Aviation Investigation Report A98H0003

1.16.6  Theoretical Emergency Descent Calculations

  1. - General
  2. - Calculations  General

Theoretical performance calculations, based on manufacturer's performance charts, were completed to assess the capability of the MD-11 in achieving an emergency landing at the Halifax airport in the minimum time possible, from FL330. The objective was to determine the point along the SR 111 route of flight at which the aircraft would have had to start an emergency descent that would result in the earliest possible landing time on Runway 06 in Halifax. The calculations were based on actual wind and other environmental factors, actual SR 111 weight and balance figures, and performance data for a fully serviceable aircraft.

When completing the theoretical calculations, flight crew decision making was not taken into account nor were the various systems-related aircraft unserviceabilities or the deteriorating cockpit environment.

The results of these calculations provide a benchmark from which to consider the limited initial cues available to the pilots and the actual decreasing flyability of the aircraft in the last minutes of its flight vis-à-vis the minimum possible time necessary to fly to the Halifax International Airport and complete a safe landing under ideal theoretical conditions.  Calculations

To determine the earliest possible potential landing time, engineering simulator data, in combination with FDR-derived actual winds, was used to calculate the aircraft's ground speed during the emergency descent. The ground speed was then mathematically integrated to derive displacement or distance travelled over time. The calculation profile assumed direct tracking to the Halifax Golf beacon from the point of the initiation of the emergency descent, followed by a straight-in segment from the beacon to the threshold of Runway 06.

The calculations identified one point along the flight path where the distance[77] from the aircraft to the Golf beacon was equivalent to the distance travelled[78] along the optimal emergency descent profile. This point coincided with a time of 0114:18. A descent initiated at that time would have required a track of 044 degrees True to the Golf beacon, and would have covered a calculated distance of approximately 62 nm. The derived winds indicated a significant tailwind component for the initial seven minutes of the emergency descent, followed by light headwinds for the remainder of this particular descent and approach.

To examine the potential for SR 111 to be able to land successfully at Halifax airport if an aggressive emergency diversion[79] had been started at 0114:18, the known significant aircraft systems-related events were transposed onto the theoretical emergency descent profile.

If the aircraft had followed the theoretical emergency descent profile, the first systems failure event apparent to the pilots—the disconnect of the autopilot at 0124:09—would have occurred on the landing approach in the vicinity of the Golf beacon, approximately 5 nm from the threshold of Runway 06. The aircraft would have subsequently experienced progressive systems failures on the approach. When the flight recorders stopped at 0125:41, the aircraft would have been at approximately 700 feet above the runway threshold elevation. The earliest estimated threshold crossing time was 0126:17, which would have been 1 minute, 35 seconds, after the pilots had declared an emergency. Approximately 35 more seconds would be required to land and stop the aircraft; therefore, the completion of the landing would have been at approximately 0127.

In reality, the crew were unaware of the existence of an on-board fire and assessed the source of smoke and fumes as the air conditioning system; the Air Conditioning Smoke Checklist does not call for landing the aircraft immediately. At 0119:50, the aircraft was 30 nm from the threshold of Runway 06, descending at about 3 300 feet per minute (fpm) through FL210, at an airspeed of 320 knots indicated airspeed (KIAS). The pilots indicated to ATC that they needed more than 30 nm.

Completion of the theoretical descent performance calculations enabled the investigation team to assess the pilots' decision that more than 30 nm was needed to complete a landing in Halifax. For comparison, if the crew had continued to the airport on the assigned heading and began an emergency descent profile from 30 nm, they would have intercepted the final approach track at an estimated 15 nm from the threshold while approaching an altitude of approximately 10 000 feet with an airspeed of 355 KIAS. The theoretical descent performance calculations, from a starting time of 0114:18 would have put the aircraft in the same position at approximately 4 000 feet and 200 KIAS. The difference in altitude and speeds would require the aircraft to lose significant altitude and speed requiring off-track manoeuvring, which can lead to a destabilized approach. Therefore, the calculations support the assessment made by the SR 111 pilots at 0119:50 that the aircraft needed more than 30 nm from a descent performance viewpoint.

[77]    Distance is based on inertial data (latitude and longitude coordinates) recorded by the flight data recorder (FDR) from the aircraft's Inertial Reference System 1.

[78]    Distance travelled along the optimal descent profile was derived from MD-11 engineering flight simulator performance data, and the FDR temperature and wind.

[79]    For the purposes of this document, aggressive emergency diversion describes a scenario in which a flight crew undertakes those actions necessary to divert and land the aircraft in the least possible time, using the optimum descent profile, and not exceeding the aircraft's approved operating limits.

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