Air France 447 - On topic only!

[Curtis Malone]

I'm incredulous that technical arguments could, after so many thousands of messages, still be going on. Given the minimal probability of having any more information, is there any time when they will end? Or will every new expert who logs on fan the embers into flame again?

Experts? Where?

[Gabriel]

Assuming also that they could afford to climb above their present cruise altitude.

What do yoyu mean?

If you apply climb and 5 deg the airplane wil do something until it settles at a given airspeed and altitude.

The airplane will not stall with 5 deg of pitch, unles there is at the same time a very steep descent (which starting from cruise speed, and with climb thrust and 5 deg nose up is unlikely). That's because the AoA is the difference/sum between the pitch and climb/descent slope. With 5 deg of nose-up pitch, it would take a descent of some 10 deg to make for an AoA of 15 deg (I don't know what's the stall AoA of an A330, lest at the Mach number they were flying, but it's usually around those values).

The risk could be overspeed due to climb thrust being too much even for 5 deg nose up, but I guess that Airbus took that into account when they made the procedure.

[Evan]

What do yoyu mean?

If you apply climb and 5 deg the airplane wil do something until it settles at a given airspeed and altitude.

The airplane will not stall with 5 deg of pitch, unles there is at the same time a very steep descent (which starting from cruise speed, and with climb thrust and 5 deg nose up is unlikely). That's because the AoA is the difference/sum between the pitch and climb/descent slope. With 5 deg of nose-up pitch, it would take a descent of some 10 deg to make for an AoA of 15 deg (I don't know what's the stall AoA of an A330, lest at the Mach number they were flying, but it's usually around those values).

The risk could be overspeed due to climb thrust being too much even for 5 deg nose up, but I guess that Airbus took that into account when they made the procedure.

MCM had posted this earlier:

The service ceiling is all good and well, but at the weight the aircraft was likely flying at it would never have had suitable performance ability to climb anywhere near that. It may have well been very close to its maximum altitude for its cruise weight. Also, in cruise it is common to sit quite near MMO - while I do not fly an airbus, in the boeing it is not uncommon to sit within .03 or so of MMO - certainly not a long way, and it can be oversped within seconds.

I assume he is referring to absolute ceiling, which requires air temperature and aircraft weight to determine. There is reason to believe the air temperature in this cell was unusual for the altitude. Is is possible for FL33 to be near the absolute service ceiling for a fully loaded A330 with much of it's fuel load still onboard? I'm sure you know what happens to airspeed at this point.

The memory items are designed to be used where UAS is generally experienced, which is not anywhere near cruise altitude, MMO or service ceiling.

[Evan]

In fact Gabriel, once you get around to the checklist items, you find that the proper pitch/thrust settings to maintain 260kts between FL25 and FL37 are 84-90% N1 and 2-3.5° pitch! If the crew performed the 5% pitch/CLB detent memory item at/near service ceiling, what would happen? Going by the chart, that is too much drag and not enough thrust. Couldn't that lead to stall?

[Gabriel]

In fact Gabriel, once you get around to the checklist items, you find that the proper pitch/thrust settings to maintain 260kts between FL25 and FL37 are 84-90% N1 and 2-3.5° pitch! If the crew performed the 5% pitch/CLB detent memory item at/near service ceiling, what would happen? Going by the chart, that is too much drag and not enough thrust. Couldn't that lead to stall?

I don't think so. As I've said the only way to stall it would be to keep 5° pitch as a descent slope of 10 degrees develop.

What I think would happen is as follows:

Say for the sake of the argument that the plane was flying, before the incident, in the following initial state:

85% N1
3° pitch
260 KCAS
flying level at FL330 (i.e. not climbing or descending)
which gives an AoA of 3 degrees (waaaaayyyyy away from stall).

Then the pilots set 5° pitch and climb thrust (let's say that's 95% of N1).

No matter what, the initial reaction of the plane, before the speed has time to change, is that it will climb with a slope of 2° (with 3° of pitch at this speed it was flying horizontally, so now with 2° more of pitch and the same speed it will fly a 2° climbing slope).

