bubbers44

No matter what the nose up trim goes to we were trained to roll instead of stall. No airline trained us that but somewhere in my background I knew if I had runaway trim just bank until the nose is where you want it. 90 degrees is fine to get started. Then pitch and roll to what ever with reduced power makes a flyable airplane again. Once stabilized set flaps so it can be landed. Full up trim isn't a problem. It seemed reasonable to me. Banking requires more back pressure so let it help you.

OK465

OG:

A good question.

Maybe under those ambient conditions full SS NU/full NU elevator at 330K comes up 'short' of +2.5G, just as it would come up 'long' of the -1G with full ND elevator.

I don't have the Va numbers handy.

But I agree at first glance it appears that something 'short' of full NU elevator would achieve +2.5 at 330K.

jcjeant

It certainly has the interest to human factors group .... ?
At the beginning of the accident pilots did not seem to have heard or considered the stall alarm .. it probably because of the surprise or overwork and stress-induced
By cons, paradoxically .. after 4 minutes during which the stress has accumulate ... they recognize (completely agree) the Pull Up alarm and respond to this alarm (pull)
Stress is selective about alarms ?

Lyman

Is that why the THS started to wind up?

I know you think I am barking up the wrong tree, OK, but something is not kosher here, yet the DFDR spells it out, AL2b forever...

I still think the THS was inhibited at -3.2 degrees (NU). Early on.

HazelNuts39

Doesn't the achieved 'gee' come in? The sidestick may have been demanding +2.5 g, but the stalled airplane, even with elevator and THS fully NU, couldn't comply?

OK465

Lyman:

I don't know.

'Inhibited', strictly speaking, I assume would not allow the 'apparent' .2-.3 or so degree change during the initial decel and initial pushover (which would have slowed the decel rate)?

This scenario apparently exited a game of inches at about 215K.

Linktrained

An Emergency Descent, for whatever reason, probably starts at Altitude and at cruising speed. Would "10 or 15 degrees ND pitch" be the kind of pitch which would be appropriate, if an Emergency Descent were required ?
Altering from a slow (unrecognised) stall with excessive NU pitch to "10 or 15 degrees ND" would feel unusual/uncomfortable in terms of Gee. (Most people seldom experience 0 Gee, except momentarily at a fairground.)

gums

@ Nuts: You may have cracked the code.

[edit] Yep, if the jet can't give you anymore, than it's the same as a conventional plane, like a J-3 Cub. you can "command" all you want, but the plane and HAL won't help.[edit]

One problem with the commercial planes is you can't fly them to the limits of the FBW "protections" or "laws" during training. Best I can figure, you can't introduce failures and such in flight. So the pilots have to depend upon sim flights, right?

Our primitive system 35 years ago allowed us to demonstrate the FBW limits on the very first flight with a newbie. Pull, pull, pull hard as you want. O.K., see AoA reach limit, but keep pulling. O.K. now command full roll. O.K. release some back pressure, even push forward. [edit] We never knew if we had reached the actual capabilities of the jet except in one case- the deep stall, where even HAL couldn't provide enuf nose down to break the stall, regardless of our stick input. Two decades later, there was an actual training program to expose pilots to those aero conditions that HAL could not handle.[edit]

[edit] The deal is that most FBW control laws never allow the jet to reach its aero limits. In a few cases, you will actually get to a condition that the "laws" and control surface limits come into play. With degraded system inputs like airspeed/dynamic pressure then all bets are off. Then we ignore AoA. So we're in a hybrid control law that may or may not "help" us. Without the traditional "feel" of more back stick or less back stick that is related to AoA/speed, then you are in trouble. The THS implementation doesn't help, IMHO. [edit]

Lyman

"Consequently, the BEA recommends that:
EASA ensure the integration, in type rating and recurrent training programmes, of exercises that take into account all of the reconfiguration laws. The objective sought is to make its recognition and understanding easier for crews especially when dealing with the level of protection available and the possible differences in handling characteristics, including at the limits of the flight envelope; [Recommendation FRAN-2012-039]"

Thirty years on, no more :

"What's it doing now?"

Owain Glyndwr

@OK465

Boy, do I agree with that one!

