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CG render of TC-JAV moments after failure of the cargo hatch.
|Date||3 March 1974|
|Type||Cargo hatch failure and control cable failures. Design flaws|
|Site||Fontaine-Chaalis, Oise, France|
|Aircraft type||McDonnell Douglas DC-10-10|
|Flight origin||Yesilköy International Airport|
|Last stopover||Orly Airport (Paris)|
|Destination||London Heathrow Airport|
Turkish Airlines Flight 981 was a McDonnell Douglas DC-10, registered TC-JAV and named the Ankara, that crashed in Fontaine-Chaalis, Oise, France, outside Senlis, on 3 March 1974. Known as the "Ermenonville air disaster", from the forest where the aircraft crashed, the accident resulted in the deaths of all 346 on board. This was the second air disaster involving a wide-body aircraft, after the Eastern Air Lines Flight 401, and the first one with more than 200 deaths.
The resulting decompression of the cargo hold caused the cabin floor above the hatch to collapse. The flight control cables for the airplane that ran through the floor were severed, leaving the pilots with almost no control over the aircraft. Problems with the latching system and the potential failure mode that led to the crash were known to Convair, the fuselage's builder, with the information passed on to McDonnell Douglas several years prior to the accident. Changes that addressed the problem had been found, but were not applied to TC-JAV, nor any other aircraft in the DC-10 fleet. McDonnell Douglas instead chose a solution less disruptive to schedules but failed to ensure that personnel were trained to follow the new procedures to ensure the hatch had locked. McDonnell Douglas's reputation and the reputation of the DC-10 were harmed.
The crash of Flight 981 was the deadliest air disaster of all time before the Tenerife Disaster event of 1977, and remained the deadliest single-airliner disaster until the crash of Japan Airlines Flight 123 in 1985. Flight 981 has the highest death toll of any aviation accident in France and the highest death toll of any accident involving a McDonnell Douglas DC-10 anywhere in the world. It is also the deadliest single plane crash with no survivors.
The aircraft, a DC-10 Series 10 (production designation "Ship 29") was built in Long Beach, California as N1337U, and was delivered to the airline on December 10, 1972 as TC-JAV. This particular aircraft could carry a maximum of 380 passengers into a single-class configuration.
Flight 981 had flown from Istanbul that morning, landing at Paris's Orly International Airport just after 11:00 am local time. The aircraft, a McDonnell Douglas DC-10, was carrying just 167 passengers and 13 crew members in its first leg. 50 passengers disembarked at Paris. The flight's second leg, from Paris to London's Heathrow Airport, was normally underbooked, but due to a strike by British European Airways (BEA) employees, many London-bound travelers who had been stranded at Orly were booked onto Flight 981. Among them were 17 English rugby players who had attended a France-England match the previous day; the flight also carried six British fashion models, and 48 Japanese bank management trainees on their way to England, as well as passengers from a dozen other countries.
The aircraft departed Orly at around 12:30 pm for its flight to Heathrow. It took off in an easterly direction, then turned to the north to avoid flying directly over Paris. Shortly thereafter the flight was cleared to flight level 230, and started turning to the west for London. Just after Flight 981 passed over the town of Meaux, controllers picked up a distorted transmission from the plane; the aircraft's pressurization and overspeed warnings were heard over the pilots' words in Turkish, including the co-pilot saying "the fuselage has burst." The flight disappeared from radar shortly afterwards.
The aircraft had disintegrated. The post-crash fires were small as there were few large pieces of the aircraft left intact to burn. Of the 346 onboard, only 40 bodies were visually identifiable. Nine passengers were never identified.
The wreckage had been so extensively broken up that immediate investigation suggested that a bomb had been placed on-board. Turkish news reports suggested that a group had intended to bomb a BEA aircraft but had switched planes along with the other passengers. Two terrorist groups soon called to claim responsibility, but this eventually became a hoax.
On the same day of the crash, a BEA VC-10 was hijacked by a terrorist group, but its 102 passengers escaped unharmed.
