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-   -   AF 447 Thread No. 10 (https://www.pprune.org/tech-log/493472-af-447-thread-no-10-a.html)

HazelNuts39 31st Aug 2012 16:07

Hi PJ2,

Yes, excellent discussion indeed!

I think the gravity argument is basically correct. But there are more changes during climb that have probably a greater effect on elevator and/or gee:
- due to fuel burned weight and c.g. changes (trim change)
- thrust reduces (trim change)
- due to reducing thrust FPA and vertical speed are constantly reducing, i.e. gee is slightly less than one, except when speed changes from constant CAS to constant Mach and FPA and V/S increase by about 68%.
- climbing at constant Mach AoA is increasing (trim change)

rudderrudderrat 31st Aug 2012 17:27

High-Altitude Upset Recovery
 
There is an interesting article in the Summer 12 edition of "focus" the magazine published by the UK Flight Safety Committee.

Radical revisions needed for pilot training, aircraft certification and simulator fidelity.
by Fred George

Investigators with the French Bureau d'Equetes et d'Analyses (BEA), the agency charged with investigating the crash of Air France Flight 447, now are focusing on a breakdown in situational awareness on the part of the flight crew and possible pilot error as contributing factors in the June 2009 mishap that killed 228 people when the Airbus A330 crashed into the South Atlantic. The latest findings broaden the scope of the inquiry well beyond a fly-by- wire flight control malfunction, possibly caused by iced-up pitot probes.

While the BEA is far from completing its investigation of the AF447 accident, its most recent progress report again focused the aviation community's attention on the perils of loss of control (LOC) incidents, especially at high attitude. This is a multifaceted challenge because improved stick-and-rudder skills are unlikely to eliminate the problem entirely.

"The Air France 447 crash was a seminal accident. We need to look at it from a systems approach, a human/technotogy system that has to work together. This involves aircraft design and certification, training and human factors. lf you look at the human factors alone, then you're missing half or two-thirds of the totaI system failure," says C. B. "Sully" Sullenberger, a 20,000-hour retired airline pilot and former fighter pilot.

Celebrated for his successful ditching of a powerless A320 in the Hudson River, Sullenberger is now a writer, aviation consultant and public speaker. He notes that there were 12 or 13 similar upset mishaps prior to AF447 in recent years, but that Air France 447 has attracted the most public interest. Sullenberger says that there needs to be a global safety reporting network that will enable the aviation industry to identify problems more quickly and find solutions.

Sullenberger says it's easy to blame the pilots in the AF447 crash while overlooking other contributing or causal factors. "l believe the transport airplane community, as a whole, would not expect the crew to lose all three speed indicators in the cockpit," he said. "That's like amputating the wrong limb in a hospital" because criticaI information was not available.

He also believes that accurate airspeed indications alone aren't the best data the crew needs to recover from an upset. That requires knowing the wing's criticaI angle of attack (AoA). "We have to infer angle of attack indirectly by referencing speed. That makes stall recognition and recovery that much more difficult. For more than half a century, we've had the capability to display AoA (in the cockpits of most jet transports), one of the most critical parameters, yet we have choose not to do it."

Training also needs improvement. "Currently, to my knowledge, air transport pilots practice approaches to stalls, never actually stalling the aircraft. These maneuvers are done at low altitude where they're taught to power out of the maneuver with minimum altitude loss." In some aircraft, they're taught to pull back on the stick, use maximum thrust and let the alpha floor (AoA) protection adjust nose attitude for optimum wing performance.

"They never get the chance to Practice recovery from a high-altitude upset," he continued. "At altitude, you cannot power out of a stall without losing altitude. "And depending upon the fly-by' wire flight control system's alpha floor protection isn't the best way to recover from stall at cruise attitude.

Maintaining situational awareness is another challenge in highly automated aircraft. "There are design issues in some aircraft that I've always wondered about," Sullenberger said. "For instance, I think the industry should ask questions about situationaI awareness and non-moving auto throttles. You lose that peripheral sense of where the thrust command is, especially in a big airplane where there is very little engine noise in the cockpit.

"ln some fly-by-wire airplanes, the cockpit flight controls don't move. That's also Part of the peripheraI perception that pilots have learned to pick up on. But in some airplanes that's missing and there is no control feel feedback," he said.

Owain Glyndwr 31st Aug 2012 18:18

PJ2

twenty years ago when checking out on the A320, one of the instructors observed that because the system maintains 1g, that it would gradually increase elevator deflection in the climb to cruise altitudes (where gravity is very slightly less, was the claim...), to maintain 1g and that a tiny ND input was required during the climb.
I'm entirely with HN39 on this one. Presumably your instructor was suggesting that since gravity is very slightly less the effective "weight" would be less leading to a slightly lower lift coefficient and therefore less up elevator needed.

