AA A321 takes off after smashing ground sign
I won't even try to dispute the technical part of your analysis. Having said that:
- The captain was (most probably) the only pilot with feet on the pedals.
- Those pedals ended up deflected to the left.
- The airplane turned to the left, and after rotation banked to the left.
- The captain commented the aircraft banked left, but not did comment the rudder pedals deflected uncommanded.
And while I cannot understand how a 20K hour pilot can make such a basic mistake, I feel it is more likely than a failure leading to an uncommanded input that wasn't recognized by the pilot.
- The captain was (most probably) the only pilot with feet on the pedals.
- Those pedals ended up deflected to the left.
- The airplane turned to the left, and after rotation banked to the left.
- The captain commented the aircraft banked left, but not did comment the rudder pedals deflected uncommanded.
And while I cannot understand how a 20K hour pilot can make such a basic mistake, I feel it is more likely than a failure leading to an uncommanded input that wasn't recognized by the pilot.
If someone has a rational explanation for that, I should be happy. Without a rational explanation that determines they are artifacts "I am troubled".
Low-frequency sampling can throw up artifacts, but the rate of input change needed to achieve such a sampling rate artifact would be somewhat beyond my creaky bones nowdays, faster than landing a Pitts S1, or downwind hover taxi of an R-22... all usually fun for tap dancing. Rough order, the guys feet would be moving between the 8Hz data points sufficient to get the surface recorded at a place that is then nowhere near the next sample of the pedal position. That is a 2-3" amplitude reversal at around 2 times sampling rate or 16Hz or so. Even Gregory Hines would have issues doing that for the Ethiopian Shim Sham Sand Dance... james Devine may be able to do it, he can get 38 taps per second out, which is toe/heel, and 2 feet, which is around 8.5Hz then.... so, there is some odd data there.
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Not so. Sensing rate for the data stream from the 429 bus is 8Hz. The timestamp is common to both data streams for the rudder and pedal.
The data shows that the rudder position appears to be feeding back to the pedals, in what should be an irreversible control system, yet, there are a number of data points that show the opposite, that the rudder has moved before there was a control input recorded. There is the potential for an artifact from the frequency of the sampling, however, there are a number of data points where it is not possible that the control input could have been made in the time to the record of the position that would result in an artifact, where the control is leading the pedal input.
I am surprised to see that in the data. I doubt that a normal person can make a control input that is so fast that it is recorded out of sequence with the following data.
The blue + is the rudder position, the orange X is the Rudder pedal position recorded in the DFDR. This should not be QAR data, so it should have no sampling errors from 429 to 717 or similar, this should be valid data and valid sampling times. In the expanded view below, there is a rational control position that could have a smoothed line on it, per the DFDR analysis done by the NTSB, but then the position of the control surface is preceding the control input, which is not something that should occur. This occurs in the mid acceleration on the runway, and again appears again in the middle of the upset. Is it an artifact? Probably, but it needs to be discounted before the firing squad lines up.
Can a hydraulic control system feedback to the control input? Normally it does not, however, there are possible internal leak cases that could do that.
To me, this looks like the hydraulic accumulator(s) got depleted, due to the extreme high flow/volume requested because of the excessive rudder application. Once the accumulator is depleted, the rudder will no longer rapidly follow the rudder pedals (only at the rate the hydraulic pumps are able to deliver). There is no longer enough hydraulic energy for the rapid "extra" rudder deflection, nor for a forced "return to neutral (or opposite deflection)". And just a few seconds later, without heavy rudder inputs, the accumulator(s) got boosted again, and the sloppy issue is over.
Does it look like that happened at the rotate? No, not immediately, however, if and it is a big if, the rudder system was compromised and feeding into the pedals, then how that would show would be dependent on how it has failed. The rudder pedal is fed back the position of the rudder as well, it also does so on both pedals.
It is possible that a rudder issue did actually occur. I doubt it, but the position of the rudder in the middle of the takeoff acceleration leading the recorded pedal position is concerning, and the same happening in the middle of the lateral excursion is also concerning.
Sensor saturation of the pedal, or rudder or both may be in play, but the data suggests a further look could be warranted. Equally, an error in the followup of the hydraulic actuator to the pedal input could be occurring.
All in all, it is an odd bit of data.
......
It is possible that a rudder issue did actually occur. I doubt it, but the position of the rudder in the middle of the takeoff acceleration leading the recorded pedal position is concerning, and the same happening in the middle of the lateral excursion is also concerning.
Sensor saturation of the pedal, or rudder or both may be in play, but the data suggests a further look could be warranted. Equally, an error in the followup of the hydraulic actuator to the pedal input could be occurring.
