the ground speed, and to the magnetic heading, used to derive the true heading. Thus the airplot given in Fig. 6 gives only an approximate impression of the aircraft motion within the final 20min. The position of the aircraft in the horizontal plane is given with respect to the final position of the aircraft. The runway 12/30 of 3300m at Mwanza Airport is plotted at an estimated position where the aircraft could have landed in the first approach. This position of the runway corresponds to the reported impact location of about 4km from the end of the runway. 2 “left 3 “left

base”: part of approach pattern flying square to the runway located to the left base turn”: part of approach pattern turning left for the final approach


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14

18

12 10

16

8 14

4 2

12

0

3

altitude, 10 ft

north position with respect to final position, 10 3m

6

−2 −4 −6

10

8

−8 −10

6

−12 −14

4

−16 −18 −6

−4 −2 0 2 4 6 8 10 east position with respect to final position, 103m

12

2 −15

−10 −5 0 5 10 15 20 position with respect to final position in direction of runway, 103m

Figure 6: Final 20min track of Boeing 707 ST-APY unplanned ditching in Lake Victoria derived from FDR readout: air plot with respect to final aircraft position (left) and altitude profile calibrated with QNH = 1015hPa with respect to runway orientation (right). Black dots indicates final aircraft position and solid straight lines approximately mark Mwanza airport runway 12/30.

Corresponding to the airplot an altitude profile of the aircraft position was calculated. The pressure altitude from the FDR was calibrated according to the International Standard Atmosphere and formula by the International Civil Aviation Organization (ICAO) (1993) with the help of the reported QNH = 1015hPa. This calibration does not account for the deviation from the standard temperature model. Together with the uncertainties of the pressure measurements this is the probable cause for the altitude after calibration being partly lower than the elevation of 3763ft of the runway of Mwanza airport. The altitude profile corresponding to the air plot is plotted in Fig. 6 with respect to the orientation of the runway 12/30. Despite the uncertainties the altitude profile clearly shows the first approach and the corresponding overshoot near the threshold of the runway. The plot also reveals the constantly decreasing altitude after the aborted second approach leading to the unplanned ditching. All in all the airplot together with the height profile visualizes the above reported flight path of the aircraft. In order to determine the flight parameters at the time of impact, the final 30s of the FDR data are evaluated and plotted (see Fig. 7). The time scale is given with respect to the end of the recording. The symbols mark those time steps where data is assumed to be reliable. The rate of change of the magnetic heading together with the final roll angle of about 13◦ to the left reveals, that in the final seconds the aircraft was changing from right turn to left turn for the final approach to Mwanza Airport. The final altitude of 3767ft is very close to the elevation of Mwanza airport of 3763ft. Regression analysis of the final seconds of the altitude over 10s to 20s gives sinking rates of 5ft/s to 9ft/s. The final indicated airspeed was 123kts and the pitch attitude about 3.8◦ . The parameters of the final seconds are reasonably assumed to be the impact conditions of the aircraft on the water. 3.2.3

Damages to aircraft structure

The reported damage to the aircraft which is noticeable from photos (see Fig. 5) of the wreckage, reads as follows: 6th International KRASH Users’ Seminar (IKUS6), 15-17 June 2009, Stuttgart

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altitude, ft

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150 100 −30 140 130 120 −30 50 0 −50 −30 10 0 −10 −30

Figure 7: Final 30s of FDR parameters readout of Boeing 707 ST-APY unplanned ditching in Lake Victoria.

• All four engines and their pylons separated on impact each taking with it about 2.4m of the wing structure forward of the front spar. There was no indication of any engine colliding with any part of the remaining aircraft structure after separation. • The left wing showed severe impact damage towards the tip (see left photo of Fig. 5). All spars and skins showed 45◦ fractures indicating that they had failed under heavy impact loading. The damage to the right wing was relatively less severe than of the left wing. The ailerons of both wings had been lost. But most of the flaps which were in landing configuration were in place, though damaged to varying degrees. • The nose landing gear was ripped of at impact. • The fuselage showed extensive damage to the underside structure. The damage starts from the nose landing gear and extends 3m backwards and to the left of the center-line (see right photo of Fig. 5). The observation goes along with the FDR evaluation of the aircraft being in a left turn with roll angle to the left and is also consistent with the damage to the left wing. All the equipment in the electronics bay fell into the lake when the lower fuselage was torn open by the separating nose landing gear. • There was no fire. • Apart from the separation of parts of the aircraft and local damages to the under fuselage no global damages to aircraft with respect to the global structure is apparent. No failure or buckling of the upper shell of the fuselage was reported or is visible in the photos of the damaged aircraft. 3.2.4

