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AF447 final crew conversation - Thread No. 1

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AF447 final crew conversation - Thread No. 1

Old 11th Feb 2012, 23:35
  #1301 (permalink)  
 
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Wouldn't hurt to go back to making real airplanes again either !!
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Old 12th Feb 2012, 00:30
  #1302 (permalink)  
 
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As long as you keep your hands off the pitch input, that aircraft will decel to the stall and keep on decelerating, and the trim will run as far up as it can
Question from SLF. Is that not a problem right there? I mean by way of a disconnect between man and machine.

The handling qualities of earlier aircraft stipulated stick force per "g", and it was up to the pilot to trim out any stick force. If disturbed the aircraft naturally sort to return to its trim speed.

It's hard to imagine the pilot of a non Airbus holding full back stick and manually trimming full nose up no matter what the circumstance.

The power of a trimable stab is such that it becomes the primary pitch control, with the elevator assuming the role of a short term pitch modification.

The pilot of old automatically knew the trim state of the aircraft because he put it there, where as the Airbus, I gather, the pilot is only aware of where the automatics have put the stab is by reference to instrumentation.
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Old 12th Feb 2012, 08:41
  #1303 (permalink)  
 
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You may be SLF, but you have made a very salient point, that the pilot knows the trim state because he put it there.
Earlier designs (thinking DC9/737) kept pilots in the loop whilst auto-trimming, by either having an irritating honking horn, or a chunky great [email protected] wheel clanking round. In this accident of course neither would have auto-trimmed, although the Turkish accident in AMS showed us another danger of auto-trimming, even on a dinosaur like a 737.
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Old 12th Feb 2012, 14:44
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Brian

It's hard to imagine the pilot of a non Airbus holding full back stick and manually trimming full nose up no matter what the circumstance.
I totally agree, Sir. However, having flown Boeing 737s, as well as Airbus 320 and 330, I would add that it is equally hard to imagine the pilot of an Airbus holding full back stick.

It seems to me that the initial error was the PF's failure to perform the Unreliable AirSpeed (UAS) drill, which is a memory item. He then pitched up to an unrealistic attitude. As the aircraft would have been in trim prior to his sidestick input, the force required to hold the sidestick fully aft would, initially, have been large. That effort would diminish as the autotrim took effect, but it would take a significant time.
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Old 12th Feb 2012, 15:22
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Originally Posted by Neptunus Rex
I totally agree, Sir. However, having flown Boeing 737s, as well as Airbus 320 and 330, I would add that it is equally hard to imagine the pilot of an Airbus holding full back stick.
Let us correct this popular misconception. Initially PF did not yank full back stick. His average aft stick was about 6 degrees out of 16 degrees possible for the first 10 seconds. What he was mostly doing was chasing a roll oscillation (PIO?).

It was not until later when the aircraft became fully stalled that he began to hold full back stick-probably to prevent the nose from falling through.
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Old 12th Feb 2012, 15:57
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Does he even need to hold in back stick to keep the nose up? or doen't this apply to the law they were in?

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Old 12th Feb 2012, 17:02
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Originally Posted by Machinbird
It was not until later when the aircraft became fully stalled that he began to hold full back stick-probably to prevent the nose from falling through.
Was he perhaps trying to maintain 15 pitch attitude?
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Old 12th Feb 2012, 18:23
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Originally Posted by HN39
Was he perhaps trying to maintain 15 pitch attitude?
What PF was doing would make better sense if his PFD had a ~ 12 degree pitch bias, but I don't think that happened. Instead, he was probably trying to maintain around 1 g and the harder he tried, the worse his problems became.

Cwatters, your diagram is essentially correct except there wasn't much good aerodynamic data available, so no protections that would have helped. Since they had no idea of Mach number (which affects stall AOA), they didn't even give the AOA system authority to control the aircraft and push the nose down, instead limiting it to shouting, "Stall" repetitively (until silenced by the A/S <60 knots).
Roll however was in Direct law so that was a whole new ball game for PF's level of training.

I am personally of the opinion that other than the airspeed system, the airframe systems performed exactly as designed.

I am not ready to cast PF into the entirely unfit to fly bucket yet. I have this uneasy feeling that many of our weaker brothers in the cockpit would have succumbed to this same scenario, particularly because there is probably something about the total scenario that we do not properly appreciate.

The human factors group at BEA will be having an interesting time if they do their job correctly. I only hope they can explain their understanding adequately.
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Old 12th Feb 2012, 19:39
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Originally Posted by HazelNuts39 View Post
Was he perhaps trying to maintain 15 pitch attitude?
Looking at that graph, I guess it's entirely & depressingly plausible.

