Habsheim
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Originally Posted by Dozy
But the claim that it should have been reached immediately in that scenario is not.
You still have provided no evidence that the difference in Alpha Max is down to the CONF setting.
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That's your claim - that the system prevented optimum performance by not getting to 17.5deg AoA quickly enough, is it not?
He didn't specify CONF 3 or CONF FULL for the demo - the assumption that there was a difference in the demo config versus that of AF296 has only come from you.
The words of your Hero is not enough ?
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Originally Posted by Dozy
That's your claim - that the system prevented optimum performance by not getting to 17.5deg AoA quickly enough, is it not?
He didn't specify CONF 3 or CONF FULL for the demo - the assumption that there was a difference in the demo config versus that of AF296 has only come from you.
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It is not - It is more specific - It still did not change.
It certainly won't efficiently as the elevators are commanded nose-down as a start.
And if the elevators are commanded nose-down to arrest the pitch-up tendency from the engines, as submitted by dozy, it is even a poorer concept.
Remember, we are still 2.5 deg short of alpha max.
And if the elevators are commanded nose-down to arrest the pitch-up tendency from the engines, as submitted by dozy, it is even a poorer concept.
Remember, we are still 2.5 deg short of alpha max.
Therefore, if the pitch attitude is limited by AoA protection and the pitch-up moment from the increase in thrust is expected to exceed that attitude, then the systems will be correct in counteracting that tendency through the elevators. As Capt. Bechet stated, this is normal behaviour, and something that Capt. Asseline should have been aware of.
Nothing in the documentation or the video states that 17.5 degrees AoA is guaranteed upon application of full back-stick. The demonstration in the video was not performed with the engines spooled down, and that would have made a massive difference to systems behaviour.
It does not have to be, what matters is the intention, the intention to efficiently deliver.
The systems were designed to keep the aircraft flying and stable even at attitudes which would be considered extreme for a transport aircraft. I'd be prepared to bet money that the combination of Asseline's inputs, had the aircraft been in Direct Law, plus the pitch-up moment from the thrust increase would almost certainly have stalled the aircraft. I'd also be prepared to bet that no human pilot would have been able to take in all the flight parameters and keep the aircraft more stable than the EFCS did.
but a contributory factor for the crash it has to be.
If CONF 3 was the setting for the video demo, why alpha max was at 15 when it was at 17.5 for Habsheim ... ?
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The point I've been trying to make is that when the aircraft is decelerating and thrust is not yet sufficient either to arrest the deceleration or power out of the projected flightpath, then either the pilot or the EFCS must take that into account when trying to keep the aircraft aloft. Commanding a pitch attitude equivalent to 15.4 degrees of AoA at timestamp X may well be the equivalent of 17.5 degrees AoA or thereabouts at timestamp X+1s.
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The Habsheim Pilot's Problem
I haven't read the complete accident report and only wish to offer a quick comment regarding this accident. What I see gives some credence to Confiture’s point of view. For common reference, I'll provide a link to a particular video of the incident and mention some key events. Crash Airbus A320-Vol 296 Air France - YouTube.
The pilot had two problems. He needed to accelerate his engines and needed to clear the trees. Jet pilots cannot forget the spool up time of their engines but at Habsheim that day, use of automation seems to have gotten in the way and created a surprise and a delay in adding power.
After rollout on heading, there are two small pitch adjustments visible. One at T=11 sec, and one at T=14 sec. These are possibly pulls on the stick intended to trigger Alpha Floor (which was not available).
The aircraft began to impact trees around T=26, or about 12 seconds after the second pitch adjustment.
If the pilot had been able to activate Alpha Floor at the second nose twitch, it appears (to me) that he would have cleared the trees.
But let’s look at the problem a little differently, from the viewpoint of what a small increment of g acceleration would have done for the flight path.
If .01 additional g was available at the second nose twitch and for the next 12 seconds, the aircraft cg would be about 23 feet higher-probably not enough to avoid an accident.
If .02 additional g was available at the second nose twitch for the next 12 seconds, the aircraft cg would be about 46 feet higher and would probably have escaped since the engines were spooling up nicely by then. (actually slightly less high since he would have converted some of his kinetic energy to potential energy and it probably would not be possible to maintain all the additional g for the duration.)
