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TAM A320 crash at Congonhas, Brazil

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TAM A320 crash at Congonhas, Brazil

Old 30th Sep 2007, 16:11
  #2561 (permalink)  
 
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I can not understand why anybody has a problem with retarding both thrust levers to idle on landing, or reverse on the bus, in spite of an inop reverser on an engine that has been locked out.
I have guesses, but they sound so ridiculous that I myself have difficulty in tryin to understand how the mistake was made.

In each case the CVR recordings alone prove that the pilots were utterly confused as to what had happened.
Bringing the throttles into idle is so intuitive that no one bothers to check when it's ommitted. Airbus SOP is there to make it as simple as possible to the critical phases of flight. Hence, regardless of manual or auto thrust, you bring BOTH to idle, then both into reverse. Whether one does the "both into reverse" is not relevant... it is the bring BOTH to idle that IS!

This is where we go into the arguments of moving T/L when on A/T or not. Again, although I prefer seeing moving T/L when on A/T, Airbus makes it simple... "Who gives a damn! bring BOTH to idle"... Besides, on A/T landing, one should (unless the procs has been modified) have one's hand on the T/Ls to ensure that idle thrust is reached when appropriate, either by monitoring the T/L position (Boeing & A300/310) or manually intervene T/Ls to idle (Boeing and Airbus)... this manual intervention is the common ground between moving and nonmoving T/L on A/T.

One of the problem is "relying on automation for the wrong things." It's the eternal problem in man/machine interface regarding automation... Take a look at this example:
Retrospectively he removed his hand compeltely from that thrust lever, it was midnight, dark cockpit.
Perhaps, one should say on a single reverse landing, "we have reverse one side only. Ensure both are idle upon touchdown."
It may be a small and seemingly useless reminder, but it reminds both of the matter and can assist in someone to check the T/L position if something is wrong on the landing roll.

The problem is very well put by the following:

So, under such pressure, 'Reverse one side only' translates in your mind as, 'Concentrate on the live one, forget the inop. bugger......."

And then the 'Retard' call stops, you get the nosewheel down OK but then there are no spoilers, no autobrakes, the runway is unreeling fast.........and there's no further warning about the throttles, they're past and gone, they don't even occur to you as the possible problem........and your off-sider is new to A320s.......twenty seconds to live.........

Can it really have been that simple?
That may be the answer to this t/l handling problem. Can it be that simple? Unfortunately, YES!

Presently, our manual prohibits the use of manual thrust except in extraordinary circumstances such as a clear intent to "practice" in a benign ATC/operational environment. Many believe this to be a fundamental mistake and have fought such prohibition for years. We finally lost. Now, one disconnects the autothrust at one's peril should anything happen. Many crews used to fly approaches manually, including manual thrust levers, all the time and I personally taught it in instructional situations and encouraged it in regular line flying. No longer; the "risk" of blame is too high. In my view, competency with the machine is thereby lost however and, with such AOM restrictions, the relative loss of competency becomes self-fulfilling problem or a vicious circle. It is a big issue, as you likely know.
Back to the automation interface problem... your sentiments shared!

Is it the same when retarding to idle in the flare - that is, you have to pull the levers firmly back, against the resistance of the 'detent' mechanism, to make sure that they are firmly seated? How much 'feel' do you actually get?
On the 320 you bring it back to the idle stop, then when going to reverse you pull the latches to enable it to go to reverse... right? For going out of reverse, bsieker has answered.

Don't forget that in aviation, the "KISS" rule strongly applies: KEEP IT SIMPLE STUPID
Golden rule!

Airline travel is statistically so safe, whatever sort of aeroplane you're flying in, that it would be half a lifetime before the figures alone showed that one particular type was significantly any less safe than another.
It's safe, but when something bad happens, it's gruesome. The fact that it's probably the safest mode of transport is also overhyped. This is probably why when an air accident happens, everyone looks at it... a high speed bus impact resulting in multiple mutilations of bodies followed by burning probably wouldn't attract so much attention.
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Old 30th Sep 2007, 18:04
  #2562 (permalink)  
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RWL;

2. Both CVR transcripts (Taipei and Congonhas), and that Madeira landing video, show the 'Retard' call being heard two or three times before the pilots actually pulled the levers back. Presumably the call is triggered at or around say 20' Radio Altitude? Possibly it's timed a bit early - and sounds rather before the pilot's eye tells him that it's time to flare and retard? No big deal, 'one of those things' on the face of it - but if that's so, it's yet another reason why the 'Retard' call may be treated as a mere 'cue' rather than any sort of warning? Especially since it then cuts out.

