Aviation Investigation Report A98H0003
1.11.3 Stoppage of Recorders (STI1-58)
Examination of various recovered aircraft system components show that the 115 V AC Generator Bus 3 was powered at the time of impact. On the base portion of the SMOKE ELEC/AIR selector that was recovered, there were indications that the selector was in the NORMAL position at the time of impact.
The CVR and the FDR both stopped because of the loss of electrical power during a 1-second time frame starting at 0125:41, which occurred 5 minutes, 37 seconds, before the aircraft struck the water. Two possibilities were examined to determine why the recorders stopped. The first was that the pilots selected the SMOKE ELEC/AIR selector to the 3/1 OFF position. The second was that a fire-related failure or failures led to the loss of electrical power to both recorders.
Selecting the SMOKE ELEC/AIR selector to the first position (3/1 OFF) would cause the two flight recorders to stop at exactly the same time, as the 115 V AC Generator Bus 3 is taken off-line.
The FDR data indicates that a brief power interruption to the digital flight data acquisition unit (DFDAU) occurred less than two seconds prior to FDR stoppage. This power interruption could not have been a result of selecting the SMOKE ELEC/AIR selector, as this would have resulted in an immediate shut down of the FDR and the CVR. A warm start re-initialization (reboot) of the DFDAU took place following the power interruption. The CVR also showed a discontinuity in recording within two seconds prior to CVR stoppage. These interruptions and discontinuities introduce variability in the relative timing between the two recordings and consequently in the precise relative stop times. It was possible to achieve a degree of time synchronization (less than one second between the CVR and the FDR). On the basis of time synchronization alone, it was not possible to determine whether the recorders stopped as a result of the SMOKE ELEC/AIR selector being selected to the 3/1 OFF position; other information was used to make this determination.
It is known that the pilots started the Smoke/Fumes of Unknown Origin Checklist by selecting the CABIN BUS switch to the OFF position. Prior to making that selection, the captain alerted the first officer about this action and received confirmation from him. The next action item in that checklist was the selection of the SMOKE ELEC/AIR selector. There are several indications that the flight recorders did not stop as a result of the use of the SMOKE ELEC/AIR selector. First, prior to the stoppage of the data recorders, the pilots made no mention of the SMOKE ELEC/AIR selector. Because the captain notified the first officer prior to selecting the CABIN BUS switch to the OFF position, the captain would likely have notified the first officer of his intention to move the SMOKE ELEC/AIR selector, as the first officer was the pilot flying and choosing the selector would have affected systems he was using. In addition, about 9 seconds after the flight recorders stopped, ATC began receiving Mode C (see Section 18.104.22.168) altitude data information from SR 111 for approximately 20 seconds. For this to have occurred, ADC-2, which is powered by the 115 V right emergency AC bus, had to be functioning. This bus would not have been powered if the SMOKE ELEC/AIR selector was in the first (3/1 OFF) position; therefore, it is very likely that this selector was in the NORM position when the recorders stopped recording.
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