On 3 March 1974, a McDonnell-Douglas DC 10 operated by Turkish Airlines, after take-off from Paris Orly, experienced an explosive depressurisation. Approximately 10 minutes after take-off, the aircraft radar signature was seen to split into two, with one part remaining stationary before disappearing from the radar screen. The second part turned left to a heading of 280 degrees. This second radar return was caused by the separation of the cargo door, which occurred at a height of approximately 11,000 feet, at a point when the cabin pressure would have been roughly equal to that of sea level. The ejection of the aft cargo door on the left-hand side followed by a sudden depressurization led to the disruption of the floor structure. The collapse of the floor rendered the number 2 engine inoperative and impaired the flight controls (tail surfaces) which led to loss of control over the aircraft. Seventy-seven seconds later, the airplane impacted terrain 37 km northeast of Paris.
This is an extract form the official report on the accident issued the French Secretariat of State for Transport, Investigation Commission:
"Three or four seconds before 1140 hrs, the noise of decompression can be heard on the cockpit voice recording, the co-pilot said: “the fuselage had burst” and the pressurisation warning sound sounded.
[…] The flight data recorder shows that, in the seconds after depressurisation, the speed of No. 2 engine dropped sharply and the aircraft turned left (9°) and went into a nose-dive attitude. This nose-down attitude increased rapidly (down to -20°) and the speed increased (360 kts) although Nos. 1 and 3 engines had been throttled back. The pitch attitude decreased progressively to -4° and the speed became steady around 430 kts."
Analysis of the DC 10 wreckage showed that one of the pieces which detached in-flight was the rear cargo door (See Figure 1). Two occupied triple seat units were ejected from the aircraft due to the depressurisation. All the horizontal stabiliser and elevator control cables, routed beneath the floor of the DC-10, were severed. The severed control cables made it impossible to control the aircraft pitch attitude.
Figure 1. Location of the DC-10 aft cargo door
Investigators found that a support plate, which prevents forced closing of the locking handle and the vent door when the door is not fully latched and locked, was not installed on this door. Maintenance records of modifications accomplished on this airplane by McDonnell-Douglas reflected that the support plate had been installed.
Investigation of the accident revealed that prior to takeoff, the cargo door was not properly latched and locked (See Figure 2) and the flight deck aft cargo door warning light went out prematurely because of an incorrectly rigged warning switch.
Figure 2. Illustration of correct and incorrect actuator shaft latching
Examination of the latch actuator revealed insufficient extension of the actuator shaft (277.5 mm versus 297 mm for full extension). This resulted in the torque tube not rotating far enough to move the latch hooks into the "over-center" position. See DC-10 Cargo Door Latch Animation for an illustration of this process. (The animation is courtesy of "FAA Lessons Learned from Transport Airline Accidents" http://lessonslearned.faa.gov/).
Additionally, the lock pins were not properly rigged to engage had the torque tube been properly rotated. The four lock pins on the lock tube were 1.6 mm short of the rear face of the restraining flanges. With the latch hooks not "over center" and the lock pins not engaged, the loads applied to the latch hooks from fuselage internal pressure were transmitted through the latching mechanism to the latch actuator support bracket, failing the two attachment bolts. When the two attachment bolts failed, the detached latch actuator enabled the latch hooks to slip off the latch spools, resulting in the sudden opening of the cargo door.
The accident report states that tests on a door with the same definition as that of accident aircraft revealed that the vent door could be closed without excessive force in case of incorrect adjustment of the lock tube and the bent link between the locking handle and the vent door shaft. The person that closed the aft cargo door stated that he proceeded as usual, without any particular difficulties, and that he did not notice any abnormalities.
The official Report identifies the following causes of the accident:
"The accident was the result of the ejection in-flight of the aft cargo door on the left-hand side. The sudden depressurization that followed led to the disruption of the floor structure, causing six passengers and parts of the aircraft to be ejected, rendering number 2 engine inoperative, and impairing the flight controls (tail surfaces) so that it was impossible for the crew to regain control of the aircraft.
The underlying factor in the sequence of events leading to the accident was the incorrect engagement of the door latching mechanism before takeoff. The characteristics of the design of the mechanism made it possible for the vent door to be apparently closed and the cargo door apparently locked when in fact the latches were not fully closed and the lock pins were not in place.
It should be noted, however, that a view port was provided so that there could be a visual check of the engagement of the lock pins.
The defective closing of the cargo door resulted from a combination of various factors:
- Incomplete application of Service Bulletin 52-37 [DC-10SC812 DOORS - Cargo- Modify and Adjust Door Mechanism Assembly].
- Incorrect modification and adjustments which led, in particular, to insufficient protrusion of the lock pins and to the switching off of the flight deck visual warning light before the door was locked.
- The circumstances of the closure of the door during the stop at Orly, and, in particular, the absence of any visual inspection through the view port to verify that the lock pins were effectively engaged. Although at the time of the accident, inspection was rendered difficult by the inadequate diameter of the view port.
Finally, although there was apparent redundancy of the flight control systems, the pressure relief vents between the cargo compartment and the passenger cabin were inadequate. Additionally, all the flight control cables were routed beneath the floor. These factors placed the aircraft in grave danger, in the case of any sudden depressurisation, causing substantial damage to that part of the structure.
All these risks had already become evident nineteen months earlier, at the time of the Windsor accident [Similar accident which occurred in Windsor, Ontario, Canada on 12 June 1972 with causal factors including depressurisation of DC-10 due opening of cargo door in-flight]. However, no efficacious corrective action had followed.
The Investigation Commission recommended that particular attention should be paid to the efficacy of the cargo closing, locking and checking systems, and also to the behaviour of the flooring in the case of sudden depressurisation of the cargo compartments.
The case Turkish Airlines DC-10 has also drawn attention to the possible consequences of damage to a control circuit, damage which should never inhibit the operation of the vital control circuits.
For the complete list of safety recommendations (page 49) see the official accident Report in Further Reading.
- The final Report published by the French Secretariat of State for Transport in French and in English.