Good theory, but in practice entirely swamped by all the other effects listed by HN39. I would add another one - if you are in gums' Viper or were in Concorde then the direction of flight (E/W or W/E) could also have an effect ;)

DozyWannabe 31st Aug 2012 18:41


Originally Posted by syseng68k (Post 7388449)
That sounds like a serious fault in the design, in that under such circumstances, the effect is a positive feedback loop that might be unexpected, not trained for and thus very difficult to recover from.

Hmm - we're talking about an infinitessimally small deviation over time, perceptible only because of the accuracy of modern instruments. I don't have the time or the wherewithal to crunch the numbers, but I suspect that the amount of time it would take for this anomaly to take the aircraft from Max cruising altitude into coffin corner would be longer than the A320 series' range could allow.

I remember someone saying early in this discussion that this anomaly was rectified in the A330/340 systems.

The evidence does not support AF447 being imperceptibly taken into coffin corner by the systems, it very clearly implies that the aircraft was controlled into the stall.

Lyman 31st Aug 2012 18:51

From Owain Glyndwr...

"This is the point I don't get. The AI system, as I understand it, seeks to maintain a commanded gee in earth axes not body axes as in your Viper."

How does the a/c orient itself when it unloads? Surprising to me was the THS cranking in when the beginning of the "momentum only" portion of the "climb" began. At the top of the trajectory, what tells the load system monitor to adjust?

Dozy?.."The evidence does not support AF447 being imperceptibly taken into coffin corner by the systems, it very clearly implies that the aircraft was controlled into the stall."


Boy howdy.....

DozyWannabe 31st Aug 2012 18:58


Originally Posted by Lyman (Post 7389178)
Surprising to me was the THS cranking in when the beginning of the "momentum only" portion of the "climb" began.

It's not rocket science - the THS is unloading the elevator demand commanded by the PF, as per design.


At the top of the trajectory, what tells the load system monitor to adjust?
The pilot's commands vs. the current aircraft attitude. Remember that in Alternate Law and below, the system is designed to defer to the human pilot and will not interfere with any command given. The flight control system has no concept of stall or operational ceiling with hard protections inoperative - even in Normal Law it only understands and acts on Alpha Max.


Dozy?.."The evidence ... very clearly implies that the aircraft was controlled into the stall."
...
Boy howdy.....
How else would you describe consecutive and sustained back-stick commands?

Lyman 31st Aug 2012 19:14

"How else would you describe consecutive and sustained back-stick commands?"

That the aircraft would have stalled without them?

Lyman 31st Aug 2012 19:47

Dozy... "It's not rocket science - the THS is unloading the elevator demand commanded by the PF, as per design."

Nope, not at full deflection of elevators....

CONF iture 31st Aug 2012 20:13


Originally Posted by DW
even in Normal Law it only understands and acts on Alpha Max.

Absolutely not, as early as alpha prot it quits trimming.

DozyWannabe 31st Aug 2012 20:19

@Lyman : Based on what evidence?

The discrepancy we're talking about here is relatively minute. Even if the tendency hadn't been corrected for the widebodies, you'd be looking at hours and hours of flight between AP disconnect at 35,000ft and reaching 38,000ft - the time it would take is certainly more flight hours than AF447 had remaining, and it would have had to escape the attention of the crew over that period of time.

In the AF447 case, from autoflight disconnect to impact is a matter of minutes. The sidestick deflection (measured at source) is consistent with the flight trajectory - ergo the evidence strongly implies that the climb, stall and subsequent actions were due to crew input.

Autotrim is commanded based on flight control position over time (i.e. a trend) - whether the elevators are currently deflected or not.

@CONF iture:

Alpha protection is a systems procedure triggered based on a variable derived from Alpha Max - the point at which it triggers is not a hard-coded value, but a function. The hard protections are implemented by a subsystem that operates outside the main flight control logic. Upon loss of Normal Law, that subsystem is inhibited. The reason the protection fires just shy of Alpha Max itself is to take into account conditions whereby Alpha Max is on the edge of the flight envelope that cannot be reliably measured by the system (e.g. tailwind).

Lyman 31st Aug 2012 20:32

You are addressing a question not asked, with a response that does not pertain...