This is from the A320 AOM, SOP's chapter:

Predetermined altitude call outs are computer generated by the Flight Warning Computer (FWC)


below 400 ft RA. Auto call out by the FWC below minimums/decision include "100", "50", "30" then

"RETARD", "RETARD" is generated if the Thrust Levers are not at idle by 20 ft (manually flying) or
by 10 ft (autoflight engaged).




AT 30 FT APPROX
Commence a gentle progressive flare and allow the aircraft to touch down without prolonged float
simultaneously moving the Thrust Levers to idle.




I have always understood and treated the "retard" call as a cue, not a "warning". In my Airbus course, we were always taught that one should not hear the "retard" call from the FWC's because the thrust levers should already be closed to the IDLE position. In practice, one, two and on rare occasions three calls may be heard if one delays the reduction to achieve a smooth landing but that is not what Airbus wants.


And a question for A320 pilots. From the Madeira video, and what I know of the manual, and Eric Parkes' notes, I gather that when coming OUT of reverse you have to make sure that you push the levers right through the 'idle detent,' and then pull them back again, to ensure positive location IN the detent as opposed to just 'nearly' in it.



Although it is not needed/indicated/required by the AOM or AI, there is the tendency among crews to do this. The material effect is minimal in the sense that there is little if any increase in engine thrust as a result of such a quick maneuver. I suppose the tendency may come from just what you say - to "ensure' that everybody is in forward thrust again.



Is it the same when retarding to idle in the flare - that is, you have to pull the levers firmly back, against the resistance of the 'detent' mechanism, to make sure that they are firmly seated? How much 'feel' do you actually get? Is it, for example, the same sort of 'feel' you get when pulling a car's gearshift back through reverse and neutral into 'drive'?



There is no need to ensure that the thrust levers are "firmly seated" when pulling them back. There is no "detent", but only a mechanical stop.



The A320 thrust levers are, despite the wide-ranging controversies and opinions, ordinary thrust levers with ordinary, pilot-expected responses. They are as benign a cockpit control as, say, the flap lever. Of this view, more in a moment...



Re "feel", there is no "reverse detent" so there is no sense that one is in "neutral" as in the metaphor you use, (car gearshift neutral position). It is merely a stop, beyond which the levers cannot move until the reverse "paddles" mounted on the front of the TL's are raised.



Unlike the A330/A340 thrust levers which have more conventional thrust reverse levers attached to the thrust levers and brought up and back in traditional manner, the A320-type reverse levers are small "paddles" which when pulled upwards release the interlocks so that the thrust levers may travel further aft into the "Reverse thrust" regime.



Re "feel" again, the thrust levers I have used on the 320 fleet type can vary in "resistance" in movement. Some I have used are tight and require a solid pull/push to move and others are quite "loose" and move fore-and-aft more than easily.



On landing, both levers are pulled smartly aft against the mechanical stop, which is then released by raising the paddles described above. The TL's then are free to move into the aft (reverse) regime. If a reverser is locked out (MEL'd), no effect other than a slight increase in N1 speed results. I believe (there is no information in the AOM on this) that no cockpit indications occur except that "REV" does not appear in green in the N1 guage on the ECAM but I stand to be corrected on this recollection.



Technically ONLY (in other words, this is to increase understanding of the system and is NOT to be taken as a technique or a recommendation. Such use is strictly against AI SOPs and is warned against!), the thrust levers could be pulled into the regime anytime before touchdown during the flare if the reverse paddles are pulled up. The system is designed such that nothing would happen until the aircraft-on-ground regime was established. This is possible because the FADECS will control the reverser and thrust sequencing. This is not the case with the A330/340 reverse thrust levers which are truly locked out, (cannot be raised up and back) until ground-regime is established by the relevant aircraft systems.



Regarding the "benign controls" comment above I would like to posit an impression that occurred to me when I first began flying the 320 in early 1992 and which hasn't left me. Almost certainly others have felt the same thing and I know it has been expressed in some form within this thread.



I wonder if there is something beyond "normal" human factors at work here which uniquely involves the A320 fleet-type. Perhaps we might call it "the illusion of complexity". I quickly add that I don't mean to dismiss the airplane's complexity by any means because it is there. By that term I mean, there exists a mode of thinking regarding the 320 which does not seem to have emerged with any other type - that the airplane is "not an airplane" and it's various systems' responses may, on some rare occasions, have to be "out-thought" and that what meets the eye/mind in some operational regimes. I wonder further, if that impression is largely latent and not approached or discussed in traditional training syllabi because of such latency. After all, in training, there is no discussion of human factors whatsoever, given economic factors in training footprints etc. Such courses are NTK - need to know, full stop.



