Disadvantages of A/THR approaches on Airbus FBW
Quote from BOAC (Post #80):
"LR - it is appropriate to note that this 'experienced' crew did not maintain power after the bounce in order to cushion the return to earth as per 'normal'."
and from CONF_iture (Post #81):"Another example of the poor A/THR response for a late wind change. A bit of thrust under manual thrust would have helped to improve the first touchdown and avoid what came next."
As a Boeing pilot (?), what BOAC may not appreciate is the difficulty involved in doing this on Airbus FBW aeroplanes when A/THR is engaged. (Have to confess to having not read the report, but am assuming that the PF was using A/THR, which is – sadly – the SOP in most airlines.) The reason for this, he will now recall, is that the non-driven throttles (thrust levers) are both stationary in the Climb-Thrust detent throughout the approach. If left there during the flare, the thrust will normally tend to rise to correct the airspeed decay. This is usually not desirable: hence the pilot normally retards the thrust levers as he/she commences the flare.
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).
On the other hand, once the thrust levers have been closed, the A/THR is disengaged automatically, and you are in conventional manual thrust. Unfortunately, most Airbus FBW pilots get little or no practice at using this (except for taxiing), so are ill-prepared if they suddenly need an increase in thrust in the flare.
This is why CONF_iture and I advocate the use of manual thrust for non-AP (visual) approaches, provided visual contact is acquired in time to carry out the slightly fiddly, time-consuming process of disengaging the autothrottle. This involves initially retarding the throttles to a point where the thrust limit matches the existing thrust (using the N1 or EPR gauges, as appropriate to the engine type) – so as to avoid any undesired thrust change – and then disengaging the A/THR. The thrust levers then work conventionally, and are second to none in my experience. The GS-Mini system helps enormously to minimise thrust changes in windshear, provided you remember that it provides a target speed, NOT a minimum speed.
Last edited by Chris Scott; 27th Feb 2011 at 13:26.