AF 447 Thread No. 12

[Lonewolf_50]

Originally Posted by PJ2

"Stalls are a pitch problem, not a power problem."

Stalls are an AoA problem, period.

Amen, deacon.

Mr Phoenix[/QUOTE] Capt. Dubois didn't comprehend why the aircraft was climbing while sidestick was forward [quote]

2 h 12 min 32,4 Dubois: No you climb there // aft -8 deg // up 5 deg
2 h 12 min 32,8 Bonin: I’m climbing okay so we’re going down // full forward 16 deg //up 6 deg
2 h 12 min 34,2 Robert: You’re climbing // full forward 16 deg // up 8 deg
2 h 12 min 39,2 Bonin: okay we’re in TOGA // forward 10 deg // 0 deg
2 h 12 min 41,9 Bonin: On alti what do we have here? // forward 10 deg // down 3 deg
2 h 12 min 43,8 Dubois: (…) it’s impossible // aft -4 deg // down 3 deg

I may recall incorrectly, but weren't they already falling by the time Captain Dubois arrived in the cockpit? I guess I need to go back to the thread and find the altitude traces to match up with your time data points there ... will get back to you when I have found it. What I recall is that around 1410 the event began, and they fell from FL 33-35 to the surface in the following four minutes. So, at 1412 the falling toward the sea surface, in a stall, was already in progress. However, I may have recalled that incorrectly.

Thanks.

[DozyWannabe]

Originally Posted by Lonewolf_50

I may recall incorrectly, but weren't they already falling by the time Captain Dubois arrived in the cockpit?

You recall correctly. They were established in the stall and the descent by the time the CDB arrived.

I suspect _Phoenix_ may have meant to refer to the aircraft being in a nose-up attitude rather than "climbing". In fact at the point Dubois arrived, the status included these aspects:

1. Bonin had his sidestick at full NU and would continue to hold it there for a further half a minute approximately
2. The nose was up at the point he arrived, but was about to swing back down
3. Unfortunately, he arrived almost at the exact point that the excessive AoA began fouling the pitot tubes and messing up the ASI readings again

[EDIT : In fact, two seconds after Dubois arrives, the VSI is indicating a descent rate of -10,000ft/min. 13 seconds later, Bonin reports that he's lost VS indication - I have to wonder if it's possible that it was indicating, but that his mental model didn't expect to see the "needle" pegged near the bottom of the scale.]

Originally Posted by Winnerhofer

Schramm: Yes, the same well, uh, approach to stall means that one must never go beyond that. They were in unchartered waters.
Anchor: Well, yes because even Airbus had never envisaged such a situation.
Schramm: Indeed, in Normal Law, Airbus is unstallable.

The "anchor" is being disingenuous here. The Airbus FCOMs have always stated that it is possible to stall the aircraft outside of Normal Law. The AF representative (M. Schramm?) is, I suspect, doing a little bit of dancing around the point. In fact, "approach to stall" training and "stall recovery" are very different beasts - one thing that became apparent in the wake of this accident was that over the previous decade or so, the airlines as a whole started dropping stall recovery training from their recurrent syllabus in favour of stall avoidance ("approach to stall"). This was the case across almost every major airline and every type - from Airbus to Boeing and the remaining MDs.

This is why Boeing and Airbus subsequently collaborated on a programme to improve understanding of and training for stall scenarios.

FO Robert had the most time on type, this is true - and he certainly seems to have initially been the closest to understanding what was happening, but the likelihood is that the last time either he or Bonin actually trained for/performed a stall recovery in a powered aircraft was during their PPL training.

[DozyWannabe]

Originally Posted by Winnerhofer

Ah, but there's no ELAC on the A330...

True, and that's an oversight - but they do get the nomenclature correct elsewhere as far as I can see. I'll give it a proper read when I have the time. A quick skim reveals a few conclusions that are at best conjecture - but this is TV after all...

Re: line check - that doesn't necessarily indicate anything on it's own. To illustrate the point, Capt. De Crespigny of the Qantas A380 which blew an engine out of Singapore (and subsequently commanded the flight to a safe landing and minimal injury) finishes his book on the subject by pointing out that he was on a checkride on that particular flight and was told by the Check Captain that he had in fact failed it (presumably because the book didn't take a situation of that gravity into account). Needless to say, he passed the next one with no problems!

