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)

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.

PJ2
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 ;)

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.

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.....

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

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.

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

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

That the aircraft would have stalled without them?

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....

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

@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).

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?

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.

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.

You've lost me - where did I use the phrase "unusual command response"?

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...

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.

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

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.

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

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.

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

One more question for the FBW flight control knowledgeable folks here:

With gains fixed for 330 knots, can you get FULL nose down elevator in a developed stall?

How would stall recovery characteristics be affected with fixed gains versus active gains?

Can one achieve sufficient ND to escape the STALL in the first place?

Because in a stable aircraft, the Nose drops on its own?

Say PF sees the error of his ways, and wants full ND, instant. Does he get it?

Would not Direct Law be a better Law in this condition?

After erasing the protection and entering Alternate Law, controls hobble a potential recovery? Madness.

Hi Lyman,
Page 90 of the final report shows the elevator at -30 degs (Max) and the stab trim at -13 degs (Max).
Why do you suppose maximum Nose Down Elevator & Stab Trim would not be available if the stick was held fully forward?
If that was not sufficient to lower the nose, then they could have selected idle power.
Neither of the above were attempted because they never diagnosed (believed?) they had stalled.

Rate. It took sixty seconds for the THS to cycle max. It took forty seconds to STALL.

Seems to me had the a/c degraded into Law Direct, the Stall happens earlier, and recovery if any, begins with more energy. Also, the likelihood of tail low entry diminishes?

With three lost ADR, the gains go to 330 knots default? Doesn't that have the effect of acclimating the flying pilot to gentle response in Pitch, and without speeds or AOA, he is lulled into confusion? , his senses are Acclimated to things happening at the airplanes rate, slowed, confident, non emergent?

I would think that full nose down elevator at 330 knots would provide for quite a bit more than -1G. :yuk:

With the load factor protection in effect, and fixed gains, wouldn't this limit the amount of elevator deflection for a full ND SS input at any actual speed?

OK465;

In the sim exercises, the SS was held full forward to achieve about 10degND pitch. The THS followed up on the command and returned to about a -3deg position. Recovery took about 40 seconds. (Note for others: I realize the sim cannot replicate full-stall conditions due absence of data but neither is the behaviour completely irrelevant).

For the exercise anyway and from my pov, there was sufficient elevator available to get the nose down, and the job was made easier by the THS following up the SS ND commands.

PJ:

Are you sure you had 330 knot fixed gains?

With full forward SS at 330 knots you get about 1/4 ND elevator deflection.

(With full forward stick, you can do an outside loop in some aircraft in 40 seconds. :))

Endlessly, monotonously, the pilot is trained to fly one gee. It is grail. Nothing can go wrong at one gee, even the pilot not flying was disturbed by "excessive" lateral inputs....

So any number less or more is to be avoided. Add a nagging suspicion of overspeed, and voila, don't ferchrissakes "lower the nose..."

Stewpid Newbie Question
 

May I ask a stewpid newbie question? OK, thanks, I will.

I have always heard/read that airliners travel "at about 600 MPH." Actual speeds are sometimes referred-to (by badly informed authors?) as 500-something MPH.

330 KTS apparently translates to ~380 MPH. What gives? I do realize that actual airspeed is different from ground speed. What am I missing here?

:confused:

OK465;

No, I don't know whether we had 330kt gains. I'm probably not providing sufficient information but the exercise began at cruise speed, (273kts), I believe the sim/check pilot failed 3 ADRs for the exercise, the pitch was increased, the sim stalled and the recovery was started at various points in the descent. I realize control systems are capable of outside loops etc but of course there is no indication of the nature of few gains for the pilots and the FCOM isn't sufficiently detailed so I can't offer an accurate assessment of what we had. I believe we had full down deflection of the elevator as indicated on the Lower ECAM Flight Control page but I'd have to review the videos to confirm. One question I would have regarding "high gains" (or 1/4 elevator only available) is, would the THS still follow through? I expect it would as this is still a trimable force exerted by the elevators which would be trimmed out, but these are esoteric areas for a non-engineer pilot of these airplanes!

Simply the difference between True Airspeed and Indicated Airspeed.
Indicated airspeed - Wikipedia, the free encyclopedia
True airspeed - Wikipedia, the free encyclopedia

The first airspeed controls many important aerodynamic control parameters such as g available and angle of attack.
The second airspeed (TAS) indicates how fast you are booking through the molecules of the atmosphere.

Recalling the picture of the PFD with the FPV visible, one would assume only 2 ADR's were failed. You would not have an FPV with 3 ADR's failed.

Variable Airspeed Drift malfunction may be a better way to do this than constrained ADR failure. The ECAM messages are ADR disagree ones (like 447) and not ADR fault.

Double ADR fail is indeed ALT2 and good for stall demos with the FPV, but it is not ALT2(B).

