Was he perhaps trying to maintain 15° pitch attitude?
What PF was doing would make better sense if his PFD had a ~ 12 degree pitch bias, but I don't think that happened. Instead, he was probably trying to maintain around 1 g and the harder he tried, the worse his problems became.
Cwatters, your diagram is essentially correct except there wasn't much good aerodynamic data available, so no protections that would have helped. Since they had no idea of Mach number (which affects stall AOA), they didn't even give the AOA system authority to control the aircraft and push the nose down, instead limiting it to shouting, "Stall" repetitively (until silenced by the A/S <60 knots). Roll however was in Direct law so that was a whole new ball game for PF's level of training.
I am personally of the opinion that other than the airspeed system, the airframe systems performed exactly as designed.
I am not ready to cast PF into the entirely unfit to fly bucket yet. I have this uneasy feeling that many of our weaker brothers in the cockpit would have succumbed to this same scenario, particularly because there is probably something about the total scenario that we do not properly appreciate.
The human factors group at BEA will be having an interesting time if they do their job correctly. I only hope they can explain their understanding adequately.
Was he perhaps trying to maintain 15° pitch attitude?
Looking at that graph, I guess it's entirely & depressingly plausible.
He selected toga as well. Toga/15° is, I believe, the first pitch/power setting on the AF uas procedure (entirely the wrong one to use for cruise, but did they know that...)
I would add that it is equally hard to imagine the pilot of an Airbus holding full back stick.
Not that hard. It is actually what Airbus wants to force in your DNA for a number of situations. no thinking, just full back stick to get the best of the protections ... That gives very good performance ... as long as data are reliable.
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As the aircraft would have been in trim prior to his sidestick input, the force required to hold the sidestick fully aft would, initially, have been large. That effort would diminish as the autotrim took effect, but it would take a significant time.
Let me disagree here again : the force required to hold a sidestick fully aft does not change a iota whatever the situation, whatever the trim setting.
It's hard to imagine the pilot of a non Airbus holding full back stick and manually trimming full nose up no matter what the circumstance.
Really ? Pop-quiz - which (now somewhat fragmented) aircraft is this then:
The crew did not take any action to re-trim the aircraft to the desired speed, so, once the nose down pressure was released on the control column, the aircraft pitch started to increase again
[...] The aircraft re-entered a stall situation (AOA reached its maximum values of around 26° at 00:41:09). Contrary to any stall recovery procedure, the control column was initially kept backward and gradually increased over the next 17”.
[hint - it's one of those hard-to-imagine non-Airbuses]
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The power of a trimable stab is such that it becomes the primary pitch control, with the elevator assuming the role of a short term pitch modification.
The pilot of old automatically knew the trim state of the aircraft because he put it there, where as the Airbus, I gather, the pilot is only aware of where the automatics have put the stab is by reference to instrumentation.
Auto-trim is damned whatever it does.
447, auto-trim stays on, pilot pulls back through stall warning, trims up in stall -> "auto trim should hand back to pilot in stall"
Perpignan, auto-trim does hand back to pilot, pilots never re-trim, leads to fatal secondary stall. Would have been much better chance of recovery had auto trim followed pilot commands. -> "auto trim should follow pilot commands in stall"
It's not just an airbus problem however:
Ethiopian, Beirut - mis-trimmed, pilots never re-trim, pulled back through fatal secondary stall. Turkish, Schipol - auto-trim does hand back to pilot, pilots never re-trim (recovery fails because they let throttles go back to idle again however) Colgan, Buffalo - auto-trim might have had a hend in setup, but they pulled back all the way down and pulled the flaps in just to be sure...
Sure, there may be a pattern there somwhere, but "airbus" ain't it.
Auto-trim is a has been with us some time and has been known for causing trouble. Pilots first began to run afoul of it with the early autopilots that would trim unbeknownst to the crew-until it then kicked out and left the crew with a handful.
Some implementations of FBW require autotrim to function properly and it can simplify the job of flying. Autotrim has teeth however, and pilots who regard it as piece of the furniture in the cockpit and thus lose awareness of what is doing stand a great chance of being adversely surprised by it.
That is why most of the modern aircraft have noisemakers associated with trim movement. But one major airframe maker skipped this step. Why is that?
And the answer to the pop quiz is the Ethiopian B-737 at Beruit. For whatever reason, PF didn't want to handle the aircraft that night, but the A/P never relieved him. Don't you guys want to earn your pay for flying?
