Aviation Investigation Report A98H0003

2.19  Remaining Few Minutes Following Stoppage of Recorders

The final 5 minutes and 37 seconds of the flight, from when the flight recorders stopped at 0125:41, were not recorded on the FDR or the CVR. To the extent possible, the events that occurred were reconstructed using information from ground-based primary radar data, full-authority digital electronic control non-volatile memory data, air traffic control (ATC) recordings, witness statements, and wreckage examination.

An analysis of the heat damage observed on the reconstructed cockpit wreckage, together with the likely fire propagation scenario, shows that the fire increased in intensity during the final six minutes of the flight. The amount of smoke, heat, and fire entering the cockpit would have continued to increase.

There are indications that at 0125:50, about eight seconds after the flight recorders stopped, the pilots switched the air data source to air data computer (ADC)-2 from ADC-1 (see Section 1.18.8.26). This was most likely done in an attempt to recover some lost flight instrumentation. The wire examination shows that the left emergency AC bus, which powered ADC-1, experienced an arcing event. The arcing would have caused it to become de-powered, resulting in the loss of ADC-1. When the pilots selected ADC-2, it temporarily restored the transponder Mode C altitude information, which showed the aircraft to be at 9 700 feet. At 0126:04, transponder information stopped being transmitted from the aircraft for the remainder of the flight. ATC radar equipment continued to record the aircraft track on primary radar until it disappeared from the radar screen about 10 seconds before the aircraft's impact with the water.

In their second-last transmission to ATC at 0124:53, the pilots reported that they were starting to dump fuel. There are some indications from witness information that they initiated fuel dumping after the recorders stopped. Also, the auxiliary tank isolation valve was found closed, which would be expected if fuel dumping had commenced. The fuel dump valves were closed at the time of impact, indicating that the fuel dumping had been stopped by the pilots.

Before the recorders stopped, the pilots indicated that they needed to land the aircraft without delay. Despite this, the aircraft continued on its southbound track away from the airport and out toward the ocean. This suggests that the condition in the cockpit quickly deteriorated to a point where the pilots were unable to effectively navigate the aircraft. They would likely have lost most of their electronic navigation capability, and the increasing amount of smoke entering the cockpit would have made it progressively difficult to see out the windscreens to navigate visually, especially in unfamiliar territory at night and with cloud layers in the vicinity.

The aircraft continued to descend in a right turn as it passed over the community of Blandford, Nova Scotia. Witnesses on the ground described hearing a noise having a repetitive beat frequency and that was generated at a constant rate and superimposed on the loud engine sound. Engine 2 was being shut down at about this time; however, no explanation for this "repetitive beat" noise could be established. People also described seeing various aircraft lights, indicating that at least some of the aircraft electrical systems were still powered. This was confirmed by the examination of various systems such as fuel pumps and fans, whose rotating components showed signs of being powered at the time of impact. Examination of components indicated that all three generator buses were being powered at the time of impact.

Shortly after passing the Nova Scotia coastline, the aircraft started a right turn. Although indications are that the captain's clearview window was likely unlocked, it is unknown when or whether the window was ever opened. At some point, the pilots selected the flaps to the pre-selected 15-degree DIAL-A-FLAP setting. When they shut down Engine 2 at about 1 800 feet, approximately one minute before the time of impact, the airspeed was about 227 knots true airspeed. The average rate of descent just prior to this time was estimated to be about 2 000 feet per minute.

The reason for the Engine 2 shutdown prior to the time of impact is unknown. One possible explanation is that the crew received a false fire warning indication. A short-circuiting of the ground wire in the Firex Handle 2 could cause both the Firex Handle 2 lights and Engine 2 fuel switch light to come on. The ground wire was not identified; however, the ground wire was installed in an area of high heat and fire damage. One of the cockpit emergency checklist booklets was found to have some minor heat distress on the page describing the "ENGINE - FIRE" procedure; however, it is unknown whether the checklist was being used during the engine shutdown. Closing the FUEL switch is part of this checklist procedure.

The passenger cabin environment would have been significantly less hostile than the cockpit environment. Although soot was noted in the attic area aft of the main fire area, there were no signs of appreciable heat in the attic aft of the first-class seats. The ceiling panels used in the passenger cabin have a significant resistance to fire and heat penetration and would have protected the passenger cabin from the effects of the fire. Some smoke would probably have been entering the passenger cabin during the last few minutes of the flight, especially at the front of the cabin.

It is unknown whether any firefighting took place using the available fire extinguishers. Based on examination, it was determined that neither of the two portable 5 lb dry chemical extinguishers mounted in the cabin were likely used. Three of the six 2.5 lb Halon portable extinguishers were not likely used; however, the charge state of the three remaining Halon extinguishers could not be determined because of the physical damage to the extinguishers. If firefighting took place, it would be expected that the M/C would have been involved; the M/C was seated at the time of impact with his seat belt fastened.

One of the passengers, who was a pilot, was wearing a life vest at the time of impact. There were no other indications that anyone else had donned a life vest, although no definitive conclusions were possible in most cases. It is unknown whether this passenger had donned his life vest by instruction or through his own initiative. If he had donned the life vest on his own, he must have been able to discern or surmise that the aircraft was over water and was in danger of ditching. If it was a result of a crew instruction, then there must have been a plan to ditch the aircraft. It would then be expected that the M/C would have been wearing his life vest, which he was not; this suggests there was no instruction to prepare for ditching.

In the last minutes, other than possibly having an electronically generated heading on DU 2, the pilots would have had no electronic means of navigating to the airport, and would have been forced to consider alternatives, such as attempting a crash landing on land or ditching into the ocean. From the wreckage examination, it is known that the fire in the cockpit created heat damage signatures of 482°C to 538°C (900°F to 1 000°F) on the forward portion of the avionics CB panel and on the air diffuser structure just above the cockpit ceiling. There was evidence that melted material had dropped down on the carpet and on the right observer's seat cover. The fire was encroaching on the pilot seat positions from the rear of the cockpit. The heavy soot deposits, and the heat-damaged condition of some of the cockpit materials, indicate that visibility would have been significantly obscured within the cockpit. It could not be determined whether the cockpit fire extinguisher had been used.

The first officer's seat was occupied at the time of impact; the captain's seat was in the egress position. Although the standby attitude display showed the aircraft to be at 20 degrees nose down, and 110 degrees right bank at the time of impact, it could not be determined whether these indications represented the actual aircraft attitude at the time of impact. The structural damage supports a nose-down attitude of about 20 degrees, and a right bank in excess of 60 degrees. If the pilots were not incapacitated and were still attempting to control the aircraft, this suggests that in the last minute of the flight they lost orientation with the horizon. This would not be unexpected, given the lack of reference instrumentation, and lack of visual cues from outside the aircraft. Regardless of whether there was pilot input at the time of impact, the aircraft was not in controlled flight.

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Date modified :
2012-07-27