Va at FL350 and 205t would be about 260 kts CAS. By definition the aircraft would stall at 2.5g at that speed. On my sums the elevator needed in those conditions would be about 9 deg without any help from the THS, so it would be a long way from coming up ‘short’ at 330 kts CAS

@HN39 & gums

That’s where I started, but so far as I can see it doesn’t explain OK’s observations.


[Note: from subsequent discussion I take this to read “with full forward SS and with the system gains set at their 330 kts values”]
The logic that if present elevator is insufficient to provide the gee demanded by stick position then the system will increase the elevator deflection until the demand is satisfied must apply in both directions.
So if the demanded gee from full forward SS in OK’s simulation exercise was satisfied by ¼ ND elevator then the answer to his original question:
is that you can get all you need to satisfy the limiting manoeuvre capability built into the control system, even if this is a lot less than full elevator travel.


However, if ¼ ND deflection was not enough to satisfy the commanded gee then following the logic that led to 30 deg NU but going in the opposite direction one should have seen more than ¼ ND elevator.


To repeat though – ¼ ND elevator seems to be nowhere near enough to provide the -1g manoeuvre limit built into the protection system.
And finally, I have just been looking at a video of an A330 simulation of AF447 stall and recovery kindly supplied by another PPRuNer, and one of the sequences shows full ND SS to be accompanied by at least ¾ ND elevator.
Help!

OK465

Actually that was really what I was trying to determine...

For a particular ADR related malfunctions, did I really get a simulated change in gains.

So first step was to check elevator deflection in Normal specifically at 330K using a full ND SS input. The logic behind checking with full ND SS as opposed to some intermediate position was that I could be sure I was duplicating the exact same SS input in Alternate. I noted the elevator deflection as a baseline for FCC commands at 330K.

Next I inserted the particular ADR malfunction I wanted to check as resulting in fixed 330K gains.

Then I applied full ND SS at both 330K & 200K, expecting to see the exact same deflection as in Normal at 330K. I indeed did. Then I checked full ND SS at 200K in Normal, expecting to see more elevator deflection with active gains. Once again I did.

I drew from this that indeed the particular ADR malfunction was also faithful to the change in gains.

Maybe my logic here is flawed.

The specific amount of elevator deflection observed was just an offshoot of the purpose of the exercise, but it brought other questions to mind. But I doubt I held the input for the up to 7 seconds as mentioned earlier, and the validity depends on simulation fidelity regarding the flight control system. Maybe not enough time for 'gee' feedback to the FCC's if in fact or to what extent they use it with gains fixed?

More work to do. I have been surprised (educated) before at various control surface deflections commanded by the FCC's in particular circumstances. Rarely do you see full control surface deflections for full SS inputs. And of course, day to day, actual flying you will probably never see full SS input. Which is a good thing.

edit: BTW if 9 degrees of NU elevator commands a delta gee of +1.5 (2.5 absolute) at 260 knots, I would think it would not be a stretch that 4 degrees of ND elevator (1/4) could command a delta gee of -2 (-1 absolute), 70 knots faster at 330K.

rudderrudderrat

Hi OK465,
I think it would be a good idea to know how much control surface defection is being applied - especially when the flight computers decide to only give me half - A320__Hamburg-Cross wind landing.pdf page 46
"During the next few seconds the aircraft rolled to a 23° left wing down attitude in spite of the full right deflection of both sidesticks and application of right rudder. The switch to Ground Law limited the effect of roll control corrections. The left main landing gear again made contact with the runway. At about the same instant, the left wingtip made contact with the runway."

Owain Glyndwr

Hi OK465

Let's see if we can put together some sort of explanation that will reconcile what we have.

Bearing in mind that your original question related to full ND in a developed stall.

If the system gains default to 330 kts values then full ND SS will give an initial elevator deflection of about 4 degrees at all speeds.
If the aircraft were genuinely at 330 kts this would be just about right to produce a delta gee of 2.0 taking the aircraft to the manoeuvre limit of -1g.
If the aircraft was in a developed stall (say 155 kts) then this elevator would, using back of envelope scaling, give a delta gee of only - 0.44.
The system would then see that this result did not conform to the expected delta gee which goes with the SS position and would apply additional ND elevator until it got a match or the elevator hit the stops.
If you cut off your observations before this second phase came into play then you would only see about 1/4 ND elevator applied.
Had you let things develop then you would have seen bigger deflections which would be consistent with what I saw on the other simulations.
If the elevator were deflected to full travel (15 deg ND) then the available delta gee would have been about - 0.75 until the THS started to move.