That mode or impression of complexity may be defined by a sense that there is "more" to the 320 "layered underneath" the controls, indicators and cautions/warnings than appears to the crew - that the airplane is more complex to operate than it "appears" - that one must have a unique type of comprehension of flying airplanes to successfully come to terms with the Airbus design; - there is a "mystique" to the 320 both by reputation, both partially-earned and partially-not.



I again hasten to add that there are indeed, "layers of complexity" in the design. It takes normal crews about six months to a year to get used to the airplane and "strap it on".



My thought here is, "how do crews react to such layers in both normal and abnormal operations?" Do they try to "out-think" the airplane and thereby make assumptions about design features or the way something will work, particularly in an abnormal situation? Such "out-thinking" is not "intentional" because pilots have learned by and large to trust designers and test pilots. Thirty to fifty million successful approaches and landings can at least attest to that, (fascinating discussion on statistics - thank you PBL and Bernd).



I hasten also to add that this is almost certainly a training matter but a deeper level of appreciation of human factors would be needed before any such considerations would obtain in normal bread-and-butter courses.



Other than traditional fbw responses which for most transitioning to the Airbus have to become used to, the airplane "is an airplane" but I wonder if there is something further here - that crews may believe that the 320 is "more than an airplane" and therefore requires more thought/consideration than the design actually needs.



This may seem an trite statement to some, but I posit it for consideration. The notion that the TAM crew may have mis-conceived the "only one reverser" comment one of them made on short final needs an explanation. Without wishing to hearken back to the earlier discussion on auto-thrust systems in this thread, was there the latent impression (vice the "certain" knowledge) that the autothrust system would "handle" the #2 engine appropriately? (I know this particular question has been beaten to death but that is not the direction I am interested in taking this question).



I know I am stating this perhaps awkwardly or even obscurely but also that these comments can seem at once "obvious", (of COURSE the airplane is complicated!!), but i'll leave it there for comment to see if it is worth pursuing at all.



PJ2
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Old 30th Sep 2007, 18:16
  #2563 (permalink)  
 
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KISS in combination with human psychology means, that the system should only assume the last command given by the human as valid, and disregard any conflicting earlier commands.

In this particular accident the last commands given to the systems were by autothrust: engines to idle, and by the humans: left hand engine to idle and into reverse. Hence, the right hand engine had no reason to spool up, even though the T/L#2 was seen in a high power setting by the computers. The last command by the human was clearly to stop the airplane, hence spoilers and brakes should activate too.

Now let's look into this a bit closer.

For example, the human activated autothrust, so he delegated thrust control to the automatics. Fine, the automatics will do the job as commanded and hence adjust thrust as needed. No change to current way of operation. Fully within the expectation of the pilot, so no problem from human psychology, too.

Now, assume, the airplane is in the flare, and autothrottles brings the engines back to (nearly) idle. At 20 feet the human retards only one thrust lever (TL#1).

What was the last command given by the human? Correct - go to idle (on the left engine), we are going to land. As a result, autothrottle disconnects. So far, so good - and so far that's what happened in this accident (as seen by the computers and the FDR recordings).

And now we are in the critical decision: what is the human intending really? The other T/L#2 is commanding a high power, does he want a go-around indeed as quite a few claimed in this thread?

So, in this conflict, why did autothrust disengage then? If the system can not decide, are we going to land or are we going around, why did the autothrust system disconnect and follow the command to land, while the other systems did not and instead setup for a go-around? This, in my opinion, is a violation of all engineering principles, I know. Based on the very same input different systems must not go into conflicting modes!

If we let the last command prevail however: the pilot wants to stop the airplane, of course, otherwise it would be inexplicable, why the one thrust lever was brought to idle.

As the system didn't sense movement of the other thrust lever (for whatever reason - be it the human forgetting the lever, be it a computer glitch as has happened before where the link between thrust lever and computers was broken, be it an electronic or any other problem), the command to put it into the CLB detent or anywhere between CLB and Idle Detent was issued before that last disengagement of autothrust and is now to be disregarded until the human makes clear, what he really wants.

Hence, the engine should not spool up as it did in this accident, but remain at whatever power setting it was producing at the time of autothrottle disengagement. The FDR tells, that the engine was at close to idle, if not idle, so that's in agreement with this pilot's expectation of the engine's behaviour. At the same time the systems to stop the airplane, spoilers, autobrakes, etc. should arm and operate upon WOW as both engines are at or close to idle, or in reverse.

Now what, if the pilot really wanted to go-around as some have claimed in this thread? It is natural for a pilot to advance the throttle levers for a go-around, and put them into TOGA. He expects and is used to, that he needs to actively issue a command to initiate the go around. So, if he did indeed advance one or more levers at this stage, then the systems would of course assume a go-around and react accordingly by increasing the according engine's thrust and disengage spoilers, brakes etc.

