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AF 447 Thread No. 12

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AF 447 Thread No. 12

Old 5th Jun 2017, 16:55
  #1441 (permalink)  
 
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In a very real way, the two manufacturers have approached abnormals and manual flight with very different approaches. The sidesticks play no part in passive CRM cues. The wheels do, and score one for B. AB fits no shaker or pusher, having been granted a waiver at certification, (because it won't Stall in Normal Law). Boeing does have these emergency devices.

With an automatic degrade (AB) into an unfamiliar mode, the pilots are at a disadvantage, as the AB flies diferently, and must be controlled with a different protocol.

Stall entry can be completely benign (AB) as the stability "accomodates" departure, instead of emphasizing it, in the interest of notifying the crew that the AC has departed....

What remains in reality is that emergency procedures are almost never needed, so much of the apparent disadvantages are moot.

Except when they're not?
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Old 5th Jun 2017, 17:24
  #1442 (permalink)  
 
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Originally Posted by Conclurs77
We need to ponder why, given seventy two "STALL STALL" alerts, none of the pilots thought to address AoA?
As the A330 on that particular evening did not have a display for AoA as it does for airspeed, vertical speed, attitude, etcetera, the more cogent question (raised during 10 of the 12 447 threads here on PPRuNe) appears to be why the more basic "pitch and power" were not the foremost concerns, in terms of being directly addressed. That question is not answerable other than by inference and guesswork, since neither of the two gents in the flying seats is alive to share with us their thoughts.
(I recall from one of the thousands of posts, memory sketchy, that on one of the pages a non-flying pilot can sort through(page 6 or 7) one will eventually get a value for AoA. I think you can appreciate why neither of the pilots went paging through that system -- their attention was on trying to fly the aircraft).
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Old 5th Jun 2017, 17:27
  #1443 (permalink)  
 
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As I recall, the AoA can be approximated using the FD, the "bird", (FPV), and a quick mental numerical subtraction? Not easy, and probably not trained?

Since this exercise utilizes computed data involving the AoA, one wonders why, if the FCM knows the AoA, how difficult or expensive would it be to display it to the only two who can save the flight?
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Old 5th Jun 2017, 17:30
  #1444 (permalink)  
 
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If it is your belief that they were trained to do that, or that the FD was fully operational even with unreliable airspeed, fine, but I think you've wandered into some Monday Morning Quarterbacking at this point.
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Old 5th Jun 2017, 18:37
  #1445 (permalink)  
 
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Originally Posted by Concours77 View Post
In a very real way, the two manufacturers have approached abnormals and manual flight with very different approaches. The sidesticks play no part in passive CRM cues. The wheels do, and score one for B. AB fits no shaker or pusher, having been granted a waiver at certification, (because it won't Stall in Normal Law). Boeing does have these emergency devices.

With an automatic degrade (AB) into an unfamiliar mode, the pilots are at a disadvantage, as the AB flies diferently, and must be controlled with a different protocol.

Stall entry can be completely benign (AB) as the stability "accomodates" departure, instead of emphasizing it, in the interest of notifying the crew that the AC has departed....

What remains in reality is that emergency procedures are almost never needed, so much of the apparent disadvantages are moot.

Except when they're not?
This, on the other hand, is a real problem.
The other real problem is that stall warning stopped when CAS went under 60kt. This design, I don't know if it has been corrected since, is completely against regulations.
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Old 5th Jun 2017, 20:01
  #1446 (permalink)  
 
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Originally Posted by Lonewolf_50 View Post
If it is your belief that they were trained to do that, or that the FD was fully operational even with unreliable airspeed, fine, but I think you've wandered into some Monday Morning Quarterbacking at this point.
If it is, or was trained it would be what is called an "admission against interest..." For AB or the line to explain how to reach an AoA value without supplied kit, it is an admission that the value is important for flight, and the Gauge is not optioned, possibly creating unsafe flight....