167 passengers flew on the Istanbul to Paris leg, and 50 of them disembarked in Paris. 216 new passengers, many of whom were supposed to fly on Air France and British Airways, boarded TK 981 in Paris. As a result the layover, normally one hour, was one hour and thirty minutes. Some other passengers cancelled their tickets due to delays or for not finding enough seats.
Most of the passengers were British. Among the British passengers were members of an amateur rugby team from Bury St Edmunds, Suffolk, who were returning from attending a Five Nations match between France and England and trade union leader James Conway. The English rugby team nearly took the doomed aircraft, but took an Air France aircraft instead. Japanese embassy sources said that a total of 49 Japanese were on board. Turkish sources said that 15 Turks were on board. Also on board was John Cooper, who won silver medals in men's 400 meter hurdles and the 4X400 meter relay at the 1964 Summer Olympics in Tokyo
Among the passengers was Dr. Wayne Wilcox, a cultural attaché of the Embassy of the United States in London, his wife, and two of his four children. 38 passengers were Japanese university graduates who were touring Europe and were planning to join Japanese firms after the end of their tour.
The flight data recorder and cockpit voice recorder showed that the first hint the flight crew had of any problem was a muffled explosion that took place just after the aircraft passed over Meaux. The explosion was followed by a loud rush of air, and the throttle for the tail-mounted No. 2 engine snapped shut at the same moment. At some point, one of the crew pressed his microphone button, broadcasting the pandemonium in the cockpit on the departure frequency.
The aircraft quickly attained a 20 degree nose-down attitude and started picking up speed while Captain Nejat Berkoz and First Officer Oral Ulusman struggled to gain control. As the speed increased the additional lift started to raise the nose again, and Berkoz called "Speed!" and started to push the throttles forward again in order to level off. It was too late, however, and 72 seconds after decompression the airliner slammed into the forest at a speed of about 430 knots (497 miles per hour, or 796 km/h) in a slight left turn. The speed of the impact caused the airliner to disintegrate.
The wreckage was so fragmented that it was difficult to tell whether any parts of the aircraft were missing. An air traffic controller noted that as the flight was cleared to FL230, he had briefly seen a second echo on his radar, remaining stationary behind the aircraft. A farmer soon telephoned in, and it was discovered that the rear cargo-hold hatch beneath the floor, portions of the interior floor, and six passenger seats (still holding dead passengers) had landed in a turnip field near the town of Saint-Pathus, approximately 15 kilometers south of the main crash site.
French investigators determined that the rear cargo hold hatch had failed in flight. When it failed, the cargo area decompressed, but not so in the passenger area above it. The difference in air pressure, several pounds per square inch, caused the floor to fail, blowing a section of the passenger cabin immediately above the hatch out through the open hatch. The control cables, which were beneath the floor, were severed, and the pilots lost control of the airliner's elevators, its rudder, and the number two engine. Without these controls, it was impossible to control the aircraft.
Lloyd's of London insurance syndicate that covered Douglas Aircraft retained Failure Analysis Associates (now Exponent, Inc.) to also investigate the accident. Dr. Alan Tetelman of Failure Analysis Associates noted that the pins on the cargo door had been filed down. He learned that on a stop in Turkey, the ground crews had trouble closing the door, which then closed effortlessly by taking less than a quarter inch off the pins. By doing so, it was proved through tests, the door yielded to about 15 pounds of pressure, while it had been designed to withstand 300 pounds.
The passenger doors on the DC-10 are of the plug door variety, which prevents the doors from opening while the aircraft is pressurized. The cargo hatch, however, is not. Due to its large radius, the cargo hatch on the DC-10 could not be swung inside the fuselage without taking up a considerable amount of valuable cargo space. Instead, the hatch was swung outward, allowing cargo to be stored directly behind it. The outward-opening design allowed the hatch, in the event of a latch failure, to be blown open by the pressure inside the cargo area. To prevent that, the DC-10 used a supposedly fail-safe latching system held in place by "over-center latches" - four C-shaped latches mounted on a common torque shaft that were rotated over latching pins ("spools") fixed to the aircraft fuselage. Due to their shapes, when the latches are in the proper position, pressure on the hatch does not place any torque on them that could cause them to open, and they actually further seat onto the pins. The latches were engaged by electric actuators, with a hand crank provided as a back-up.
