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AF 447 Thread No. 7

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AF 447 Thread No. 7

Old 31st Mar 2012, 00:33
  #1101 (permalink)  
 
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Doze:

That G limitation only applies in Normal Law however.
The 2.5 g limit applies in all laws, including Boyle's, Charles's, and Burke's (Gene Barry) law and the law of supply and demand.

In one particular law you can actually exceed it. Starts with a 'D'.
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Old 31st Mar 2012, 00:52
  #1102 (permalink)  
 
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You know what I meant - the aircraft's systems will no longer prevent a pilot from exceeding it outside of Normal Law.
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Old 31st Mar 2012, 01:09
  #1103 (permalink)  
 
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Neofit , thanks. some ride.
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Old 31st Mar 2012, 02:02
  #1104 (permalink)  
 
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Still with significant nd and thrust at idle ? Think 20k ft was intimated in an earlier post which is what caught my eye. With a startled crew in poor conditions one minute isn't very long to interpret and initiate a course of action.

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Old 31st Mar 2012, 02:04
  #1105 (permalink)  
 
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@PJ2 - not sure where your post's gone, but...

Going back to my notes, it took about 16-18,000ft to recover the way we did it. My writeups are on the last page of thread 6 and the first page of this thread.

We did not reenable ADR or FPV during the recovery, so I don't have the numbers available that you do. This was purely seat-of-the-pants stuff, applying ND until the THS returned more-or-less to the centre and holding the dive to gain speed, correcting the adverse roll with rudder inputs and gently pulling out of the dive once we figured we were going fast enough to be flying again.

Our experiment was purely qualitative - proving that the aircraft could be recovered if the situation was correctly diagnosed in time and that the aircraft's systems would in no way impede the recovery.
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Old 31st Mar 2012, 02:07
  #1106 (permalink)  
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Re altitude loss and other data, Dozy, what was your experience in the sim? Our sim exercises usually took about 20,000+ ft. for recovery.

Dozy, edited/re-posted for clarity.

With weights, CG, SAT mirroring AF447 and a bit of turbulence, following loss of airspeed (all 3 ADRs out), the sim was pitched up at FL350 and held in the climb until stalled, (THS reached 13.6deg). Shortly after the stall we returned the ADRs for use during the balance of the exercise, (to see the FPV during the stall).

Post-apogee (approx FL360), full forward stick was applied and held.

At FL330 the pitch was 8deg ND.

At FL310 the AoA (using FPV) was approx 40deg and the VSI was 18000fpm +. Pitch was about 14deg ND which was all the pitch that could be obtained.

Pitch slowly reduced to about 10degND still with full forward stick. As it was held the THS unwound and returned to normal settings.

We could watch the AoA reducing as the FPV slowly climbed "up" the PFD from past the red ND warning arrows below 30deg pitch marks.

Thirty seconds after the first Stall Warning passing through FL270 the AoA had reduced to 30deg, descent rate was 16000fpm.

Ten seconds later at FL255 the AoA was 12deg, CAS was 250kts, VSI was 7400fpm.

At FL245 the stall warning stopped 40 seconds after it began, the AoA was 10degND, M0.658, VSI 7000fpm down, CAS 278kts.

From an AoA of 40deg to 10deg took 24 seconds and about 6000ft. This exercise took about 22000ft; some were less.

Overspeed was never a problem nor was a secondary stall if one was gentle, (took about another 6000ft IIRC)
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Old 31st Mar 2012, 02:09
  #1107 (permalink)  
 
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Sorry to butt in but can i ask how you judged how much pitch up to apply?
Hmm post back again...

The unfolding sequence over 2 years has given the profession plenty of time to consider how one would have dealt with this situation so in a sense you must have figured out what you hope you would have done. if not 'primed' for action allow a minute to stabilize the brain and that must add another 10k ft at the rod experienced. i realise the issue was how not to get to that situation in the first place but understanding the option space does inform any potential mitigations .