[infrequentflyer789]

Originally Posted by PJ2

"Stalls are a pitch problem, not a power problem."

Stalls are an AoA problem, period.

For me, as a non-pilot who once knew a bit about making stuff that flies, the single most scary thing from all the 447 and other stall threads has been the number of apparent pilots who don't seem to know this. That and the realisation that many of them don't even have an AoA indicator in the cockpit (which is probably connected to previous point).


[Actually, for several years I might have said that stall is a boundary layer separation problem, it just happens at a particular AoA - but that depends on your point of view. By the time the pilot is in control, the engineering is done, and AoA is the difference between flying and falling. ]

[DozyWannabe]

Originally Posted by infrequentflyer789

That and the realisation that many of them don't even have an AoA indicator in the cockpit (which is probably connected to previous point).

To be honest, it's fairly reasonable to state that if the aircraft is descending while nose-up and/or apparently unstable in pitch and roll, the odds are good that it is in a stall.

[Mr Optimistic]

Agreed re boundary layer.

[_Phoenix_]

@ DozyWannabe and Lonewolf_50

I suspect _Phoenix_ may have meant to refer to the aircraft being in a nose-up attitude rather than "climbing".

No, WE know that aircraft was falling in a nose-up attitude.
But, Capt. Dubois mental picture was different: "why the aircraft was climbing while sidestick was forward"... HE was right!

2 h 12 min 32,4 Dubois: No you climb there // aft -8 deg // up 5 deg // No Stall Warning

Then, he might saw Bonin pushing sidestick at full forward stop, the stall warning goes off... "nice! as expected"

there was no sustained effort on the part of either PF or PNF to get the elevators to nose down and hold them there

Again, PF kept sidestick forward for 10 sec. He was a glider pilot too, imagine that input for a glider

..sometimes, I'm wondering why Airbus doesn't replace sidestick with a touch screen ADIRU, linked to a tablet, so the capt. can fly the plane from rest area or lavatory.

[flyingchanges]

Then, he might saw Bonin pushing sidestick at full forward stop, the stall warning goes off... "nice! as expected"

And the airplane gave he impression that the correct input was wrong.

I still contend that if they had not touched the airplane at all, there would have been a better outcome.

[_Phoenix_]

@flyingchanges

Actually, DozyWannabe is right, they should nailed the sidestick at forward stop, since for 10sec forward input the elevator rotated from -30deg NU to -15deg NU, THS remained at -15deg NU - constant as gravity

What is the ideal crew for AF447? A pilot and a dog.
The pilot is there to feed the dog, and the dog is there to bite the pilot in case he tries to touch anything

[tdracer]

There are some (who know their aerodynamics) who have done some work on this and who believe that the airplane could be recovered, even from quite low altitudes, (thicker air). The horizontal stabilizer was never fully stalled and the airplane demonstrated the ability to pitch down when ND stick was held those rare times in the ND position.

Thanks, that's the bit of information I was looking for. Clarification, when I say "deep stall", I don't mean to imply an unrecoverable stall condition (such as what happed on the BAC 111), but rather a sustained stall condition where the aircraft is going down as fast (or faster) than it's going forward. As I understand it, on the BAC 111 (and some other 'T' tail aircraft), if you get into a deep stall, the spoiled airflow from the wing effectively blanked the tail and literally prevented pushing the nose back down to recover.


I've been on flight tests where they did dozens of stalls and "wind up turns". I suspect most of you are unfamiliar with that term so I'll explain - for a 'normal stall test, the pilot holds a constant altitude while allowing airspeed to decay (throttles at or near idle) until the aircraft stalls. These stalls are pretty gentle (at least on the 767 I was on) - a little forward push and the airplane falls out of the stall and recovers (although if the pilot had to use a big rudder input to keep it straight and level, the rudder would tend to shear the line to the 'trailing cone' Pamb sensor - in which case we got to go home early ). A 'windup' turn was used to test the inlet at very high angles of attack with the engines at takeoff power - hold constant altitude but pull the turn ever tighter until the aircraft stalled and fell out of the turn. It's basically a way to test the engine inlet reaction to a takeoff over rotation, without doing it a few feet off the ground. Three hours of windup turns is also the only time I've gotten airsick (straight stalls didn't bother me, but windup turns did).