Of course, flying a climbing slope of 2° requires more thrust that flying horizontally at the same speed. Would the change from 85% to 95% be enough? I don't know, and I don't care much really. Two scenarios:

95% N1 is more than needed to keep a climb slope of 2° at the same speed:

The airplane will not only climb but also accelerate. Speed will increase, and with it lift will tend to increase, but what will happen in fact, because the pilots are actively keeping 5° of pitch, is that the climb slope will increase and hence, at the same time, the AoA will be reduced, hence keeping lift=weight (ok, there will be a small transients increase of lift needed to bring the plane from level to climb, but it won't last long)

So now we have the airplane climbing, accelerating, keeping the pitch but increasing the climb slope. This can't go forever. The increasing climb slope requires more thrust and the increased speed, with the related increased drag, also requires more thrust to be sustainable. A point will be reached where the required thrust equals the available thrust and the plane will initially settle at one given airspeed and climb slope. It could be something like this:

95% N1
5° pitch
285 KCAS
Climbing slope of 3°.
which gives an AoA of 2° (even more away from stall).

The question here is whether 285 KCAS (or whatever results from this) would be beyond the overspeed speed. I guess that Airbus took that into account when setting the memory items, and still there is a pretty comfortable margin between the Vmo/Mmo (which is tha max speed at which the pilots are allowed to fly intentionally) and Vd/Md, which is the max structural speed demonstrated in flight tests. The plane is required to be fully controllable and withstand 2.5Gs up to Vd/Md, the reason for Vmo/Mmo to be quite lower is that with a prescribed upset at Vmo/Mmo, it is required that the upset is recovered before reaching Vd/Md. In the case of the A330 the difference between Mmo and Md is 0.08 or 0.09 of Mach, which is quite a bit, plus a few points more because the plane was surely not flying at Mmo in the initial state, but (slightly) slower. The planes are optimized to fly at cruise speed, ad drag increases a lot above it, especially as you get close to Md (wave drag), so I think it's quire unlikely that the additional thrust will be enough to exceed Md.

Yet, this will not last forever either. While the plane achieved a steady state initially, there are variables changing outside the plane, basically the air density. As the plane climbs through the flight levels the available thrust will be less and less ("climb thrust" is less thrust as you climb), hence the speed will slowly diminish, the climb slope will diminish and the AoA will increase until the plane settles at some given state like this one:

95% N1
5° pitch
240 KCAS
flying level at FL 370 (i.e. not climbing or descending)
which gives an AoA of 5 degrees (higher, but still waaaaayyyyy away from stall).

The final speed will necessarily be less than the initial one, because it takes less speed to fly level with an AoA of 5° than with 3°.

It's impossible that the plane will slow down so much that it will stall here. By keeping 5° of pitch and having enough initial performance to sustain a climb like in this scenario, the plane will eventually slow down only until reaching 5 of AoA and flying level, and whichever speed makes that possible it can't be close to stall because 5° of AoA is way away from stall.

95% N1 is not enough to sustain a climb slope of 2° at the same speed:

In this case the plane will slow down and the climb slope will start to reduce from the initial 2°. Because you are holding 5deg, the AoA will start to increase.

As the climb slope diminish, less thrust is required to hold the shallower climb slope. But there is another factor: drag:

As the plane slows down, the drag will initially diminish, but you might reach the point of minimum drag, and slowing down further will in fact increase the drag.

So on one hand you need less thrust for the shallower climb, but you might need more or less thrust for the reduced speed. So now we have two sub-scenarios:

The thrust needed to fly slower is less, or it is more but that additional thrust is more than offset by the lower thrust required for the shallower climb:

In this case the plane might settle at something like this state:

95% N1
5° pitch
250 KCAS
climb slope 1°
which gives an AoA of 4° (still waaaaayyyyy away from stall).

Note that this state is in fact a state that the plane would have eventually gone through in the first scenario. Again, as the plane climbs less thrust will be available and the plane will eventually settle at 5° of pitch and flying horizontally. The final state will be something like this:

95% N1
5° pitch
240 KCAS
flying level at FL 350 (i.e. not climbing or descending)
which gives an AoA of 5 degrees (higher, but still waaaaayyyyy away from stall).

That is, the same final state than in the first scenario, but at a lower final altitude.

The thrust needed to fly slower is more and not offset by the shallower climb:

In this case, the plane will keep slowing down and reducing it's climb slope which will turn a descent slope. As the slope keeps going down, less and less thrust is needed to keep the plane flying at this slower and more draggy speed. The question is, will this ever settle into a safe state, or as the plane slows down and more thrust would be needed the increased descent slope will not be enough to offset the additional thrust required and the plane will keep slowing down until it stalls?

Of course, I cannot answer that for sure, but I feel confident that it will settle in a safe state like this one:

95% N1
5° pitch
240 KCAS
descending with a slope of 5°
which gives an AoA of 10° (closer, but still safely below stall).

Why am I confident?

