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. |
Can one achieve sufficient ND to escape the STALL in the first place? 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! |
Originally Posted by Organfreak
What gives? I do realize that actual airspeed is different from ground speed. What am I missing here?
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. |
I believe the sim/check pilot failed 3 ADRs for the exercise... 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). ...but these are esoteric areas for a non-engineer pilot of these airplanes! |
@OK 465
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? I would think that full nose down elevator at 330 knots would provide for quite a bit more than -1G. 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? With full forward SS at 330 knots you get about 1/4 ND elevator deflection. |
Previous quote:
I think this would lead the system to apply less elevator (to drive the pitch acceleration) than it would normally use at these low speeds, Where does this come from please? So it would, but (a) the aircraft was not at 330 kts... In any airplane I ever flew, I certainly never claimed to have all the answers, but I always strived to have all the questions. :) |
Originally Posted by OG
Difficult to say when negative load factor protection would have kicked in because the THS movement and general speed increase go together.
Originally Posted by OK465
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.
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@OK 465
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 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. @HN39 Does negative load factor require negative AoA (zero lift at about -2°)? |
Sorry, I don't recognise what you mean by 'SD' Systems Display (normally central lower display unit MFD), shows the position of all flight control surfaces at any time, as well as FCC status. 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. I understand you're saying that you can. I may be misinterpreting what I'm seeing. Always possible. edit: How about the THS, did it move at all? |
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
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. 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? |
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