Aviation Investigation Report A98H0003

2.20.4  Theoretical Emergency Descent Calculations

By coincidence, the time at which an emergency descent would have needed to begin to achieve the optimum theoretical emergency descent profile to land at the Halifax International Airport coincided with the actual time of the Pan Pan radio transmission. Any delay in descending would mean that the aircraft would be above the ideal descent profile. During the Pan Pan transmission, the captain requested a diversion and suggested Boston. It was not until about 1 minute and 25 seconds later that the following events were completed: the controller offered Halifax as an alternative diversion airport, the pilots evaluated and accepted Halifax, and the pilots commenced a non-emergency but rapid descent.

During that time, the aircraft was travelling in the general direction of the Halifax International Airport at a ground speed of more than 8 nautical miles (nm) per minute. From the actual descent start point, it would not have been possible for the pilots to position the aircraft for a landing on Runway 06, without some form of off-track manoeuvre to lose altitude and slow to the appropriate speed. In a best-case scenario, the extra manoeuvring would have added two or three minutes to the landing time. More likely, a manoeuvre such as a 360-degree turn would have been necessary, or they would have had to switch to a different runway. Either choice would have added several minutes to the earliest possible landing time, and the effects of the fire would have negated the possibility of completing a safe landing.

At about 0125, when the fire condition became distinctly evident in the cockpit, the aircraft was about 25 nm from the airport, at an altitude of about 10 000 feet, and at an airspeed of about 320 knots. It was flying in a southerly direction, away from the airport. In optimum circumstances, from that point it would have taken a minimum of about six minutes to get to the runway.

Theoretical calculations confirm that from any point along the actual flight path after the aircraft started to descend, it would not have been possible for the pilots to continue maintaining control of the aircraft for the amount of time necessary to reach the airport and complete a landing.

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