Several reasons:
First, the airplane reached the initial state (cruise) somehow, so obviously the required performance is there.
Second, I guess, again, that Airbus took this into account when setting the memory items.
Third, at say 240 KCAS, that's a TAS of some 400 KTS, that with a descent slope of 5° gives a descent rate of about 3500 fpm. That equals to so much thrust that a plane descending from cruise usually keeps its speed and some 3500 fpm with the throttles idled!, not with climb thrust!

In fact, now that I think of it again, I think that the plane, if it descends at all, will settle at a much shallower descent than 5deg.

So, the plane initially settles in a steady descent, but as it goes down the flight levels climb thrust is more thrust, so the descent slope slowly diminishes until it reaches again a final state of flying horizontally with 5° of pitch and AoA, albeit at a lower altitude than the initial one.

So I'd summarize all this in two sentences:

1) I feel pretty confident that setting climb and keeping 5 deg of pitch would have saved them.
2) If you apply and keep climb thrust and 5 deg the airplane will do something until it settles at a given airspeed and altitude.

But I had already done that, hadn't I?

[Evan]

Gabriel, everything you just posted is very interesting but it all relies on having 95% N1 thrust selected. But the memory items do not state an N1 setting, only a CL detent thrust lever position. Levers at CL in AT can mean actual N1 values anywhere between 0-ish% (ok, not proposing that) and 90-ish%. What if they were in a retarded AT thrust level when this occurred?

Just try to visualize this scenario... ECAM tells the crew of UAS... AT disconnects... Thrust Lock engages... crew goes with memory items... Thrust levers are most likely in CL detent already, although actual thrust may be in a reduced setting from the last AT command before THR LK... memory items say CL thrust, ok, so pilots leave the thrust levers where they are... since the only way to get out of thrust lock is to move the thrust levers, they are still is thrust lock, and perhaps dealing with too much ECAM and chop to get around to noticing the actual N1 reading, which may be in the 70% range or lower... they apply 5° pitch, when 2-2.35 is actually called for at their current alt and weight (they are heavy)... they are well below the desired N1 value and have no IAS data or thrust lever position to clue them... and now they have added too much drag... they are sinking... but they believe they are at the correct memory item settings... in range to maintain safe margin and altitude, and they move on to working the ECAM messages. Distracted, shook up and flying in the dark with no visual horizon, they don't notice the vertical speed or altitude until it is too late to recover. This would result, not in a stall, but in a wings level, high rate of descent impact.

It seems to me that there is a possibility that the A330 UAS memory items do not mix well with a non-servo driven thrust lever and a thrust lock function at cruise altitude in the dark in the middle of the ocean.

It seems to me that stating the CL detent position as a memory item here instead of an actual N1 value is a bit short-sighted...

[Gabriel]

Gabriel, everything you just posted is very interesting but it all relies on having 95% N1 thrust selected. But the memory items do not state an N1 setting, only a CL detent thrust lever position. Levers at CL in AT can mean actual N1 values anywhere between 0-ish% (ok, not proposing that) and 90-ish%. What if they were in a retarded AT thrust level when this occurred?

Just try to visualize this scenario... ECAM tells the crew of UAS... AT disconnects... Thrust Lock engages... crew goes with memory items... Thrust levers are most likely in CL detent already, although actual thrust may be in a reduced setting from the last AT command before THR LK... memory items say CL thrust, ok, so pilots leave the thrust levers where they are... since the only way to get out of thrust lock is to move the thrust levers, they are still is thrust lock, and perhaps dealing with too much ECAM and chop to get around to noticing the actual N1 reading, which may be in the 70% range or lower... they apply 5° pitch, when 2-2.35 is actually called for at their current alt and weight (they are heavy)... they are well below the desired N1 value and have no IAS data or thrust lever position to clue them... and now they have added too much drag... they are sinking... but they believe they are at the correct memory item settings... in range to maintain safe margin and altitude, and they move on to working the ECAM messages. Distracted, shook up and flying in the dark with no visual horizon, they don't notice the vertical speed or altitude until it is too late to recover. This would result, not in a stall, but in a wings level, high rate of descent impact.

It seems to me that there is a possibility that the A330 UAS memory items do not mix well with a non-servo driven thrust lever and a thrust lock function at cruise altitude in the dark in the middle of the ocean.

It seems to me that stating the CL detent position as a memory item here instead of an actual N1 value is a bit short-sighted...

Two things:

1- All this doesn't change what I've said that setting climb thrust and 5° of pitch would have likely saved them. Leaving the levers at untouched at climb after the thrust being locked in another setting doesn't count as setting climb thrust.

2- Are you sure about what you are saying? In particular, are you sure that...