Perhaps you suspect that even that that small amount of additional g was not available, but consider that the video shows an alignment turn seconds before the nose twitches up. Was he decelerating that fast that there was absolutely nothing left to maneuver at the end?
In other words, if a measly .02 additional g had been available, the Habsheim event likely would not have been an accident. Would an additional.02 g have been available if the maneuver was flown in direct law and the pilot had pulled just slightly past optimum AOA and then eased off to optimum? That is definitely faster than an asymptotic approach to applying g and is the way most experienced manual pilots would lay on rapid g.
Looking at the Gordon Corps video, it seems there might well be some additional g potential/ AOA in the A320 Normal law being held back as a “safety margin”.
The pilot had two problems. He needed to accelerate his engines and needed to clear the trees. Jet pilots cannot forget the spool up time of their engines but at Habsheim that day, use of automation seems to have gotten in the way and created a surprise and a delay in adding power.
After rollout on heading, there are two small pitch adjustments visible. One at T=11 sec, and one at T=14 sec. These are possibly pulls on the stick intended to trigger Alpha Floor (which was not available).
The aircraft began to impact trees around T=26, or about 12 seconds after the second pitch adjustment.
If the pilot had been able to activate Alpha Floor at the second nose twitch, it appears (to me) that he would have cleared the trees.
But let’s look at the problem a little differently, from the viewpoint of what a small increment of g acceleration would have done for the flight path.
If .01 additional g was available at the second nose twitch and for the next 12 seconds, the aircraft cg would be about 23 feet higher-probably not enough to avoid an accident.
If .02 additional g was available at the second nose twitch for the next 12 seconds, the aircraft cg would be about 46 feet higher and would probably have escaped since the engines were spooling up nicely by then. (actually slightly less high since he would have converted some of his kinetic energy to potential energy and it probably would not be possible to maintain all the additional g for the duration.)
Perhaps you suspect that even that that small amount of additional g was not available, but consider that the video shows an alignment turn seconds before the nose twitches up. Was he decelerating that fast that there was absolutely nothing left to maneuver at the end?
In other words, if a measly .02 additional g had been available, the Habsheim event likely would not have been an accident. Would an additional.02 g have been available if the maneuver was flown in direct law and the pilot had pulled just slightly past optimum AOA and then eased off to optimum? That is definitely faster than an asymptotic approach to applying g and is the way most experienced manual pilots would lay on rapid g.
Looking at the Gordon Corps video, it seems there might well be some additional g potential/ AOA in the A320 Normal law being held back as a “safety margin”.
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If the pilot had been able to activate Alpha Floor at the second nose twitch, it appears (to me) that he would have cleared the trees.
That is definitely faster than an asymptotic approach to applying g and is the way most experienced manual pilots would lay on rapid g.
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Originally Posted by Machinbird 
...but at Habsheim that day, use of automation seems to have gotten in the way and created a surprise and a delay in adding power.
I think I need to clarify that there was apparently a mental 'expectation' that Alpha Floor was going to kick in, thus even though he may have forgotten his specific autothrottle configuration, the expectation that it would operate caused confusion and a delay in actually adding power.
The point is that his point of attempted Alpha Floor activation was not entirely unreasonable from a maneuvering standpoint. It completely failed because he failed to allow for the unexpected non-response.
When maneuvering near the ground, there really isn't much time for goof ups and when doing atypical flying, there is a need for an exceptional level of planning and practice. That was not done in preparation for the Habsheim flight.
Dozy, I'm going to have to tell you to check your 6 'cause you are dealing with a fighter pilot who could lay on extreme g during violent maneuvers without departing the aircraft
.
Consider basic servo theory for how quickly the output can match a step input. Remember that a human is a variable gain computer, and although not error free, can error correct once trained in error correction.
A computer must use conservative control strategies since it is supposed to operate independently, thus the asymptotic (essentially an overdamped servo approach) control of AOA.
A human can use a variable gain underdamped approach to a step input and even though they may overshoot the ideal AOA they will correct and arrive at the desired AOA sooner, particularly if there is some sort of feedback mechanism for them to observe (such as mild airflow separation causing a slight vibration.) The key is training and practice.