He selected toga as well. Toga/15 is, I believe, the first pitch/power setting on the AF uas procedure (entirely the wrong one to use for cruise, but did they know that...)
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Old 12th Feb 2012, 19:54
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Originally Posted by Neptunus Rex
I would add that it is equally hard to imagine the pilot of an Airbus holding full back stick.
Not that hard. It is actually what Airbus wants to force in your DNA for a number of situations. no thinking, just full back stick to get the best of the protections ...
That gives very good performance ... as long as data are reliable.

As the aircraft would have been in trim prior to his sidestick input, the force required to hold the sidestick fully aft would, initially, have been large. That effort would diminish as the autotrim took effect, but it would take a significant time.
Let me disagree here again :
the force required to hold a sidestick fully aft does not change a iota whatever the situation, whatever the trim setting.
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Old 12th Feb 2012, 20:09
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Machinbird

The poor PF was not only the PF, he was also the deputy Captain at the time.

He was trying to correct a PIO when by the book he should (as Capt) have called Unreliable Airspeed.

PNF didn't realise that all this was way too much for the PF. He could have done more to save the ship.

I suspect, without proof, that AF procedures (seemingly assuming that nothing would go wrong) contributed a lot that night.
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Old 12th Feb 2012, 20:35
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Originally Posted by Brian Abraham View Post
It's hard to imagine the pilot of a non Airbus holding full back stick and manually trimming full nose up no matter what the circumstance.
Really ? Pop-quiz - which (now somewhat fragmented) aircraft is this then:
The crew did not take any action to re-trim the aircraft to the desired speed, so, once the nose down pressure was released on the control column, the aircraft pitch started to increase again
[...]
The aircraft re-entered a stall situation (AOA reached its maximum values of around 26 at 00:41:09). Contrary to any stall recovery procedure, the control column was initially kept backward and gradually increased over the next 17.
[hint - it's one of those hard-to-imagine non-Airbuses]

The power of a trimable stab is such that it becomes the primary pitch control, with the elevator assuming the role of a short term pitch modification.

The pilot of old automatically knew the trim state of the aircraft because he put it there, where as the Airbus, I gather, the pilot is only aware of where the automatics have put the stab is by reference to instrumentation.
Auto-trim is damned whatever it does.

447, auto-trim stays on, pilot pulls back through stall warning, trims up in stall -> "auto trim should hand back to pilot in stall"

Perpignan, auto-trim does hand back to pilot, pilots never re-trim, leads to fatal secondary stall. Would have been much better chance of recovery had auto trim followed pilot commands. -> "auto trim should follow pilot commands in stall"


It's not just an airbus problem however:

Ethiopian, Beirut - mis-trimmed, pilots never re-trim, pulled back through fatal secondary stall.
Turkish, Schipol - auto-trim does hand back to pilot, pilots never re-trim (recovery fails because they let throttles go back to idle again however)
Colgan, Buffalo - auto-trim might have had a hend in setup, but they pulled back all the way down and pulled the flaps in just to be sure...


Sure, there may be a pattern there somwhere, but "airbus" ain't it.
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Old 12th Feb 2012, 21:25
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Auto-trim is damned whatever it does.
Auto-trim is a has been with us some time and has been known for causing trouble. Pilots first began to run afoul of it with the early autopilots that would trim unbeknownst to the crew-until it then kicked out and left the crew with a handful.
Some implementations of FBW require autotrim to function properly and it can simplify the job of flying. Autotrim has teeth however, and pilots who regard it as piece of the furniture in the cockpit and thus lose awareness of what is doing stand a great chance of being adversely surprised by it.

That is why most of the modern aircraft have noisemakers associated with trim movement. But one major airframe maker skipped this step. Why is that?

And the answer to the pop quiz is the Ethiopian B-737 at Beruit. For whatever reason, PF didn't want to handle the aircraft that night, but the A/P never relieved him. Don't you guys want to earn your pay for flying?
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Old 12th Feb 2012, 21:33
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Cwatters, your diagram is essentially correct except there wasn't much good aerodynamic data available, so no protections that would have helped.
I realise that but..

For me the important bit is that neutral side stick means "maintain this attitude". eg It's not necessary to hold in back stick to keep the nose up once it's up.