... the memory items call just for climb detent?
... that leaving the levers untouched in the climb detent would be considered conforming the procedure?
... that after the AT disconnected itself the thrust went into thrust lock?
... that this means locking itself in the last setting commanded by the AT before disconnecting itself, regardless of the position of the levers?

Not that I know better, but it sounds very strange to me, just like to you.

[Evan]

1- All this doesn't change what I've said that setting climb thrust and 5° of pitch would have likely saved them. Leaving the levers at untouched at climb after the thrust being locked in another setting doesn't count as setting climb thrust.

The distinction is between setting manual MAX CLIMB thrust and setting CLIMB autothrust mode. The QRH seems to call for setting the former, but both would result in the exact same thrust lever position. If the crew were disoriented and distracted, mode setting could be mistaken for actual thrust setting.

2- Are you sure about what you are saying? In particular, are you sure that...

... the memory items call just for climb detent?

You will find the chart on page 69 of the first interim report. The memory item is 5°/CLB. In normal autoflight cruise, the levers would already be set at the max CL(B) detent.

... that leaving the levers untouched in the climb detent would be considered conforming the procedure?

There is nothing to indicate any other requirement, but perhaps there is more specific training for the procedure. Testimony I've read suggests that there wasn't. But no, I'm not sure.

... that after the AT disconnected itself the thrust went into thrust lock?

It is designed to do this.

... that this means locking itself in the last setting commanded by the AT before disconnecting itself, regardless of the position of the levers?

This is the function of thrust lock.

The Airbus autothrust uses a series of thrust limit modes. In normal cruise, the thrust is set at the CL (climb) detent, and the AT operating range is between flight idle and the mode limit of maximum climb thrust (or maximum continuous power if one engine is inop). One key difference from the Boeing system is that the levers DO NOT MOVE in autothrust mode, so there is no way to determine actual N1 by looking at the levers when AT is engaged.

When the AT self-disconnects due to failure, the thrust lock function engages, locking the thrust at the current N1 value, regardless of actual thrust lever position (which is always set at the upper limit of the mode). If, and only if, the pilot then moves the lever out of the detent, thrust lock is defeated and N1 becomes manually set to the thrust lever position.

I can see where, in the confusion that must have ensued, this could become anti-intuitive and could confuse situational awareness of actual thrust setting.

[Gabriel]

Yes, I know that the AT will agust thrust as needed between flight idle and the TL setting, as long as the TL are set at or below the CL detent. Above CL the AT will "honor" the thrust commanded by the TL, which is to say it's as if the AT was not engaged.

What I didn't know or remembered is that after an AT disconnect the thrust would be kept at the last setting commanded by the AT. It sounds reasonable in an AT system that lacks TL movement feedback, but then the pilot should be very familiar with this feature and know like second nature that leaving the TL in CL is not CL thrust. Yet, it's another potential point of failure.

One of the things that I like least of the Airbus flight control philosophy is this lack of control movement feedback that applies both to the thrust levers and the sidestick, and that in my opinion is a contributor to the famous "what is it doing now" (not that the availability of that feedback helped much in the Turkish case, though).

So I agree with you: having "move the TL away from CL" is a counter-intuitive instruction within the "setting CLB thrust" procedure. But then so it is "if you stall and the nose goes down and the plane start to fall from the sky, then lower the nose to bring the plane back up". I guess that enough training and practice (together with education and understanding) can make intuitive things that were once not intuitive.

Training will likely be a key factor in the final report (even if the black boxes are never found problems with training will likely be investigated and addressed).

For the rest, I'm afraid that even when you and me don't like it much, the Airbus flight control philosophy won't change much after this accident.

[EconomyClass]

Experts? Where?

Isn't everyone here but me an expert? That's why I'm loath to even ask a question because when in the company of gods, silence is a sign of reverence.

[TheKiecker]

they got it!!!

http://www.bbc.co.uk/news/world-europe-12953432?utm_source=twitterfeed&utm_medium=twitter

[Evan]

they got it!!!

http://www.bbc.co.uk/news/world-euro...medium=twitter

Whoa there! They got something. No indication yet of how much, let alone the flight recorders. The wreckage could be very spread out in a very challenging environment. This could be an isolated find. Still, this is very welcome news.

[AVION1]

yeap, they found some parts today....hope the CVR/FDR are in there...

http://www.foxnews.com/world/2011/04/03/searchers-pieces-crashed-plane-atlantic/

[busfan]

Good news. Hopefully the rest of the wreckage is nearby.

[AVION1]

Some information from the B.E.A. website (French NTSB)

http://www.bea.aero/en/enquetes/flight.af.447/sea.search.ops.phase.4.php