To ensure this rotation was complete and the latches were in the proper position, the DC-10 cargo hatch design included a separate locking mechanism that consisted of small locking pins that slid behind flanges on the lock torque tube (which transferred the actuator force to the latch hooks through a linkage). When the locking pins were in place, any rotation of the latches would cause the torque tube flanges to contact the locking pins, making further rotation impossible. The pins were pushed into place by an operating handle on the outside of the hatch. If the latches were not properly closed the pins would strike the torque tube flanges and the handle would remain open, visually indicating a problem. Additionally, the handle moved a metal plug into a vent cut in the outer hatch panel: if the vent was not plugged the fuselage would not retain pressure, eliminating any pneumatic force on the hatch. Also, there was an indicator light in the cockpit that would remain lit if the cargo hatch was not correctly latched, controlled by a switch actuated by the locking pin mechanism.
In 1972, American Airlines Flight 96, another DC-10, had its cargo hatch open and separate in flight. In the ensuing investigation it was discovered that the handlers had forced the locking handle closed in spite of the fact that the latches had not fully engaged, because of an electrical problem. The incident investigators discovered that the rod connecting the pins to the handle was weak enough that it could be bent with repeated operation and some force being applied, allowing the baggage handler to close the handle with his knee in spite of the pins interfering with the torque tube flanges. Both the vent plug and cockpit light were operated by the handle or the locking pins, not the latches, so when the handle was stowed both of these warning systems indicated that the door was properly closed. In the case of Flight 96, the airliner was able to make a safe emergency landing as not all the underfloor cables had been severed, thus allowing the pilots limited control.see 'loading'
In the aftermath of the Flight 96 incident, the NTSB made several recommendations. Its primary concern was the addition of venting in the rear cabin floor that would ensure that a cargo area decompression would equalize the cabin area, and not place additional loads onto the floor. In fact, most of the DC-10 fuselage had vents like these: it was only the rearmost hold that lacked them. Additionally, the NTSB suggested that upgrades to the locking mechanism and to the latching actuator electrical system be made compulsory. However, while the FAA agreed that the locking and electrical systems should be upgraded, the FAA also agreed with McDonnell Douglas that the additional venting would be too expensive to implement, and the FAA did not demand that this change be made.
TC-JAV had been ordered three months after the service bulletin was issued, and been delivered to Turkish Airlines three months after that. Despite this, the changes required by the service bulletin (installation of a support plate for the handle linkage, preventing the bending of the linkage seen in the Flight 96 incident) had not been implemented. The interconnecting linkage between the lock and the latch hooks had not been upgraded. Through either deliberate fraud or oversight, the construction logs nevertheless showed that this work had been carried out. However, an improper adjustment had been made to the locking pin and warning light mechanism, causing the locking pin travel to be reduced. This meant that the pins did not extend past the torque tube flanges, allowing the handle to be closed without excessive force (estimated by investigators to be around 50 lbs) despite the improperly engaged latches. This matches the comments made by Mohammed Mahmoudi, the baggage handler who had closed the door on Flight 981, who noted that no particular amount of force was needed to close the locking handle. Changes had also been made to the warning light switch, so that it would turn off the cockpit warning light even if the handle was not fully closed.
The fix that was implemented by McDonnell Douglas after the American Airlines Flight 96 incident was the addition of a small window that allowed the baggage handlers to visually inspect the pins, confirming they were in the correct position, and placards were added to show the correct and incorrect positions of the pins. This modification had been carried out on TC-JAV. However, Mahmoudi had not been advised as to what the indicator window was for. He had been told that as long as the door latch handle stowed correctly and the vent flap closed at the same time, the door was safe. Furthermore, the instructions regarding the indicator window were posted on the aircraft in English and Turkish, but the Algerian-born Mahmoudi, who could read and write three languages fluently, could not read either language.