But this time, it is not a decision on its own based on an outdated (forgotten) thrust lever setting, but on a current lever movement clearly indicating the pilot's intentions. And yes, that go-around would be initiated even with T/L#1 still in reverse and reverser deployed on engine#1, so the human would not deprived of that option in the case, he feels it is safer to abort than to continue the landing despite the deployed reverser.

Now, how would that system react to an engine problem and the pilot putting the according thrust lever into the idle detent?

Same way. Autothrust would turn off, the other engine would continue on its current thrust setting, and the troubled engine would spool down to idle. Pilot moves the other thrust lever into the MCT detent or commands a slightly higher manual thrust setting to compensate for single engine operation, and everything works as normal (autothrust engages again to command the other engine only or other engines spools up to manually commanded thrust setting).

No brainer, as fas as I am concerned, in the technical design of the systems. But it puts the pilot back into control, and does not confront him with a scenario, where he gets taken at surprise by an accelerating engine in a very critical moment of flight without him commanding that acceleration (as far as the pilot is concerned, even though the throttle lever may be in a position commanding a high thrust output).

Even if - which may be an outcome of the investigation, too - the links between the thrust lever and the computers were lost (and hence the computers no longer registered any movement of the lever, as it has happened on quite a few flights already) or the pilots really forgot to retard that lever, this would not have ended in the catastrophic outcome as in this crash. And the pilot would still have had the option to initiate a go-around on single engine with one thrust reverser deployed on the other engine. All the pilot would need to command that go around, advance (move) the other thrust lever.

This solution would be equivalent to the moving thrust levers in other plane types, too. As the autothrust system on those types would move those throttles to idle during the flare, the engine would remain at the idle setting even if the human forgets to pull the throttles back to idle. Upon autothrust deactivation no engine would spool up on those types.

Well, what, if the lever was forgotten in the CLB detent and the pilot actually now requires it to be in the CLB detent to resolve the resulting situation? Just move it out of the CLB shortly and back in. That would be a natural move for the human anyway, as he'd assume the lever in the IDLE detent and thus grab it and move it forward, then develops the feel of a wrong position and probably looks down to get in sync with the current position again.

What happens, if the pilot forgets both levers in the CLB detent? During the flare autothrust would command both engines to idle. As a result, spoilers and brakes would arm and operate. If the pilot now wants to abort the landing, it would be natural for him anway to advance both levers to TOGA, which would increase engine thrust, stow spoilers and disengage brakes. Works as with any other plane, even those whose autothrust systems move the thrust levers, except perhaps that the hand wouldn't find the thrust levers in the IDLE position.

Now, let's assume, the pilot forgot one or all levers in the CLB detent, autothrust remained active though spoilers and brakes operated due to both engines at idle and WOW, now the airplane is about to taxi off the runway, and at this point the pilot moves the levers to increase thrust for taxi. Now the engines would respond, of course - but now it's absolutely clear to the pilot, that his last action was to command thrust, something went wrong with this command, and he would immediately turn his attention towards that last command and correct, whatever went wrong, especially he'd immediately pull the throttles to idle (which effectively turns off autothrust and brings the thrust levers into the needed position and engines back to idle). Pilot and machine are immediately back in synchronisation again.

Now, a look at the normal way of moving from autothrust to manual thrust: the pilot moves the levers into a position that is consistent with the current thrust output of the engines, then turns autothrust off. Bingo, works as we are used to. The engines would remain at their current thrust output until the levers are moved again. Exactly, what we want.

So my solution to this would be: always adhere to the last command given by the pilot.

Once autothrottle deactivates, leave the engines at their current thrust setting, regardless of lever position, and wait where the humans move the thrust levers to. If engines are at or close to idle and weight is on the wheels, allow activation of spoilers and brakes.

I believe, such a system would support the human - without needing an alert at a critical phase of flight, which may be blanked out by the human brain concentrated on other tasks. And I believe, the system would be correct in the principles of control circuits, match human psychology and provide a solid, easy and simple man/machine interfaces compatible with all existing plane types. An absolutely intuitive solution consistent with KISS.

Have I overlooked something, that would not be covered in such a scenario?

Servus, Simon
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Old 30th Sep 2007, 18:23
  #2564 (permalink)  
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Originally Posted by archae86
There was much prominence given in the United States in the middle 1980s to incidents in which drivers asserted that their cars accelerated wildly while the driver was "standing on the brakes".
I am very familiar with those incidents. They do not constitute statistics.

