TNAG-2856-FCO40-4109-Hong-Kong-compensation-claim-regarding-Korean-Air-Lines-Flig-1993 — Page 125

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2.16

2.16.1

The attack phase based on the CVR and the DFDR

As a result of the damage caused by the missile the aircraft initially pitched up and the vertical acceleration increased to approximately 1.2 g over three to five seconds. During this period, the aircraft rolled very slightly right wing down.

2.16.2

The Boeing 747-200 aircraft had four separate hydraulically powered elevator control surfaces. Each outboard elevator was driven by the inboard elevator on the opposite side by means of a cross-over cable. Simulations by the aircraft's manufacturer suggested that the initial pitch up was probably caused by damage to the cross-over cable from the left inboard elevator to the right outboard one that caused the cable to unravel over a few seconds. The failure of the cable would command the right outboard elevator to move up to the maximum angle that it could attain with the force available from its hydraulic actuator.

2.16.3

Although autopilot A remained in the Command mode through this period, there was no control column movement to correct the pitch up. The aircraft's manufacturer concluded that this was probably due to a loss of hydraulic system No. 3 that powered the autopilot A actuator in the elevator control system. Hydraulic system No. 3 also provided half of the power to the hydraulic actuators driving the inboard left and outboard right elevator surfaces. With only half the power operating the latter surface, in the flight condition at which the attack occurred, the maximum available deflection would be about 5o.

2.16.4

Eleven seconds after the CAM recorded the first sounds of the attack, the sound of the cabin altitude warning was heard. Since any initial airflow out of holes in the pressure cabin at high altitude would be choked i.e. with a local speed of sound at the restriction, and as long as that condition persisted, it was possible to estimate the approximate total area of the holes which would result in a decompression and subsequent cabin altitude warning after eleven seconds. An estimate, taking into account the output of the air conditioning packs, indicated holes with a total area in the order of 1.75 sq.ft.

2.16.5

The vertical acceleration remained at approximately 1.2 g for about seventeen seconds with the aircraft entering a climb at 7 000 ft/min. At this time, the autopilot tripped or was selected to OFF and the sound of the autopilot disconnect warning occurred. The control column then moved forward significantly. From the lack of response of the aircraft to this control input, it was concluded that there had probably also been failures of hydraulic systems No. 1 and 2. Such failures would have disabled the right inboard and left outboard elevators. Loss of systems 2 and 3 would have also disabled the yaw damper systems. This could account for the presence of oscillations in the lateral acceleration measurements and the roll attitudes from the start of the attack phase at a frequency close to that of the dutch roll of the undamped aircraft.

2.16.6

With the wing flaps up, lateral control of the Boeing 747 aircraft was achieved with the inboard ailerons and the five outer spoiler segments on each wing. When the only hydraulic power available was system No. 4, control was reduced to the right inboard aileron and the innermost of the spoiler segments on each side. Simulations by the aircraft's manufacturer showed that this reduction could account for most of the lack of aircraft response in roll to the control wheel inputs that were applied.