Machinbird

Clandestino, As a line pilot, you are not supposed to be taking your aircraft to any points in the envelope the test pilots have not already demonstrated. That is part of what certification is all about.

Automatic systems should not assist you in taking the aircraft outside the demonstrated envelope. If they do, they are improperly designed.

DozyWannabe

But autotrim under manual control is not "automation" in the classic sense of the term, it is entirely slaved to the demands of the pilot in control, which is why you have to be more careful with the protections out. Holding the stick back that far for that length of time is the antithesis of "careful".

I think we're going to hit something of a semantic argument here where those who are sympathetic to the Airbus FBW design will consider the fact that autotrim gives effectively complete trim control through the sidestick to redefine "primary flight controls", whereas those of a more traditional bent will insist there be a separation.

If you take the definition of "primary flight controls" to be anything you can do with the sidestick or rudder, then is is a simple matter of pushing forward on the stick to recover the trim position to normal while in the process of unstalling the aircraft.

Clandestino

Exactly! Pulling further into the stall when faced with stall alarm makes a whole lot of the assumptions on which certification is based nil and void.

Machinbird

Dozy,
Why would you be holding a control back for that length of time. It is because the aircraft was not performing the commanded action (It couldn't).
It may be a natural human instinct to re-select or hold a control when you do not get the expected action the first time, but as we see that this can be very dangerous. Things can be happening unseen (like the trim running nose up).

OK465

(my bold)

MACH: The elevator alone took the aircraft out of the demonstrated flight envelope. The THS behavior you find objectionable occurred as a result of being outside the envelope.

The flight test stalls performed in the aircraft were done at specific weights in specific configurations with specified CG's and at particular altitudes.

The three test data points for each stall were onset of the stall warning, onset of buffet and a termination point which was generally associated with a 'break' in the VSI occurring, a result of either 'pitchdown' or 'pitchup' depending on config. They are very structured and after that termination point you are 'out of the envelope'.

RetiredF4

Thank you for confirmation.

Another bit i found in the FDR traces and i like to point to:
Look at Page 111 of the BEA 3. interim report second trace from bottom. It is the normal acceleration graph.
02:10:28 - 02:10:50 below 1.0g, average g is 0.8, peak value is 0.6
02:10:50 - 02:11:10 above 1.0g, peak is 1.2
02:11:10 - 02:12:00 below 1.0g, average is about 0.9, peak is 0.7

In the timeframe from 02:10:28 up to 02:12:00, where the elevators and the trim moved to full NU (ordered by loadfactor demand from SS), only 20 seconds from the total of 92 seconds had been with a loadfactor of 1 g and more. All the other 72 seconds the actual loadfactor was below 1 g, giving the crew the feel of unloading (which actual was reality!) and doing the right thing. The SS NU inputs in this phase could be explained as controling the unloading.

By the way, you will be amazed, how much the body can feel an unload from 1 g to 0.8 g.

I´m fully aware that most of this loadfactor was created by the increasing vertical component of the flight. Did the crew know as well?

I´m open to other explanations.

franzl

Machinbird

OK465
3rd BEA report
Not really disagreeing, but I believe the application of thrust at that point finished the job of taking the aircraft out of the envelope. They would likely have been able to do it with elevator alone as well, it would have just taken a little longer, particularly since PF was making inputs in both directions at that time. If they had not put in TOGA thrust, it may have kept struggling along until the Captain got back or the airspeeds returned.

Note: The THS had piled on at least one degree of trim by the time the aircraft left the envelope. If it had been stopped at stall warning, it would have been a bit harder to pull all the way into the stall.

I guess I find it objectionable because it was unnecessary and had the effect of digging the hole the aircraft was in deeper. Not only do you have to rotate the aircraft nose down to recover, you have to run the trim back down to have a successful recovery and that would not be a quick process unless done manually. The fact that the aircraft was driven further into the stall by the THS again made things still worse.

The trim is essentially an AOA control if I understand the relationship between trim position and pitching moments properly. The more nose up trim you have, the more AOA you want the aircraft to fly at. The closer you trim to a stall AOA, the easier it is to pull into a stall.

How often does the Bus need to trim 12+ degrees nose up anyway? Aren't the flaps down when you do it and the cg is well forward as well? Would logical configuration related limits on trim position be appropriate?

Lyman

Notwithstanding the last bit of kick into STALL, the THS promised a long trim effort whilst Nose Down, subtracting prodigious amounts of altitude, adding breathtaking velocity at PULL OUT, with an aircraft that stubbornly limits G, (Probably a good thing, for that matter).

With that much NU, one expects less than docile 'stability' in the dive?

HazelNuts39

Did you take into account the variations of 'gee' due to buffet shown on page 43, together with the associated Note?

Lyman

So they are actually identifying it as "buffeting"? Because if you read the addend closely, you will see how BEA works to make a "could be" an actual fact. So, I have been reading the BEA this way from the git; it is the creation of a separate reality before one's eyes.

"How did the crew disregard the STALLWARN, and the blatant BUFFET.....etc. etc. etc.........."

Numptification by proxy, and nary an harrumph save one. You fellas are too easy.

Grip/a/Get

Clandestino

If one take this phase out of context, this explanation is plausible. However, on pages 29-31 are some parameters traced more precisely than in appendix and they don't confirm that theory. First part, where load is slightly below 1G is where CM2 somewhat heeds CM1 advice to go down. Sidestick input is nose down, pitch goes from 12 to 6 ANU but aeroplane is still climbing and bleeding speed. Second phase, with G slightly above 1 is when stall warning goes of and CM2 reacts by pulling up again to 17.9° ANU - there are brief periods of pitch down command, quickly superseded by pulling again. G below 1 in third phase you mention is aeroplane stalling with oceanward acceleration accouting for Nz<1 till terminal velocity is attained. Stick is hovering around slight nose-up, to eventually move to full nose up.

