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AF447 Thread No. 3

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AF447 Thread No. 3

Old 27th May 2011, 14:55
  #61 (permalink)  
 
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For interested layman:

Angle of Attack Indicator

If I hold the pitch of my airfoil constant in a fixed wing aircraft (I'll do that by controlling the nose pitch attitude, either via visual reference to the horizon, or on the attitude indicator based on a gyro of some sort)

and then I vary my airspeed,

my AoA will change: increase as I go slower, decrease as I go faster.

Depending upon model of aircraft, when the fuselage is roughly level compared to the ground, you may have a few degrees of pitch up, relative to level to the ground on the airfoil. (There is also on some aircraft "twist" from wing root to wing tip, which would induce the wing root to stall before the wing tip at high angles of attack ... )
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Old 27th May 2011, 14:55
  #62 (permalink)  
 
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Could the physical sensation of a rapid descent combined with doubt about the veracity of the flight information and the lack of outside visual references simply trick the brain into misreading the situation they were in?
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Old 27th May 2011, 15:05
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It can. 3-D disorientation is a common phenomnon in instrument flying.

Part of the purpose of instrument training is to get you to focus on your instruments, regardless of spatial disorientation, and to develop scan patterns that allow your brain to sort out what the aircraft is doing. It's a skill that is useful to practice in case you need it. If you have some instruments that are of doubtful accuracy, most scan training teaches you to cross check other instruments to see what the aircraft is doing. (I called this "partial panel scan" in another thread but that may not be the current term of art).

Now, if you are stalled, the flight controls don't respond as they normally do, getting unstalled on instrument scan (the first time you do it) is a bit tougher than simply flying maneuvers on instruments. If you are stalled and rotating, the degree of difficulty goes up an order of magnitude or so. (First time I ever did a training spin "under the bag" in instrument training was an eye opener. It took a few tries to get the hang of what I needed to do, and to actually do it. Some years ago in a single engine trainer).
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Old 27th May 2011, 15:07
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Originally Posted by Lonewolf_50
Vertical stab isn't the THS, but thank you for the load factor.
Good catch!... I didn't see it while typing/posting that.
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Old 27th May 2011, 15:07
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Which leads to a post I made last week - why don't they fit AOA instruments?
The difference between "the bird" (the flight vector symbol) and the pitch reference on the PFD gives you the angle of attack - if you are aware of it.

If stablizer trim is full nose up (why is not important at this point) and TOGA is applied - does the elevator have the ability to overcome the consequences if full nose down input is made and held (without changing stabilizer trim)?
No - for any aircraft with this configuration, the stabilizer is more effective than the elevator, and easily overpowers it, especially with the added pitch up moment from high thrust.

There's a stall warning at 2:10:51, then the report notes that the stall warning horn stopped at 2:11:40, the clear implication being that the horn was sounding throughout the intervening period. So the instruments could not have been telling the PF he is overspeeding, as the instruments clearly thought he was stalling -- and saying so.
Stall warning comes from the angle of attack sensors, not the airspeed indicators - so a stall warning with an indicated high speed is possible.

Or is pitch calculated from the [email protected] ring gyro and AoA via an aerodyamic sensor?
That's right - in the pic below (this one from a Boeing, but airbus use the same systems) you see two pitot tubes which detect airspeed, and between them the angle of attack vane, which detects the direction of the airflow (and thus the angle of attack).


The inertial reference system also supplies vertical speed information, along with ground speed - so the vertical speed is independent of any air sensing failure.
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Old 27th May 2011, 15:09
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What is also interesting is that the pilots discussed the wx ahead. They told the cabin crew that here would be an increase in turbulence. How did they know that? Were they planning to fly through some of the returns? Flying through echo's at level 350 is a bit iffy, not something I would do voluntarily and not something I can imagine the captain leaving to the two FO's.

So here is the retrospective catch-22. The wx could not have been that bad, otherwhise the captain would linger a bit longer in the flightdeck. However the wx was bad enough for the FO's to warn the cabin crew. Was it that bad that there was no space to properly avoid the echo's? In that case, why was the captain away? The PNF suggested tentatively a turn, but only a turn of 12 degrees was made. What were they avoiding?

I have never flown across the ITCZ, so maybe others can comment, but again I feel as if we are missing a lot of information.
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Old 27th May 2011, 15:09
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Verdict not in, but a question or two

Hey, TK??? Or others, for that matter.

