Thanks Meikleour, for picking up my argument (#94), and carrying it further. I always advised that the only way to select idle from A/THR was to close the thrust levers rapidly, without trying to modulate the reduction in thrust: the FADECs do that for you. It's impracticable to find the angle at which the reducing thrust limit (as you retard the levers) matches the existing thrust when you are eyes-outside in the flare. Accurate modulation of thrust changes by the pilot can only be achieved in manual thrust; and this has to be established much earlier, at a safe height.
Quote from PJ2 (#119):
"Normally, if the ATHR is engaged, the speed will not decrease below Vls but there may be circumstances (quartering, slightly increasing tailwind) which may require a small, momentary increase in thrust and setting the thrust levers above the CL detent is one way - disconnecting and flying manual thrust all the way to touchdown is another."
This is the technique I referred to in my post (#94):
"The difficulty arises in the case of significant airspeed loss just before the flare, before A/THR is disengaged. There is a crude logic which enables the PF to apply a burst of thrust by pushing the levers forward for a very short time (assuming he/she does not elect to go around)."
The question is: why does the pilot need to intervene? The A/THR should already have recognised "speed below target and falling", and have called for extra thrust. If it is a gross speed deficit (say, > 5 kt), the FADECs should be accelerating the engines as fast as possible. But the new target thrust is likely to be well below CLB, and will be reduced as soon as IAS approaches target.
In the procedure PJ2 describes, on the other hand, the pilot is calling for thrust in excess of CLB, albeit momentarily. The trick is: how long is that? The curve of thrust versus time is (roughly) exponential, so there is a significant risk that target IAS will be well exceeded before the levers are returned to the CLB detent AND the big fans have had a chance to slow down again. That would be likely to necessitate a go-around, particularly on a short runway in a beam wind, which might otherwise have been unnecessary.
It would be interesting to hear how successful PJ2 and others who have used this expedient have been in avoiding overkill. I would also like Airbus to explain if and why, with a serviceable A/THR, it should be necessary for the PF to intervene, except to go around. In the early years, many of us commented on sluggish autothrottle response, and it seems to be little better today. Bernard Ziegler should have provided us a better one, particularly as he insisted on static (non-driven) throttles.
But manual thrust, assisted by GS-MINI, is a delight for visual approaches...
Chris
Last edited by Chris Scott; 1st Mar 2011 at 23:05.
Reason: Syntax and punctuation