Recommendation Date
Recipient Name
Ansett NZ
Text
Review Ansett's practice of setting MDA once established on the approach, with a view to implementing a procedure which will not set the MDA before it is safe to descend to that altitude.
Reply Text
Ansett New Zealand has reviewed the practice of setting MDA (in the ALT SEL) and has concluded that the practice is indeed appropriate.
Given that the ALT SEL cannot be "disconnected" from the Auto Flight Guidance System, it is consequently always "live", and as a result will warn of deviation from, or approach to any set altitude. The system is designed to provide a protection in respect of cleared altitude.
Setting the ALT SEL to any other altitude than MDA (i.e. missed approach altitudes, commencement altitude, or any intermediate altitude within 1,000 feet of setting), would result in continuous warnings that would have to be ignored by the flight crew. Ansett New Zealand considers that any practice which has that effect is itself patently unsafe.
The setting of MDA provides a real warning when approaching MDA, and Ansett New Zealand notes that such a warning would have occurred on the accident flight.
It is noted that during a precision approach the system is automatically disabled to preclude inappropriate warnings.
Further the ALT SEL plays no part in the instrument scan, and neither should it. The system is set to provide a warning approaching the cleared altitude, in this case the MDA.
Instrument approach profiles are not flown by reference to ALT SEL, and neither is MDA referenced from the ALT SEL, in this or in any other airline of which Ansett New Zealand is aware. MDA is referenced by the altimeter and the safe achievement of any MDA, on any kind of approach requires adherence to the published profile for that approach.
Therefore, following its review of the practice (as recommended by this safety recommendation), Ansett New Zealand concluded that to the extent that the recommendation suggests the implementation of a procedure which would not involve the setting of MDA "before it is safe to descend to that altitude" that such suggested procedure would in practice be detrimental to flight safety, with the result that Ansett New Zealand having reviewed the matter, has concluded that it will not adopt any recommendation to implement such a procedure.
Given that the ALT SEL cannot be "disconnected" from the Auto Flight Guidance System, it is consequently always "live", and as a result will warn of deviation from, or approach to any set altitude. The system is designed to provide a protection in respect of cleared altitude.
Setting the ALT SEL to any other altitude than MDA (i.e. missed approach altitudes, commencement altitude, or any intermediate altitude within 1,000 feet of setting), would result in continuous warnings that would have to be ignored by the flight crew. Ansett New Zealand considers that any practice which has that effect is itself patently unsafe.
The setting of MDA provides a real warning when approaching MDA, and Ansett New Zealand notes that such a warning would have occurred on the accident flight.
It is noted that during a precision approach the system is automatically disabled to preclude inappropriate warnings.
Further the ALT SEL plays no part in the instrument scan, and neither should it. The system is set to provide a warning approaching the cleared altitude, in this case the MDA.
Instrument approach profiles are not flown by reference to ALT SEL, and neither is MDA referenced from the ALT SEL, in this or in any other airline of which Ansett New Zealand is aware. MDA is referenced by the altimeter and the safe achievement of any MDA, on any kind of approach requires adherence to the published profile for that approach.
Therefore, following its review of the practice (as recommended by this safety recommendation), Ansett New Zealand concluded that to the extent that the recommendation suggests the implementation of a procedure which would not involve the setting of MDA "before it is safe to descend to that altitude" that such suggested procedure would in practice be detrimental to flight safety, with the result that Ansett New Zealand having reviewed the matter, has concluded that it will not adopt any recommendation to implement such a procedure.
Related Investigation(s)