...but at Habsheim that day, use of automation seems to have gotten in the way and created a surprise and a delay in adding power.
How so? Bearing in mind they were not using automation at any point during the approach or flypast, nor were they intending to.
The flight plan included complete disabling of A/THR (and thus A. Floor) - that this step of the procedure was forgotten in the event doesn't alter that.
When maneuvering near the ground, there really isn't much time for goof ups and when doing atypical flying, there is a need for an exceptional level of planning and practice. That was not done in preparation for the Habsheim flight.
While taking into account deceleration and the pitch-up movement from slamming thrust back on? I very much doubt a human pilot could have done better.
.Consider basic servo theory for how quickly the output can match a step input. Remember that a human is a variable gain computer, and although not error free, can error correct once trained in error correction.
A computer must use conservative control strategies since it is supposed to operate independently, thus the asymptotic (essentially an overdamped servo approach) control of AOA.
A human can use a variable gain underdamped approach to a step input and even though they may overshoot the ideal AOA they will correct and arrive at the desired AOA sooner, particularly if there is some sort of feedback mechanism for them to observe (such as mild airflow separation causing a slight vibration.) The key is training and practice.
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Originally Posted by Dozy
Therefore, if the pitch attitude is limited by AoA protection and the pitch-up moment from the increase in thrust is expected to exceed that attitude, then the systems will be correct in counteracting that tendency through the elevators.
Can you prove that the extra 2.5 degrees of pitch would have avoided the crash?
We don't know what the CONF setting was for either.
You're getting some signs of confusion now. Maybe time to take a rest.
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Originally Posted by Machinbird
I think I need to clarify that there was apparently a mental 'expectation' that Alpha Floor was going to kick in, thus even though he may have forgotten his specific autothrottle configuration, the expectation that it would operate caused confusion and a delay in actually adding power.
I think I need to clarify that there was apparently a mental 'expectation' that Alpha Floor was going to kick in, thus even though he may have forgotten his specific autothrottle configuration, the expectation that it would operate caused confusion and a delay in actually adding power.
At 12h 46' 26'' (Crash - 13 seconds)
CDB "Bon Je vais bien la debrayer l'auto-manette."
(Google translate = "Good I'm fine to disengage the auto-throttle").
Radio Altimeter shows 40 / 50 / 40 feet.
I agree. It also seems to me as though he was going to "demonstrate" how clever the aircraft was and that Alpha Floor was going to prevent a crash, despite the thrust levers being closed.
It would explain the crews insistence that they didn't deliberately fly below 100 radio and did not hear the radio call outs because they had both ears covered by their headsets. (From a crash investigator TV program)
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Originally Posted by Machinbird
I think I need to clarify that there was apparently a mental 'expectation' that Alpha Floor was going to kick in, thus even though he may have forgotten his specific autothrottle configuration, the expectation that it would operate caused confusion and a delay in actually adding power.
It has his own annotated version of the final 60 seconds of the CVR transcript, how he himself remembers the accident flight. Although he seems to be wrong about at least one detail, it may still give an idea about what he was planning to do and what he was expecting. Mentionable items:- When PNF said "Bon, t'arrives à 100 pieds là fais, fais", this was well above 100 ft, and Asseline is certain that after he levelled off, his pressure altimeter indicated 100 ft exactly.
- For the fly by, he was mainly focused on keeping the vertical speed at zero.
- In the moment when he said "Bon, j'y vais bien là, débrayer l'auto-manette", he pushed the auto-thrust disconnect buttons and advanced the thrust levers to some intermediate thrust setting. (Here the DFDR disagrees: No movement of the thrust levers yet, both still at idle.)
- His original plan for the fly by was to continuously adjust thrust to maintain his intended speed and to apply go-around thrust when passing the control tower. But since his speed was still 160 kts after levelling off, there was initially no need to add thrust.
- Just before passing the control tower, he becomes aware that the engines haven't spooled up from idle. Also he realizes that what looked like just some vegetation of a darker shade of green at the edge of the airstrip is actually a forest.
