Originally Posted by John Farley
My guess is that they ignore it because the precise effect would be too difficult to quantify due to the many variables that would be involved between different landings. So therefore they say if you can’t actually quantify the benefit it has no place in the manual.
Anybody know?
Any civil testers out there care to comment?
John, the answer is partly in your guess. It is very difficult to quantify; manufacturers may even have different or even opposing techniques amongst their aircraft types. It is not just about improving the published stopping distance, which in normal (regulatory approved) operations should have sufficient safety margin, but also about many other aspects of control on the runway, which may not have such generous margins, e.g. crosswind.
Civil testers? Well at least two have commented.
I have sat in many meetings similar to those related by
MFS; in addition to manufacturers asking ‘why did they do that’, chief pilots should consider ‘could their pilots do that?’. Many people cannot contemplate the range of situations they could encounter or put themselves in, or the behaviors that pilots might (or in fact do) exhibit. Even when individuals consider ‘could it ever happen to me?’ a realistic answer may only be available in hindsight through analysis and understanding, only then is the error provoking situation or the personal behavior seen to be hazardous.
Part of this discussion is about discipline, the need to follow procedures and avoiding hazardous attitudes such as ‘I can do’. Similarly, there is the need to resist peer pressure, which is not aided by well intentioned suggestions that are promoted beyond responsible boundaries.
Discipline is the foundation of airmanship; this thread also relates to other qualities such as skill, knowledge, situation awareness, and judgment.
In seeking an end to runway overruns, pilots’ need thinking skills in addition to those of flying, they need greater knowledge of the regulatory assumptions to identify those situations where the margins in landing distance are significantly degraded, or the unreliability of braking coefficient values that could lead to misunderstanding the situation. These aspects have been discussed in related threads.
An overview of accident reports identifies two fundamental causal themes, either the crew did not understand the situation, of if they did, then they chose the wrong course of action. If pilots have to consider alternative techniques on the runway then they have failed in the first instance, misjudging the situation, the need for a go around, as well as not applying their skills to land at the correct speed or position; if they had been successful in these aspects then there should be no doubt about stopping. However, having made such mistakes and arrived on the runway, a back stick technique is not guaranteed to recoup the hazardous situation and may make it worse.
A quick and un-scientific assessment of recent accidents (before the facts are known) suggest that a small increase in braking effectiveness at high speed would be unlikely to have prevented the result; it might have alleviated some of the consequences, but equally it could have resulted a lateral deviation into far greater hazards (N.B. Toronto wind shift and off-runway hazards).
Of the 5 or so overrun accident investigations that I was associated with, none would have been prevented by a small increase in braking effectiveness. The majority had root causes involving human behavior in the air, the others, human error on the ground, which involved perception and incorrect use of retardation devices.
Thus, we already have large variability in human behavior before anyone adds more from aircraft control techniques.
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