Aviation Investigation Report A98H0003
1.6.21 Landing Performance
Landing distances at various aircraft weights were calculated to determine whether the occurrence aircraft could have stopped safely on Runway 06 at the Halifax International Airport. Calculations were completed for the occurrence aircraft with all systems operating normally, and with certain technical malfunctions.
The horizontal distance necessary to land an aircraft and come to a stop on a level, smooth, dry, hard-surfaced runway is called the landing distance. This distance is based on the aircraft being in the landing configuration on a stabilized landing approach at a height of 50 feet (15 m) above the landing surface (usually the runway threshold). For normal operations at destination and alternate airports, regulations require that this full stop landing be accomplished within 60 per cent of the available runway length, with spoilers, and anti-skid operative, but without use of thrust reversers.
The Swissair MD-11 AOM contains landing graphs that flight crew can use to calculate anticipated landing distances. These graphs provide landing information for 35-degree and 50-degree flap settings, predicated on aircraft landing weight, airport elevation, wind component, and runway surface conditions. For unscheduled landings, the regulations do not require any operational reserve or safety margin as would be included when calculating the runway length for normal operations (1.67 multiplied by the landing distance).
The atmospheric conditions that existed at the time of the occurrence for a landing on Runway 06 at the Halifax International Airport were taken into account. For situations where all aircraft systems are operating normally, the calculated landing distances for various weights are shown in Table 10.
|Aircraft Weight||Flaps 35 Degrees Landing||Flaps 50 Degrees Landing|
|199 580 kg||4 725 ft.||4 236 ft.|
|218 400 kg||5 118 ft.||4 725 ft.|
|230 000 kg||5 316 ft.||4 920 ft.|
If certain technical malfunctions occur, additional horizontal stopping distance will be used by the aircraft; therefore, a correction factor would need to be applied to estimate these increased landing distances. The Swissair AOM lists correction factors that must be added to the landing distance for various possible malfunctions. As indicated in Section 126.96.36.199, the wreckage revealed that the slats were retracted; if the pilots were aware of this anomaly, they would be required to land the aircraft with 28 degrees of flap, which is the certified landing configuration with slats retracted. Also, fire damage to the upper avionics CB panel resulted in several systems failures being recorded before the flight recorders stopped. The ground sensing CB is located in the area adjacent to the systems that were recorded as faults. If the ground sensing circuit was compromised because of the fire, the aircraft, once on the runway, would not have auto ground spoilers or the brake anti-skid feature. These additional factors would need to be added to the calculated landing distance. The minimum landing distance the SR 111 aircraft would have required under conditions of no slats, inoperative spoilers, and anti-skid brakes is shown in Table 11.
If the flight crew were unable to select 28 degrees of flap and landed with 15 degrees of flap, the landing distances would increase by approximately 12 per cent, as shown in Table 11. If the flight crew were able to get the flaps to 50 degrees and decided to conduct a landing in this unconventional configuration, then the above landing distances would be reduced by approximately 10 per cent.
Table 11: Estimated Landing Distance With Technical Malfunctions
|Aircraft Weight||Flaps 15 Degrees,
Auto Ground Spoilers
|Flaps 28 Degrees,
Auto Ground Spoilers
|Flaps 50 Degrees,
Auto Ground Spoilers
|199 580 kg||10 700 ft.||9 600 ft.||8 700 ft.|
|218 400 kg||11 800 ft.||10 600 ft.||9 500 ft.|
|230 000 kg||12 400 ft.||11 100 ft.||10 000 ft.|
A caution in the AOM landing graphs states that for every 5 knots above the ideal approach speed, the landing distance will increase by 1 000 feet. The SR 111 flight crew was dealing with smoke and fire in the cockpit and failed aircraft systems and displays, and at some point was flying on standby instruments. Therefore, it is likely that the aircraft would not have been at the ideal position and speed for landing over the threshold, which could further increase the landing stopping distance. Thrust reversers, if available, would reduce this distance slightly.
Considering all of the factors, the SR 111 landing would likely have required more runway than the 8 800 feet available on Runway 06 at the Halifax International Airport.
 The remaining 40 per cent of runway length is known as operational reserve or safety margin.
 The certified procedure for landing if slats are retracted is to land with flaps at 28 degrees. The landing distance graphs do not provide correction factors with slats retracted for flap settings of 15 degrees or 50 degrees; therefore, these calculations are approximations and are provided for illustration only.
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