In addition to BOAC's input, it is possible that the changes, particularly the combination and/or timing, were beyond the capability of the autoflight system – a design limit, which requires pilot intervention.
Autopilots have limited capability when adapting to rapidly evolving situations. Thus, when demanding a descent, the pitch channel will lower the nose and there is a tending to increase speed. Auto trim may subsequently run nose-down for the speed increase.
If the autothrottle is not well matched to the pitch command, then the thrust reduction (due increasing speed) is slightly delayed and when it occurs, may add to the nose-down pitching moment (force), which together with the new trimmed condition (also nose down force) may exceed the opposing auto pitch control (nose-up force), due to lack of autopilot ‘muscle’.
Thus, speed will increase until the nose-up control and delayed auto trim input (always delayed to prevent oscillatory motion) takes effect. During the intervening time, it is possible to exceed Mmo, particularly when cruising at high speed.
The effect may be more noticeable in VNAV if that mode uses a ‘soft ride’ feature – reduced autopilot ‘muscle’ and/or rate of control application, and also if the cg is nose-heavy and/or the airbrake causes an adverse pitching moimnet - often a dominant feature (cf Avro RJ operators).
The situation has to be understood as a limit in the capability of automation, which pilots are expect to know, detect, and correct, but which they might only learn from by encountering the situation – experience!
Thus the incident may have been a limit of expertise.
Blame should not be allocated; the situation is an opportunity to improve understanding. Thus, both the crew and the management should have learnt something – the crew about aircraft / autoflight limits; management about the need for training, and the need to gain experience – often via small errors.
We all suffer error, many occurrences during each flight. Generally, they are inconsequential because the source of the error is detected (understood) or the resultant, adjusted in time to avoid a consequence.
We should only worry about the big errors; most of those originate from management.
In this instance, it appears that the management error is the lost opportunity to learn – about investigating incidents, about aircraft systems, about human factors, and the concept of a no-blame culture.
P.S. If you ever consider the need to ‘override’ the autopilot, always disengage the system and fly manually. Manual flight provides a quicker and more precise feel for the situation; particularly for aircraft trim.
Some aircraft may have nasty surprises when attempting to mix auto/manual flight – the trim may run against you, and this can make the situation very much worse. The aircraft / trim muscle may equal your muscle!
Last edited by safetypee; 7th Mar 2010 at 20:05.