Likely if the practice is known, it is an informal "workaround", shared informally amongst A330 crew who are rostered on one of the cheaper models.

A football player or coach who does not review film and stats after the game, on ANY day, is not a winning player or coach. imo.
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Old 5th Jun 2017, 21:55
  #1447 (permalink)  
 
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Originally Posted by KayPam View Post
This, on the other hand, is a real problem.
The other real problem is that stall warning stopped when CAS went under 60kt. This design, I don't know if it has been corrected since, is completely against regulations.
I'm not sure 'bout that--the Bombardier business aircraft cancel the stall warning at 60 knots and are FAR 25.
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Old 5th Jun 2017, 22:18
  #1448 (permalink)  
 
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There is a line in the certification specifications that is very specific indeed :
The stall warning should not stop until the airplane is clear of the stalling situation.
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Old 5th Jun 2017, 23:43
  #1449 (permalink)  
 
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Kay Pam

In defense, an aircraft which is exempted by demonstration does not have to adhere to that which is exempted, nor to related prose in the regulation.

IOW, AirBus aircraft demonstrated an aversion to aerodynamic Stall sufficient to satisfy the Regulator. This also takes into account the airframe's static stability, longitudinally, to and through Stall.

Given that, it is a short step to assume the requirement to maintain the Stall aural warning below 60 knots is less stringent than the rather important exemption from shaker/pusher.

I have found "Low Energy Awareness Program", and it is related, but cannot load it, perhaps it is proprietary.

There may be, at the root of this apparent warning "failure", a very good reason the Stall Warn is inhibited below 60 knots IAS. EFCS, once Stalled, may have an exemption related to "recovery" that requires the Warning be inhibited. Whether or not it is related to Low Energy warnings, I have no idea?
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Old 6th Jun 2017, 06:08
  #1450 (permalink)  
 
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Originally Posted by KayPam View Post
There is a line in the certification specifications that is very specific indeed :
The stall warning should not stop until the airplane is clear of the stalling situation.
I do not think this is a correct statement, at least in FAR 25. Effectively, as per FAR25.207(c): "Once initiated, stall warning must continue until the angle of attack is reduced to approximately that at which stall warning began."
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Old 6th Jun 2017, 13:10
  #1451 (permalink)  
 
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It may be different now. But only a few years ago every aircraft I had worked on cancelled the stall warning below around 60kts. That's Boeing, Airbus, Cessna, all of them.

The understandable assumption was that you should know that you're stalled below 60kts. Further to this, at below 60kts, the airflow over the vane is not reliable enough to provide accurate AoA information and also I was told, to stop erroneous stall warnings on, for example, undulating runways during TO.

I know Airbus has introduced a MOD to keep the stall warning below 60kts.
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Old 7th Jun 2017, 15:55
  #1452 (permalink)  
 
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Originally Posted by Bidule View Post
I do not think this is a correct statement, at least in FAR 25. Effectively, as per FAR25.207(c): "Once initiated, stall warning must continue until the angle of attack is reduced to approximately that at which stall warning began."
The wording is a bit different but the idea is the same.
Thanks for digging that out.

In the case of AF447, the stall warning stopped even though the angle of attack had increased way further than stall warning aoa.
This is in contradiction with the abovementionned statement.

Even if Airbus was authorized to design it like that, this can be criticized.

Mono : thanks for this info.
However, a better idea than a below 60kt condition for deactivation would have been a weight on wheel condition.
If you're in flight below 60kt with your airbus, you're almost 100% sure that you're stalling (or in 0G flight but that seems unlikely) so I think it would deserve to ring..
Aoa is not reliable under 60kt ? if you stall so deeply that your CAS goes under 60kt, that's a very good reason to make the stall warning ring. Not stop it.
(if no weight on wheels)
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Old 7th Jun 2017, 17:46
  #1453 (permalink)  
 
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KayPam

Would have been interesting to be present at the last programmer's meeting prior to certification trials.

"Yeah, at less than 60 knots, the aircraft is taxiing, right?" AoA vane inop, so Stall Warn is inhibited.

