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Old 27th May 2011, 17:12
  #108 (permalink)  
Checkboard
 
Join Date: Aug 1998
Location: Ex-pat Aussie in the UK
Posts: 4,485
Is there an elevator offload to the stab? By this I mean does the aircraft auto trim in the direction of input to centralise the elevator? and would a prolonged nose up pitch input lead to the stab auto trimming itself nose up?
Of course - that's how trim, and auto trimming works. A constant input on the stick (or from the autopilot) is trimmed out using the Trimmable Horizontal Stab.

Neglecting for the moment how the aircraft entered the stall, do the A330 pilots here believe such a stall was recoverable, allowing for the THS trim (correctable? ), cruise aft CG, and aircraft weight and altitude?

Possibly recoverable, or routinely recoverable?
The aircraft (with the possible exception of the pitot tubes) was fine - the situation should have been routinely recoverable.

If there was some pitot pressure and he climbed with blocked static then that would have indicated a reduction in IAS.
Climbing with blocked pitot results in the airspeed displayed increasing from the speed displayed when the blockage occurred, not decreasing.

In the Perpignon disaster, from the final report:

Originally Posted by page 28

When the aeroplane exceeds certain attitude thresholds, roll, angle of attack or speed, the system uses a specific law [abnormal attitudes law]. The display on the PFD is identical to that for alternate law. This abnormal attitudes law breaks down into two phases:

ˆ For the first phase, the law used in pitch corresponds to the alternate law without auto-trim and with only the load factor protection. In roll, a direct law with maximum authority is used. The yaw is controlled mechanically. This first phase should make it easier for the crew to return to more usual attitudes.
Originally Posted by page 86

When the stall warning sounded, the Captain reacted by placing the thrust levers in the TO/GA detent and by pitching the aeroplane down, in accordance with procedures. The nose-down input was not however sufficient for the automatic compensation system to vary the position of the horizontal stabilizer, which had been progressively deflected to the pitch-up stop by this system during the deceleration. The Captain controlled a left roll movement, caused by the stall. The aeroplane’s high angle of attack and the roll movements generated asymmetry, and a speed variation between ADR 1 and 2 appeared. This increasing divergence caused a rejection of the three ADRs by the FAC then the ELAC. The flight control system then passed into direct law. [i.e. No auto trimming in Direct Law] It is likely that the crew did not notice this due to the emergency situation and the aural stall warning that covered the warning of a change of flight control laws. The Air New Zealand pilot, by saying “alpha floor, we’re in manual” likely considered that the alpha floor function had triggered and that in fact the autopilot had disconnected.
Originally Posted by page 87

The aeroplane attitude increased sharply and its speed dropped to the point that rendered it practically uncontrollable, the flight control surfaces becoming ineffective due to the low speed and the high angle of attack. The aeroplane stalled again, this time irrecoverably, bearing in mind the aeroplane’s altitude and without any crew inputs on the trim wheel and the thrust levers.

The loss of control was thus caused by a thrust increase performed with a full pitch-up horizontal stabilizer position. This position and the engine thrust made pitch down control impossible. It should be noted that the PF made no inputs on the horizontal stabilizer nor reduced the thrust and that the PNF did not intervene. This seems to indicate that none of them were aware that the automatic trim system, which relieves the pilot of any actions to trim the aeroplane, was no longer available.
Originally Posted by page 93

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 twenty-five 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.
[emphasis and notes mine]

Last edited by Checkboard; 27th May 2011 at 17:30.
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