Analysis
4.1 Analysis of the process of ejection of the aft cargo door on the left-hand side
The initiating factor in this accident was the opening and ejection of the aft cargo door
during flight.
Expert examination of the wreckage of the door, of which the closing system (latches, latch actuator and operating mechanism) was found in good condition, has revealed various deficiencies. The latter were such that the aircraft’s take-off was undertaken with the latches very near to their correct position when closed, but at the same time the links which control the latches had not achieved over-centre (fig 2) and as a natural consequence the lock pins could not have been engaged.
Under these conditions, the chain of operation between actuator and latches is not irreversible: any force exerted on the latches is re-transmitted to the actuator instead of being absorbed by the 4 stops provided for that purpose (fig 1).
The actuator withstood the compression force without displacement of its shaft, since it is irreversible. It therefore transmitted the force from the 4 latches to the two bolts (titanium D.25 inch in diameter) which attach its fixed part to the door structure.
The force on the latches is directly proportional to the difference between the pressure inside the fuselage and atmospheric pressure. The force is nil on take-off and increases progressively with altitude up to about 22,400 feet.
There is no way of knowing what measures were taken by the flight crew as regards to cabin pressurization. No malfunction had been reported and the flight level of 240 chosen for cruise and the fact that the aerodromes of departure and arrival had the same 'elevation give reason to assume that pressurization was under automatic control. In that case, at 12,000 feet the cabin altitude was close to sea level and the fuselage pressure differential must have been between 330 and 360 mb (4.7 and 5.2 psi). It should be noted that these are figures of the same magnitude as those estimated in connection with the previous accident at Windsor (Ontario).
The force transmitted to the actuator attachment bolts is the product of the force on the latches and the position of the lever arms of the system. (When the link arm pivots are at dead centre the force on the actuator bolts is zero; the force increases as the arms . move away from the dead centre position.)
The door therefore remained closed as long as the two bolts attaching the actuator to the door structure withstood the increasing pressurization force.
When the two bolts gave way, the latch opened and the door opened suddenly after breaking the top shaft of the door actuator.
As a result of the sudden stress on the fuselage combined with the dynamic pressure of the air, the door broke into severa3 pieces and became detached from the aircraft
For the configuration of the door of TC-JAV, the studies undertaken have shown that the incomplete closing of the latches resulted from incomplete extension of the actuator shaft.
The tests and research on the parts recovered from the wreckage failed to establish the process with certainty.
Either the control switch was maintained in an active position for too short a time (the modification contained in SB 52-44' had not yet been applied to T.C-JAV so that a visual light indicator showing that the shaft had reached the end of its travel was not available to the operator).
Or the extension of the actuator shaft stopped too soon because of:
the slip of its torque limiter;
the normal operation of the thermal protection trip device of the electric motor;
accidental cut-off of the electrical power supply.
Since the electric motor of the actuator was not found after the accident, it is impossible to establish which of the above reasons was the cause of the actuator’s malfunction.
Finally, it should be noted that the door had had to be closed by the manual drive tool on numerous earlies occasions, which would conf¿ the hypothesis of erratic functioning of the actuator.
4.2 Consequences of the ejection in flight of the aft cargo door on the left-hand side
The loss of the door caused an almost instantaneous drop in the pressurizetion, established in the cargo compartment beneath the passenger cabin floor.
The various pressure relief vents between the cargo compartment and the passenger cabin are not of a size to accommodate a discharge of air as large as that which passed through the door which had suddenly opened. As a result, there was an instantaneous excess pressure above the floor of the order of 36 KPa (about 3.6 tonnes/m2) ie the same order of magnitude as in the case of N 103 AA (cf paragraph 3.16.7).
In the case of TC-JAV, this excess pressure, added to the normal stresses on the floor, caused damage such that parts of passenger seats were ejected from the aircraft together with six passengers probably occupying two triple seat units in line with and above the cargo door. This damage was therefore clearly more substantial! than in the case of
N 103 AA in which the inifia1 floor loading was lighter.
Studies were undertaken in an atternpt to reconstruct the damage sustained by the controls, but the impairment of their functioning could not be established in precise detail. Nevertheless, because all the horizontal stabilizer and elevator control cables are routed beneath the floor of the DC-10 and because of the priority assigne4 in this aircraft to each of these mechanical controls, the state of airworthiness of TC-JAV after the loss of the cargo door and the disruption of the floor structure must have been such that the crew were left with no means of regaining sufficient control of the aircraft.
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