Agreed, but they do fall into the category of nuts and bolts. Hard to keep an old Flight Engineer trained guy like me from pursuing this kind of 'trivia'. Remember the old Burtek Boards? :)

@OK 465

Sure - why not? The gains, as I tried to explain, relate to the transient response characteristics; that is to say to the first 3 or 4 seconds of any manoeuvre. If the elevator deflection is not enough to satisfy the pilot's demand the system goes on winding in elevator until the demand is met. The 'gains' are irrelevant in that context. At the speeds involved in a developed stall there would be no hinge moment restriction on control deflection or rate of movement.

Not at all - it is the steady state conditions that will dictate stall recovery not the entry transient.

So it would, but (a) the aircraft was not at 330 kts - more like 155 kts and (b) you have to allow for the deflected THS opposing the down elevator.

No, it is the other way around. One can look at it several ways, but if you go back to the graph of AoA vs equivalent elevator angle that I posted way back you will find that with 13 deg NU THS (-19.5 deg equivalent elevator) and full ND elevator (+15 deg) giving an equivalent -4.5 (NU) elevator deflection, the aircraft would settle out at about 4 or 5 deg AoA. At the speed then in existence the 1g AoA would have been about 10~12 deg I think, so the best one can hope for until the THS starts to move back down is around 0.5g (ish). Difficult to say when negative load factor protection would have kicked in because the THS movement and general speed increase go together.

Where does this come from please?

Previous quote:

Then you mean this only as a transient with the eventual elevator deflection for a sustained full ND SS input at 'slow' speed being the same in both Normal & ALT2(B)?

SD Flight Control page with sustained full ND SS input at both 330K & 200K in ALT2(B). It is constant, and the same in Normal at 330K. I don't even see full elevator deflection in Normal at 200K with a full ND SS input.

But the FCC's were. I'm still trying to reconcile this with what I've encountered. I'm just not seeing what you're describing.

In any airplane I ever flew, I certainly never claimed to have all the answers, but I always strived to have all the questions. :)

Does negative load factor require negative AoA (zero lift at about -2°)?

How about the THS, did it move at all?

@OK 465

Yes

Sorry, I don't recognise what you mean by 'SD'

We may be a bit at cross purposes. I was referring to the aircraft response to actual full ND elevator deflection. Were you perchance referring to full ND SS command? My point was that at low speed you will need a lot more deflection for a given delta gee than you will at high speed.

@HN39
Yes - I agree zero lift at about -2 deg.

Apologies,

Systems Display (normally central lower display unit MFD), shows the position of all flight control surfaces at any time, as well as FCC status.

Yes

Exactly, but can you get it when the FCC's continuously think you're at 330K?

I understand you're saying that you can. I may be misinterpreting what I'm seeing.

Always possible.

edit: I think I see what you're getting at, as far as elevator effectiveness, but THS 'specifically' not noted. Another 'question' to ponder over a 3 day weekend.

OK465, I can't speak to the method used to bring about the UAS event. The FPV was available during the descent as, (I believe) one ADR was reinstated.

We just had chalk and a board for the DC8 plus some stuff from the maintenance manuals and other sources. The later CBTs were initially pretty dry but they got better. Like you I prefer understanding what I'm doing in my aircraft. I prefer nuts-and-bolts (or bits and bytes) just because it's fun.

If the simulator experience is to be believed then for whatever technical or aerodynamic reasons it demonstrated that the aircraft was recoverable with full ND stick held in until the stall warning ceased. Secondary accelerated stall was not a problem in the pull-out. "Why?" again in technical and aerodynamic terms , is an excellent question which is why contributions on this topic are such excellent reading. O.G. has previously offered varying recovery scenarios and the numbers for the higher-altitude recovery weren't that different from what we observed. We never tried recoveries below about FL300 and when I get a chance I'm going to try them.

Excellent discussion.

A question if I may.

If after years and thousands of point counterpoint, an internet group of obviously intelligent/knowledgable people don't understand just how the THS/pitch control behaves in all circumstances, does anyone really expect pilots to know how the aircraft behaves after only a day of flight control systems training?

And a point........this question would never come up with a DC9 :ok:

@OK465

No, I think it is I who may have it wrong :ouch:
I see what you are getting at now. I had been assuming that only the C* gains defaulted to their 330 kt values, but from what you describe it sounds as if the steady state gain (delta gee/unit SS deflection) may default as well. In that case full ND sidestick would only produce the elevator deflection to give (I suppose) -1g at 330 kts so that at 155 kts the available gee would be much less. [But in that case how did they get 30 deg NU elevator on AF447?]

If the maximum ND elevator were only 5 deg one could only get down to about 15 deg AoA until the THS came off its stop. OTOH, given the insistence of many contributors that professional pilots would be unlikely to push beyond 10 or 15 deg ND pitch, perhaps consideration of -1g is a bit academic?