Anyway, back to the point - in a deep stall, with forward airspeeds well below that necessary for level flight - even if the horizontal stab isn't stalled, it's not going to provide much authority - simply not enough Q. It's going to take a long time to push the nose down enough start gaining sufficient airspeed to regain control.
Back during the original 777 flight test, an unnamed FAA pilot somehow managed to get into a deep stall at high altitude - even though the right seat Boeing pilot immediately took control, they reportedly lost over 10,000 feet before he was able to regain control (second hand info - I wasn't on that flight and I'm pretty sure I'm glad I wasn't ).

Again, PF kept sidestick forward for 10 sec. He was a glider pilot too, imagine that input for a glider

Yea, while 10 seconds probably felt like an eternity, in a deep stall I suspect at least 30 seconds just to get the nose down and get airspeed up to a reasonable, controllable level.

For me, as a non-pilot who once knew a bit about making stuff that flies, the single most scary thing from all the 447 and other stall threads has been the number of apparent pilots who don't seem to know this.

Ditto! And the idea that one can power out of stall by advancing the throttles has a potentially fatal flaw - engine inlets are not designed to provide high flow at those angles of attack. Inlet separation and engine stall is a distinct possibility (reportedly one engine stalled/surged during the Birgenair 757 plunge)

[PJ2]

tdracer;

Thanks for the elaboration on your meaning of "deep stall". Davies discusses the phenomena in Handling the Big Jets - what you've described re the BAC111 is how he describes it.

AF447 was descending a bit faster than it was going forward, (12,000fpm, or about 130kts) but although reduced, the elevator still had authority.

While the stick was never held in the ND position for "10 seconds", there is a time period when it oscillates between NU & ND but is mainly ND for about 10" at around 17000ft in the descent and the pitch can be seen to change to ND and the AoA moves from about 43deg to about 38deg; then the stick returns to NU, essentially until impact.

While the sim does not have stall algorithms from actual flights past the approach-to-stall, going from memory the sim took even longer than the time you've estimated, (~45" from beginning of recovery at around FL350 to recovery at FL220, approx). The stick was held in the full ND position for the entire time, the descent rate reached 20,000fpm. The flight path vector symbol moved up very slowly from it's initial indication of ~ -35deg; the THS began moving from its full -13deg (NU, tail down) position a few seconds after the stick was held in ND position - the pitch was controllable throughout, though very slow to respond.


Recovery is 30" away...

As you know, air is thicker lower down and control may have been better and recovery time a bit shorter. I'll bet they're worked on algorithms to simulate an actual full stall. Still, the sim was pretty good - shake, rattle and hum, with the "Stall, stall, stall" message blaring - oddly it never quit until the sim became unstalled so not sure the NCD loss of the warning situation is reproduced.

One last item - I had thought the same thing regarding the engines - that high incidence angles would upset the engine to the point of putting the fire out but they both remained operational (and at high power) throughout, until impact.

[DozyWannabe]

Originally Posted by _Phoenix_

But, Capt. Dubois mental picture was different: "why the aircraft was climbing while sidestick was forward"... HE was right!

2 h 12 min 32,4 Dubois: No you climb there // aft -8 deg // up 5 deg // No Stall Warning

Hmm - I think there's a degree of doubt regarding translation - as far as I can tell, the distinction between "climb/descend" or "pitch up/down" can be a bit tough to decipher from the French without context.

The altimeter would have been unwinding at a significant rate, so unless he made that statement before he'd checked the altimeter display, I feel a little dubious about making the call.

Originally Posted by _Phoenix_

they should nailed the sidestick at forward stop

Once in the stall yes, but if flyingchanges means leaving the controls alone from initial AP disconnect, then I'm inclined to agree with him.

To be sure, I'm not saying that pilots shouldn't use the controls in such situations, but a pilot should be sure what they intend to accomplish and how they intend to accomplish it before acting, unless in a split-second "life-or-death" situation.

[Bpalmer]

"Stalls are a pitch problem, not a power problem."

Stalls are an AoA problem, period.

Well, yes, of course. I didn't mean to imply they were a pitch attitude problem, but a problem resolved with pitch control, not thrust levers.

[Bpalmer]

the Crew's Stall Training

but the likelihood is that the last time either he or Bonin actually trained for/performed a stall recovery in a powered aircraft was during their PPL training.

The accident report details that they each had stall training in their A320 initial training-as it was the full course for that airplane. However, since their A330/A340 training was a differences course, it did not include stalls.