Cwatters, your diagram is essentially correct except there wasn't much good aerodynamic data available, so no protections that would have helped.
I realise that but..
For me the important bit is that neutral side stick means "maintain this attitude". eg It's not necessary to hold in back stick to keep the nose up once it's up.
What does the equivalent diagram look like for other aircraft? Do you have to hold the stick back to maintain a nose high attitude or will neutral stick do it?
What does the equivalent diagram look like for other aircraft? Do you have to hold the stick back to maintain a nose high attitude or will neutral stick do it?
For conventional non-FBW aircraft and Boeing FBW, it looks like the well known phugoid where the nose drops as you slow from trim speed. (But let me preface this with the statement that my Boeing FBW understanding is theoretical.)
Auto trim may be nice to make flying easy but it doesn't do much to help piots fly when things go amuck. These pilots were not the brightest so let it lead them into a deep stall because they were stupid.
We all know why they crashed. They were incapable of flying an aircraft with unreliable airspeed. All aircraft have charts and pilots should know approximate attitudes and power if airspeed goes bad. Nobody pulls full up on the stick hoping the automation will save them but they did. When they went into alternate law the airbus was no longer idiot proof.
(But let me preface this with the statement that my Boeing FBW understanding is theoretical.)
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Iceman. My first impression was that you had done your sim evaluation in a Boeing type sim, but I see you list yourself as a 'Bus pilot, seat 0A, so will give you the benefit of the doubt.
Very condescending!
Perhaps you could also grace us with your practical / hands on knowledge and experience of the flying the Airbus. ( An Airbus pilot TRE/IRE, + 20 years military- Rotary(piston/gas turbine) fixed wing (piston and fast jet), 6 years Boeing (757/767) and 16 years Airbus flying A340/A330)
What does the equivalent diagram look like for other aircraft? Do you have to hold the stick back to maintain a nose high attitude or will neutral stick do it?
Positive aircraft stability means that when you let go of a control, the nose of the aircraft returns to the speed that you had when you first moved the control. This speed is called the trim speed. Neutral pitch stability means that when the nose is moved, the pitch angle remains where you put it.
When the Wright brothers started developing aircraft a century ago, they initially worked with the premise that aircraft should exhibit positive stability in yaw but be neutrally stable in pitch and roll. As the first test pilots, they quickly discovered that it is easier to fly if you have positive stability in pitch. If you pull or push on the stick in pitch and let go, the aircraft returns to the trim speed and settles down. This is particularly helpful in turbulence. If a gust displaces the nose in pitch (or yaw) the plane simply returns to the trim speed without help from the pilot. Accidentally bumping the stick or yoke is no problem.
Some PPRuNers have asked why the AF447 PF didn't simply let go of the side stick and let the plane resume the trimmed speed. Why indeed? In every plane those PPRuNers had flown, letting go would have allowed the plane to return to trim speed. In the AB system, letting go, which neutralizes the stick, does nothing but stop the movement in pitch. It does not cause the aircraft to resume the previously trimmed speed because the autotrim has set a new trim speed. The trim speed set by the autotrim in AF447 (in response to the PF's side stick input) was below stall speed.
With the AB flight control system, because it incorporates autotrim in pitch, the aircraft essentially becomes neutrally stable in pitch because the autotrim follows the sidestick command. The airplane does still retain positive stability in pitch when the nose is moved by external aerodynamic forces, but if you move the stick in pitch, the nose stays at the pitch angle commanded.
With a century of flight experience under our belts, why did AB decide to alter the flight control philosophy? Good question.
With a century of flight experience under our belts, why did AB decide to alter the flight control philosophy? Good question.
They didn't. The "philosophy" and control laws they used were already tried and tested in military a/c (and the odd NASA vehicle I believe...). The behaviour you are referring to is apparent neutral speed stablity, and is (I think) a fairly fundamental characteristic of the c* ("c-star") control law. Airbus just applied existing FBW control knowledge to a civilian a/c.
Boeing, on the other hand, for the 777, invented a completely new control law, c*U, to give the effect/illusion of speed stability. As far as I know this was completely new control law/philosophy, never tried on any previous a/c, and arguably therefore much higher risk.
As to which turns out "better", I guess maybe we'll see as FBW spreads beyond A&B - already in Embraer I think, and filtering down to the bizjets. Have they chosen A-style or B-style ? [I don't know, haven't looked].