- This is when he pulled the thrust levers all the way back and pushed them all the way forward again. (The CVR records this as three clacks of the levers moving across their detents.)
What I make of this:
- If you're surprised by a forest in your intended flight path, there's something seriously wrong with your flight preparation. Duh.
- It looks like Asseline made one serious omission (meant to adjust thrust right after levelling off, but didn't), but could never admit it.
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This is when he pulled the thrust levers all the way back and pushed them all the way forward again. (The CVR records this as three clacks of the levers moving across their detents.)
Jet engines decelerate very rapidly, and if there was any RPM above idle prior, that was undoubtedly lost and that would cause/contribute to a long engine accelerate time.
Time to get the engines spooled up was one of the critical problems on that flight.
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I think I need to clarify that there was apparently a mental 'expectation' that Alpha Floor was going to kick in, thus even though he may have forgotten his specific autothrottle configuration, the expectation that it would operate caused confusion and a delay in actually adding power.
Dozy, I'm going to have to tell you to check your 6 'cause you are dealing with a fighter pilot who could lay on extreme g during violent maneuvers without departing the aircraft.
.
EDIT : In fact, the whole reason Airbus (as opposed to AF, who were responsible for this flight) used the high alpha demonstration to show off and sell the A320 was because no other airliner at the time could perform that manoeuvre precisely enough to be safe! In terms of risk, barrel-rolling the Dash 80 - while a superlative example of aircraft control - was a Sunday stroll compared to a low-speed high-AoA manoeuvre without EFCS backup.
@noske: While the Captain undoubtedly made mistakes as a result of the improvised approach, at least part of the reason the forest was "surprising" can be laid at the door of AF's operations department - they only had one copy of the airfield chart, so the chart supplied to the crew was a B&W photocopy. Unfortunately, the graphics indicating trees at the far end of the grass strip were of a light grey which did not transfer to the photocopy.
Last edited by DozyWannabe; 9th Jan 2014 at 15:20.
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Dozy,
In the right hands and without all the "protections," your genteel A320 could be made to do a low g airshow on the order of what Bob Hoover did with his Aero Commander.
To make it fly in the genteel mode, they have provided some safety margin from the performance boundaries. What people are not talking about is how big the safety margins are (that is closely held data because in the wrong legal hands, it could be said that AI took away a pilot's maneuvering margins and thus caused an accident.
But I'll bet that there was .02 g available from the airframe that the computers were not letting the pilot obtain in the Habsheim accident.
Surely you must recognize the need for some safety margin from an engineering standpoint. There is variability between aircraft after all. Surface contamination of the wings does happen.
What a properly trained pilot can do is to operate an aircraft right up to the boundaries and if the boundaries are slightly exceeded, he will maintain control and correct back toward the actual boundary. (Unless an intervening computer limits this performance.)
In the right hands and without all the "protections," your genteel A320 could be made to do a low g airshow on the order of what Bob Hoover did with his Aero Commander.
To make it fly in the genteel mode, they have provided some safety margin from the performance boundaries. What people are not talking about is how big the safety margins are (that is closely held data because in the wrong legal hands, it could be said that AI took away a pilot's maneuvering margins and thus caused an accident.
But I'll bet that there was .02 g available from the airframe that the computers were not letting the pilot obtain in the Habsheim accident.
Surely you must recognize the need for some safety margin from an engineering standpoint. There is variability between aircraft after all. Surface contamination of the wings does happen.
What a properly trained pilot can do is to operate an aircraft right up to the boundaries and if the boundaries are slightly exceeded, he will maintain control and correct back toward the actual boundary. (Unless an intervening computer limits this performance.
)
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[QUOTE=DozyBut disabling A/THR (and thus Alpha Floor) was central to the entirety of the attempted demonstration - there would have been no expectation of Alpha Floor operating.[/QUOTE]
I might suggest that you modify 'would' into 'should' in the above sentence. It is probably closer to the truth.
I might suggest that you modify 'would' into 'should' in the above sentence. It is probably closer to the truth.
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To make it fly in the genteel mode, they have provided some safety margin from the performance boundaries.
