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AF447 Thread No. 3

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AF447 Thread No. 3

Old 15th Jun 2011, 13:21
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Lazerdog: As they were descending in the the stall through 10,000 feet, the THS was trimmed up automatically."
Chris: All of us are occasionally running into problems of ambiguity, due to the inherent limitations of written language. That's why the FBW design engineers have to avoid it. I'm sure you don't mean to imply that the autotrim was still changing the THS trim position as AF447 was passing 10,000ft?
Lazerdog: "Would full forward side-stick have caused immediate stall recovery, or would the PF have to wait until the THS auto-trim caught up with nose-down trim?"
Chris -- Short answer is: don't know. But I think we can expect that the AoA would have started to reduce immediately. Unless the THS motor stalled, which would be unlikely at such a low airspeed (low loading), the THS would have started moving immediately. I don't know its maximum rate.
Chris, I think some of the ambiguity that has people concerned is that even with the THS being ~13 deg at apogee, other info from BEA based on FDR describes pilot nose down inputs at various times during the event, with apparently little or no influence on the THS position. (Ah, I see RetiredF4 has made an analysys that makes some sense to me).

Concern -- were control surfaces unable to respond (??).

Concern -- without creating an error message, (which a stalled THS motor would generate??) did properly operating functions (alt law speed stability, which you addressed earlier) or Abnormal Attitude law (at present conclusion is that it wasn't active) provide trim commands in opposition, or as a "correction," to the nose down commands? I would think it wouldn't, but if Airspeed sensing is out the window, what is speed stability using as a reference to direct an assist toward "better" nose pitch?

Since pilot can override the speed stability functions, Lazerdog's kinesthetic comments IIRC don't come into play with side stick forces, but can factor in to "seat of the pants."

(In unusual attitudes, in night instrument conditions, descending in stall, the "seat of the pants" fast becomes a false reference ...)
Idea: pilot is putting in nose down input, stability is either over helping (doubtful, as pilot overrides) or sensing speed at the other extreme, so that as soon as pilot input stops, it helpfully moves THS in the wrong direction. That (if the THS isn't stalled) might be felt as turbulence, or not at all if there is already turbulent air as the environment.

But that idea may be nonsense.

There are still nose down commands from the sidestick of unknown duration. RetiredF4's post finally puts a finger on something I was trying to ask: were they at some point futile?

Or, were the pilots reverting to back stick inputs to get the aircraft to respond at all. (Seen something similar to that in spin training with a pilot mentally behind the aircraft ... ) RetiredF4's post explores that nicely.

The skeletal nature of the reported timeline (needs a bit more meat on the bone, hopefully to come in time) leaves the question of "why didn't nose down commands (such as they were) change the THS position from the reported ~13 deg from apogee to the surface?"

If I am saying roughly what you are saying, my apologies for the repetition.

Back to one of our other posters, who described a while back how long it took for the nose to respond in stall recovery ... per lazerdog's kinesthetic comments. I will add to that thought the sense of tempopral distortion that frequently arises during disorienting events.

I've experienced it myself, and it ain't pretty.

By temporal distortion I mean a sense of it all happening in slow motion, where the sense of time passage is lost. This can influence actions and thoughts. Sample:

"I pushed the stick, nothing happened, I need to do something else" so another control input is made.

The decision/act cycle (or the OODA loop) happens in compressed time scales (fractions of a second) as compared to normal flying of a passenger jet, where smooth and deliberate is the normal pacing. (OT: That smooth and deliberate approach is much appreciated by the passengers, I can assure you! )

Last edited by Lonewolf_50; 15th Jun 2011 at 13:50.
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Old 15th Jun 2011, 13:32
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29% or 37%?

From the above, it seems fuel transfer to the tail is automatic once in cruise. There is no mention in the BEA info that fuel was manually transferred forward, so how did the CG get to 29% in latest BEA note, from 37% in initial reporting?

37% CG would be a big difference from 29% in manual pitch control. It's real easy to over control with aft CG.
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Old 15th Jun 2011, 13:38
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One particularly prescient poster, or an industry doomed to repeat history until it learns from it ?
Another quote from Collinson:

"The high integrity, full authority FBW can provide the automatic
stabilisation required to enable a civil aircraft to be designed
with reduced stability margins under certain loading configurations
and flight conditions. This enables a lighter aircraft to be produced
with an increased performance and payload capability compared with a
conventional design".