All in all, it is an odd bit of data.
......
Not to say, the ailerons/spoliers may have become sloppy too, given these are having the pressure benefit from the same accumulators.
Q: Do rudder, elevators, ailerons and spoilers use all the same accumulator or are the 3 hydraulic systems working in parallel ? The latter would imply, all accumulators were depleted at the same time.
All in all, it could very well be, the captain was right about the "malfunctioning" of the rudder/ailerons, IE operating in unknown territory, though he did cause this issue himself. The handling transfer to the other side was a decision correct for the malfunctioning experience he encountered.
On hydraulics, the sizing of hydraulic supply has considerable reserves of volume for operations, [§25.1435(c)], separately, Subpart C covers loads but not rates.
Back a ways the AS350 had a nasty little habit of having a jack stall in one particular manoeuvre... another helicopter on one occasion happened to return back to the manufacturers ramp after a funny, which distorted the rotor blades, the company owner being the driver. Hydraulics can have anomalies, but in this case, my concern is that there is a curiosity in the data, the full data set should exist still and could provide some further insight into whether this was an artefact or a control anomaly.
Each of the 3 systems have accumulators, and a priority valve prioritises flight control supply over all other services. there are also had fuses that could cause anomalies to supply volume. The data should readily show transients, but the data in th public docket is an inadequate subset to assess deeper. All 3 systems, green, blue and yellow, power the rudder by recollection on that bus. A single transient would likely not result in an anomaly of much note, but more could become an issue, and could be latent. This is a wild card but the data suggests that there is a funny that should be explained. Weird stuff doesn't just happen to marines.
Back a ways the AS350 had a nasty little habit of having a jack stall in one particular manoeuvre... another helicopter on one occasion happened to return back to the manufacturers ramp after a funny, which distorted the rotor blades, the company owner being the driver. Hydraulics can have anomalies, but in this case, my concern is that there is a curiosity in the data, the full data set should exist still and could provide some further insight into whether this was an artefact or a control anomaly.
Q: Do rudder, elevators, ailerons and spoilers use all the same accumulator or are the 3 hydraulic systems working in parallel ? The latter would imply, all accumulators were depleted at the same time.
Last edited by fdr; 30th Jul 2022 at 12:11.
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(simple fly-by-steelwire PPL here, no clue about big kite hydraulics)
Last edited by spornrad; 7th Aug 2022 at 07:44.
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Pedal position does not follow rudder position, it's the other way around. If there's hydraulic pressure rudder will be kept neutral if pedals not touched.
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So when the pilot lets the pedal go, rudder being centered, pedals stay deflected? Don't think so...
(Edit: learned there is a conventional mechanical link between pedal position and rudder PCU in the A321. So rudder following pedal and vice versa, both ways.)
(Edit: learned there is a conventional mechanical link between pedal position and rudder PCU in the A321. So rudder following pedal and vice versa, both ways.)
Last edited by spornrad; 25th Aug 2022 at 09:55.
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You mean like Air France flight AF11 where control issues on final to CDG were also due to an aircraft malfunction? Right...... I'm more inclined to believe either the Pilots had something incorrectly set up or failed to react to the conditions they were experiencing....
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A320 written off after pilot-induced wingtip strike on takeoff
Don't know if this final report from a couple of months ago has been mentioned before. An American Airlines A320 at JFK, April 2019. Aircraft understoodf to have been scrapped due irreparable damage to wing structure - left tip bent approx 6ft higher than right.
https://data.ntsb.gov/carol-repgen/a...port/99240/pdf; summary here Accident: American A321 at New York on Apr 10th 2019, wingtip strike and collision with runway sign during departure (avherald.com)
There's an interesting recreation of it here.The first "cover image" shows the wrong wing in contact wiht the ground though. I have no idea how respected the creator "mentour" is but he seems well informed.
https://data.ntsb.gov/carol-repgen/a...port/99240/pdf; summary here Accident: American A321 at New York on Apr 10th 2019, wingtip strike and collision with runway sign during departure (avherald.com)
There's an interesting recreation of it here.The first "cover image" shows the wrong wing in contact wiht the ground though. I have no idea how respected the creator "mentour" is but he seems well informed.
Last edited by slast; 14th Sep 2022 at 17:26. Reason: link added
The PCU may lag the rudder pedal slightly, however what it won't do is lead it. That is the "funny" in the data. The data rates are fast enough that it is not likely but not impossible that an extremely rapid input and reversal occurred, but of all of the control axis, the rudder is the most unlikely to have that occur, our legs just don't dance that fast (Fred Astaire, we ain't)
