IMHO more frequent use of manual braking would add to safety, particularly in reducing the risk of an overrun. The ‘training’ problem appears to reside with the level of knowledge about the auto-brake systems, and the wider ranging aspects of their use and effect on landing performance.
Re: “Autobrakes provide a known rate of acceleration, which means that it can be used to calculate stopping distance. …” (# 2)
This is only true for a very narrow range of conditions.
The auto-brake deceleration should give a consistent
ground distance for the landing weight provided that the touchdown is made at the ‘correct’ speed, on a defined surface (composition and texture), and with known frictional qualities.
The actual landing distance involves an additional airborne distance and a transitional distance for the time taken to deploy spoilers and activate brakes – the total ‘stopping distance’.
For certified performance these additional distances are included in the measured performance landings.
For ‘advisory’ data (unfactored), the total landing distance may assume a fixed airborne distance from the threshold to touch down and a fixed transition time at touchdown speed; it may also assume the use of reverse thrust whereas certification might not.
In normal operation, landings may not match the assumed airborne distance, spoiler/ brake application timing, or achieve the correct touchdown speed (Vref/wind errors). These contribute significant errors in landing distance (ref
AC 91-79)
If the runway frictional qualities are not as assumed due to a different runway texture or water depth, then the ground distance may vary. The auto-brake brake will apply up to the maximum antiskid brake effort in attempting to achieve the required deceleration, beyond that more distance is required.
In limiting conditions the increased distance may significantly reduce the landing safety margin provided by the LDR, thus there is an increased risk of overrun.
Some thrust reverse use is normal during landing. Any deceleration provided by reverse enables the auto-brake to reduce the brake effort while still meeting the required deceleration; operators who buy brake time and use reverse may have a poor deal – are you paying for brakes that you don’t use.
Thus reverse might be considered to add a safety margin, minimizing the risk of overrun. However, this could be a false argument where the additional safety margin from reverse becomes a norm for operations and crews may become dependent on the ‘expected’ availability of reverse i.e. they use of lower than required auto-brake settings because previous landings (with reverse) ‘were OK’.
The use of higher power reverse with auto-brake could mask occurrence of some risky landings due to errors in the factors above.
This suggests that where landing conditions are good, a long non limiting runway, and reverse is to be used, then auto-brake provides few operational or technical benefits - except providing a smooth feel due to the constant deceleration. If pilots are equally capable, then why not let them use the brakes and add to their experience; brake for safety, not for comfort.
During a manually braked landings, pilots can relate to the deceleration from the applied brake effort via foot force (pedal deflection). During auto-brake operations pilots do not have this feedback which is an important cue aiding ability in judging the actual stopping distance in relationship to the runway conditions and speed at touchdown – opportunity for more experience.
This again is a safety argument for manual braked landings in suitable conditions.
In adverse conditions or on limiting runways (and RTOs) there is a more balanced safety case for using auto-brake at the higher settings. However, without manual braking experience, the problems of ‘masking’ the applied brake level from the crew might appear ‘surprising’ when cancelling reverse in marginal operations.
Furthermore, some auto-brake/landing SOPs might unintentionally rely on reverse to provide a consistent safe landing distance; if so, then in conditions where certificated performance might assume reverse use (EU/JAA contaminated perf) then there could be some nasty surprises.
Re: “… brakes coming on at the moment of touch down …”
Not in some auto-brake systems, which at lower settings delay or ramp-up the brake pressure to give priority to thrust reverse deceleration at high speed.
See:
Chap 8.4, page 2.
Re: “...Autobrake leads to less wear on carbon brakes...”
Is this fact or myth? Decelerating to a stop requires the dissipation of energy – the same level from identical touchdown speeds, same wt, etc; the variable is the rate of dissipation of energy. If the manual braking level is the same as the auto-brake, then brake temps/wear should be equal.
Perhaps the inequality / asymmetric braking problems originate from the lack of manual practice.