Weasel words. Saying that stability can be traded against cost and
performance, but are current designs going too far in this respect ?.

Note: "reduced stability requirements". This implies that when
the systems gives up, the aircraft will be more difficult to control
manually, than a non fbw aircraft. I think someone already mentioned
elsewhere that the a/c would be very sensitive to control inputs. Just
what's needed when confronted with an emergency situation with ambiguous
or unreliable data from the flight control systems and a crew with no
training or experience of such conditions.

On balance, correct me if wrong, once AF447 stalled, only a test pilot
with a lot of experience would have had any chance of recovery...
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Old 15th Jun 2011, 13:50
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Suppose an APU turbine with afterburner; Suppose it´s nozzle pointed 45° down.

How many pounds of thrust you typically need to "restore pitch" in order to start flying again in this class of a/c? To "restore a decent AoA"

A drag chute is unthinkable in an airliner. An special APU may save?
I suppose the lack of additional information leads to idle minds wandering off into "la-la land"... the idea of something as dangerous as an afterburner equipped APU with a nozzle at 45 degrees down, well, let's just say I have horrible visions of aircraft tumbling end-over-end due to inadvertent engagement. Assuming such a monstrosity were fitted (and at what cost), how often would this find any application and how well might it function?

Personally, I suspect the great cost of the extra training such a dangerous devices would require, would be far better spent on upset training, either by putting regular line-pilots into a glider or acrobatic aircraft for 4+ hours a year (pick a number), or funding enhanced simulator development to include the extended stall regimes (and before the masses jump down my throat read this -> http://www.idt-engineering.com/image...an_25JUL09.pdf ).
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Old 15th Jun 2011, 13:52
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Speaking of Relaxed Stability...

Just yesterday I talked with a retired aero engineer who had worked on the design of the MD-11 THS. He said its small size was predicated on the assurance (from Honeywell) that the LSAS, Longitudinal Stability Augmentation System, would meet 10 <-9 probability of No Undetected Failure. It didn't, of course, and many MD-11 crashes have been related.

Could the A330 be operating on similar assumptions?
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Old 15th Jun 2011, 13:55
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Originally Posted by CONF iture
As anticipated, PJ2 never wrote anything like it.
I oversimplified the description, but it seemed pretty clear to me that with autoflight off, the only thing that can move the THS is pilot input, PJ2 agreed with that.

As you don’t seem to realize what’s the function of a trim, if you have a chance, go in a flying club and see by yourself if you are able to apply a full deflection before you feel the urgency to trim.
Chris Scott and Lonewolf_50 are more diplomat than I am, but what you wrote simply don’t make sense.
No need to be insulting, I know what the function of aircraft trim is. In normal circumstances that's what it's used for. Under Alt law, the autotrim is designed to follow the pilot's sidestick commands. This gives the pilot regular trim control through the sidestick, but has the additional side-effect of effectively giving them full pitch authority with little or no interference from the computers.

Chris has been kind enough to state that his "G" theory is just that - a theory. I'll try and track down some information over the next couple of days to confirm or refute that theory - I've also sent some PMs to people who might know.

If you want to get personal, I could say that at least I'm not trying to defend a man guilty of the manslaughter of three people who to this day is still arrogantly unrepentant about not taking responsibility for those deaths.

Trim wheel has priority over the electrical control but does it 'disable autotrim for the duration of the flight' ?
Now, you won’t mind if I ask to back up that one …
A330 Flight Deck and Systems Briefing

Page 5.13

The control wheels are used in case of major failure (Direct Law or mechanical back-up) and have priority over any other command
The implication is that manual trim will have priority and can be used for the duration of the flight if the pilots are not happy with what autotrim is doing. Svarin claimed in an earlier thread that some airlines train pilots to avoid use of the wheel for that reason (which IMO is a dreadfully myopic practise).
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Old 15th Jun 2011, 14:04
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JD-EE
Quote:
Originally Posted by JD-EE
Kinetic energy is 1/2 mV^2. Potential energy is mgh. So the mass washes out of the equation. So a 3000' climb would give "gh" = 1/2 v^2 = 96000 ft^2/s^2 equals about a 438 '/s velocity change or about 300 MPH.

