TC-JAV, the aircraft destroyed in the accident, taxiing at London Heathrow Airport on 6 May, 1973.
|Date||3 March 1974|
|Summary||Cargo door failure due to aircraft design flaw, leading to explosive decompression, destruction of control systems, and loss of control|
Fontaine-Chaalis, Oise, France
|Aircraft type||McDonnell Douglas DC-10-10|
|Flight origin||Yesilköy Int'l Airport
|Destination||London Heathrow Airport
London, United Kingdom
Turkish Airlines Flight 981 was a regularly scheduled flight from Istanbul Yesilköy Airport to London Heathrow Airport with an intermediate stop at Orly Airport in Paris. On 3 March 1974, the McDonnell Douglas DC-10 operating the flight crashed into the Ermenonville Forest, outside Paris, killing all 346 people on board. The crash was also known as the Ermenonville air disaster, from the forest where the aircraft crashed. At the time, Flight 981 was the deadliest plane crash in aviation history, until three years later when 583 people perished in the collision of two Boeing 747s in Tenerife on 27 March 1977.
The crash was caused when an improperly secured cargo door at the rear of the plane broke off, causing an explosive decompression which severed cables necessary to control the aircraft. Because of a known design flaw, left uncorrected before and after the production of DC-10s, the locking system on the cargo hatches did not latch reliably. There was also a flaw in the manual procedures designed to ensure that the hatches were locked correctly. Problems with the hatches had occurred previously, most notably in an identical incident that happened on American Airlines Flight 96 in 1972, the so-called "Windsor Incident". Investigation showed that the manual handles on the hatches could be improperly forced shut without the latching pins locked in place. It was noted that the handle on the hatch that failed on Flight 981 had been filed down to make it easier to close the door, resulting in the hatch being less resistant to pressure. Also, a support plate for the handle linkage had not been installed, although manufacturer documents showed this work as completed. Finally, the latching had been performed by a baggage handler who did not speak Turkish or English, the only languages provided on a warning notice about the cargo door's design flaws and the methods of compensating for them. After the disaster, the latches were redesigned and the locking system significantly upgraded.
The Smithsonian Institution has featured the crash--and the related American Airlines Flight 96 mishap--in the "Behind Closed Doors" episode of Air Disasters, a documentary series on its basic-cable channel, and a book about the accident, titled The Flight 981 Disaster: Tragedy, Treachery, and the Pursuit of Truth, written by Samme Chittum, was slated to be published by its Smithsonian Books bibliographical imprint in early October 2017.
The aircraft, a DC-10 Series 10 (production designation Ship 29), was built in Long Beach, California, under the manufacturer's test registration N1337U, and leased to Turkish Airlines as TC-JAV on 10 December 1972. The plane, together with four other DC-10-10's, were owned by Mitsui, and were originally intended to be purchased by All Nippon Airways, but the Japanese airline declined the aircraft in favor of the Lockheed L-1011 TriStar. Three of those 5 Mitsui planes went to Turkish Airlines, while the two remaining went to Laker Airways.
The accident aircraft had 12 six-abreast first-class seats and 333 nine-abreast economy seats, for a total of 345 passenger seats. At the time of the accident there were only two people seated in first class, while economy class was fully occupied. The flight crew were all Turkish. Flight attendant nationalities included four from the UK, three French and one Turkish. Flight 981's Captain was Nejat Berköz, age 44, with 7,000 flying hours. First Officer Oral Ulusman, 38, had 5,600 hours flying time, and Flight Engineer Erhan Özer, 37, had 2,120 flying hours experience.
Flight 981 departed from Istanbul at 7:57 am local time and landed at Paris's Orly International Airport at 11:02 am local time, after a flight time of just over 4 hours. The aircraft was carrying 167 passengers and 11 crew members in its first leg, and 50 of these passengers disembarked in Paris. The flight's second leg, from Paris to London Heathrow Airport, was normally underbooked, but, due to a strike by British European Airways employees, many London-bound travellers, who had been stranded at Orly, were booked onto Flight 981, delaying the flight departure by 30 minutes.
The aircraft left Orly Airport at 12:32 pm, bound for Heathrow Airport, and took off in an easterly direction, before turning north. Shortly after take off, Flight 981 was cleared to flight level 230 (23,000 feet (7,000 m)) and started turning west towards London. Just after Flight 981 passed over the town of Meaux, the rear left cargo door blew off and the sudden difference in air pressure between the cargo area and the pressurised passenger cabin above it, which amounted to 2 pounds per square inch or 14 kilopascals, caused a section of the cabin floor above the open hatch to fail and blow out through the hatch along with six occupied passenger seats attached to that floor section. The fully recognizable bodies of the six Japanese passengers who were ejected from the aircraft were found along with the plane's rear cargo hatch, having landed in a turnip field near Saint-Pathus, approximately 15 kilometres (9.3 miles) south of where the remainder of the plane was found. An air traffic controller noted that, as the flight was cleared to FL230, he had briefly seen a second echo on his radar which remained stationary behind the aircraft; this was likely the remains of the rear cargo door.