Since the Audi 5000 incidents some 20+ years ago, notice that there has been a lack of reports of cars accelerating while drivers were "standing on the brakes".

Apart from a couple of well-known incidents with police cars, in which people were injured by police drivers. Investigated by the NTSB. Turns out to have been a unintended consequence of a drive-train mod that some police districts undertook.

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Old 30th Sep 2007, 18:42
  #2565 (permalink)  
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Austrian Simon,

you are new to this thread. Some of us have seen thousands of posts consisting of amateur human-factors psychology "explaining" what pilots would have thought and done.

Do you have any professional expertise and credentials in engineering psychology that you could share with us?

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Old 30th Sep 2007, 19:11
  #2566 (permalink)  
 
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Originally Posted by Rob21 Maybe this can explain why on "nice" runways (POA) is SOP (TAM) both TLs to reverse and also (maybe) there is another "SOP" selecting only one TL to reverse on the "good" engine when runway is slippery (CGH).
If you are correct, on the preceding track (CGH-CNF), the landing at Confins should have be done with both TLs to reverse, because of the runway's lenght there.


Originally Posted by PBL AI's position on (5) will be that, until the Congonhas accident, there was little evidence that failing to reduce thrust to idle on flare was an oft-repeated error. There were two incidents on record in (what AI claimed in its deposition) over 30 million fleet landings in 19 years.
Originally Posted by PBL Now we come across is a deeper reason that one cannot do what marciovp and Rob21 are suggesting. We cannot derive any meaningful *statistical* conclusion. That is: the number of total events non-X that one has registered is too small. Any Tests of Significance (I capitalise it because this is another technical term which I suggest that people look up) would show that any correlation derived from a sample of two are not significant.
It is right. You can't look at the event from a purely statistical point of view, since the kind of causality in this kind of event is not direct, but systemic. In other words, it is the well known theory of the holes and the layers, or the slices of cheese. Two three, or even ten events of this type are not by themselves statistically significant, but, if we see the things from the point of view of the relevance of the event in terms of possible consequences (or his quality), the story changes. If the runway at Taipei had been shorter and there were buildings at his end, this event could be far more tragic. After two accidents with a similar dynamic, and probably some more mistakes without consequences, it should be expected that AI at least made mandatory at that time only one procedure with locked-out reverser, the one with both TL to idle and then both to rev, i.e. the usual and simplest one.
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Old 30th Sep 2007, 20:14
  #2567 (permalink)  
 
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Originally Posted by Austrian Simon
KISS in combination with human psychology means, that the system should only assume the last command given by the human as valid, and disregard any conflicting earlier commands.
Now where does that come from?

I share PBL's sentiment.

If you want to voice your opinion, to which you are entitled, it is a good idea first to get the facts straight. I'll point out some of the numerous factual errors in your post. There are others.

In this particular accident the last commands given to the systems were by autothrust: engines to idle,
Wrong.

and by the humans: left hand engine to idle and into reverse. Hence, the right hand engine had no reason to spool up
A command was given to the left engine. What does that have to do with the right engine?

Giving a pilot something other than what he has commanded is hardly "keeping it simple".

Now, assume, the airplane is in the flare, and autothrottles brings the engines back to (nearly) idle. At 20 feet the human retards only one thrust lever (TL#1).
Wrong. Autothrust maintains approach speed. Except in the case of Autoland, Autothrust does not know about reducing thrust to idle in the flare. (Even in autoland (A/THR in RETARD mode), levers must be retarded to idle, only difference being the timing of the RETARD! callout (20ft manual, 10 ft autoland).

What was the last command given by the human? Correct - go to idle (on the left engine), we are going to land.
Wrong. Only one engine to idle could also be done to avoid excessive vibrations, among other things.

As a result, autothrottle disconnects.
Wrong. Autothrust remainded active.

So far, so good - and so far that's what happened in this accident (as seen by the computers and the FDR recordings).
Wrong again. Go back and read the FDR graphs yourself and look for the moment of A/THR disconnection (hint: you need to read more than one page).

So, in this conflict, why did autothrust disengage then?
Take a look at my Why-Because-Analysis to see why, how and when A/THR disconnected.

Hence, the engine should not spool up as it did in this accident, but remain at whatever power setting it was producing at the time of autothrottle disengagement.
It did.

When it spooled up, A/THR was still active.

This solution would be equivalent to the moving thrust levers in other plane types, too. As the autothrust system on those types would move those throttles to idle during the flare,
Would it? How many types have you flown? In all aircraft you put the levers to idle in the flare. Moving or non-moving.

What happens, if the pilot forgets both levers in the CLB detent? During the flare autothrust would command both engines to idle.
No. It would maintain approach speed and float the aircraft along the length of the runway.

