Cause
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. Owing 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 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.
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, written after 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.
Read more about this topic: Turkish Airlines Flight 981