When the door blew off, the primary as well as both sets of backup control cables that ran beneath the section of floor that blew out were completely severed, destroying the pilots' ability to control the plane's elevators, rudder, and Number 2 engine. The flight data recorder showed that the throttle for Engine 2 snapped shut when the door failed. The loss of control of these key components resulted in the pilots losing control of the aircraft entirely.
The aircraft almost immediately attained a 20-degree pitch down and began picking up speed, while Captain Berköz and First Officer Ulusman struggled to regain control. At some point, one of the crew members pressed their microphone button broadcasting the pandemonium in the cockpit on the departure frequency. Controllers also picked up a distorted transmission from the plane and the aircraft's pressurisation and overspeed warnings were heard over the pilots' words in Turkish, including the co-pilot saying "the fuselage has burst!" As the plane's speed increased, the additional lift raised the nose again. Berköz is recorded calling out, "Speed!" and pushed the throttles forward in order to level off. Seventy-seven seconds after the initial door hatch gave way, the plane slammed into the trees of Ermenonville Forest, a state-owned forest at Bosquet de Dammartin in the commune of Fontaine-Chaalis, Oise. At the point of impact, the aircraft was travelling at a speed of approximately 430 knots (490 mph; 800 km/h) at a slight left turn, fast enough to disintegrate the plane into thousands of pieces. The wreckage was so fragmented that it was difficult to determine whether any parts of the aircraft were missing before it crashed. Post-crash fires were small because there were few large pieces of the aircraft left intact to burn. Of the 346 passengers and crew on board, only 40 bodies were visually identifiable, with rescue teams recovering some 20,000 body fragments in all. Nine passengers were never identified.
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, BEA, Pan Am, or TWA, boarded TK 981 in Paris, resulting in a 30-minute departure delay. Some passengers cancelled their tickets because of delays or a lack of seats.
The majority of the passengers were British, including members of an amateur rugby team from Bury St Edmunds, Suffolk, who were returning from a Five Nations match between France and England. Notable people on board were John Cooper, who won silver medals in men's 400 metres hurdles and the 4 × 400 metres relay at the 1964 Summer Olympics in Tokyo, and Jim Conway, general secretary of the Amalgamated Engineering and Electrical Union.
There were 177 Britons, 48 Japanese, 44 Turkish, 25 Americans, 16 French, 5 Brazilians, 3 Argentine, 2 Australian, 2 Indians, 1 Belgian, 1 Cypriot, 1 West German, 1 Irish, 1 Moroccan, 1 New Zealander, 1 Pakistani, 1 Senegalese, 1 Spaniard, 1 Swedish, 1 Swiss, and one South Vietnamese.
The French Minister of Transport appointed a commission of inquiry by the Arrêté 4 March 1974, and included Americans because the aircraft was manufactured by an American company. There were many passengers on board from Japan and the United Kingdom, so observers from those countries followed the investigation closely.
The Lloyd's of London insurance syndicate which covered Douglas Aircraft retained Failure Analysis Associates (now Exponent, Inc.) to also investigate the accident. In the company's investigation, it was noted that during a stop in Turkey, the ground crews filed the pins down to less than a quarter of an inch (6.4 millimetres), when they experienced difficulty closing the door. Subsequent investigative tests proved the door yielded to approximately 15 psi (100 kPa) of pressure, in contrast to the 300 psi (2,100 kPa) that it had been designed to withstand.
The passenger doors on the DC-10 are inward-opening plug doors, designed to prevent opening while the aircraft is pressurized. However, due to its large radius, a cargo hatch on the DC-10 could not open inside the fuselage without taking up valuable cargo space, so the hatch was designed to open outward, allowing cargo to be stored directly behind it. The outward-opening design presents the risk of the hatch being blown open by the pressure inside the cargo area if the latch were to fail during flight. To prevent this from happening, the DC-10 uses a latching system whose main security principle is an "over-center concept": four C-shaped latches mounted on a common torque shaft are rotated over latching pins ("spools") fixed to the aircraft fuselage. The rotating movement of the torque shaft is brought about by an electric actuator, through a linkage which includes a crankshaft that ensures the "over-center" position of the whole system. Due to the shape of the latches and the over-center design, when the latches are in the correct position, internal pressure on the hatch not only produces insufficient torque to open the hatch, but it also makes the whole system safer because the over-center safety principle is increased. The system has a hand crank provided as a backup.
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, controlled by a switch actuated by the locking pin mechanism, that remained lit unless the cargo hatch was correctly latched.