I am probably being a bit thick, but does this relate to the aircraft in question and if so, are you saying the aircraft lost 300mph in its climb?
Roughly speaking - I cheated on the math somewhat. Rudderrat posted the more correct equation from which mass drops out. I presumed a drop to zero velocity which is a cheat. It gives the general idea, though. A plane drops a staggering amount in the 3000' climb with no additional power input through the energy tradeoff.

Since we're working on the difference between the squares of two velocities it does vary. I'm not sure of the actual velocity of the plane to apply this to the AF447 case. (Too lazy to look up speed of sound at 37000'.) If the plane's physical speed was around 300MPH it'd have been dead stopped. If it started at 475 MPH the drop is probably around 120 MPH.

Again, the key takeaway is that the mass isn't part of the equation until you worry about flying bricks vs flying pillows of equal size and shape.

Last edited by JD-EE; 15th Jun 2011 at 14:26.
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Old 15th Jun 2011, 14:09
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Trim wheel has priority over the electrical control but does it 'disable autotrim for the duration of the flight' ?
Now, you won’t mind if I ask to back up that one …
But you don't!

Using the TRIM WHEEL does NOT disable autotrim for the duration of the flight.

What it does is override the instantaneous demand and allows the direct pilot input to take effect. The computers continue to track the trim state.
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Old 15th Jun 2011, 14:12
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@GY - Unless I'm mistaken, they track it, but they don't change it.
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Old 15th Jun 2011, 14:14
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icehart says, "(hope that does not apply to Airbus)".

Sadly, it does. Solder pot connections don't work very well over time. Crimps work very well over time.

To see why solder pots (not wires wrapped around terminal lugs or the like) fail find a 6" or so length of 1/16" or solder. Secure it around a heavy spoon by one end of the solder. Secure the other end of the solder to a door knob or the like. Let it percolate a few days. Odds favor the spoon waking you up at night as it clatters to the floor when the stretched solder breaks. The weight of the spoon will stretch it. (You may have to adjust the size of the spoon so it doesn't visibly stretch as soon as the weight of the spoon is on it.)

With old bar solder you simply clamped one end of the bar to the table top with the other end hanging over. It would become Dali-esque over time. The 1/8" solder I had at one time would stretch itself and break with only its own weight on a 3' length given some time.

The magic of the crimp is the tight mechanical connection that actually cold welds the two objects, wire and connector pin, together. Wire wrapped around terminals for a secure mechanical joint also performs well because of the good mechanical joint. Solder pot connections have no mechanical strength to speak of. So over time the connection can break if there is any stress on it.
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Old 15th Jun 2011, 14:16
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FCOM 1.27.30 ALt LAW
"In flight, the alternate law pitch mode follows a load-factor demand law much as the normal law pitch mode does, but it has less built-in protection (reduced protections)."
Originally Posted by DozyWannabe
I oversimplified the description, but it seemed pretty clear to me that with autoflight off, the only thing that can move the THS is pilot input,
I don't think so.
With stick free, the aircraft will attempt to fly 1G (either level or climbing etc.) and if the speed is reduced the stab will be auto trimmed to more nose up aircraft attitude.
(Modified Attitude stable flight law rather than Speed stable flight law.)

Last edited by rudderrudderrat; 15th Jun 2011 at 15:38. Reason: extra text = Modified
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Old 15th Jun 2011, 14:16
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From GB's link:
Loss of control following an upset is the largest single cause of airline crashes and fatalities.


Stick shakers and pushers are warnings of last resort, and pilot attitudes or experience may lead to such indications being ignored or wrongly attributed, resulting in no or incorrect actions.

Training issue, which appears to be what the paper intended to address.
Pre-stall indicators do not provide sufficiently timely or accurate cues to alert the crew to a developing situation. More flexible types of indicators, which can be integrated into primary cockpit displays, have been developed which provide dynamic information on the aircraft’s attitude and velocity with respect to its normal flight envelope. These could provide the cues needed to alert pilots to developing situations, and to provide guidance on aircraft handling and performance during recovery manoeuvres.