- Don't have the full manual, only some flight control law pages. So...

Is the THS manual trim wheel connected to the THS via a mechanical means versus simply another electric command to the servoactuators/jackscrew motor/whatever?

- Is the THS position displayed in the cockpit?

- Any mention of "runwaway trim" procedures?
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Old 27th May 2011, 15:12
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AoA vs Pitch

Got it, thanks ( and tanaka too )
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Old 27th May 2011, 15:14
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high incidence

Hi there,
The BEA note states that
- most of the pilot actions were pich up commands
- @ 02:10:51: incidence around 6° (when the stall alarms sounded), TOGA, pich up commands. 15 sec later, incidence around 16°, FL380, pich up commands again
- @ 02:11:40: end of stall alarms, FL350, -10 000 fpm, pich angle 15°, N1=100%, large (40°) roll excursions, pilot commands: roll compensation & pich up (during 30 s)
- @ 02:12:02: FP says "no longer any reliable indications", N1=55 %, 15 s later: pich down commands, incidence is decreasing, IAS reliable again, stall alarms
- @ 02:13:32: FP says "we are going to get under the FL100"

the incidence has always exceeded 35°, and seen from the pilots, the altitude was decreasing rapidly (a bit more than -10 000 fpm) with an inertial piching attitude reaching 15°: how can we explain that most of the pilot commands were to pich up the plane except around 02:12:17 (15 s after 02:12:02) ? Did the crew apply the AF SOP in the early phase (around 02:10:51) by setting TOGA thrust and a pich up at high altitude ? It is only 1 min 11 sec after that THR is on IDLE, the N1 is on 55 %, and that pich down commands are applied.

the stall is not recovered and the terminal vertical velocity is around -10 000 fpm.

why a crew would respond to the early stall alarms by a nose up ? spatial disorientation ? confusion/stress due to multiple conflicting indicators ? or tricky/misliding/lengthy procedures ?

Last edited by Hyperveloce; 27th May 2011 at 15:21. Reason: correction
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Old 27th May 2011, 15:15
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Originally Posted by BOAC
The only observation I can make is that I have seen a full and maintained nose-up demand from a pilot in a different aircraft before, in reaction to a large (and unexpected) rate of descent while stalled.
You know what? That makes absolute sense.
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Old 27th May 2011, 15:16
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Is the THS manual trim wheel connected to the THS via a mechanical means versus simply another electric command to the servoactuators/jackscrew motor/whatever?
Yes, cable connection to the hydraulic servo motor (in the A320 at least) with a placard position gage next to the wheel. If you have hydraulic power, you have trim.
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Old 27th May 2011, 15:16
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A few thoughts after reading this prelimanary report, and the many questions it raises. I write in a modest way, as it has been some years since I flew those planes, and I have forgotten a lot.
My very sad and friendly thoughts to the pilots of this flight.And everyone aboard, of course.
The THS (trimmable hydraulic stabilizer) auto trims the plane, the PRIM 1 is normally in charge of that job. The report does not say if the PRIMs (there are three) were lost, which is possible after losing speed information, and it does not say (understandably, it will come later) if from normal law, then alternate, the plane ended in direct law, then changed back etc…(possible resettings, speed indications coming back…)
If in direct law, there is not automatic trim, the horizontal stabilizer stays where it is and you have to use manually the trim wheel which a warning on the PFD, among very many other warnings in that case, tells you to do.(I am reminded of the Perpignan accident where the captain fought with the plane but if I remember correctly, never trimmed manually, with a very UP THS, exactly as described here, though for other reasons of course).
The THS is very effective, and even with sidestick applied fully down, if it stays at 13 degrees up, I think lowering the nose will be difficult…
From what I understand, having been familiar with this company, the captain went to take a rest, asked the most experienced of the two copilots (who had the appropriate qualification) to take his seat (left one) and the copilot in the right seat was PF. This is quite normal. If, and I do not believe it for a second, the PF was « lost » and applying full aft sidestick, while on the instruments, all kinds of understandable things happened, with chimes, alarms, synthetic voices etc…the copilot in the left seat , and even the captain coming back and sitting between them, had no way at all of knowing this, and could believe that he was applying the correct inputs, unsuccessfully. In the old days (and I am a very old-timer), putting your hand on the trimwheel was a habit…Nowadays, you don’t even know you have one, except for checking its position at take-off, which is not the same as using it in manual flight. Rolling it manually far forward, and that is always possible, in all modes or laws (correct me if I’m wrong, active bus drivers) and applying cruise thrust might have solved the problem, but that is easy to say from where I sit. I am quite sure of one thing, as you all are, but it bears to be repeated : thos pitot probes were changed by an efficient management, many months before, in another company, after an icing incident. The management of Air France truly thinks, not having done anything, that they did everythin very well, since it was not mandatory. Food for thought. May they sleep in peace.
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Old 27th May 2011, 15:17
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Cool