Analysis of accident from ditching point of view

The key factors for surviving a ditching event are the approach conditions, the configuration of the aircraft, the environmental conditions and the circumstances for the rescue. There were several positive key factors contributing to this “successful” unplanned ditching in Lake Victoria:


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• The aircraft weight was relatively low, i.e. well below the certified maximum landing weight of 122t (Department of Transportation Federal Aviation Administration, 1984), as the aircraft was supposed to pick up cargo at Mwanza airport and had burned some fuel during the flight. • With flaps set to landing configuration, the final airspeed of about 123kts was close to a normal landing speed. • The rate of descent of the aircraft of about 5ft/s to 9ft/s was in the order for a normal landing or ditching. • The aircraft had a nose-up pitch attitude of about 3.8◦ . That is favorable, although it is by far smaller than the pitch attitude of 12◦ recommended in the ditching investigations of this aircraft type (Thompson, 1955). • The moderate damages to aircraft caused no direct threat to the health of the crew and enabled a long floatation time. • There were no indications of wind, thus it is reasonable to assume that the aircraft ditched into calm water. • Immediate rescue of the crew was possible due to the near-shore location of the impact. • The reported air temperature of 25◦ C as well as the water temperature in the order of 25◦ C (Mangeni and Ngirane-Katashaya, 2005) constituted no immediate threat to the health of the crew waiting for rescue or swimming to the shore. But also some key factors threatened the “successful” outcome of this unplanned ditching: • The large 13◦ roll angle of the aircraft lead to an asymmetrical impact with the water surface. Although the left wing apparently hit the water first, the aircraft must have leveled during the landing phase, as damages occurred also on the right side of the aircraft to wing and fuselage. As reported, also the right engines with their pylons were ripped off, thus they must have come to water at a phase of the landing with a significant forward speed. • The landing gear was extended for the landing at Mwanza airport, which is opposed the recommended procedure for a planned ditching for this aircraft type (Thompson, 1959). • The accident occurred during darkness, usually making rescue operations more difficult. • As the aircraft apparently did not sink, all crew members should have awaited rescue on the aircraft wing instead of taking the risk of trying to swim to the shore. Thus it may be concluded from this accident that an aircraft designed with respect to planned ditching also has a good chance to survive an unplanned ditching under circumstances close to the planned ditching conditions. Apparently even ditchings with a significant roll angle are not unsurvivable as the aircraft levels itself during the landing phase. Probable reason is the partly planing on the engines which reduces the sensitivity to a role angle. 3.2.5

Comparison of ditching tests for this aircraft type to the unplanned ditching accident

The model ditching experiments conducted for the Boeing 707 by Thompson (1955) considered pitch attitudes of 12◦ , 9◦ and 6◦ resulting in landing speeds of 100kts, 104kts and 114kts respectively for an aircraft weight of 59t in full scale and flaps set to 50◦ landing configuration. Also tests were carried out with no flaps for the respectively full scale landing speeds of 119kts, 127kts and 146kts. Flaps and engines were mounted with scaled strength, and the fuselage bottom was investigated in undamaged and scaled strength, conditions. The conclusion from these tests was, that “the aircraft should be ditched in a nose-high attitude of about 12◦ with landing flaps down”, because of the lower landing speed going along with this condition leading to less fuselage damages. It was further anticipated that “the engine nacelles will probably be torn away and the fuselage bottom will most likely be damaged enough to cause rather rapid flooding”. Also the failure of flaps was noted in the tests. Further ditching tests by Thompson (1959) in calm water and head waves were undertaken for the Boeing 707 with the favored pitch attitude of 12◦ at a full scale landing speed of about 120kts necessary for a full scale aircraft weight of 116t and flaps set to 50◦ landing configuration. The ditching test with landing gear extended and retracted were carried out. For extended nose landing gear it was noticed, that it always was torn off in calm- or rough-water ditchings. The test also showed, that “there was no appreciable difference in behavior or


