Engines and latch TC-JAV

Examination of No 2 engine

No 2 engine, General Blectric CF6-6D, serial number 451-200, was examined at the Centre d’essais des propulseurs de Sac1ay. It examination gave rise to the following conclusions:

the mechanical damage found on examination was due to the impact;
the engine was not lit at the time of impact;
the engine was running down at the time of impact;
there was no trace of fire.
Examination of the servo controls and of a horizontal stabilizer jackscrew

The sarvo controls (four for the elevator, two for the rudder and four for the ailerons) were recovered and found to be only very slightly damaged, they could therefore be examined.

The examination was made in the UTA hydraulics laboratories at Le Bourget. The electrical characteristics of the electro-hydraulic flow control valves were checked and no abnormality detected, either in the hydraulic part or in the electrical part.

An examination was also made of one of the two jackscrews for the control of the hori-zontal stabilizer; on this screw the nut was found to have jammed at the time of impact.

On examination of the jackscrew and its nut, the number of exposed threads between the bottom of the screw and the bottom of the nut could be measured. These measurements made only on this one screw gave for the stabilizer angle on impact a value very close to the position recorded before the loss of the door.

Examination of the latch actuator of the aft cargo door on the left-hand side

The irreversible actuator was taken off the door, The two bolts which attach the bracket on which it is mounted, were found sheared: the shearing force was estimated at 4,700 daN.

The shaft extension was 277.5 mm measured between the axis of the connection of the actuator to the structure and the axis at the end of the actuator ram (where it is con-nected to the torque tube). Normal extension required for correct closing of the latches is 297 mm.

COCKPIT CALL SYSTEM SWITCH: Satisfactory functioning

UNLOCK LIM1T SWITCH: Satisfactory functioning

CLOSE LIMIT SWITCH: Satisfactory functioning
LOCK LIMIT WARNING SWITCH: Did not function (arm distorted)‘
3.16.5 Check of the adjustment of the locking system
(a) Lock 5mit warning switch
The following repairs were made to the wreckage of the door:
straightening of the lock tube;

straightening and putting back into position the mounting of the lock limit warning switch;

replacement of the lock limit warning switch damaged during the crash, by a new switch of the same type.

After re-assembly of the parts, the following observations were made:

The striker P/N 7797-3 located at the end of the lock tube had ten shims, ie:

1 S4im PlN ADA 7773-1
8 shims P7N ADA 7773-50l
1 shim Pft4 ADA 7773-503
with a total thickness of 15.9 mm.

When the lock tube was pushed towards the locked position, the switch switched off the flight deck warning light, although the ends of the lock pins were still 3 millimetres away from the restraining flanges (fig 3). It should be noted, moreover, that according to the Maintenance Manual, the ends of the lock pins in the unlocked position must not be more than 2 mm away from the flanges.

In conclusion, the adjustment of the lock limit warning switch was defective and caused the flight deck warning light to go out when the latches were not necessary dosed.

(b) Adjustment of the lock tube

The two rods of adjustable 1ength by means of which the extreme positions.of the lock tube can be varied were straightened and their adjustments measured (the lockwire locking the adjustment nuts had remained in position).

The adjustments were as follows:

link P/N ADA 7366: distance between centres = 302.79 mm push rod P/N ADA 7372: distance between centres: 914.38 mm

These adjustments were then made to the corresponding rods on a door of the same type loaned by McDonnel Douglas, from which the support plate specified by SB 52-37 had been removed, thus making it conform to the definition of the door with which TC-JAV was equipped.

Tests on u door with the same definition as that of TCJAV

The results of examination of this door adjusted as describe above in accordance with the measurements made on the wreckage were as follows:

(1) Latches dosed, locking handle dosed

The four lock pins were engaged behind the restraining flanges and prevented the opening of the latches, but they were only partly engaged. The ends of the 1ock pins were 1.6 mm short of the rear face of the flanges (fig 4).

The official adjustment documentation – Maintenance Manual, Revision 4, January 1973 – stipulates that the ends of the lock pins, in the locked position, must protrude for
6.35 mm beyond the rear face of the flanges (fig 5).

In consequence, with this adjustment the lock tube in its locked position was 6.35 + 1.6 = 7.95 mm short of the correct locked position.

(2) Latches open

When the latches were open, the movement of the handle towards the closed position WR8 stopped when the lock pins came up against the front faces of the flanges.

Tests carried out on the same door, with varying adjustments of the extreme positions of the lock pins, showed that the force which has to be applied to the handle in order to force its closure depends on the extreme position (locked) to which the lock tube is adjusted.

When this adjustment is in accordance with the manufacture a requirement, ie when the ends of the lock pins protrude for 6.35 mm beyond the rear faces of the flanjges, it is physically impossible to force the handle even in the absence of the support plate for the vent door shaft (SB 52-3?).

