BBB,
Thanks a lot for your input. Very good to have differing opinions here. We need to consider all things.
[scenario with one TL in reverse, brake pedal inputs, but not wanting to stop ...]
Originally Posted by TripleBravo
Yes, there is.
1) If you can have a TL inadvertently in CLB position at touchdown, you can also have a TL inadvertently in REV during other flight phases, where this is not desired (e. g. climb or go around).
Yes it had crossed my mind, that, although the scenario would be considered extremely unlikely, so was the scenario we seem to have in this accident. Yet it did occur.
2) Usage of rudders can imply inadvertant activation of brake sensors by deflection of the pedals with the toes.
I was aware of this one. I'm curious about it. If the FDR graphs really show degrees of brake pedal deflection, it has a stop at 80 degrees, which seems a lot. I don't think that inadvertent brake-pedal inputs would be anywhere near that range, even in flight phases where larger rudder inputs may be needed, e. g. cross-wind go-around with turbulence and asymmetric thrust, etc ...
So to make additions to GS logic that will actually be useful, we have to be very careful not to make things worse instead.
Data mining through quick access recorder data will be useful, to see what maximum magnitude "parasitic" brake inputs while using the rudder can have. Try to find out if there might be bigger inputs still for reasons yet unidentified (slippery pedals, inappropriate footwear, ...), and add a sufficient safety margin. And see if that is still well below the normal brake pedal input used for braking during landing and rejected take-off.
Am I right in assuming that the desired mode of operation is to move the rudder bar with the heels, and the brake pedals with the toes?
=> This can lead to the scenario that there was something wrong with your TL (being in REV while still on your approach path), you want to abort and sort this out while airborne, because of asymmetric thrust you have to use rudder - and you're almost automatically falling from the sky, because the spoilers are deploying, because your toes applied a bit too much of a pressure.
When somebody argues this scenario to be not likely - so was leaving the TL at CLB position during landing roll!
And this scenario is indeed very rare, and would probably have been considered "unthinkable" by most type-rated pilots prior to the well-known occurences.
Absolutely. What's more, it makes it even harder for the crew to understand in complex situations and therefore reducing the chance of successful troubleshooting in time.
Worth considering. But any addition here is to
reduce the need by the pilot to analyse the logic, and rather to offer him one more way out of a pinch which he will resort to
without thinking:
firm manual braking.
Without needing to know that this is one of the possible GS deployment conditions. The thrust lever position, in an emergency, may not be the obvious way to look for to stop the aircraft (although maybe it should).
Brakes are.
Once again, as we saw in this thread, the non-deployed ground spoilers did not have such a big effect on the landing roll distance that it would have turned things much. Therefore I am not sure if it helps a lot when we consider the spoiler's logic to be changed. I would rather concentrate on what might have caused the crew to leave the thrust lever where it was. Wordings in paperwork like MEL and training awareness seems to be a good starting point as well as tactible feedback to the crew like moving levers (thrust or spoilers).
We witness that brakes even without ground spoilers did reach about 1.3m/s^2, And since GS have a three-fold effect ---(1) increasing normal force, thus friction force, thus deceleration, (2) avoiding hydroplaning, thus increasing friction coefficient, thus friction force, thus deceleration, and (3) increasing aerodynamic drag, thus deceleration, especially at high speeds--- its effect may have been considerable, and while not preventing the overrun, may have reduced it to a much slower one. We'll never know exactly.
But granted, training awareness is paramount.Early onset of manual braking combined with ground spoilers would have done the trick. I'm not sure the MEL wording was a problem, but it has to be chosen carefully. And MEL procedures should be subject to the same rigorous risk analysis as other procedures.
For sure the most effective spoilers cannot override an engine running at some 75% thrust (which was more than the approach thrust, by the way).
Is 1.19 EPR indeed 75% thrust? (i. e. 75% force? And what is 100%? CLIMB or TOGA?)
But you are right, and I stand corrected, it is significantly more than the thrust during final approach, which was between 1.05 and 1.1 EPR.
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Originally Posted by Lemurian
The slow spool-up in cruise /speed has to do with the AI speed stability algorythm which prioritises fuel consumption / pax comfort over instantaneous speed-keeping accuracy. B has another philosophy.
If I recall correctly, A320 also has "Soft altitude" mode, allowing deviations from the flight level by up to 50ft, in order to reduce thrust changes and save fuel.
During approach, thrust variations are quicker, in "approach autothrust".
Bernd