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damage to the fuselage bottom whether the model was ditched with the nose gear extended or retracted”. For the main landing gear the conclusion was: “it appears that, if the main gear fails, there is little choice between ditching with the gear extended or retracted; but, if the main gear does not fail, a more dangerous behavior results. Therefore, it is recommended that the gear remain retracted in a ditching”. The landing speed as dominant parameter in ditching in the above ditching tests were almost the same as in this ditching accident, whereas the pitch attitude was considerably lower in the ditching accident. The failure of all four engines, some of the flaps and ailerons and the damage to the bottom of the fuselage in this unplanned ditching go along with the above cited conclusions of the ditching tests. Also the failure of the nose landing gear was predicted by the tests. However, the good floatation capability of the aircraft, even in damaged condition, was not foreseen.

3.3

Planned ditching of Boeing 737 in 2002 in Bengawan Solo River

On 16 Jan. 2002 at about 09.24 UTC (16.24 local time) the Boeing 737-300 with registration PK-GWA operated by PT Garuda Indonesia as flight GA421 ditched in the Bengawan Solo River, Central Java, Indonesia. The aircraft had suffered a dual engine flame out attributed to excessive hail and rain ingestion. At least two relight attempts on both engines failed and the aircraft suffered run-out of electrical power. There were 6 crew members and 54 passengers on board the aircraft. One cabin crew member was fatally injured in addition to one other cabin crew member and 12 passengers which were seriously injured. The Boeing 737-300 is a short-to-medium-range, single-aisle, narrow-body aircraft with a length of 33.40m and a span of 28.88m. The fuselage has a height of 4.01m and a width of 3.76m. The aircraft has a low wing and a conventional tail design. Propulsion is provided by two under-wing turbofan engines. The ditched aircraft was configured with 104 passenger seats: 22 in business class and 82 in economy class. An aft-facing, doubleoccupancy flight attendant jump-seat was located by the forward passenger door and single occupancy flight attendant jump seats were mounted behind the left-hand and right-hand aft lavatory. The accident investigation was carried out and reported by the National Transportation Safety Committee Department of Communications - Republic of Indonesia (2006). Assistance was provided by the National Transportation Safety Board (NTSB) (2005) leading to own safety recommendations. Aircraft, aircraft component and engine manufacturer participated in the investigation. The DFDR and CVR readout was performed by the Air Accident Investigation Branch (AAIB), Farnborough, UK. The accident was only briefly reported in the international press, e.g. Ionides and Learmount (2002). 3.3.1

History of flight

En-route from Ampenan to Yogyakarta, Indonesia the B-737 flight entered an area covered by Cumulonimbus cells while descending from cruise altitude at FL310 to FL190. The crew prepared the aircraft and passengers to enter turbulence. Both engines flamed out in severe turbulence and heavy precipitation. Total electrical power loss was noted by the flight crew after at least two engine restart attempts and one attempt of APU start. The engine flame-out has been attributed “to excessive water/hail ingestion on the engines which is beyond the engine certified capabilities.” The relight attempts were unsuccessful since the aircraft was still in precipitation beyond the engines’ certified capabilities. The power loss was due to the battery inability to maintain sufficient power because of inadequate battery maintenance procedures. Because of the total power loss CVR and DFDR immediately ceased recording. The pilot spotted the Bengawan Solo River from about 8000ft altitude and decided the ditch the aircraft on the river. Hence, this accident is a planned ditching. The flight crew is reported to have “announced to the flight attendant to prepare emergency landing procedure”. The passengers were already advised to wear seat belts before because of the flight through severe turbulence. One flight attendant is reported to “not expect to ditch on a river”. It is not given in the accident report whether and which parts of the ditching procedures were followed by the flight and cabin crew. The ditching was executed into the Bengawan Solo River in upstream direction between two iron bridges located approximately 2km apart. At the ditching site the smooth flowing river is approximately 75m wide and 1 to 5m deep. The aircraft settled down on its belly in partly submerged position (Fig. 8). Key parameters of the ditching relevant water-impact condition are the aircraft’s weight, speed, and orientation with respect to the water surface. Due to the unavailability of DFDR data for the water impact, the impact


