Going back to flying basics if we found ourselves in a runaway trim or unusual attitude putting your airliner in a high nose up pitch attitude rolling into a 60 degree bank to drop the nose back down to the horizon was part of the recovery if necessary. I know the Airbus 320 has a restricted bank angle so the pilot cannot not exceed that bank.
In Direct Law those restrictions do not apply and you can roll the A320 into a spiral dive straight into the ground should you so desire. Even in Normal Law you can get a bank angle of up to 67 degrees, so the maneouvre you describe above is perfectly do-able in the A320.
Last edited by DozyWannabe; 26th Feb 2009 at 08:49.
Lem. Yes manual pitch trim would be required. That is if it was working at the time. As I said still surprised that the elevators do not have enough authority.
May I assume that in simulator training on the A320 that the student only takes it to "recognition" of a stall and not a full stall? Is the sim able to simulate a full stall?
Our training is firstly the "Normal Law" charateristics of Approach to Stall, and the "protections" and the recovery actions. Secondly the Alternate / Direct Law where you now get a Stall Warning (audio) and the Recovery Actions. You do not, IIRC, "stall it" fully, and doubt the sim would reproduce this accurately?
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Are you A320 qualified?
My First Officer's would probably say not. 5 years command... and current now.
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you should not get a Stall Warning in Normal Law
There's a caveat in the FCOM: you can get stall warning in the normal law if AoA probe is damaged.
My reading of the FCOM is that the "Stall Warning" is still available, just in Normal Law the protections should avoid you getting to that stage. In this case, some Normal Law malfunction (?) AoA Probes? seems to have led to a genuine Normal Law Stall Warning (maybe from the 3rd AoA probe not on the FDR?)
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Methinks that shock from experiencing the stall in "unstallable" aeroplane may prevent crew from noticing "USE MAN PITCH TRIM" caption on top of their PFDs and acting accordingly.
One would hope that people flying Test Profiles are suitably briefed and trained... for what eventualities are possible with each test area...
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It seems to me that in the last few seconds that the 2 elevators (green* line on p. 34) reversed to what the stick input (blue) was.
Anybody have any idea why ?
My guess is to limit the pitch rate / 'g' as per Altn Law demands...
In Direct Law those restrictions do not apply and you can roll the A320 into a spiral dive straight into the ground should you so desire. Even in Normal Law you can get a bank angle of up to 67
For anyone not wanting to do the math: 67 degrees bank angle corresponds to a co-ordinated turn at the load limit of 2.5G normal (i. e. downwards relative to the aircraft) acceleration.
(The 33 degree limit, to which the bank angle returns from higher angles at stick release corresponds to a comfortable 1.2G turn.)
Which means... At 67°AOB your pitch-up capability is nil At 33° AOB you can pitch up at 1.3G
Initial AOB during the final pitch-up was 45° ....
... which only corresponds to 1.4G, leaving 1.1G for pitch-up.
(Although the values for 33 and 67 seem to imply otherwise, the relationship is not linear. At 84.26 degrees the load is 10G, at 90 degrees it becomes infinite.)
I think that this pulls together some of the previous suggestions.
Given the decision to do the alpha floor test when they did:
- AoA sensors stuck so alpha prot fails
- stall warning
- throttles forward
- control law to direct : failure or automatically or manually so horiz. stab (THS) trim stuck full up
- fail to consider (THS) which is full nose up
- rest just follows
- mush mush mush
- nose drops eventually
- insufficient alt to recover within load limit (2/2.5g)
[all the while various wings are dropping and being recovered]
Clearly the computers are doing some yet to be explained things but there seems a possibility that above points are the substantial explanation for the aircraft behaviour.
So we need three slices of cheese
- Poor decisionmaking regarding the test conditions
- AoA probes faulty in some way
- THS trim, rarely used manually, was required to be operated in this case if recovery was to be effected.
It appears that the flight controls were functioning reasonably, except for perhaps the THS trim after stall warning. What the design behaviour of the THS trim is and whether that design is appropriate or whether it performed to that design is way beyond my capacity to consider.
In Normal and Alternate Law the autotrim moves the THS to keep the a/c in trim longitudinally. This ensures elevator authority in both directions. In Direct Law the autotrim does nothing - the pilot moves the THS using the trim wheel. In Abnormal Attitude Law (>50° NU pitch or <60 KT CAS in this accident) the pitch law is Alternate but without autotrim. Once the pitch is <50° and CAS above 60kt the autotrim should function again. However at 15:45:57 the pitch attitude exceeds 30° nose DOWN and the Abnormal Attitude Law again applies - so no autotrim. Thus from Direct Law (15:45:15) to approx 15:45:53 the trim wheel is the only way to move the THS, then there was about 3 or 4 seconds with autotrim enabled then back to the trim wheel only.
When the a/c goes into Direct law an ECAM warning informs the crew, and on both PFD's the message "USE MAN PITCH TRIM" appears in amber. TP
The elevators were set "full aircraft nose down" trying to limit normal load at 2.5G
The aircraft was pitching nose up with approx 2.5G normal load (~5 deg/sec).
