• DECEL or ‘decel’:
If the call was based on auto-brake DECEL light it suggests that auto-brake was engaged and that the deceleration was at least 80% of what ever was selected. There are too many unknowns here to be of use, other than the assumption that at the time of the call the system was working.
Alternatively, if the SOP was to call ‘decel’ based on the PFD speed trend, then the conclusion is that aircraft was decelerating. However, without knowing what value the SOP uses, when the SOP is used (manual braking only), or the specifics of the PFD speed trend – IAS or IAS inertial mix (probably not pure deceleration in a GS sense – N.B. tailwind), then again, no great value in understanding what is happening.
• Re manual braking if auto-brake is perceived to have failed:
How is auto brake failure deduced? Is manual braking used when there is no DECEL lamp / call, if so what level of braking is advised?
If brake failure / ineffectiveness is expected to be judged by human senses, then both auto- and manual braking depend on ‘seat–of-the-pants’ assessment (inertial deceleration); manual has the additional cue from foot-force (position) feedback of the demanded level of braking.
Anyone use a brake pressure gauge, if so what does it show?
• Reduced (failed) braking:
I disagree with the assertion that “The kind of deceleration we are talking about here will be felt, …” (#296).
If the runway condition involved low friction, then the majority of the deceleration would come from reverse. After cancelling reverse, the relative ineffectiveness of the brakes could be perceived as a failure. The decel level from braking is less ‘negative’ than reverse, thus the change of body sensed acceleration is in a positive sense, which could be deduced to be a system failure opposed to a human attribute.
Problems in this area stem from training / experience, where IMHO crews are not familiar with maximum manual braking on a wet runway; auto-brake removes an important cue, and with everyday use of reverse, it masks the brake contribution to the total deceleration. (see ref – Boeing briefing)
Full manual braking on a moderately wet runway can result in an uneasy feeling of not stopping where you expect. Furthermore many pilots have misgivings about the use of maximum braking with fears of tyre burst etc – there should be none. Braking systems are certificated for maximum use at any time; if pax comfort is quoted then the safety priorities are incorrect.
The only system failure in the above of the scenario is that of human perception – a known weakness, which can be exacerbated by poor training, inadequate SOPs, and low experience. There is no evidence of the latter contributions, but the human factor is always present.
Ref:
Flt Ops / Flying Technique … Landing on Slippery Runways.