Investigation
The flight data recorder and cockpit voice recorder showed that the first hint the flight crew had of any problem was a muffled report that took place just after the aircraft passed over Meaux. That sound was followed by a loud rush of air, and the throttle for the tail-mounted Number 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 its speed increased, the additional lift started to raise the nose again. Berkoz called "Speed!", and once more started to push the throttles forward, 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; 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, and the cargo area had decompressed. The resulting difference in air pressure between the cargo area and the pressurized passenger cabin above it, which amounted to several pounds per square inch, caused a section of the cabin floor above the open hatch to fail and blow out through the hatch, along with the passenger seats attached to the floor section. Control cables that ran beneath the section of floor were severed, and the pilots lost the ability to control the airliner's elevators, rudder, and Number 2 engine. Loss of control of these key components was then catastrophic to the pilots' ability to control the entire aircraft.
Lloyd's of London insurance syndicate, which covered Douglas Aircraft, retained Failure Analysis Associates (now Exponent, Inc.) to investigate the accident as well. In the company's investigation, Dr. Alan Tetelman noted that the pins on the cargo door had been filed down. He learned that on a stop in Turkey, the ground crews had had trouble closing the door. After less than a quarter of an inch was taken off of the pins, they were able to close it effortlessly. It was proven by tests that the door subsequently yielded to about 15 pounds of pressure, in contrast to the 300 pounds of pressure that it had been designed to withstand.
Read more about this topic: Turkish Airlines Flight 981