Machinbird;
The
BEA Final Report into the XL Airways A320 accident at Perpignan makes for interesting reading, bearing in mind what information has been provided thus far on the THS design, stall warnings, normal, alternate and direct laws and so on. (The G-THOF incident has been brought up here before).
We know that the THS is the long term follow-up to elevator input and so with AF447 and here the trim dutifully followed the sidestick commands as the stall exercise at Perpignan was approached. Some here have offered the notion that the THS should stop trimming at some point but what are the all the effects of such a design change? Should it be stopped at a stall indication? In these two accidents that would have been too late as the THS was already at the NU stop. Ironically, for AF447, stopping the autotrim function at that point would remove the ability of the autotrim to follow a ND stick order and reduce from -13.6 back to normal as the trim did in our little sim exercise, as it would have for AF447. However, for the XL A320 dropped from normal to alternate and then quickly to direct law where manual trim was required but by that time things were happening too fast. There's an animation on the BEA site, (with a caution not to use it to come to specific conclusions...a caution for ALL animations!)
TTex600, re the autotrim and THS, you told only part of the story!
The trim went back to its original setting when you fiddled with it because that's what it is and does...it's a full-time autotrim in normal and alternate laws. Manual (mechanical) trim IS available at all times just as the FCOM says,
when needed, (such as in direct law or a G+Y Hyd flr), it
will stay in the position the pilot puts it in. That's just part of knowing one's airplane. Hey
gums...no worries.
From the BEA Report, p.92-93:
2.4 Functioning of the Automated Systems
When the real angle of attack increased, the blockage of AOA sensors 1 and 2 at similar values caused the rejection of the ADR 3 anemometric values, even though these were valid. This rejection was performed by vote without any check that the parameters were consistent with each other. The crew was not aware of this rejection, except indirectly through the loss of CAT 3 DUAL approach capacity.
The low values of the limit speeds did not attract the crew’s attention. Due to the blockage of the AOA sensors, calculation of the limit speeds was erroneous and the triggering of the AOA protections in normal law was rendered impossible. The values of the speeds corresponding to angle of attack protections (Vaprot and Vamax) were proportional to the computed airspeed of the aeroplane (see 1.16.2). The display of the amber CHECK GW message on the MCDU(58), a consequence of the gap between weights calculated on the one hand by the FAC, based on the angle of attack, and on the other hand by the FMS, based on the takeoff weight and the fuel consumption, would have allowed this anomaly to be detected. This message is however associated with no aural warning, which contributes to reducing its importance.
On approach to stall and taking into account the dynamic of the flight and of the complexity of the displays, the automatic changes in the control laws can fail to be perceived and their consequences can sometimes be misunderstood by pilots. In this case, the passage to direct law rendered the auto-trim function inoperative. Even if the amber USE MAN PITCH TRIM flag was displayed on the two PFD artificial horizons, the crew did not notice the position of the stabilizer and did not command the trim wheel manually during the twentyfive seconds in direct law between 15 h 45 min 15 s and 15 h 45 min 40 s. From this time on and for the rest off the flight, as a result of passing into abnormal attitudes law, the amber USE MAN PITCH TRIM flag was no longer displayed. The systems thus functioned in a degraded manner, without the real overall situation of the aeroplane being known by the crew.
The necessity to trim the aeroplane manually can occur in a situation that is already degraded, as was the case during the accident. This then leaves the crew no time to analyze the situation, especially since, on this type of aeroplane, the crew was used to not performing this task in normal operations. One of the only circumstances in which a pilot can be confronted with the manual utilisation of the trim wheel is during simulator training. However, in this case, the exercises generally start in stabilized situations. It should also be noted that the technique for approach to stall does not remind crews of the possible need to have recourse to the trim wheel in direct law. This absence de reference to the use of the trim is also mentioned in AAIB report into a serious incident to a Boeing 737 on 23 September 2007(59). In addition, the angle of attack constitutes essential information to characterize the situation of an aeroplane on approach to stall, while the speed information is that which is always used.
(59) http://www.aaib.
gov.uk/publications/
bulletins/june_2009/
summary___
aar_3_2009___
boeing_737_3q8__g_
thof.cfm