What does the equivalent diagram look like for other aircraft? Do you have to hold the stick back to maintain a nose high attitude or will neutral stick do it?
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Old 12th Feb 2012, 22:33
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What does the equivalent diagram look like for other aircraft? Do you have to hold the stick back to maintain a nose high attitude or will neutral stick do it?
For conventional non-FBW aircraft and Boeing FBW, it looks like the well known phugoid where the nose drops as you slow from trim speed. (But let me preface this with the statement that my Boeing FBW understanding is theoretical.)
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Old 12th Feb 2012, 22:55
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Auto trim may be nice to make flying easy but it doesn't do much to help piots fly when things go amuck. These pilots were not the brightest so let it lead them into a deep stall because they were stupid.

We all know why they crashed. They were incapable of flying an aircraft with unreliable airspeed. All aircraft have charts and pilots should know approximate attitudes and power if airspeed goes bad. Nobody pulls full up on the stick hoping the automation will save them but they did. When they went into alternate law the airbus was no longer idiot proof.
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Old 12th Feb 2012, 23:00
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Machinbird

(But let me preface this with the statement that my Boeing FBW understanding is theoretical.)
Iceman.
My first impression was that you had done your sim evaluation in a Boeing type sim, but I see you list yourself as a 'Bus pilot, seat 0A, so will give you the benefit of the doubt.
Very condescending!

Perhaps you could also grace us with your practical / hands on knowledge and experience of the flying the Airbus. ( An Airbus pilot TRE/IRE, + 20 years military- Rotary(piston/gas turbine) fixed wing (piston and fast jet), 6 years Boeing (757/767) and 16 years Airbus flying A340/A330)
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Old 12th Feb 2012, 23:04
  #1318 (permalink)  
 
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Originally Posted by cwatters
neutral side stick means "maintain this attitude"
I believe that this is correct provided a thrust is set that maintains arspeed for the chosen trajectory.
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Old 12th Feb 2012, 23:11
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A Bit of History

What does the equivalent diagram look like for other aircraft? Do you have to hold the stick back to maintain a nose high attitude or will neutral stick do it?
Positive aircraft stability means that when you let go of a control, the nose of the aircraft returns to the speed that you had when you first moved the control. This speed is called the trim speed. Neutral pitch stability means that when the nose is moved, the pitch angle remains where you put it.

When the Wright brothers started developing aircraft a century ago, they initially worked with the premise that aircraft should exhibit positive stability in yaw but be neutrally stable in pitch and roll. As the first test pilots, they quickly discovered that it is easier to fly if you have positive stability in pitch. If you pull or push on the stick in pitch and let go, the aircraft returns to the trim speed and settles down. This is particularly helpful in turbulence. If a gust displaces the nose in pitch (or yaw) the plane simply returns to the trim speed without help from the pilot. Accidentally bumping the stick or yoke is no problem.

Some PPRuNers have asked why the AF447 PF didn't simply let go of the side stick and let the plane resume the trimmed speed. Why indeed? In every plane those PPRuNers had flown, letting go would have allowed the plane to return to trim speed. In the AB system, letting go, which neutralizes the stick, does nothing but stop the movement in pitch. It does not cause the aircraft to resume the previously trimmed speed because the autotrim has set a new trim speed. The trim speed set by the autotrim in AF447 (in response to the PF's side stick input) was below stall speed.

With the AB flight control system, because it incorporates autotrim in pitch, the aircraft essentially becomes neutrally stable in pitch because the autotrim follows the sidestick command. The airplane does still retain positive stability in pitch when the nose is moved by external aerodynamic forces, but if you move the stick in pitch, the nose stays at the pitch angle commanded.

With a century of flight experience under our belts, why did AB decide to alter the flight control philosophy? Good question.
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Old 13th Feb 2012, 00:17
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Originally Posted by Smilin_Ed View Post
With a century of flight experience under our belts, why did AB decide to alter the flight control philosophy? Good question.
They didn't. The "philosophy" and control laws they used were already tried and tested in military a/c (and the odd NASA vehicle I believe...). The behaviour you are referring to is apparent neutral speed stablity, and is (I think) a fairly fundamental characteristic of the c* ("c-star") control law. Airbus just applied existing FBW control knowledge to a civilian a/c.

Boeing, on the other hand, for the 777, invented a completely new control law, c*U, to give the effect/illusion of speed stability. As far as I know this was completely new control law/philosophy, never tried on any previous a/c, and arguably therefore much higher risk.

As to which turns out "better", I guess maybe we'll see as FBW spreads beyond A&B - already in Embraer I think, and filtering down to the bizjets. Have they chosen A-style or B-style ? [I don't know, haven't looked].

[There might be another complication - I vaguely recall Boeing having patented the whole c*U concept, in which case they might be able to keep it Boeing-only even if it is "better". Love the patent system...]
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