As a result, spoilers and brakes would arm
Spoilers and Autobrakes are armed by the flight crew, they don't arm themselves.

Have I overlooked something, that would not be covered in such a scenario?
Too much to mention. Start with the points above, which show some lack of understanding of the A320 systems.


Bernd

Last edited by bsieker; 30th Sep 2007 at 22:02. Reason: Corrected minor procedural error.
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Old 30th Sep 2007, 22:32
  #2568 (permalink)  

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Old 30th Sep 2007, 23:02
  #2569 (permalink)  
 
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Despite all the excellent analysis available on this thread there is one nagging issue to which I have yet to see the answer (my apologies if it has already been answered and I have missed it).

My question is: under what circumstances would a pilot put one (or more) of his aircraft's engines into MAX REVERSE if his intention was not to make a full stop landing?
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Old 30th Sep 2007, 23:34
  #2570 (permalink)  
 
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I think no one here has access to it but anyway:

Does anyone have the logic diagram(s) of the A320 autothrust system in speed mode ?
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Old 1st Oct 2007, 00:29
  #2571 (permalink)  
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Originally Posted by Avionista
My question is: under what circumstances would a pilot put one (or more) of his aircraft's engines into MAX REVERSE if his intention was not to make a full stop landing?
Yes, Avionista, you're quite right to ask that in my view - it is a question Airbus will eventually have to address very seriously. Indeed, I hope that it is ALREADY being addressed. There is a real hole in their control logic.

As I mentioned earlier, the 'logic' - or, in this case, 'lack of logic' - of the flight control system appears to be:-

1. On the one hand, it appears to 'assume' that the pilot intends to land to the extent of allowing the deployment of reverse thrust.

2. On the OTHER hand it appears to 'assume' that the pilot does NOT intend to land in a much more serious way, by preventing the deployment of ground spoilers and the operation of autobraking.

And there is no manual override available to the pilots, especially in the vital area of spoilers.
One remedial measure - the panel warning that one T/L is above idle - has already been put forward by Airbus, and adopted by TAM. But plenty of people on here seem to feel that 'extra warnings' may not work. And the basic anomaly - that once the wheels are down, the pilots can select reverse thrust even though some forward power has been left on, but NOT ground spoilers - will remain.

Personally, I'd favour returning manual control of the ground spoilers to the pilot once the aeroplane is on the ground. Selection of reverse thrust clearly implies an intention to land/pull up.

As a matter of interest, is the A320 the only Airbus design where extension of the ground spoilers cannot be carried out manually, and even auto-deployment is dependent on both throttles being at 'idle'?

Or are all their types designed that way?
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Old 1st Oct 2007, 00:44
  #2572 (permalink)  
 
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gpvictor,

As far as I could see, the landings registered on the FDR I had access were CGH, POA and CGH. I don't see any landings at Confins on the FDR. That's why I said that maybe there is an "SOP", or a special non official procedure for landing on that slippery 35L (It's downhill too, does anyone here remembers that?).

BTW, according to the news, now is prohibited to land in CGH with "locked" reversers and also there is a weight limitation (no info on values). The only "clue" is the limit of 130 passengers. Which a/c type, no info so far...

Rob

Last edited by Rob21; 1st Oct 2007 at 00:46. Reason: typo/spelling
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Old 1st Oct 2007, 00:52
  #2573 (permalink)  
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Originally Posted by Austrian Simon
Once autothrottle deactivates, leave the engines at their current thrust setting, regardless of lever position, and wait where the humans move the thrust levers to. If engines are at or close to idle and weight is on the wheels, allow activation of spoilers and brakes.
Sorry, Austrian Simon, omitted to mention that you had posted along similar lines. I'm afraid that I get 'battle fatigue' nowadays reading really-long posts.

Obviously I agree, except that there is no logical reason, in my view, for the 'system' to check whether engines are 'at or close to idle.' That seems to be just about the reason why this accident happened, the 'system' interpreting two conflicting bits of data in conflicting ways.

Reverse thrust, spoilers, and brakes are three legs of the same landing 'stool' - once the wheels are down, I'd say if the pilot selects one he should get access to the others. If he makes a later decision to go around (very unlikely) his first action would be to cancel the reverse thrust - it would be relatively simple to 'teach' the systems to re-interpret that action appropriately.
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Old 1st Oct 2007, 01:02
  #2574 (permalink)  
 
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Avionista & RWA,

Maybe I can add that pilots are instructed NOT to attempt a go around after reverse is engaged.

IMHO, in this accident, the computer had more indications that the crew wanted to stop, than indications they wanted to continue flying. I know, it's not for the computer to decide that. But it did. Did not deploy ground spoilers and did not activate auto brakes.