Hi,

The BEA has kept its promise
He submitted a series of factual facts in chronological order
He announced that this would be incomplete .. it does not disappoint.
Upon reading the end of the release .. Lambda can conclude that:
The aircraft responded to control of the pilots and it was structurally capable
That the pilots did not follow the guidelines of Air France in effect has the time for this kind of event.
As this release is incomplete and is filled with shadows .. full of scenarios can be envisaged.
I am sure members of this forum will describe possible scenarios
Conclusion:
Much work awaits the BEA and the court of Paris

Last edited by jcjeant; 27th May 2011 at 15:40.
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Old 27th May 2011, 15:19
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Few remarks come to mind

Apparently those 3 well trained pilots did not manage to recover from a high altitude stall with a fully functional and structurally sound aircraft. They even recovered reliable speed measurements about half way in their descent but still ended up in the water. This is really puzzling and I believe we might still be missing an important piece of the equation.

For the fairly high tech measurements and recording equipments on board it seems that there is in fact not that much information being saved. I would venture to say that with modern FBW planes we should have all inputs covered as well as some "trace / log" of what the software is actually doing.

Although rare pitot icing is not unheard of. As other have mentioned it would be desirable to add some different airspeed measurement devices, as FWB can really become confusing with unreliable airspeed.
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Old 27th May 2011, 15:20
  #75 (permalink)  
 
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why a crew would respond to the early stall alarms by a nose up ? spatial disorientation ? confusion/stress due to multiple conflicting indicators ? or tricky/misliding/lengthy procedures ?
... or simply being constantly told during training "You can't stall an Airbus." as an absolute statement, reinforced by full back & side stick climbs at the lowest possible speed in the simulator to demonstrate "How the aircraft won't let you stall."
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Old 27th May 2011, 15:21
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Gums: take a look here > SmartCockpit - Airbus 330

I believe the flight controls section has what you might need.

My understanding is the trim wheel has mechanical linkage.
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Old 27th May 2011, 15:21
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Manual pitch trim (2)

takata wrote : If Airbus didn't want someone to ever use the trim wheel, the very simple way was not to put one in the cockpit at the first place. They would even have made the electrical imputs to have a precedence over the pilot mechanical control which was, of course, made the other way.
In the Perpignan accident, manual pitch trim was not used.
In this accident, it would appear it was not used either.

The fact that the manufacturer left this manual pitch trim wheel in the cockpit does not exclude the fact that in real life, in real airlines, on board real airplanes, pilots are actually taught not to use it, and never use it, according to design.

The piloting quality one would get using for the first time in a highly dynamic, critical situation this item of equipment would likely be very poor, anyway.
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Old 27th May 2011, 15:22
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Hi gums,
Originally Posted by gums
Is the THS manual trim wheel connected to the THS via a mechanical means versus simply another electric command to the servoactuators/jackscrew motor/whatever?
Yes. mechanical control, it is all you have in direct law.
Originally Posted by gums
- Is the THS position displayed in the cockpit?
Yes.
Originally Posted by gums
- Any mention of "runwaway trim" procedures?
Yes. indications written on trim wheel are for take-off (not related to flight position). I'll sent you that part later (no time to dig in now).
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Old 27th May 2011, 15:24
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Does anyone remember why the US Navy set up Top Gun? Maybe it's time airlines start renting Cessnas, Robins, and Pipers.

That, and fixing pitot tubes that ice up.
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Old 27th May 2011, 15:25
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Question

.... While it's nice to have some "factual" information, for a change, I'm certainly none the wiser for it. .... I hope it's not ages before we get the rest of the story. Conversations in the flight deck could be somewhat illuminating...
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