[DozyWannabe]

@Bpalmer:
The stall-related training on the A320 initial course seems to have been approach to stall only.

From Final Report Section 1.18.3.5.1:

Stall phenomena are covered during the initial A320 type rating, according to the same philosophy of the manufacturer and the operator. They are not reviewed during the long haul passage, in CCQ 330, or during recurrent training.
At the time of the accident, the immediate actions were: simultaneously reducing angle of attack and applying TOGA thrust from the first signs of the stall (Stall warning / buffet onset). A minimal loss of altitude was expected.

(Note that it went to stall onset/warning - not recovery from a developed stall condition.)

and 1.18.3.6:

Note: Stall and stall recovery exercises are undertaken during initial pilot training (in particular basic training, private pilot, professional pilot, etc.) but not during type rating training.

[flyingchanges]

Once in the stall yes, but if flyingchanges means leaving the controls alone from initial AP disconnect, then I'm inclined to agree with him.

Yes, my exact thought. The machine was trimmed and stable the last time there was valid information presented. I would be inclined to not do a whole lot more than keep the wings level until we had moment to ponder the situation. At that moment, it is important to not kill yourself before you have time to figure things out. I know exactly where the trim and throttles (physically) need to be on my machine to stay out of trouble long enough to get things sorted out. In this situation, time is your friend, figuring out how to make your time last is the true solution.

[Bpalmer]

Stall phenomena are covered during the initial A320 type rating,
and

Note: Stall and stall recovery exercises are undertaken during initial pilot training (in particular basic training, private pilot, professional pilot, etc.) but not during type rating training.

Seems they can't both be true.

Further more the tendency of the flight control law to want to pitch down at stall would be dependent on if the low-speed stabilities were active while in Alternate law, which in AF447's case, they were not (due to lack of IAS). So unlike the α-prot (aoa command) in normal law, or when the low-speed stabiliites are active, the pitch command was still centered on g-load/pitch rate (aka C*). I believe you can see the elevator response (up) to the g load going below 1 as the stall progresses and the airplane starts to accelerate downward. This requires pilot action - stick forward - for as long as it takes to get the nose to drop and the aoa to reduce. I also found that in the simulator manual forward trim was required, as even a full down elevator was not enough to counteract a full up stab as airspeed started to recover.

[DozyWannabe]

@Bpalmer:
I think we're looking into technical specifics a little deeply there in terms of the discussion we were having on training.

Reading between the lines, I think when the report is saying "Stall phenomena are covered during the initial A320 type rating", the important word is "phenomena". This covers a few areas, but I think the clue is in the following context - namely "(Stall warning / buffet onset)"

The second paragraph in full said (sorry for repost, but we're across the default page boundary):

At the time of the accident, the immediate actions were: simultaneously reducing angle of attack and applying TOGA thrust from the first signs of the stall (Stall warning / buffet onset). A minimal loss of altitude was expected.

This is an "approach to stall" action set, not that of recovery from a fully-developed stall, and the key phrase is "A minimal loss of altitude was expected". This refers implicitly to the industry's change of focus from stall recovery to avoidance, because recovery from a developed stall necessitates an appreciable loss of altitude.

Regarding your technical point, while the low-speed stability soft protection was not available in Alternate 2B - I think the pitch attitude readout from the DFDR implies that the airframe has a natural tendency to pitch nose-down as the speed drops off regardless of any extra impetus from the flight control system. Of course, a manual nose-down input will enhance that tendency and make a recovery happen more swiftly, but the FCOM (http://www.bea.aero/docspa/2009/f-cp...nexe.11.en.pdf) at the time seemed to cover this with the instruction "PITCH ATTITUDE...REDUCE".

[PJ2]

Bpalmer;

Thanks - read it differently; agree with your meaning as explained.

[Machinbird]

Some have discussed keeping hands off completely in the period immediately after the A/P disconnected and the switch to Alt 2b law.

Just a personal opinion, but that course would have a high probability of ending in a spiral dive. There is nothing to level the wings in Alt 2b but the aircraft's natural roll stability (which isn't all that good in most jets).

Bonin had to get on the stick, but only in roll, and not with such a heavy hand. If he had used a gentle touch in roll and stayed off the pitch axis, AF447 would have ended in Paris.

The problem was that Bonin had never seen that control configuration in training and so used a highly inappropriate control technique which appears to have generated a roll PIO.