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Figure 8: Boeing B-737 PK-GWA resting in Bengawan Solo River, Indonesia after ditching in 2002 with no visible damages to the primary overall aircraft structure apart from the distorted engine mountings. Photos (left, right): Indra (2002), Lim (2005).

conditions can only be estimated from other data and information. Based on the damage pattern the aircraft presumably impacted the river in nose-up attitude. The flaps and slats were in retracted position when ditching and were found largely intact. This clean configuration at ditching gives rise to a higher approach speed compared to normal landings with flaps. The maximum lift coefficient at landing for the B-737-300 is given by Brady (2007) to CL max, landing = 2.88 and from the data given by Englar (1996) the maximum lift coefficient in clean configuration of the B-737 is estimated to about CL max, clean = 1.25. Following the approach presented in section 3.1 expressing the relation of lift coefficient to speed (1), the kinetic energy at landing with flaps would have been about 43% of that at landing in clean configuration. The approach speed could have been in full flaps configuration only 66% of that in clean configuration. While the intact right-wing fuel tank was found with 2800 liters of fuel, the leaking left-wing fuel tank was found with 400 liters. No information on the filling of the center fuel tank is given in the accident report. Assuming that both wing tanks carried the same amount of fuel and the center tank was almost empty at impact, the aircraft had about 4.5t of fuel on board which is about 1/4 of the fuel capacity. With only slightly more than half of the passenger seats being occupied the weight of the aircraft at impact can be estimated to be in the order of 80% of the aircrafts maximum landing weight. 3.3.2

Analysis of damages

The primary overall aircraft structure appeared undamaged apart from distorted engine mountings (Fig. 8). The left engine is reported being still attached to the pylon, while the right engine was detached from its pylon. As both engines were damaged to the same degree, the aircraft likely impacted the water without a considerable roll angle. The nose landing gear and the right hand main landing gear were found detached from their attachments approximately 50m from the aircraft final rest. The wings and control surfaces were largely intact and the leading edge of the horizontal stabilizer suffered light water impact damages. The auxiliary power unit (APU) and battery as well as the CVR detached from the aircraft and were found 200-300m downstream of the aircraft. The fuselage was heavily damaged at the bottom aft of the right hand cargo door. Here a hole opened which was large enough for the flight attendants seated in the rear of the aircraft to be pulled out of the plane at water impact. The upper fuselage shell in the aft just in front of the vertical stabilizer showed skin buckling. The forward part of the fuselage was also heavily damaged between the cockpit section and the front wing spar bulkhead (see Fig. 8 left photo). The accident report clearly attributes the damages of the aft fuselage section to the impact in the ditching with a pitch-up attitude. The buckling of the aft upper shell can be assumed to be related to the upwards bending moment introduced at impact of the aft fuselage section. The damages reported and depicted in photos of the interior of the aircraft (Fig. 9) match those described above for the overall aircraft structure. The largest damages occurred in the aft section of the fuselage where the left and right hand lavatory and the lower part of the galley were ripped apart from the aircraft. The floor in this


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section of the aircraft collapsed and opened a large hole. In the front part, the floor aft of the cockpit door buckled. At the second row of the business class, the floor on the aisle collapsed. At least partly related to the disintegration of the floor is the detachment of seats. In line with the fuselage deformation, some ceiling and wall panels separated.