IMHO, CM2 was not controlling the unloading by pulling, he was bent on pulling ever since he lost airspeeds and autopilot, for reasons not picked up by the CVR.

Ouch. You don't trim for pitch, you trim for AoA. How many units of trim is needed for given AoA? Depends on speed, weight, config and CG position. A330 is long aeroplane with large speed range, so having powerful THS is necessary. I wouldn't suspect Airbus designers of being wasteful and putting too strong THS on their airframe. DP Davies has it all neatly explained.

Machinbird

Clandestino, that wasn't a pitch attitude, it was a trim setting. Sorry if I wasn't clear.

I think my point is that the higher trim settings are rarely used unless the cg is way forward and the flaps are down. What is the highest trim setting you could encounter in a clean aircraft within the normal flight envelope?

CONF iture

As we got little extra time in the sim today, we did experience a full stall from FL350.
Here is what I can report from the experience :

• From the STALL warning we kept a light aft pressure on the sidestick
• It was not long before we got a negative vertical speed of 15000ft/min
• THS went to 12 deg UP under STALL warning
• As we decided to exit the stall, full fwd pressure on the sidestick was applied
• But we were unable to lower the nose
• THS did not move
• THS was then manually rolled fwd
• Nose came down
• Exit was then possible

I can't remember all the details, too much stuff to look at.
Thrust was kept at idle all the time.

Early fwd pressure on the sidestick at initial STALL warning should prevent a stall to develop.

Capn Bloggs

I'm not an aerodynamics expert, but I can't believe that 10° of huge horizontal stab is equivalent to 15° of the relatively little elevator.

Organfreak

This seems to be breakthrough information in re the THS!
Stupid questions:
Why wasn't TOGA tried? Did somebody stop you?
Why has it been stated (someplace?) that the sims won't reproduce stall conditions?

Machinbird

Organ guy, since I'm still awake, the answers are simple once you understand the principles:

TOGA is counterproductive in a stall. The low thrust line of the engines relative to the cg forces the nose even higher at high thrust levels. That is real bad for stall recovery.

The sims only accurately reproduce areas of the flight envelope that have had data collected by the manufacturer. Until AF447, no one had flown an A330 aircraft that far outside the allowed envelope. I expect the AF447 data is being used to make this terra incognita a bit more accurate, but it is not a complete data set as I understand.

DozyWannabe

@CONF

Was this an A320 or A330 sim session? Apologies, but I don't know which type you're actually on at present.

Zorin_75

You can determine the aerodynamic behaviour also through calculations and wind tunnel testing (which is BTW also all you've got up to the point of the first flight of a new aircraft, and they usually do pretty well out of the box nowadays). So the sim will of course do stall conditions (the other question being how much effort has at all been put into getting the model right for conditions that far out of the envelope), but to know how accurately your simulation reflects the real thing you need validation data from test flights and until now there hasn't been much data from free falling A330s to go on...

RetiredF4

I looked at those too, and i came to a differetn result and i´m not giving up yet. Those pages miss one vital information, the outcome of the actions in relation to airframe loadfactor. As the SS is a "loadfactor commanding device" (non technical expression) and the aircrew is feeling the loadfactor result, this value is important in judging the SS inputs the PF made.
agreed

Second phase, with G slightly above 1 is when stall warning goes of and CM2 reacts by pulling up again to 17.9° ANU - there are brief periods of pitch down command, quickly superseded by pulling again.

There we need to look at the TOGA power input as well, because that would have had a great deal in increasing pitch and increasing g-load. And as it looks like, an unexpected one.
Agreed, but again, the g load was less than 1 g, what would we expect a pilot doing, when the stall is not yet recognized and PF felt unloading ( and maybe even acceleration due to SI) for some time already? It was also the phase, where the altitude came down to the assigned FL350 again. The nose up limit stop could be the input to level off in FL 350.

At that point go back to Page 111, at 02:11:45 the THS and elevators and SS input all reach full nose up, and the pitch drops from 12° nose up to 12° nose down within 10 seconds and stays below the horizon until 02:12:15. I interpret that phase, that the THS was stalled and could not keep the nose up anymore. Look further to the loadfactor. The loadfactor increases despite the fact, that the nose drops (or because of it?) In that phase the THS got effective agin.

Must be a funny feeling, positive pitch unloading, negative pitch loading?

My analysis is different.
After the initial unfortunate pullup and the following unloading hey did not recognize the stalled state, as it was against any training. They sure did not expect to be stalled with a loadfactor below 1 g. Their concern was the roll and not to unload too much, leading to the SS inputs. The application of TOGA - after the stall warning sounded- complicated their situation and led to an increase of pitch, loadfactor and altitude. Correction followed, again the loadfactor was being kept below 1 g to get the nose slightly down and recover the altitude FL350. But vertical descent rate picked up (unseen) and the pitch remained high. When approaching FL350 the level off attempt with full NU SS and THS and elevators also full NU the THS stallled and the nose dropped violently. The THS unstalled due to the pitchchange and grabbed air again, load factor got positive.

At that point i dont want to go any further at the moment.

I think it is not fair to say, they pulled all the way from the beginning. There where mistakes, big mistakes like the initial pull and like not recognizing the stalled situation, but the handling of the SS had different motivations than sensless pulling.

HazelNuts39

Could that be the result of the thrust levers moving from TOGA to IDLE?