Is it correct to say that
Should there be sufficient altitude (so they did not crash into water at the lowest point of the dive), aircraft would pass zero pitch (horizontal) and continue pitching upwards at the same rate (~5 deg/sec, incorrectly assuming unchanged airspeed)?
That is, unless trim was moved to neutral. Because full pilot authority to control in pitch axis was giving about 2.5G normal load "pitching up"?
Dear Pax2908: On any complex aeroplane, AoA data is primarily used for stall protection, A320 not being an exception.
As for stall warning in normal law methinks that FCOM is referring to spurious warning triggered by damaged AoA vane. IIRC Vsw (altn and direct laws) is near Valpha prot (normal law) and it's a bit greater than Valpha max and stall warning really should be inhibited in normal law - otherwise one would get continuous warning during hard WS and GPWS escape maneuvers.
I plotted the vertical trajectory of the last 41 secs of the flight and found that in the last 30 secs the vertical acceleration (relative to ground) was -0.43 G* or that the experienced vertical G* level was 0.57 G*.
* : Relative to the earth. Horizontal accelerations not included.
I'm really surprised that it was constant for so long (30 secs).
What is the precise source of your data, and what did you do with it? If you post the derivation then others can perhaps comment.
I now agree that the altitude plot looks very like a parabola. i.e. a ballistic trajectory (like a rock). However the pitch attitude together with the airspeed plots makes that difficult to understand. (for me).
At 15 h 45 min 40 s, the control law for pitch passed from direct to alternate. The bank angle reached a maximum of 59° to the left and the normal load factor dropped below 0.5 g.
At 15 h 45 min 50 s, the normal load factor exceeded 0.5 g.
It may be that the only load factor readout is above/below 0.5g - no info.
alph2z
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I'm really surprised that it was constant for so long (30 secs).
Rusty maths but I suspect you have calculated an average value over time, not a constant value - also the pitch rates that the a/c experienced in the final 15 seconds are not consistent with constant 0.57g. I agree about the parabola over the top - typical ultra-low-speed behaviour. TP
Stall Warning is only inhibited on the ground. From FCOM 1.34.10:
An aural stall warning is triggered when the AOA is greater than a predetermined angle. This angle depends on the:
- Slats/Flaps position
- Speed/Mach
- F/CTL law (normal, alternate/direct)
In other words Vs will vary with F/CTL law - with (my guess) a higher value in alternate law. When we train low speed protections in normal law the stall warning does not sound - the protections prevent the a/c getting to that AOA.
Edited to correct stupid mistake!
Last edited by TyroPicard; 27th Feb 2009 at 12:18.
.....the other protections prevent the a/c getting to that AOA
Not looking for an Airbus FBW lesson here, but could you explain that a little further? Are Auto Throttles on all the time in 'normal'? If not and you close the throttles and try and hold altitude, what happens? Would the airplane just continue to oscillate down?
DC, It does not matter which Flight control law the A/C is in, (as long as the Autothrust is not u/s), it will kick in at the pre determined angle of attack and give Toga thrust, even with the thrust levers in the idle position.
TYRO, How do you teach stall recovery in Normal law. It can only be demonstrated with the appropriate stall audio warning in Alternate law (or Direct).
High AOA protection enables the PF to pull the sidestick full aft in dangerous
situations, and thus consistently achieve the best possible aircraft lift. This action
on the sidestick is instinctive, and the high AOA protection minimizes the risk of
stalls or control loss. High AOA protection is an aerodynamic protection:
· The PF will notice if the normal flight envelope is exceeded for any
reason, because the autopitch trim will stop, the aircraft will sink to
maintain its current AOA (aPROT, strong static stability), and a significant change in aircraft behavior will occur.
If the PF then pulls the sidestick full aft, a maximum AOA (approximately corresponding to CL Max) is commanded. In addition, the speedbrakes will
automatically retract, if extended.
In addition to this aerodynamic protection, there are two more energy features:
· If ATHR is in SPEED mode, the speed cannot drop below VLS, even if the target speed is below VLS
· If the angle-of-attack still increases and reaches ALPHA Floor threshold, the A/THR triggersTOGA thrust and engages (unless in some cases of one engine-out).
In case of an emergency situation, such as Windshear or CFIT, the PF is assisted in order to optimize aircraft performance via the:
· A/THR: Adds thrust to maintain the speed above VLS
· ALPHA FLOOR: Provides TOGA thrust
· HIGH AOA protection: Provides maximum aerodynamic lift
When flying at amax, the PF can make gentle turns, if necessary.
The PF must not deliberately fly the aircraft in alpha protection, except for brief periods, when maximum maneuvering speed is required.
If alpha protection is inadvertently entered, the PF must exit it as quickly as
possible, by easing the sidestick forward to reduce the angle-of-attack, while
simultaneously adding power (if alpha floor has not yet been activated, or has
been cancelled). If alpha floors has been triggered, it must be cancelled with the
disconnect pushbutton (on either thrust lever), as soon as a safe speed is
resumed.
Note:There is a nice graphic representation of the protections activation sequence, unfortunatelly cant figure out how to paste it to the post ; sorry