(1) (13) (46)

Figure 9: Boeing B-737 PK-GWA ditching in 2002 in Bengawan Solo River, Indonesia: Overview of interior damages and occupant injuries (left). Damages to floor, seats, and wall panels of business class (upper photo) and to ceiling panels, aft lavatories, and aft galley (lower photo). Graphic: Accident report by National Transportation Safety Committee - Department of Communications - Republic of Indonesia (2006). Photos: Moch (2008). The B-737-300 has 6 exits: on the left hand side the forward and aft passenger doors, on the right hand side the forward galley door and the aft service door, and the left and right hand overwing exits. Both aft doors were unusable due to structural deformation of the fuselage and also because of the adjacent floor collapse. The forward passenger door was jammed closed as well. Thus only half of the installed exits were usable for the evacuation. The cockpit door was only slightly jammed and could be kicked open by the flight crew. 3.3.3

Evacuation and injuries

The two uninjured flight attendants seated in the front part were able to open the forward galley door on right hand side. This was the only immediately available exit. The respective escape slide inflated automatically. The uninjured flight crew assisted in organizing the passenger evacuation. The other two flight attendants seated in the aft were pulled out of the plane during the ditching. One sustained serious injuries and the other fatal injuries attributed to the impact of the aircraft with the water and the ground of the river. As depicted in Fig. 9 of the 6 passengers seated in the business class 5 were seriously injured. These injuries can be correlated to the damages in the front part of the fuselage, especially the disintegration of seats. There


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is no obvious pattern regarding the distribution of the 7 seriously injured passengers among the 48 passengers seated in the economy class. Due to the total power loss the flight crew was not able to inform the air traffic control of the emergency situation and the decision to ditch the aircraft. With the help of his mobile phone the pilot notified the Garuda Operations Centre in Jakarta. Right after the ditching, a fisherman on the river offered his help. With his assistance from the outside the right-hand side overwing exit could be opened. The seriously injured business-class passengers were evacuated through the forward galley door assisted by the flight attendants. The economy-class passengers were evacuated through the right-hand side overwing exit. Some passengers did not leave the aircraft immediately; instead they tried to retrieve their personal belongings stowed in the overhead bins. Villagers from around the accident site helped with the evacuation, temporarily sheltering, and the transportation of seriously injured persons to nearby hospitals. The evacuation process of all passengers to save locations was completed approximately one hour after the ditching. Official and professional personal arrived at the crash site hours after the crash. 3.3.4

Analysis of accident from ditching point of view

This planned ditching with only one fatality out of 60 persons on board shows that modern type aircraft can be successfully ditched. Contributing key factors to the positive outcome were: • The aircraft impacted the water in a nose-up attitude. • The aircraft had a moderate weight during the water impact. • The level of the loads and accelerations at water impact were within a survivable limit regrading injuries to the persons on board and damages to the aircraft. • Due to the water depth at the ditching site the floatation capability of the aircraft was not an issue. The usage of water survival equipment was not necessary as well. • Third persons provided almost immediate help assisting in the evacuation, conducting first-aid treatment, lodging the rescued passengers, and transporting the injured persons to nearby hospitals. • The weather and daylight eased the execution of the ditching and the evacuation of the persons on board the aircraft. On the other hand there were also factors hampering a successful outcome of this ditching: • There were only a couple of minutes between total engine flame-out and ditching leaving only a short time for the flight crew to inform the cabin crew and passengers and to prepare the aircraft for ditching. • The approach speed at ditching without slats and flaps was considerably higher than the respective landing speed with slats and flaps. • The damages to the fuselage shell would have caused rapid flooding leading to significant reduction of floatation time in case the aircraft came to rest in deeper water. • The detachment of seats very likely contributed to the injuries of the passengers. • The immediate availability of only one exit could have been a possibly fatal trap for the persons on board in case of necessarily quick evacuation because of rapid flooding or fire and smoke. • The breakage and collapse of the floor likely caused additional injuries and slowed the evacuation process. • The arrival of professional search and rescue forces took hours, thus leaving the evacuation and first-aid injury treatment to local private persons. This planned ditching was obviously well executed ensuring the overall integrity of the aircraft structure being responsible for the almost totally successful ditching regarding the fatality rate. On the other hand, the failure of subsystems such as seats and parts of the floor and the unavailability of exits could have led under slightly different boundary conditions to a considerably more fatal outcome of this ditching event.