It was said here that the computer can't read the pilot's mind. But here we have the proof that the computer "guesses".
And in this case, it "guessed" wrong...

Rob

Last edited by Rob21; 1st Oct 2007 at 01:38. Reason: add another phrase
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Old 1st Oct 2007, 01:30
  #2575 (permalink)  
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Yes Rob21, that as well. Which just makes it even MORE anomalous that the systems are apparently 'designed to leave' ('capable of leaving' is maybe a better choice of phrase) the go-around option open even though, in terms of normally-prudent flying, that option has been ruled out by the pilot selecting reverse thrust.

Last edited by RWA; 1st Oct 2007 at 01:33. Reason: increased clarity
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Old 1st Oct 2007, 01:40
  #2576 (permalink)  
 
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Austrian Simon,

The AutoThrust kept engaged after the left engine reverse was fully deployed for c. 2 secs as per design (as a consequence of a comparision between target FMGC EPR and target FADEC EPR at reverse). It disengaged by letting eng. 2 at c. 1.18 EPR, far above idle.

As it was an involuntary disconnection the general consequence would be a thrust lock at the last EPR commanded (or achieved) by the Autothrust regardeless of the right engine TLA (aside from the max. thrust limit set by this TLA).

The crew should be warned of this thrust lock by a flashing (ECAM ?) with the message "ENG THRUST LOCKED" (and the messages AUTO FLT A/THR OFF in amber and THR LEVERS...MOVE in blue) with a single chime and the Master Caution Light flashing every five seconds (or something similar to that).

The problem with this warning as I could learn from a discussion some X posts ago was that it is (supposedly) inhibited after touchdown and when (at the same time) the a/c speed is above 80 knots.

There must be a logic diagram of this warning as there probably is one for the autothrust in speed mode behaviour. In the case of the warning someone probably saw in the logic diagram that "phase 8" and above 80 knots were inputs to a NAND gate in the diagram (or something equivalent).

Maybe this participant could help us in this quest for the logic diagram of the A320 autothrust system.

Last edited by xulabias_bent; 1st Oct 2007 at 02:47.
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Old 1st Oct 2007, 02:00
  #2577 (permalink)  
 
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Rob, the computer guessed wrong, as you said, that the pilots really wanted to stop and have ground spoilers and auto brakes. This has gone around way too many times but why did they program the computer that way just because it sensed the #2 TL wasn't at idle?
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Old 1st Oct 2007, 06:51
  #2578 (permalink)  
 
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Avionista & RWA,
My question is: under what circumstances would a pilot put one (or more) of his aircraft's engines into MAX REVERSE if his intention was not to make a full stop landing?
Maybe I can add that pilots are instructed NOT to attempt a go around after reverse is engaged.
Yes, but if you decide to go around, and command TOGA on the T/L, the FADEC on the 320 would remain at forward idle until the reverse sleeve mechanism's hydraulics have fully depressurized...

It is possible, but definitely not recommended.

One scenario for going around after reverser selection is, touchdown, select reverse, derotate and while the aircraft is still at high speed, something enters the runway... If you got aircraft on both sides of the runway, what would you like to do? Wish for a go-around?

Personally, I'd favour returning manual control of the ground spoilers to the pilot once the aeroplane is on the ground. Selection of reverse thrust clearly implies an intention to land/pull up.
Indeed, but it still doesn't help much when you got 1 TL in positive forward... You're still gonna run off the end of CGH that day if that was the case.

Brakes and positive thrust isn't wise, spoilers and brakes assume the need for idle or reverse. Brakes heat up quickly, have positive thrust and your numbers are varied. Is one suggesting an idle override?

How to go-around then? BOTH in TOGA? Would that be for single engine too? or just 1 TL into TOGA? *aihhh... too much to think off*

Bye bye simulated single engine take off on a real plane then, and single engine go-arounds... (where you put the simulated failed engine into idle). Or that "idling due to excessive vibration" and then you need to do a late go-around? Override that and go into land?

---

AustrianSimon,
Any jet transport aircraft require you to have engines at idle when touching down. This is to enable ground spoiler deployment. Any forward thrust would auto-stow the spoilers as the system assumes you're about to go around or has finished the landing with a full stop.

So if the plane touches down with all levers in CLB, on the 320, spoilers wouldn't deploy! Simple no?

Once autothrottle deactivates, leave the engines at their current thrust setting, regardless of lever position, and wait where the humans move the thrust levers to. If engines are at or close to idle and weight is on the wheels, allow activation of spoilers and brakes.
Again, see above!