"The discrepancy we're talking about here is relatively minute. Even if the tendency hadn't been corrected for the widebodies, you'd be looking at hours and hours of flight between AP disconnect at 35,000ft and reaching 38,000ft - the time it would take is certainly more flight hours than AF447 had remaining, and it would have had to escape the attention of the crew over that period of time."

Does a/p fly attitude, airspeed, or load... Hands off, what does the a/c fly in AL2? What rate?

What do you devine is "unusual command response"? You have a source?

PJ2 31st Aug 2012 20:45

HN39, O.G., my thanks for clarifying yet another item. For a pilot this is really interesting.

Apart from understanding how all this works, it leads me closer to conclusions that such knowledge remains valuable to our craft for those flying transports and that "nuts-and-bolts" courses, far from being replaced by notions such as "managing the aircraft", are still relevant to what we do (and did!) for a living. Continuous education is the responsibility of all professionals but it would be interesting to revisit curriculum design for pilot courses for the Airline Transport License and at the airlines where training footprints, concerning fbw (C*), autoflight and aircraft performance topics. Anyway, that's for another thread! Once again, thank you.

DozyWannabe 31st Aug 2012 21:01


Originally Posted by Lyman (Post 7389336)
Does a/p fly attitude, airspeed, or load... Hands off, what does the a/c fly in AL2? What rate?

Autopilot "flies" flightpath, constantly correcting itself if deviation is detected (see the "zipper" trace you were interested in earlier). In Alternate 2, the aircraft holds commanded flightpath in pitch and is roll direct.

The discrepancy in the A320 being talked about is something on the order of 1 foot every 20 minutes, depending on external conditions.


What do you devine is "unusual command response"? You have a source?
You've lost me - where did I use the phrase "unusual command response"?

Lyman 31st Aug 2012 22:00

You didn't, BEA did....

"Autopilot "flies" flightpath, constantly correcting itself if deviation is detected (see the "zipper" trace you were interested in earlier). In Alternate 2, the aircraft holds commanded flightpath in pitch and is roll direct.

How does Pilot determine which is commanded, and which is flightpath? Does he have to assume the displayed flightpath is his command? Trusting soul...

DozyWannabe 1st Sep 2012 00:25

If autoflight is on, then autoflight commands flightpath. If it isn't, then flightpath is manual and determined/commanded by the crew. Both are commanded - as far as the flight control logic is concerned it doesn't matter whether the command comes from the autopilot or the human crew.

Some may disagree, but in commonly understood terminology autotrim is not automation in the classic sense because it does not determine flightpath independently. Instead it is slaved to the control inputs of the autopilot or the human pilot depending on which is in control.

If the sidestick traces indicated neutral or nose-down pitch throughout the sequence and the aircraft pitched up into climb and stall, *then and only then* would the systems logic deserve to be under scrutiny, but that's not what happened. The sidestick traces clearly indicate repeated and sustained nose-up inputs which correlate closely with the aircraft flightpath.

CONF iture 1st Sep 2012 01:14


Originally Posted by Dozy
Alpha protection is a systems procedure triggered based on a variable derived from Alpha Max - the point at which it triggers is not a hard-coded value, but a function. The hard protections are implemented by a subsystem that operates outside the main flight control logic. Upon loss of Normal Law, that subsystem is inhibited. The reason the protection fires just shy of Alpha Max itself is to take into account conditions whereby Alpha Max is on the edge of the flight envelope that cannot be reliably measured by the system (e.g. tailwind).

As usual, you're moving air around with some stuff made on your own, but don't address the point in question.

bubbers44 1st Sep 2012 01:18

A Boeing yoke works the same way. We don't have protections, just warnings. Try pulling full back on any Boeing, which probably never happened, and see what happens because the other pilot would slap him on the side of his head and say I have the aircraft. Yes it is unfair because the other Boeing pilot can see he is doing something really stupid and intervenes.

CONF iture 1st Sep 2012 02:15


Originally Posted by bubbers44
Try pulling full back on any Boeing, which probably never happened, and see what happens because the other pilot would slap him on the side of his head and say I have the aircraft

It did happen already, and without the other pilot intervening ...

bubbers44 1st Sep 2012 03:05

Yes, all aircraft types can have incompetent pilots. Seeing what the other pilot is doing by a yoke gives you the ability to smack him on the side of the head quicker. With incompetent FO's it probably doesn't matter. Airplanes shouldn't operate with a pair like that. Guess they do though.

HazelNuts39 1st Sep 2012 10:02


Originally Posted by Dozy
Alpha Max is on the edge of the flight envelope that cannot be reliably measured by the system (e.g. tailwind).

I'd be interested to know how tailwind enters into the equation?


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