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4

Assessment of planned and unplanned ditching events

The listed planned ditching accidents (Table 1) show that this type of accident is rare but does exist. The survival rate for the individual accidents range from completely successful to totally fatal. The damages range from minor local damages to the shell structure of the fuselage to complete breakup of the fuselage in several pieces and massive local damages and rapture. An average of one ditching in six years can be deduced for passenger transport jet aircraft planned ditchings. Further statistical evaluation of the number of water impact accidents would have to be conducted taking into account the constantly increasing over-water flight operations. Even without these detailed statistics it is evident, that there is no increase in ditching events as predicted by earlier water-impact accident studies. This goes along with the constantly increasing safety of air transport. In order to focus on passenger transport jet aircraft, the present study does not consider the planned ditching on 6 Aug. 2005 of the twin-turboprop ATR-72-202 with registration TS-LBB operated by Tuninter. The high-wing aircraft suffered a dual engine flame-out after running out of fuel and ditched in the Mediterranean. Of the 4 crew members and 35 passengers on board, one cabin crew member and 15 passengers suffered fatal injuries and the remaining occupants sustained minor to serious injuries. The fuselage broke fore and aft of the wing section into three parts. This event shows remarkable similarities to the accidents analyzed here. Together with the findings from the accident investigation into the Airbus A320 ditching on 15 Jan. 2009 into the Hudson River these accidents can supplement the presented case studies. The compiled lists of actual ditchings provide some aspects which should be addressed within the design and certification of an aircraft. All planned ditchings of passenger transport jet aircraft listed above (Table 1) as well as those of the ATR-72 in 2005 and the A320 in 2009 had to be carried out with all engines out. Despite all safety measures concerning the reliable operation of the engines, also the recent ditchings show that there are circumstances leading to an all-engines-out situation and the necessity to ditch the aircraft. Along with the loss of engine thrust other systems important for the ditching maneuver such as flaps and navigational instruments may fail. These systems are directly linked to the speed and thus energy at water impact, and to the possibility to reduce this energy during the final approach. Hence, this possibility should be addressed within the ditching investigation during design and certification of aircraft. Also the ditching procedures and training given to the flight crew should cover this issue (see reports on ditchings of B-767 in 1996 and ATR-72 in 2005), because the skill and performance of the flight crew is a key factor in a ditching maneuver. A crucial aspect already marked by earlier water-impact accident studies is the communication on board of the aircraft in the preparation phase for the ditching. As the ditchings of the B-767 in 1996 and of the ATR-72 in 2005 show, it is important that the flight crew informs the cabin crew and the passengers. Additionally, both ditchings point at the necessity of the cabin crew to inform the passengers about the situation and assist with seat-belts and life-vests. As the three case studies show, the structural integrity and the fatality in ditching accidents are directly correlated. Common reason is the state of motion including the kinetic energy of the aircraft at water impact. The accelerations during water impact are responsible for impact injuries to the persons on board. For the global loads acting on the aircraft the overall motion of the aircraft is decisive. Further fatalities are caused if the fuselage is disrupted due to excessive loads. The rapid flooding in case of disintegration of the fuselage structure is reason for further fatalities. Thus, the most important issue in ditching is to minimize the kinetic energy. The motion of the aircraft and the related global forces are to be investigated within studies of aircraft ditching by tests and numerical simulations. Despite large differences concerning circumstances and type of aircraft, the ditchings of the B-767 in 1996 and the ATR-72 in 2005 show remarkable similarities regarding accelerations and global loads exceeding the fuselage strength. This can be concluded from the type of injuries and the fuselage breakage pattern. Loads on smaller parts of the aircraft such as engines, flaps, and landing gear, including their possible separation, have to be accounted for as well if these loads effect the motion of the aircraft substantially. As the unplanned ditching of the B-707 in 2000 shows, local damages to the shell of the aircraft are of secondary influence to the success of a ditching. Despite local damages the floatation time was not critically reduced in this case. The presented analysis of the ditching accidents reveals that not only the overall integrity of the aircraft structure is decisive regarding survivability and injuries, but also the crashworthiness of interior subsystems such as for example seats, seat-belts, floors, hatracks, galleys, and exits. The importance of cabin subsystem design and integration has been stressed already in earlier ditching studies (National Transportation Safety Board (NTSB), 1972) and remains an issue as the ditchings of the B-737 in 2002 and of the ATR-72 in 2005 show. Three of the six planned ditchings were carried out in rivers with the advantage of quick rescue possibilities.


