From the pictures I've seen of the AF447 wreckage, I don't think anything from around the THS has been found that would indicate its trimmed position. I'm betting it was trimmed full nose up. Machinbird
If you're betting on that, then what is behind your assumption?
mm43
mm43
Thank you for the lead in.
When an aircraft crashes in a deep stall, it is reasonable to assume that it was also in a deep stall at higher altitudes, and that it transitioned to this condition at still higher altitudes.
Agreed, there are a number of ways to transition an airliner into a stall, but in general, more dynamic stall entries leave residual inertial moments on the airframe and work against developing a stable stall. It appears that AF447 hit the water in a stable stall configuration.
I have been looking at scenarios where an A330 could decelerate into a stall while maintaining normal attitudes. The Perpignan accident involving A320 D-AXLA, although a different Airbus model, indicated to me the amount of pitch authority Airbus builds into their THS designs and how an aircraft could be left with full nose up trim and only the manual trim wheel available to reduce this trim. When added to a triple pitot tube freeze up, and a full trim tank, the resulting cockpit “pinball show” could easily confuse an unsuspecting aircrew until it was too late. I also suspect cockpit design made it too hard to get the critical information needed (actual power setting, THS position, actual angle of attack) and as a result, the aircrew was unable to develop the big picture of what was happening. I also suspect that the A-330 can be held in a deep stall by full nose up trim and an aft CG and yet be stabilized in a relatively stable normal flight attitude by the flight control system.
Admitted, these are a lot of unproven suspicions at this point but none particularly illogical.
Consider these paragraphs from David P Davies book, “
Handling The Big Jets”. Mr. Davies was Chief Test Pilot for the Air Registration Board and carried out certification testing of a large number of jet transport aircraft.
From the section on “ The Super Stall” commencing on the bottom of page 120 in my second edition copy, bold print is my emphasis, italics, the author’s. (The previous paragraphs explained why swept wing aircraft tend to pitch up after a stall.)
“Having explained the elements of the deep-stall let us now look at the complete picture. Figure 5.20 illustrates pitching moment and lift coefficient against angle of incidence. This shows that as incidence is increased toward the stall there is a stable nose down pitching tendency. Notice particularly the increase in stable slope of Cm just before the stall. This is typical of some aircraft types and reflects the success of the designer in achieving an inboard stall first, thus producing a nose down tendency. After the stall, however, the aeroplane pitches up at increasing angles of incidence until it reaches a more or less stable state at around say 30 degrees. So far so good, for a purely academic illustration
. In real life, however, the manoeuvre is more treacherous than the illustration suggests, for two reasons stemming from the same source. It has been explained previously that below, say roughly 1.4 Vs, an increase in incidence causes drag to increase faster than lift and thus the aeroplane tends to sink. It is most important to realize that this increasing sinking tendency
, at a constant pitch attitude, results in a rapid increase in incidence as the flight path becomes deflected downwards. Furthermore, once the stall has developed and a lot of lift has been lost, the aeroplane will start to sink rapidly and this is accompanied by a rapid increase in incidence.
This matter of a downward inclining flight path producing rapid increases in incidence compounds the entry to, and the progression of, the super-stall. It must be emphasised again that this can occur without the need for an excessively nose high pitch attitude. It can happen on some types a an apparently not abnormal pitch attitude, and it is this quality that can mislead the pilot because it looks very similar to the beginning of a normal recovery.”
Now skipping down a half paragraph, “There is no point in discussing the irrecoverable case any further, except perhaps to say that those aeroplanes which have been lost in such manoeuvres finally reached the ground substantially level laterally, having defied all efforts to roll or spin them out of the stabilized condition; only slightly nose down in
pitch, with little or no forward speed; at an extremely high incidence; rotating only very slowly in yaw; with (in one case) all the engines flamed out because of being exposed to such massive angles of incidence; and finally with an enormous vertical velocity.”
I would especially appreciate comment from A330 knowledgeable individuals on the availability of the 3 pieces of information that I suspect are not continuously displayed in the cockpit.