I wonder how many pilots disagree with that bolded part? This bit looks toward tools, rather than training ... but training (personal bias here) is the higher payoff target. You get another tool, you just added to your training requirement.
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Old 15th Jun 2011, 14:28
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Re: The "G" theory

I wouldn't take this as read, but here's an interesting post from someone purporting to be a former A330 pilot on another forum. (emphasis mine)

Ok, as the aircraft decelerates from say, 250 knots to 210 knots from a level flight with no bank, the aircraft starts with a given pitch of x, and a 1G corrected for pitch. As the aircraft decelerates, the full deflection of the sidestick (which should normally give you a load factor of +2.5G) will not be able to provide you such G load. So, as the speed decreases, the command is no longer neutral stick=1G, but it become neutral stick=0 pitch rate. This means that you start having a ever so slight change in V/S. It is almost not noticeable, and as a matter of fact, most pilots new to Airbus may not even notice that they are slowly correcting the pitch to maintain level flight (they just actually do it without noticing because it is intuitive for a pilot to do this as an aircraft decelerates in level flight, and in the airbus this corrections are very minute compared to a conventional aircraft). Yes, techinally you wouldn't be maintaining exactly 1G, as an increase in V/S (decent) would translate to a change from 1G to something less than 1G (almost negligible in my humble opinion). Sorry for the confusion. The bottom line is the aircraft will switch from G-factor command with the sidestick deflection to pitch-rate command with the sidestick deflection as speed reduces.
There's some other interesting stuff there, link below.

Air France 447 - On topic only! - Page 60 - JetPhotos.Net Forums - The Friendly Way to Fly
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Old 15th Jun 2011, 14:33
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Originally Posted by Lazerdog
As they were descending in the the stall through 10,000 feet, the THS was trimmed up automatically. Would full forward side-stick have caused immediate stall recovery, or would the PF have to wait until the THS auto-trim caught up with nose-down trim? (How long would that take if so?)
I don’t think that stall recovery could have been immediate. With the aircraft “deep” into the stall (AoA >40 degrees ?) it must be pitched down over a considerable arc: 10, 20 or 30 degrees, would that take 5, 10, or 15 seconds? I don’t know, but I believe one should think in terms of those delays.

Perhaps the graphs posted earlier would help to answer questions about AoA and pitch.
The green line in Figure 1 shows the total energy (expressed as the equivalent FL) at five instants at which the BEA update provided altitude and airspeed. Total energy is the sum of:
Potential energy = m*g*h, and
Kinetic energy = 0.5*m*TAS^2,
where m=mass, g=acceleration of gravity, and h=height.
The red line shows the vertical acceleration assumed to calculate the actual FL (shown in blue), by integrating vertical acceleration to obtain vertical speed, and integrating that again.

Figure 2 shows the vertical speed and altitude, and also the airspeed calculated from the kinetic energy, i.e. from the difference between altitude and total energy FL shown in figure 1.

Figure 3 shows the air-based flight path angle calculated from TAS and vertical speed, and the AoA that corresponds to the lift force that would produce the vertical acceleration at the calculated CAS. The pitch angle is then the sum of FPA and AoA. The two red nearly parallel lines show the alpha-max for the Mach-number at each instant (full line), and the stall warning threshold (dotted line).

How does one recover from a stall without reliable airspeed and without AoA display?

AP and A/THR disconnected when one or more pitots iced up at 2h10min05 at FL350 and M=0.8. Less than one minute later the airplane’s climb peaked at FL380. The airplane stalled some seconds earlier, and the PF apparently made only a half-hearted attempt to prevent that. His pitch-down input reduced the pitch to about 7 degrees up, and the v/s from 7000 to 700 fpm. Maybe if he had stopped the rate of climb, and reduced pitch to below 5 degrees, that would have been enough to prevent stalling. Given the inadequate control commands in that first minute it is perhaps unlikely that, even if the PF had been aware that the was in a fully developed stall, he would have made the more agressive commands that were needed for recovery from “deep” into the stall.

The inconsistency between speeds 1 and 3 lasted less than one minute in this case, but in one documented cases of UAS as much as 3 minutes and twenty seconds. So I asked myself: how does one recover from a stall without reliable airspeed and without AoA? I would think that the proper action would be a determined nose-down push, maintained until the stall warning stops, and then trying to maintain an AoA on the edge of s/w, gently allowing the nose to raise until s/w occurs, then a small nose-down correction to silence it, etc., until back in approximately level flight at a reasonable pitch attitude.
EDIT:: Can anyone be expected to do that succesfully without being trained for that eventuality?