[There might be another complication - I vaguely recall Boeing having patented the whole c*U concept, in which case they might be able to keep it Boeing-only even if it is "better". Love the patent system...]
As to which turns out "better", I guess maybe we'll see as FBW spreads beyond A&B - already in Embraer I think, and filtering down to the bizjets. Have they chosen A-style or B-style ? [I don't know, haven't looked].
Linked sidesticks, C* law but with softer protections in some areas, an AOA limiter, back driven autothrottles, and a different way of doing some things. Looks to be a very pragmatic approach.
Perhaps you could also grace us with your practical / hands on knowledge and experience of the flying the Airbus. ( An Airbus pilot TRE/IRE, + 20 years military- Rotary(piston/gas turbine) fixed wing (piston and fast jet), 6 years Boeing (757/767) and 16 years Airbus flying A340/A330)
Iceman, I apologize for stepping on the toes of a real Airbus Pilot , however you must admit that in the light of subsequent discussion, your statements were not quite accurate.
I am a retired USN F-4 carrier pilot with ~5000 hrs of civil and military time-virtually all of that involved actual manipulation of the flight controls. I've been the Safety Officer in two squadrons, Maintenance officer in one. I've seen first hand a number of accidents, investigated a number of them, and read thousands of accident and incident reports. I've been studying your Airbus flight control system for the last 2 1/2 years and I think I understand many of the Engineering concepts behind it.
I came to the conclusion 2 years ago that AF447 stalled at altitude and descended in a stall. When I realized how little actual hands-on flying time you folks are getting presently, I knew it must have consequences.
I did not go with the airlines because I realized that in order to make a small fortune in aviation, you had to start with a large fortune.
Apology accepted from one naval aviator to another!
I feel the discussion has got way too "technical" and away from what an ordinary "line pilot" would or needs to understand. I too had 20 years of no A/P nor Flight Director where you had to "fly" the aircraft. What we have here though is what appears to be a basic airmanship / piloting error, after a relatively minor emergency, as unpalatable as that may be. Blaming the aircraft or airbus for this failing is not good enough. Basic pilot skills of attitude and power would and should have been enough to solve the problem. There also appears to have been a failure in monitoring by the PM and by not taking control and commanding the situation. The auto-trim did not cause the stall it was trimming the aircraft to assist the pilot, until it cut out. The upset training we carry out always puts emphasis on the fact that the aircraft may be out of trim and manual input may be required to assist in the recovery. The effort required on the side-stick to achieve the pitch attitude and "zoom" climb was something that should never be applied to any "airliner" at FL350! My very unscientific "experiment" achieved 13,000/min rod from 'stalling" at 38,000'. But with 15 degrees nose down, some trim, then thrust aircraft recovered by 29,000'.
I feel the discussion has got way too "technical" and away from what an ordinary "line pilot" would or needs to understand. I too had 20 years of no A/P nor Flight Director where you had to "fly" the aircraft.
Looks like the PPRuNe Administrators have fixed the problem by moving the thread into tech section. It is a natural progression for long term threads it seems.
Iceman, the problem as I see it is that the majority of new guys hitting the airlines do not have the same background and experience that we have acquired, nor will they be getting it in the typical airline flying environment. It is FUBAR, but that seems to be the way things are going. The AF447, and Colgan accidents are representative of this newly developing trend. Where it ends, who knows? In the meantime, the airframe manufacturers had better be looking for ways to protect this new type of pilot better. Whenever I see an area that needs improvement, I try to point it out.
I assume you would be perfectly comfortable, given a sufficiently maneuverable aircraft, to perform aerobatics by instruments at night by with no visible horizon. I get the feeling that the majority of the new airline input would look at you like you were crazy if you asked them to do that.
Different times with new economic factors at work since you and I entered aviation.
Thanks for the link on the Embraer Legacy 500/450 FBW. The audio briefing is very interesting.
You mention "Linked sidesticks" but that's not what it is.
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Embraer also incorporates a vibrating tactile, aural and visual warnings in the case of an accidental dual input on the sidestick. In this instance, the inputs from pilot and co-pilot are summed together.
Sidesticks are still independants (Airbus way) Audio describes it between minutes 13 and 21.
As you mention, on the autothrottle side, Embraer did not follow Airbus, and even mentions 2 airbus accidents to justify their choice.
As I understand it also, autotrim under manual flying.
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