The cargo door design flaw, and the consequences of a resulting in-flight decompression, had been noted by Convair engineer Dan Applegate in a 1972 memo. The memo was written after American Airlines Flight 96, another DC-10 experienced a rear cargo door failure identical to the one that occurred on Flight 981, also causing an explosive decompression. Fortunately, even though the pilot's ability to control Flight 96 was compromised by some severed underfloor cables in the blown-out section of the plane, they were able to land in Detroit without further injuries. The NTSB's investigation into Flight 96 determined that baggage handlers forced the locking handle closed, and the latches did not engage fully 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 force, allowing the baggage handler to close the handle with his knee even when the pins interfered with the torque tube flanges. 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 devices indicated that the door was properly closed. In the case of Flight 96, the plane was able to make a safe emergency landing because not all of the underfloor cables were severed, thus allowing the pilots limited control. This greatly contrasted with Flight 981, where all of the underfloor cables were severed in the decompression and the pilots completely lost control of the plane.[note 1]
In the aftermath of Flight 96, the NTSB made several recommendations. Its primary concern was the addition of vents in the rear cabin floor that would ensure that a cargo area decompression would equalise the cabin area, and not place additional load onto the floor. In fact, most of the DC-10 fuselage had vents like these, only those in the rear of the aircraft lacked them. Additionally, the NTSB suggested that upgrades to the locking mechanism and to the latching actuator electrical system be made compulsory. Despite this, the FAA also agreed with McDonnell-Douglas' assessment that additional venting would be too expensive to implement and did not demand that this change be made.
Flight 981, named TC-JAV or "Ship 29", had been ordered from McDonnell-Douglas three months after the service bulletin was issued, and was delivered to Turkish Airlines three months later. 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. Through either oversight or deliberate fraud, the manufacturer construction logs however showed that this work had been carried out. In reality, 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 pounds-force or 220 newtons) despite the improperly engaged latches. These findings concurred with statements made by Mohammed Mahmoudi, the baggage handler who had closed the door on Flight 981; he 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.
After Flight 96, McDonnell-Douglas added a small peephole that allows baggage handlers to visually inspect the pins to confirm they are in the correct position, and information placards to show the correct and incorrect positions of the pins. This modification had been applied to Flight 981's plane, however, Mahmoudi had not been instructed about the purpose of the indicator window; 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 safely latched. Furthermore, the instructions on the plane regarding the indicator window were printed in English and Turkish, but Algerian-born Mahmoudi, who was fluent in three other languages, could read neither of these.
It was normally the duty of Turkish Airlines flight engineer or chief ground engineer to ensure that all cargo and passenger doors were securely closed before takeoff. In the case of Flight 981 however, the airline did not have a ground engineer on duty at the time of the departure, and the flight engineer for Flight 981 failed to check the door. Although French media outlets called for Mahmoudi to be arrested, the crash investigators stated that it was unrealistic to expect an untrained, low-wage earning baggage handler who could not read the warning sticker to be responsible for the safety of the aircraft.
Issues related to the latch of the DC-10 include human factors, interface design and engineering responsibility. The control cables for the rear control surfaces of the DC-10 were routed under the floor; therefore, a failure of the hatch resulting in a collapse of the floor could impair the controls. If the hatch were to fail for any reason, there was a very high probability the plane would be lost. To make matters worse, Douglas chose a new type of latch to seal the cargo hatch. This possibility of a catastrophic failure as a result of this overall design was first discovered in 1969, and actually occurred in 1970 in a ground test. Although Convair, who had been sub-contracted to manufacture the door, informed McDonnell Douglas of the potential problem, Douglas ignored these concerns, because rectification of what Douglas considered to be a small problem with a low probability of occurrence would have seriously disrupted the delivery schedule of the aircraft, and caused Douglas to lose sales. Dan Applegate was Director of Product Engineering at Convair at the time.
After the crash of Flight 981, the latching system was completely redesigned to prevent them from moving into the wrong position. The locking system was mechanically upgraded to prevent the handle from being forced closed without the pins in place, and the vent door was altered to be operated by the pins, thereby indicating when the pins, rather than the handle, were in the locked position. Additionally, the FAA ordered further changes to all aircraft with outward-opening doors, including the DC-10, Lockheed L-1011, and Boeing 747, requiring vents be cut into the cabin floor to allow pressures to equalise 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, later given to a Boeing 777-300ER (TC-JJP, MSN: 40797). Turkish Airlines still flies to London, but the route is now non-stop, and flown with either an Airbus A330 or the Boeing 737.
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An inward-opening passenger door, also called a "plug door", is inherently more resistant to blowing open than an outward-opening cargo hatch. In flight, the air pressure inside the aircraft is greater than that outside, and pushes outward on the door with the effect of forcing it open; in the case of a plug door, this seals the door more tightly. On the other hand, the cargo hatch relies entirely upon its latch to prevent it from opening in flight, making it particularly important that the locking mechanisms be secure.
Aircraft other than DC-10s have also suffered catastrophic cargo hatch failures. The Boeing 747 has experienced several such incidents, the most noteworthy of which occurred on United Airlines Flight 811 in February 1989, when a cargo hatch failure caused a section of the fuselage to burst open, resulting in the deaths of nine passengers who were blown out of the aircraft.
The NTSB's recommendations, following the earlier Flight 96 incident, were intended to reduce the risk of another hatch failure, but were not implemented by any airline. As a result, the NTSB now communicates its recommendations for safety improvements directly to the FAA, which, though not obligated to take any subsequent action, may then issue Airworthiness Directives based on those recommendations.
The crash was featured in Season 5 of the Canadian made, internationally distributed documentary series Mayday, on the episode "Behind Closed Doors", which also covers the similar incident on American Airlines Flight 96.