Last edited by Mr Optimistic; 31st Mar 2012 at 02:27.
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Old 31st Mar 2012, 02:25
  #1108 (permalink)  
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Mr. Optimistic;

Normal cruise pitch attitude is between 2.3 and 3deg depending mostly upon weight. A pitch up to 5deg pitch attitude (+2.5deg) results in about an 800 to 1500fpm climb and a gradual loss of energy if held long enough. The UAS QRH checklist and the FCTM cautions strongly against holding such pitch attitudes for long and advises to get the QRH out quickly and set pitch and power. The FCTM also states that the Memory Items are not to be done if the immediate safety of the aircraft is not impacted.

For the purposes of the exercise there was no "judging" of how much to pitch up. We pitched up high enough to stall the aircraft. Fifteen degrees would do it, sometimes we were higher.

The overriding impression of these sessions was how quickly things occurred and how fast was the altitude loss.
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Old 31st Mar 2012, 02:29
  #1109 (permalink)  
 
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Sorry Pj2, cross posted. Thank you.
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Old 31st Mar 2012, 02:58
  #1110 (permalink)  
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Mr. Optimistic;

Does "option space" refer to the time available to assess a situation, decide upon a course of action, assess the response and secure the airplane? I hadn't heard of the term but likely it applies!

The danger of subtlely being taken in by hindsight bias is always there even when one really tries to avoid it; - we simply know what happened beforehand. The best that can be hoped for in these practical experiments is some sense of possible recovery time. "What would one do?" is not part of the exercise because we already really, really know what we would do!, and we weren't there... ;-)

But two questions remain unanswered: "Why the initial pitch-up?", and "Why wasn't the stall warning heeded post-apogee?". We've all seen various explanations as to why and are waiting to see how the BEA HF people come to terms with these two inexplicable matters and how such compares with our own thoughts and suggestions.
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Old 31st Mar 2012, 03:23
  #1111 (permalink)  
 
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Yep that is what i had in mind about option space, the time and actions available to the people required to act. I understand that the lhs did the wrong thing, and the rhs should have spoken and acted, but the safety of the industry cannot rely on people always being on top form or of analysing to the degree bystanders like us can.
If you are going to design a clever aircraft, need to help the crew in situations like t
his.
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Old 31st Mar 2012, 03:29
  #1112 (permalink)  
 
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Mr. Optimistic:

Other way round - the PF was in the RHS on this occasion. I'm not fully understanding your point either - the aircraft's systems told them that they were approaching stall, and continued to warn them as they went into the stall regime itself. It warned them that they were in Alternate Law 2 without protections. The ADI told them that they were too nose-high, the speed indications (when they came back) indicated they were too slow and the altimeter was unwinding at an alarming rate.

How much more help can you expect the aircraft to give?
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Old 31st Mar 2012, 03:31
  #1113 (permalink)  
 
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Recovery altitude and "touch"

Salute!

If I recall, we determined that the jet was not in a "deep stall" such as the Viper had or the T-tail jets. In other words, the cee gee and center of aero pressure was much like most airplanes.

So I have a hard time thinking it would take more than 10,000 feet to recover.

Granted, I was used to flying at the limits and sometimes over the limits, but I can forgive the basic heavy pilots in that regard.

As PJ has asserted, being smooth and getting the nose back up after reducing the AoA is the key to minimum altitude loss. You can't do this as a "mechanic". You have to have "touch" and be in "touch" with the jet ( I can just see Doze snorting, heh heh). I don't see any way to do this in a sim.

A reliable AoA indication seems a great thing to help you get back to acceptable flight conditions. The FBW implementation on the 'bus appears to have a flaw with regards to AoA. In other words, you can override the "system" and get into the situation that AF447 did. I don't get it. I lost one friend over the desert that buried the nose too far on an attack pass and the gee/AoA limiter gave him the best aero possible but he was just too low/steep. Sierra happens.

I also speak from personal experience in straight wing jets and bent wing ones and deltas. The mach buffet feels different than the stall buffet. One is more of a buzz and the other is more of a "shake". Then there's aileron reversal if it's a mach problem as others have described here about the shock waves.

Go read about Yeager's experience and his use of the horizontal stab. Once supersonic his elevators didn't work, so he manually cranked the stab. Seems an engineer warned him about that possibility( I heard the story directly from Yeager one day in 1979 at the 16th Sqd, First in the F-16) So all the USAF jets after that had the solid horizontal stab that worked for elevator and kept working in the transonic/supersonic regime.