On the other hand, when this distance of 6.35 mm is decreased, the force required fer forced closure also decreases. It becomes theoretically rdl when the end of the lock pin is in line with the front face of the flange.

During the tests carried out (with the lock tube adjustment 7.95 mm short of the correct position) the handle could therefore be closed (and the vent door apparently closed) with a force of 22 daN (about 50 1bs) (fig 7),

This closure was possible only because of deformation of the mechanism providing control transmission for the operation of the lock tube. The principal deformation affected the vent door shaft. The additional support plate specified by gR 52-37 (fig 6) was designed precisely to prevent 'such deformation.

It should be noted that the partial engagement of the lock pins as reproduced on the new door corresponds to the friction marks which they made on the edges of the es of the door of TC-JAV, which occurred only on the leading half of the edges (fig 8).


The new door

Because of the defective adjustment of the extreme positions of the lock tube, the handle could be closed without excessive force, although the 3atches were not completely closed.

Defective closure could not be detected from the external appearance of the handle, vent
door and cargo door, unless a visual inspection was made through the view port provided
for that purpose.

Door of TC-JAV

The thickness of the striker shims made any correct adjustment of the lock pins impossible; any attempt to bring the ends of the lock pins to the correct distance of 6.35 mm beyond the rear face of the flanges inevitably led to damage to the lock limit warning switch.
The incorrect adjustment of this switch caused the flight deck warning light to go out when the door was still unlocked.

Because of the insufficient protrusion of the lock pins, the locking handle and vent door could be closed without excessive force, although the lock pins were stopped by the front face of the flanges (fig 7).

From the production and inspection documents received by the National Transportation Safety Board from Dougias, the aircraft manufacturer, it appears that the adjustments of the lock pins on fuselage Ho 29 (TC-JAV) gave a minimum pin protrusion of 0,25 inches (6.35 mm), account being taken of the accomplishment and verification of the work specified in EO ADA 7797 Change A.

These documents merely provide guarantees in accordance with prescribed forms, but the adjustments in question were also those shown in the Maintenance Manual (in particular
Revision 4, January 1973) and had to be verified or applied again by the operat.or when-ever any work was carried out on the door.

In brief, examination of the wreckage of the door of TC-JAV revealed defective adjustment of the lock limit warning switch. In addition, after the rods of a new door of the same definition had been adjusted to the dimensions measured on TC-JAV, it could be judged that the adjustment of the lock pins was likewise incorrect in the locked position.

The adjustment reconstructed in this way is consistent with that of the lock limit warning switch and also with the friction and other marks found an the lock pins and flanges. Finally, it provides an explanation of why the locking handle could be operated without excessive force although the latches were not fully engaged.

3.16.7 Research: accident on 12 June 1972 to DC-10-10, N 103AA near Windsor, (Ontario)

In the course of its work, the Commission of Inquiry studied the report of the National Transportation Safety Board, dated 28 February 1973, relating to the loss in flight of a similar aft cargo door on the left-hand side from an American Airlines DC-10-10.

Although the course of events and some of the causes are not exactly the same, nevertheless that accident presents points in common with the accident to TC-JAV:

The latches were not fully closed and the latch lock pins were not in place.

The flight deck warning light had gone out before effective locking had occurred.

The altitude reached by the American Airlines DC-10 was of the same order as that of TC-JAV when the door opened and the two bolts (connecting the fixed part of the latch actuator to the door structure) failed under the same conditions.

In the absence of pressure relief vents of adequate size between the passenger cabin and the aft cargo compartment, the sudden decompression in the cargo compartment caused damage to the cabin floor and its structure. This damage was less severe than in the case of TC-JAV in which the floor was more heavily loaded, but the functioning of the control cables was impaired in various ways, although it did not become completely impossible to control the aircraft.

In the conclusions of the NTSB report, it is stated that the probable cause of the accident
was the incorrect engagement of the latching mechanism and the design characteristics of
the system which permitted the door to be apparently dosed when the latches were not
fully engaged and the lock pins were not in place. Two recommendations had been issued
by the investigators:

modification to the locking system to make it physically impossible to position the external locking handle and vent door to their normal door-locked positions unless the lock pins are fully engaged;

the installation of pressure relief vents between the cabin and the aft cargo compartment to minimize the pressure loading on the cabin flooring in the event of sudden depressurization of the cargo compartment.

The first of these recommendations had given rise to the modifications specified in Alert Service Bulletin 52-35 and in Service Bulletins 52-27 and 52-37 (in the case of 52-37, only a start had been made with its application to TC-JAV).

Other modifications of the door closing system and methods of mitigating the effects of sudden depressurization of the aft cargo compartment were still under study at the time of the accident to TC-JAV.

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