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Additionally, the importance of aircraft floatation and personal floatation devices is reduced. Two out of the six planned ditchings (B-767 in 1996 and the B-737 in 2002) have the impact with obstacles combined with the water impact in common with overrun water impact accidents. The impact with obstacles in the water increase the deceleration of the aircraft and increase the damages to the aircraft partly up to the disintegration of the fuselage structure. Thus, this type of accident directly poses additional threats to the persons on board. Additionally, the damages caused by impact with obstacles considerably increase water ingress and consequently reduce floatation time. The presented case study of the B-707 unplanned ditching in 2000 is representative for a typical controlled, although unplanned, water contact during final approach. Examples of similar situations and similar outcome exist, e.g. unplanned ditching of the B-727 in 1978 investigated by the National Transportation Safety Board (NTSB) (1978). The reason for these unplanned ditchings being generally so successful is the fact that during normal final approach the speed is reduced corresponding to the flap setting, the sink rate is moderate, and the weight is reduced by the amount of fuel burned during the flight. Hence, if the aircraft has a nose-up pitch attitude these conditions are close to the generally recommended approach conditions for ditching. This fact supports the justification that the investigation and design of aircraft with respect to planned ditching also cover likely unplanned ditching situations. An extraordinary fact about the B-707 ditching in 2000 is that the unplanned ditching was successful despite a considerable roll angle. The conclusion is that the roll angle was, in this case, of minor importance to the overall ditching and does not necessarily lead to fatal ditching events.

5

Conclusion

As one pillar within the framework of the “triangle of aircraft ditching investigation methods” this paper focuses on the “Ditching Accident Analysis”. The analysis consists of the compilation of key facts of planned ditching events of passenger transport jet aircraft and the presentation of three ditching accident case studies. The overall absolute and relative small number of planned ditching accidents of passenger transport jet aircraft may raise the question, how much the aircraft design and operation procedures should account for this situation. As, within the certification of an aircraft, ditching is one aspect (paragraph “25.801 Ditching” by the European Aviation Safety Agency (EASA) and the Federal Aviation Administration (FAA)), the overall question whether it is worthwhile to account for ditching at all can not be raised unless challenging this regulation. Although the ditching regulations basically focus on the planned ditching scenario, the intention is also to cover, at least partly, the unplanned ditching case. As the number of unplanned ditchings is higher, this increases the value of covering ditching within the design, certification and operation of aircraft. All in all the aspect of ditching should not be over-emphasized compared to more frequently occurring hazards. Thus, issues solely relevant for ditching should be balanced within the overall safety introduced in aircraft design and operation. The distinct feature of aircraft ditching is the transition of loads acting on the airframe and parts of it within the four ditching phases Fig. 1: inertia loads, aerodynamic loads, hydrodynamic loads and hydrostatic loads. Due to the density of water the hydrodynamic forces dominate the external forces. Thus, the correct modeling of the hydrodynamic forces taking into account the high forward velocity is decisive for aircraft ditching modeling. As the aircraft deforms under the external loads, modeling of aircraft ditching is a fluid-structure interaction problem. Thus, an adequate and matching level of detail for the modeling of the hydrodynamic loads as well as the structure dynamics is required. This means that the level of physical modeling as well as the disretization in space should be selected appropriately. With respect to the aircraft motion and global loads the hydrodynamic modeling is more important than the structural modeling. The modeling of local deformations is difficult and the significance with respect to loads and motion of the aircraft during ditching is not obvious. But force modifying the separation of aircraft components such as engines and flaps should be modeled. A challenging task is the modeling of the combined water and (soft) ground impact as seen in two of the ditching studies. As the ditching event case studies show, there are no clearly defined initial conditions for ditching of aircraft. Moreover, ditching is influenced by various parameters. Consequently, a trade-off between parameter studies to cover these parameter envelopes and detailed physical modeling has to be chosen. Detailed physical modeling requires respective computational effort and thus can be carried out only for a restricted number of cases. Hence, a combination of applying detailed physical modeling in selected cases for validation purposes and using reduced order modeling for a variety of practically relevant cases appears desirable. 6th International KRASH Users’ Seminar (IKUS6), 15-17 June 2009, Stuttgart

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