Aviation Investigation Report A98H0003
2.18 Known Technical Failure Events
The first indication to the pilots of a systems-related failure was the disconnect of the autopilot at 0124:09 (see Section 18.104.22.168). Twenty-four seconds prior to this, at 0123:45, the captain had selected the CABIN BUS switch to the OFF position. This selection is the first action item in the Swissair Smoke/Fumes of Unknown Origin Checklist. Until that point, it appears that the conditions in the cockpit were such that the pilots perceived that they were dealing with smoke from an air conditioning source.
The airflow testing showed that with the recirculation fans off, as would be the case after the CABIN BUS switch was selected to the OFF position, the predominant airflow in the forward attic area reverses direction so that instead of flowing aft toward the fans, much of the air flows forward into the cockpit attic area, and then down through the cockpit into the avionics compartment below the cockpit.
Between the time the captain selected the CABIN BUS switch to the OFF position at 0123:45 and when the flight recorders stopped recording at 0125:41, the fire-related effects in the cockpit began. This is confirmed by the rapid succession of systems-related failures. The environmental conditions in the cockpit also began to deteriorate rapidly, with an increasing amount of smoke, heat, and fire entering from overhead.
The systems failures up to that point, would have reduced the ability of the pilots to control and navigate the aircraft, especially at night, with smoke in the cockpit, and in instrument meteorological conditions. The loss of the autopilot would have added to the pilots' workload, and the associated warbler warning tone that sounded until the end of the CVR recording would have been disconcerting. The master caution light would have illuminated with the loss of flight control computer 1, Channel A at 0124:57; it is unknown whether the pilots reset the master caution light. The loss of the left emergency AC bus at 0125:06 would, in part, have caused the loss of the captain's display units (DU) 1 and 3. DU 2 would show a red X, and the master caution light would illuminate. Again, it is unknown whether the pilots reset this caution light; however, if they did, the loss of the captain's pitot heat, about 10 seconds later, would have triggered the master caution light again. These failures would have been accompanied by numerous fault messages, cues, and alerts. Dealing with such a barrage of faults and messages would have been confusing, distracting, and difficult to cope with.
The loss of all three of the first officer's DUs at about 0125:30 would have forced him to use the standby instruments to maintain aircraft spatial orientation (see Section 22.214.171.124). The transition to the standby instruments would have been challenging because of their small size and positioning, relative to each other, especially in the deteriorating conditions (increasing smoke and heat) of the cockpit.
At that point, although the captain may have restored all primary flight display information, (such as aircraft attitude, airspeed, heading, and altitude) on DU 2, DUs 1 and 3 had failed and it would have been impossible to restore these two displays. Although all three 115 V AC generator buses were functional at the time of impact, fire damage to distribution buses, wires and cables, and CBs disrupted the electrical power to some, or all (if DU 2 was lost) of the systems that provided primary attitude information, navigation, communications, and various other functions. Consequently, the pilots would have been dealing with a multiplicity of tasks, many of which were highly abnormal, while the cockpit environment was rapidly deteriorating.
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