Last edited by HazelNuts39; 15th Jun 2011 at 14:44.
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Old 15th Jun 2011, 14:35
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@RetiredF4: Your analysis of the likelihood of recovering from a fully developed stall is based on some flawed assumptions, principally that a stalled aerofoil has "stopped working". Lift coefficient doesn't just fall to zero after the stall AoA is reached, it falls gradually (and may even have a second peak that's not that far off the stall value). At 60 deg AoA you may still have a lift coefficient half the primary stall value, so a stalled HS can still be generating a lot of lift. In addition the wing pitching moment (nose down) typically increases significantly with AoA post-stall. In principle a conventional design is still recoverable, given reasonable lateral control and enough height.
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Old 15th Jun 2011, 14:38
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I am probably being a bit thick, but does this relate to the aircraft in question and if so, are you saying the aircraft lost 300mph in its climb?
JD-EE:
Roughly speaking - I cheated on the math somewhat. Rudderrat posted the more correct equation from which mass drops out. I presumed a drop to zero velocity which is a cheat. It gives the general idea, though. A plane drops a staggering amount in the 3000' climb with no additional power input through the energy tradeoff.
For those having a hard time beleiving a massive (true) speed drop from the climb consider a cart on a roller coaster going fast at the bottom of the first drop, as it reaches the top of the next hill it's velocity drops to close to 0 then accelerates again as it goes down.

The a roller coaster cart has no thrust and less friction/drag losses than an airplane however the principle is the same: trade speed for altitude.

Note, ignoring drag no energy is "lost" just converted from kinetic to potential, at the top of climb most of it is still available to convert to speed if the nose is pointed down.
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Old 15th Jun 2011, 14:40
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syseng68k

You mention a very important part of the puzzle. At the critical time of a/p drop, the PF had a sensorial challenge. I believe he read the panel and made corrections consistent with his overall and immediate assessment. If the a/c is in ROLL Direct, but with Trimming Pitch, his ability with the ailerons will need to be immediately adjusted to inputs with Elevator.

The pic supplied by Machaca of the engineer on the ladder working on the RCU shows the massive HS and its actuator jack screw. A/C operate in three planes, and when one starts reacting substantially differently when it still needs to be used in co-ordination with the others can cause that focus you note.

you say:

"...Note: "reduced stability requirements". This implies that when
the systems gives up, the aircraft will be more difficult to control
manually, than a non fbw aircraft. I think someone already mentioned
elsewhere that the a/c would be very sensitive to control inputs..."

How often did the pilots train to the discrepancy in "touch" to Roll compared with Pitch? How necessary was the Rudder, if at all, to settle the Yaw produced by Roll excursion? I believe firmly the accident began at loss of a/p, and the corrections input by PF. It is very easy to entertain getting a bit behind, which makes it not difficult to question whether they caught up, and if not, perhaps the a/c and Pilots started down different paths? In a general way, I think this will be the fulcrum of the findings.

Reading the A320 thread (Unreliable Air Speed Flaw), there simply seems to be an uncomfortable lack of understanding between crew and a/c in off normal regimes.

Lonewolf Re: Boyd. Coram makes mention of very high drag in ACM, in his biography, which has an excellent Index.

Are you sure aircrews should study OODA? With patience and calm being part of the Air Bus check list, how will you fold AGILITY into the Loop?
 
Old 15th Jun 2011, 14:41
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Graybeard

Automatic fuel transfer normally starts at FL 255 in the climb and FL 245 in the descent. The CG position is monitored continously and fuel transferred to give optimum CG for best fuel economy.

However, you don't get something for nothing. At 205 tonnes, compared with 29%, a CG of 37% will give a reduced Buffet Margin of more than Mach 0.02.

Fuel can be transferred forward at any time by the pilots using a switch on the fuel panel.
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Old 15th Jun 2011, 14:49
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Just asking

GY,

I suppose the lack of additional information leads to idle minds wandering off into "la-la land" May be this is happening with other people

I am just asking the required thrust (and if this could "save").

I agree in general with the other comments you made.

I just would like to known if a typical APU class turbine could fit the thrust requirements (augmented) or if we would need much bigger ones for the task.
It´s just a "quantitative" question.
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Old 15th Jun 2011, 14:51
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Originally Posted by Neptunus Rex
At 205 tonnes, compared with 29%, a CG of 37% will give a reduced Buffet Margin of more than Mach 0.02.
You sure? I thought it was the other way round.
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