Does this make sense?

DozyWannabe

I'm a little cautious about going too far off-topic when the conversation has suddenly become interesting again, but given my basic understanding, the function from which the alpha protection AoA is derived provides a small buffer zone between that value and alpha max - the reason being that alpha max is the highest AoA the wing can achieve while continuing to provide sufficient lift (as an aside, the alpha floor trigger function computes an AoA between the alpha prot AoA and alpha max).

If the Normal Law protections were to stop dead on alpha max, then even a relatively minor change in the airflow over the wing could conceivably cause the AoA to exceed alpha max and consequently stall the wing. I used tailwind only as an example of change in airflow.

OK465

Yes, it certainly does. Thanks OG.

Timing comparisons with active versus fixed, good idea...

(As they say, timing is everything....getting to the second phase....chrono start....patience, patience. )

HazelNuts39

OG, OK465:

Just for comparison:

In the QF72 involuntary pitch-down 10° ND elevator was applied during about 1 second (2 seconds including ramps). The airplane pitched down from 2.1° NU to 8.4° ND and the normal acceleration reached -0.8 g.
Flight condition FL370, M.815, 265 kCAS, c.g. 25%.

CONF iture

Negative.
Alpha Max is the limit Airbus gave to its protective system, and they were not fool enough to put that limit on the edge.
If they wanted it on the edge they would have put Alpha Max at Alpha Stall where the lift coefficient is the highest.

For the tailwind ... I don't see where you're going ...

DozyWannabe

Probably not the greatest source, but the best I could find at short notice:

A320 Flight Controls flashcards | Quizlet

OK, I'll try to illustrate by way of example. As I understand it, a large amount of following airflow can be sufficient to reduce the amount of lift generated by a wing, which is why the third phase of a microburst encounter is just as, if not more, dangerous than the second phase where the downdraught forces the aircraft down. If you have an aircraft with a wing at the max lift coefficient which subsequently encounters a strong airflow from behind, then the lift may no longer be sufficient to keep the aircraft aloft.

If I've got this backwards, fine - but it seems to make sense to me...

[EDIT - I wasn't kidding about going too far off-topic, I really don't want to derail a discussion on an accident where the protections were inhibited by talking about protections...]

roulishollandais

Hi Dozy,

You know that in the closed loop with feedback like the autotrim system, in automation we use to say (in french) : "on pilote l'erreur" said in common but acurate language "the input is not the human command, but the difference between the command and the output", and that input is not human but computed.

So I suggest to use here in PPRuNe the word "automation" for the autotrim, which is more similar from the common sense from "stupid" pilots !!!

DozyWannabe

I'd say that's not entirely accurate. The *output* of the autotrim system is computed based on three primary inputs - elevator position, THS position and command from the flight controls. When hand-flown, the last of these inputs is indeed human.

"Common sense" is by its very nature subjective, and if the discussion here is any guide there seems to be no significant barrier to pilots understanding the terminology.

That statement is loaded anyway - no part of the Airbus FBW system, and that goes for the PFC design, digital flight control system and the protections exists because anyone thought pilots were "stupid". The press liked to play that angle up because stirring up distrust between pilots and engineers means controversy, and controversy means more column inches. It exists in part because technology reached a point whereby the more mind-numbing aspects of aviation could be performed reliably, thus reducing workload on a two-man crew, and that in the case of a non-technical emergency, the pilot could perform manoeuvres safely without worrying about staying within the envelope. For management there were commercial benefits too, but that's outside the scope of what we're discussing here.

Anyway, that aside - I think autotrim should be considered distinct from the more traditional definition of automation because when under human control it simply determines the optimum placement of the THS based on the inputs it is given. Unlike autoflight it does not attempt to alter flightpath on its own. An admittedly rough analogy would be like power steering on a car.