Airbus crash/training flight

[J.O.]

FSF Critical of Prosecutorial Interference

http://www.flightsafety.org/news/nr09-04.pdf

[jimjim1]

suggested key events

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.

[TyroPicard]

THS - Trimmable Horizontal Stabiliser

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

[AlexGG]

Considering that at the point of impact

1. The stabilizer was set "full aircraft nose up"
2. The stick was commanded "full aircraft nose up"
3. The elevators were set "full aircraft nose down" trying to limit normal load at 2.5G
4. 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"?

[pax2908]

Several posts mention that faulty AoA sensors may have been a factor. How likely is it that this fault was not detected earlier during the flight?

[Clandestino]

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.

[jimjim1]

THS

Tyro,

Thanks.

"THS" - I had realised what it did but I could not work out the TLA

This ensures elevator authority in both directions.

I had not figured out in such clear terms that that was the aim of the trimmer. It was not like that on the C150 or Chipmunk as far as I recall.

Well I am reasonably convinced by myself now, especially after your clarifications.

Very sad if that turns out to be near the case.

[alph2z]

From the preliminary report.

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).

Any ideas ?



.

[jimjim1]

alph2z,

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).

[TyroPicard]

From the report pages 21/22:

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

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

[TyroPicard]

Stall Warning

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!

[DC-ATE]

TyroPicard -

.....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?

[raven77]

Alpha Floor

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).

Raven

[Yebo]

AOA Protections

Extract from Airbus flight crew training manual:

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

· Automatic speedbrake retraction: Minimizes drag.

Operational Recommendations:

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

[DC-ATE]

raven77 -

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.

OK...thank you for that info.

Does "u/s" mean Unservicable, or 'Deferred'? If so, what happens with the throttles in idle?

[TyroPicard]

Raven77
Thanks! Slip of the pen - I meant "when we train low speed protections in normal law". Post edited.
TP

[OFSO]

Attitude, mental not aircraft

Moving slightly off-thread.

A question for aircrew doing check or test flights.

When performing test flights, as you go through the list of checks, in what frame of mind are you ?

When performing an action, do you expect a normal response, or are you prepared for an abnormal response, or are you prepared for "anything to happen" ?

Back to the Perpignan thread: is it possible that towards the end of a checkflight where everything has been routine, the crew were in a frame of mind expecting that all responses would continue to be routine/as expected ?

I'm thinking back to commissioning many newly-launched spacecraft, and working with ground stations, where it was esssential to be in the correct frame of mind when doing systems checks, and how after the initial burning-in phase where one was very wary, one "got used" to all responses being nominal.

And therein lurked the danger.....

[Strongresolve]

The aircraft went straight to Direct Law while performing an ALPHA PROT at low altitude in landing config, I think that this is the final reason of the crash.

We dont need to go much futher to know what caused the accident.

The Control Laws didnt catch the plane while performing the deep Stall and later the dive. Instead, the loss of auto trim in Direct Law and the full up position of the stab limited de manouver envelope for a recovery when the control transited to Alternate Law.

Probably it get to the limit of 2,5G during the pull up, this is pretty scary in a commercial jet, but it wasnt enough.

I dont know if the plane structure would have resisted 2,6 or 2,7 Gs, but I prefer to wreck the plane rather than being wrecked by the plane.

[John Citizen]

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

I am not sure about the above. This is my interpretation :

Alpha Floor protection (TOGA thrust and AOA "stall" protection) only works in NORMAL LAW !!

Alternate Law has a Low Speed Stability which may be overridden and the aircraft may stall

Direct Law has no such protections as all, and once again, you may stall !!

[guiones]

"

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."

Raven 77:

You are absolutely wrong, the protection you are speaking about is ALPHA FLOOR and it is ONLY available in NORMAL LAW.

G