So, you do a steep unstable descent with A/T, you somehow manage to flare, and disconnect the A/T using the D/C buttons on the TLs or ATHR button on the MCP (you gotta be nuts to do the MCP d/c in flare)...

So now you got weight on wheels, engines probably at idle during disconnection, but TLs are in CLB... You're going to land or G/A? How does the computer know?

Again, the computer doesn't know of the weight on wheels is because the aircraft is on a runway, taxiway, grass, road, highway, parking lot, whatever. D/Cing the A/T and allowing spoiler extention and brake activation on the scenario above but on grass (let's assume the crew became unstable and tried to go in and end up undershooting, which isn't the first time!), would actually spell more trouble than if the spoilers and brakes didn't engage... Why? U're dumping lift and trying to create wheel friction on an unrated surface unintentionally... you could end up ripping off the wheels, digging the engines etc and write off the plane, You add more thrust, but it's not quite at TOGA... how does the computer know when to override? whereas, if U're on the current system of the 320, you could getaway with it because no lift is dumped.

Keep It Simple! The current system looks dauntingly complicated, but it is so to make it simple to operate... ie: when landing, bring both tls to idle and then reverse... simple no?

Bubbers,
I thought the computers aren't supposed to guess, it's supposed to think and understand, and when it doesn't understand, it gives it back to the pilot.

PK-KAR
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Old 1st Oct 2007, 08:33
  #2579 (permalink)  
 
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Originally Posted by xulabias_bent
The problem with this warning as I could learn from a discussion some X posts ago was that it is (supposedly) inhibited after touchdown and when (at the same time) the a/c speed is above 80 knots.
The warning display "ENG THRUST LOCKED" on the E/WD is inhibited on the ground, i. e. in all flight phases before liftoff, and all flight phases after touchdown.

There must be a logic diagram of this warning as there probably is one for the autothrust in speed mode behaviour. In the case of the warning someone probably saw in the logic diagram that "phase 8" and above 80 knots were inputs to a NAND gate in the diagram (or something equivalent).
Phase 8 is defined as between touchdown and 80kts during rollout.

Maybe this participant could help us in this quest for the logic diagram of the A320 autothrust system.
I don't know exactly what you mean by this. If you're looking for the precise algorithms and parameters of the feedback control loop for autothrust speed mode, I doubt that we will get it here.

On of the few things one can infer from the FCOM is this:

Originally Posted by FCOM 1.22.30, P65
Approach Autothrust
Below 3200 feet radioaltitude, with at least CONF 1, the A/THR logic is modified to be more responsive to speep variation. This is referred to as approach autothrust.
When managed (i. e. the FMGC sets the speed target for the autothrust, and not the pilot directly), the speed target varies during approach, it is computed using the so-called "ground speed mini function", which takes into account ground speed, tower reported wind, Vapp, etc. to ensure ...

the energy of the aircraft is mainained above a minimum level ensuring standard aerodynamic margins versus stall.
[...]
The minimum energy level is is the energy the aircraft will have at touch down if it lands at VAPP speed with the tower reported wind as inserted in the PERF APPR page.

The logic for activation of the different modes can be reverse-engineered from the FCOM (assuming it is correct).

Assuming autothrust is active and ALPHA FLOOR protection is not active, then SPEED/MACH mode engages if:

- Neither FD nor AP is engaged
OR
- FD/AP is active in one of the following pitch modes:
. V/S
. FPA
. ALT*, ALT
. ALT CST*, ALT CST
. ALT CRZ*, ALT CRZ
. G/S*, G/S
. FINAL, FINAL APP
. DES (geometric path)

It goes to RETARD mode, if the autopilot is in FLARE mode.

It goes to THRUST mode if autopilot is active in any other pitch mode.


Bernd
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Old 1st Oct 2007, 10:49
  #2580 (permalink)  
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Originally Posted by PK-KAR
Indeed, but it still doesn't help much when you got 1 TL in positive forward... You're still gonna run off the end of CGH that day if that was the case.
Endeavouring to 'keep it simple' in my turn, PK-KAR.

Had they been able to deploy the spoilers manually, 'worst case' is that they'd have come off the end at much lower (probably surviveable) speed. 'Best case' is that they'd have had more time to work out what had gone wrong and maybe even manage to stop on the runway.

Originally Posted by PK-KAR
I thought the computers aren't supposed to guess, it's supposed to think and understand, and when it doesn't understand, it gives it back to the pilot.
That's exactly what the 'puter DIDN'T do - as Bubbers and I pointed out. It just made things a damn sight worse by making sure that they didn't have spoilers OR autobrakes - and then didn't even give them any sort of warning as to what was wrong.......
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