Thanks for the info
kwick; are there any comments on runway grooving?
OutOfRunWay, re autobrake “it wouldn’t have made any difference if had been a bus or a boeing”.
I agree, but in you ‘criticism’ of pilot behaviour (pilots should notice these things #96) you may wish to consider the many aspects of human factors and for example, how a pilot’s perception might be effected by daily use of autobrake, particularly those systems which demand deceleration level.
First consider operations without autobrake. With experience pilots develop a relationship between deceleration and brake pedal foot force; this is modulated depending on runway length, surface condition, landing weight, etc. On long dry runways minimal braking is required, but with increasing wetness/contamination and/or reduced runway length more braking is required, requiring higher foot forces.
From this relationship a pilot develops a sense of how the landing deceleration is progressing (judgment); if the deceleration is not as expected more braking is applied; the pilot might conclude that the runway has less friction than expected. In extreme (erroneous) situations maximum braking and full reverse can be used to salvage a deteriorating situation.
With autobrake, pilots might not acquire (or refresh) their sense of braking as there is no relationship between foot force and the applied braking.
When autobrake is used in conjunction with reverse the pilot is further ‘detached’ from the aircraft. It is not possible to determine which of the retarding devices is providing the braking effort. In normal use on a dry or minimally wet runway, the autobrake may only provide a small component of the required deceleration, reverse contributes the larger amount. The sum of decelerations meets the required (selected) autobrake deceleration level. This is described in the first ref link.
Problems can occur with reducing runway friction. The autobrake provides a greater proportion of the braking so that in a limiting case the brakes and reverse only just meet the required level of deceleration. If an inappropriately low autobrake setting is used, a pilot might not detect the limiting situation until it is too late for any corrective action to be effective (max brake / full reverse). Pilots might perceive this condition as a failure in a retarding system, but actually the autobrake is working as set by the crew.
Similarly for an unexpected tailwind or a wet ‘contaminated’ runway (deep water, no grooves or rubber deposits, where the pilot misjudges it as only ‘wet’), the limiting retardation is quickly achieved and there is a significant risk of an overrun.
The ability of the pilot to detect the limiting deceleration requires exposure to such situations (experience); this must be supported by guidance materials (SOPs training), and an appropriate decision when the runway situation is assessed as marginal.
Pilots who only use autobrake require an in-depth understanding of autobrake operation and its interaction with reverse (know – what), and greater exposure to a range of deceleration situations to aid their judgement in the approach and landing decision (know – how).
However, these pilots may not get opportunity to gain experience in limiting situations, particularly as in normal operations we strive to avoid them. Thus the provision and use of guidance and procedures in limiting situations is of heightened importance. This in turn involves situation awareness/judgment training (operator responsibility) and supporting activities from airports and regulators.
Summary:
Manual Brakes, foot force demands deceleration – feed back
– Dry runway: Reversers are additive
– Slippery runway: Reversers are additive
Autobrake, little or no foot force feedback
– Dry runway: Reversers NOT additive
– Slippery runway: Reversers may be additive
Refs:-
Study the Boeing diagrams carefully; many place deceleration level on the x axis, which should not be confused with landing distance – deceleration is ‘more’ or ‘less’ stopping capability.
Flight Ops - 'Landing on Slippery Runways'.
Also see the links
in post # 23.
Avoiding an overrun: what should be trained?
An unrelated video, but useful for training.