Logical, but as with all things, the exception breaks the "rule".

So as with "Low energy awareness program", the committee thinks similarly....

Low Altitude only. Sometimes the 'committee' is an ass. Otherwise bright people defer to "consensus".......
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Old 7th Jun 2017, 18:25
  #1454 (permalink)  
 
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It is standard design practice to inhibit indications when that indication is considered to be unreliable - and the AOA below 60 knots is not considered to be a reliable indication. That a "professional pilot" might manage to stall the aircraft so badly that airspeed dropped below 60 knots while stalled was probably not considered to be a credible scenario.
Until it happened...
Now that the designers know that there are "professional pilots" out there that will stall an aircraft that severely, the designers can take that into account with creative logic - e.g. when stall warning "true" and airspeed greater than 60 knots, if airspeed drops below 60 knots while stall warning remains true, keep the alert active.


However my personal opinion is that Bonin simply shut out the stall warning indication - literally didn't hear it - because it didn't fit what he thought was happening. If correct, something like a stick shaker would have been more effective...
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Old 7th Jun 2017, 21:16
  #1455 (permalink)  
 
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It didn't happen. The actual 'airspeed' never 'dropped below 60 knots' only the ADR sensed airspeed.....
Fair enough, I should have specified indicated airspeed...

.....Would you give them credit then for being at least 'semi-professional'?
No, not Bonin. What he did demonstrated a complete lack of understanding of basic aerodynamics.


Note that I spent ~40 years on the design side of things - and prior to AF 447, if someone had told me I had to design my system to account for an aircraft that was in air and was stalled so badly that indicated airspeed dropped below 60 knots (and with the PF taking no corrective action to get out of the stall), I probably would have laughed out loud...
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Old 7th Jun 2017, 21:51
  #1456 (permalink)  
 
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if someone had told me I had to design my system to account for an aircraft that was in air and was stalled so badly that indicated airspeed dropped below 60 knots (and with the PF taking no corrective action to get out of the stall), I probably would have laughed out loud...
Looking at your location, it appears you didnít design an aircraft system without feed back between the pilot's controls, and with a stabiliser auto trim system which continues to trim whilst stalled.
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Old 8th Jun 2017, 00:45
  #1457 (permalink)  
 
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Airspeed

In descent, 447's airspeed (velocity) was seldom below 300 knots. Indications were what put them in the grinder in the first place. So any discussion of indications or "How bad was the Stall", are meaningless.

She impacted with a velocity around 240 knots.

"sixty knots" is extraneous information. A pilot who confuses a reoccurring Stall Warn and increasing velocity with decreasing Pitch such that he pulls back, is too in the weeds to fly. Neither of the other two could have done any better, using available data from CVR.

jmo
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Old 8th Jun 2017, 05:14
  #1458 (permalink)  
 
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@tdracer
You have a slight advantage over me in that I only spent 37 years on the design side, but I would have been laughing along with you. With the caveat that it was allowing the aoa to develop to over 50 degrees that caused the indicated airspeed to drop below 60kts I agree with everything you wrote

Last edited by Owain Glyndwr; 8th Jun 2017 at 05:28.
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Old 8th Jun 2017, 07:39
  #1459 (permalink)  
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Neither of the other two could have done any better, using available data from CVR.

.. many of us would have thought that it just needed someone to look at the AH for a moment (obviously, there was no semblance of an instrument scan in the cockpit at the time ..) and wonder why the body angle was up where it was ... not rocket science for those who have a good grounding in stick and rudder I/F, I would think.
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Old 8th Jun 2017, 13:25
  #1460 (permalink)  
 
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Originally Posted by Owain Glyndwr View Post
With the caveat that it was allowing the aoa to develop to over 50 degrees that caused the indicated airspeed to drop below 60kts
Only active participation of the Autotrim was allowing the aoa to develop to over 50 degrees ... Not much to laugh on that one.
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