I have to admit that this is a sad story, and I pray that I fly as a SLF with a crew that does more than drag us from DFW to DEN without flying a Citabria every now and then.

Gums
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Old 31st Mar 2012, 03:51
  #1114 (permalink)  
 
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I guess like me that you are an engineer. yes we all discuss the evidence available but look at the outcome. The 'system' failed. Sorry about the lhs/rhs issues, it has been discussed so often i forget where the pendulum stopped.

How do you train aoa instrumentation if it is never to be used unless in upset!

The warnings they had didn't tally with the situation they thought they were in so the significance of the warnings was lost. The stall warnings in their minds were erroneous and had to be filtered out.
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Old 31st Mar 2012, 08:07
  #1115 (permalink)  
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Re, "How do you train aoa instrumentation if it is never to be used unless in upset!"

Precisely.

There is far more to this suggestion than just installing the guage.
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Old 31st Mar 2012, 09:24
  #1116 (permalink)  
 
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Originally Posted by gums
The mach buffet feels different than the stall buffet. One is more of a buzz and the other is more of a "shake".
If you are responding to my post #1084, I suspect we are talking about different things. The airplane encountered "low speed" buffet at 02:10:53 at Mach 0.68. I believe that buffet was caused by "an oscillatory interaction between local shock waves and airflow separation", rather than the stall buffet as you describe it..
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Old 31st Mar 2012, 11:22
  #1117 (permalink)  
 
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I believe that buffet was caused by "an oscillatory interaction between local shock waves and airflow separation"
Or, a combination of Center of Lift [max] and roll angle causing the lift separation and 5 second pitching, which persisted for about 15 seconds before pitch and AoA parted company.
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Old 31st Mar 2012, 14:58
  #1118 (permalink)  
 
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Sim Stalls

PJ2 Sim Stalls #1107
I assume that these were reproducing 447's performance using TOGA. A lower power ( Flight Idle ?) might have helped in the earlier stages.
How in the limited time (which WE now know was very limited) could PF have explained what he was having to do, to PNF. There was no further Drill to be actioned.
Of course he should not have stalled... But having got there, someone had to try to get out of it.
The Captain might have come to the Flight Deck without being called, due to the change of apparent gravity with the change of pitch of the aircraft - not I think, noticeable on a simulator.
( None of my Training or being checked was done on a simulator - we just had to use the real thing, with four fans and a Flight Engineer.)
LT

Last edited by Linktrained; 31st Mar 2012 at 15:43.
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Old 31st Mar 2012, 15:00
  #1119 (permalink)  
 
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How do you train aoa instrumentation if it is never to be used unless in upset!
With all respect, how do you train someone to use an oil pressure gage?

When necessary, you assess whether or not it is in the marked proper range and note whether it is increasing, decreasing or steady. You're trained.

What you do then is up to you.
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Old 31st Mar 2012, 15:44
  #1120 (permalink)  
 
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Originally Posted by gums View Post
You have to have "touch" and be in "touch" with the jet ( I can just see Doze snorting, heh heh). I don't see any way to do this in a sim.
Why do you say that? I don't scoff at the idea that pilots should have a feel for the aircraft in the slightest, I just don't agree with the idea that the Airbus FBW control design makes that impossible. In fact all jetliners have had artificial feel of some sort from the DH Comet onwards.

In other words, you can override the "system" and get into the situation that AF447 did.
No-one "overrode" anything - the system dropped into a failure mode without protections because the airspeed sensors temporarily failed, and the evidence points strongly towards departure from controlled flight as being pilot-induced due to escalating misreading of the situation.

Originally Posted by Linktrained View Post
How in the limited time (which WE now know was very limited) could PF have explained what he was having to do, to PNF.
The point is that from the moment the UAS indication appeared, the PF did not in fact immediately have to do anything. They were at cruise, their speed was OK, they had a little bump from turbulence, but that wasn't going to send them into a significant dive. The correct thing to do according to CRM would have been to let the aircraft ride itself out of the turbulence and then discuss what to do with each other while monitoring the aircraft's response. Instead, the PF seems to have instinctively yanked the aircraft into a climb and developed a near death-grip on the sidestick until they hit the water, without telling anyone that he was doing so.
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