What people are not talking about is how big the safety margins are (that is closely held data because in the wrong legal hands, it could be said that AI took away a pilot's maneuvering margins and thus caused an accident.
But I'll bet that there was .02 g available from the airframe that the computers were not letting the pilot obtain in the Habsheim accident.
Surely you must recognize the need for some safety margin from an engineering standpoint. There is variability between aircraft after all. Surface contamination of the wings does happen.
EDIT - re:would/should, I think the wording is reasonable as it stands - the crew were briefed to have Alpha Floor disabled in order to perform the manoeuvre, and while the approach was improvised and the Captain's physical SA seems to have been compromised by that, he seemed to be pretty sure that A.Floor was out of the equation.
Last edited by DozyWannabe; 9th Jan 2014 at 17:42.
While I disagree with 'bird about using "direct law", I find a flaw in your logic, Doze.
You point out two factors that are essential to understanding the final outcome.
- "improvised" ???? Huh? Flying a multi-zillion $$$ jet without having done a dozen practice profiles? BEAM ME UP!!!!
- thinking that the magic FBW system with all the "protections" and limits and such would "save" him and all the folks on-board? Give me a break.
- Not using manual throttle throughout the flyby? Sheesh. All the automation and such can only compensate for so much.
- blaming pictures of the field? Good grief!!! Just two or three practice missions would have made a huge difference.
I enjoyed flying my first fully-FBW jet, and we always had one place to hang our hat upon - max AoA was always there if we pulled full back on the sidestick. No auto-throttle, but that was up to us. The jet gave you everything it could.
and while the approach was improvised and the Captain's physical SA seems to have been compromised by that, he seemed to be pretty sure that A.Floor was out of the equation.
- "improvised" ???? Huh? Flying a multi-zillion $$$ jet without having done a dozen practice profiles? BEAM ME UP!!!!
- thinking that the magic FBW system with all the "protections" and limits and such would "save" him and all the folks on-board? Give me a break.
- Not using manual throttle throughout the flyby? Sheesh. All the automation and such can only compensate for so much.
- blaming pictures of the field? Good grief!!! Just two or three practice missions would have made a huge difference.
I enjoyed flying my first fully-FBW jet, and we always had one place to hang our hat upon - max AoA was always there if we pulled full back on the sidestick. No auto-throttle, but that was up to us. The jet gave you everything it could.
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Since Gen 317 seconds RA<52 Ft, the longitudinal law is pitch law until Gen 327 (RA <32 Ft), and then incidence law with derotation. Both are nearly direct and no more Nz law.
There is a problem between Gen 312 and 315 due the trees BEFORE the runway. (These trees have been cut very quickly after the crash!) The expert Max Venet said at the first trial they did not exactly knew how much time was needed to switch the law to "flare" (incidence law with derotation).
It seems descending 2 Ft is enough to switch the law. But Machinbird is talking about 36 Ft which is much more due to loss of. 02 g. We know that stall is achieved on the A320 with g<1 (plane descending!) due to beeing "protected against stall" (rudderruddererrat link post 269). But AF447 learned us that the A330 (and probably the A320) DOES STALL. Using the stall definition with g<1 (.94?, .97?) is transfering ≈50 ft or more from the pilot's margin toward system margin, as Machinbird was arguing.
Contradiction accepted but wih proofs.
There is a problem between Gen 312 and 315 due the trees BEFORE the runway. (These trees have been cut very quickly after the crash!) The expert Max Venet said at the first trial they did not exactly knew how much time was needed to switch the law to "flare" (incidence law with derotation).
It seems descending 2 Ft is enough to switch the law. But Machinbird is talking about 36 Ft which is much more due to loss of. 02 g. We know that stall is achieved on the A320 with g<1 (plane descending!) due to beeing "protected against stall" (rudderruddererrat link post 269). But AF447 learned us that the A330 (and probably the A320) DOES STALL. Using the stall definition with g<1 (.94?, .97?) is transfering ≈50 ft or more from the pilot's margin toward system margin, as Machinbird was arguing.
Contradiction accepted but wih proofs.
Last edited by roulishollandais; 12th Jan 2014 at 17:04. Reason: Ref to "rudderruddererrat post 269" link instead "Chris Scott link"