It was normally the duty of either the airliner's flight engineer or the chief ground engineer of Turkish Airlines to ensure that all cargo and passenger doors were securely closed before takeoff. In this case, the airline did not have a ground engineer on duty at the time of the accident, and the flight engineer for Flight 981 failed to check the door personally. Although French media members called for Mahmoudi to be arrested, the crash investigators stated that it was unrealistic to expect an untrained, low-paid baggage handler who could not read the warning sticker (due to the language difference) to be responsible for the safety of the aircraft.
|This unreferenced section requires citations to ensure verifiability.|
The latch of the DC-10 is a study in human factors, interface design and engineering responsibility. The control cables for the rear control surfaces of the DC-10 are routed under the floor, so a failure of the hatch could lead to the collapse of the floor, and disruption of the controls. To make matters worse, Douglas chose a new latch design to seal the cargo hatch. If the hatch were to fail for any reason, there was a very high probability the plane would be lost. This possibility was first discovered in 1969 and actually occurred in 1970 in a ground test. Nevertheless, nothing was done to change the design, presumably because the cost for any such changes would have been borne as out-of-pocket expenses by the fuselage's sub-contractor, Convair. Although Convair had informed McDonnell Douglas of the potential problem, rectifying what the aircraft manufacturer considered a small problem with a low probability of occurrence would have seriously disrupted delivery of the aircraft and cost sales so Convair's concerns were ignored. Dan Applegate was Director of Product Engineering at Convair at the time. His serious reservations about the integrity of the DC-10's cargo latching mechanism are considered a classic case in the field of engineering ethics.
After Flight 981, a complete re-design of the latching system was finally implemented. The latches themselves were re-designed to prevent them from moving into the wrong positions in the first place. The locking system was mechanically upgraded to prevent the handle from being able to be forced closed without the pins in place, and the vent door operation was changed to be operated by the pins, so that it would properly indicate that the pins were in the locked position, not that the handle was. Additionally, the FAA ordered further changes to all aircraft with outward-opening doors, including the DC-10, Lockheed L-1011, and Boeing 747, requiring that vents be cut into the cabin floor to allow pressures to equalize in the event of a blown-out door.
The name given to the crashed DC-10, "Ankara", is still used on an Airbus A340-300 (TC-JDL, MSN: 57) in Star Alliance Livery.
The story of the disaster was depicted in the fifth year of Canadian National Geographic Channel show Mayday (known as Air Emergency in the US, Mayday in Ireland and Air Crash Investigation in the UK and the rest of world). The episode is entitled "Behind Closed Doors".
It was also featured in Loose Change 9/11: An American Coup.
|This unreferenced section requires citations to ensure verifiability.|
Outward-opening cargo hatches are inherently not fail-safe. An inward-opening hatch (a plug door) that is unlatched will not fly open, because the difference in air pressure between the aircraft cabin and the air outside will seal the hatch shut. However, an outward-opening, non-plug type hatch needs to be locked shut to prevent any unwanted opening. This makes it particularly important that the locking mechanisms be secure. American Airlines Flight 96 experienced the same problem before the accident of flight 981 happened, but the NTSB's recommendations to prevent it from happening again were not implemented by any airline. As a result, now whenever the NTSB comes up with recommendations to prevent certain accidents from happening, they talk to the FAA, consequently, the FAA may issue an Airworthiness Directive to help prevent certain types of accidents from happening. However, NTSB and FAA are two independent Federal agencies, and the FAA is not obligated to act on any NTSB recommendation. Aircraft types other than the DC-10 have also experienced catastrophic failures of a hatch. The Boeing 747 has experienced several such incidents, the most noteworthy of which occurred aboard United Airlines Flight 811 in February 1989, when the cargo hatch failed and caused a section of the fuselage to fail, causing the deaths of nine passengers who were expelled from the aircraft. A somewhat similar problem led to a cockpit window being blown out of British Airways Flight 5390.
|Pre-crash photo at Airport Hamburg taken from Airliners.net courtesy of M. Maibrink|
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