By: Chris Kilroy

The disaster that had been feared since the start of the jumbo jet era, a non-survivable crash involving a heavily loaded wide-bodied transport, left commercial aviation reeling for more than a year. This accident has been used by industry critics a as an example of corporate ineptitude, design shortsightedness and government laxity.

At the center of the controversy was a serious defect in the McDonnell Douglas DC-10 aircraft. The fault lay not in its performance or handling, but something far more mundane, namely the locking mechanism of its rear cargo door. Previously, most jetliner doors had been of the `plug` type, opening inwards and held firmly inplace by cabin pressure while the aircraft was in flight. But the door on the DC-10, which was built by the Convair division of General Dynamics, opened outwards.

Due to the constant force being the exerted against the door while operating at high altitudes, a durable and foolproof locking system was an absolute necessity. The door was designed to be closed with a switch. An electrically powered actuator was used to turn a torque tube to which four latches were attached. As the tube revolved during the locking process, the talon of each latch snapped over a corresponding spool. An external lever was then pulled down, driving a locking pin into place on the outside of each latch. Any jamming of the lever should have been an immediate indication that the latches were not in the correct over-centre position.

There had been previous problems with the cargo doors on DC-10s, one of which was on board an American Airlines flight from Detroit in June 1972. Shortly after take off, as the jet climbed through 12,000ft, the rear port-side cargo door blew off. It caused severe damage to the aircraft's hydraulics and control systems, but the pilot managed to make a safe landing and none of the 67 persons on board were injured.

Investigation showed that the door had been improperly shut by a handler on the ramp at the airport. He had forced it shut and had bent the internal rods and tubes without properly locking the door. Instead of the locking pins fitting neatly around the latches they just jammed against the lugs, and when the cabin reached a critical pressure where the door could not hold the force any more it simply just tore itself away.

Three service bulletins were released for modifications to be carried out on the aircraft. One such aircraft, TC-JAV, was flying with only two of the requested modifications. TC-JAV, a DC-10-10 owned by the Turkish carrier THY, was scheduled to ferry passengers back from Orly Airport in Paris, France to London's Heathrow Airport on the day of March 3, 1974. It was being used because of a strike by British Airways, so fans returning from an internantional rugby match in Paris decided to switch to the Turkish carrier. The aircraft was loaded almost to capacity upon departure, and more than half of the passengers were British.

The weather at the time was ideal, with only scattered clouds at about 3000ft. Cleared for ascent to flight level 230, the wide-bodied jet airliner   was observed on radar as it assumed a north-north-westerly direction. Shortly afterwards the primary echo was seen to split  into two, with one part remaining stationary before disappearing from the radarscope. The second part turned left onto a heading of  280 degrees before it too vanished.

What the air traffic controller had observed was the separation of the cargo door, which occured at a height of 11,000 feet over the village of Saint-Pathus - at a point when the cabin pressure should have still roughly equalled that at sea level. As with the American Airlines DC-10, the loss of the door caused a sudden depressurisation, which was followed by the failure the cabin floor. However with the extra weight imposed on the structure, the collapse was more extensive, and six occupants in two triple-seat units were ejected through the opening.

The was serious damage not only to the elevator and rudder cables, but to the no.2 engine thrust levers as well. The nose of the aircraft dropped to about 4 degrees before the DC-10 ploughed into a forest at more than 500 knots, and while banking slightly to the left, disintegrating into a huge fireball. All 346 persons aboard, including 12 crew members perished. The main wreckage was strewn over an area approximately 2,300 feet long and 300 feet wide, some 25 miles north-north-east of the French capital. There were only a few small post crash fires, as there were virtually no pieces large enough to burn.

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The duration of the flight was about 10 minutes, and only 77 seconds expired between the time of the door failure and the final impact. The remains of the cargo door revealed various defficiencies. It was obvious that the latches had not achieved the over-center position. The forces against the door generated by the increasing pressure in flight had been transmitted back to the actuator which withstood the compression, and then to the two bolts that attached it to the door structure, which did not. When the bolts gave way, the latches lost their support, causing the door to open and the top shaft of the actuator to break. The door then shattered into several pieces and became detached from the aircraft.

Studies indicated that, once again, the improper securing of the latches was attributed to the incomplete extension of the actuator shaft. This was believed to have resulted from the intentional cut off of power when the generator did not hold the switch long enough, or unintentionally, due to slippage of the torque limiter, the normal action of the thermal protection trip device, or the accidental stoppage of the electrical power supply.

Company records indicating that the suggested modifications had been completed on TC-JAV, prior to its delivery to the airline in December 1972, proved to be erroneous. Although adjustments to the lock limit warning switch were made, the work was not in accordance with aeronautical standards. The installation of the viewing port, one modification that had been carried out, could alone have prevented the tragedy, had somebody used it to make a visual inspection prior to the takeoff. The warning placard was also in place, but of no use for two reasons. First it had been printed in English, which the Algerian born baggage handler could not read, and perhaps more importantly, the design of the mechanism and the shodiness of the modifications made it possible to pull down the locking lever, bending the internal components, without the use of any abnormal force. The faulty design also accounted for the fact that a warning light on the flight engineers panel had failed to illuminate, indicating that the door was not locked.

Following the disaster, the F.A.A. issued an Airworthiness Directive mandating a closed loop system on all DC-10 cargo doors. Similar to that used on the Boeing 747, the mechanism is designed so as to prevent closure of the vent door unless the locking pins are correctly in place. Subsequently, the government agency took action to further enhance safety in the DC-10, 747, and L1011 Tri-Star. Cabin floors were reinforced and venting improved so as to increase survivability of the aircraft in the event of a major decompression or structural failure. McDonnell Douglas slowly recovered from this black mark on their record, but the DC-10 would be involved in several accidents later in the decade that would put the airplane in the public eye again.