Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Sully's Flare on the Hudson: Airbus Phugoid Feedback

Tech Log The very best in practical technical discussion on the web

Sully's Flare on the Hudson: Airbus Phugoid Feedback

Old 23rd May 2017, 14:39
  #81 (permalink)  
Thread Starter
 
Join Date: May 2017
Location: San Diego
Posts: 65
Originally Posted by zzuf View Post
Quote Airbus response to NTSB
"On the last 10 sec in the air of Flight 1549 , DFDR data show that pitch attitude is increasing and CAS decreasing. "
---- Hey, Airbus, thats called a "flare". LOL Are they saying he flared too early? I don't think he did flare early, as his descent rate was so high, he needed to start mushing it in close to maxCL (max lift) before entering ground effect.
Originally Posted by zzuf View Post
Quote Airbus response to NTSB: "Then, the phugoid damping terms are non null and are acting in the sense to decrease the finally commanded AoA vs. the stick command, in order to prevent the Aircraft from increasing the phugoid features."
---- This a red herring argument by Airbus. It is incredibly important to say that impact is imminent, and no phugoid ever has time to develop. Airbus loyalists, apologists, and lawyers love that statement. Truthy folks don't.


Originally Posted by zzuf View Post
Would appear that Airbus have no enthusiasm for the "turn off" phugoid damping just prior to the flare scenario.
The NTSB did at first, then backed away under the slightest pressure from the BEA and Airbus. Snowflakes populate the NTSB.

You can go on and on about how Sully was too slow up higher than 100' or so, yet no aircraft automatic feedback should ever exacerbate the situation by nosing down in critical flare, seconds before impact. We don't punish pilots & passengers that way. Instead, we do what we can. Obvious to most.

This water ditching just uncovered similar issues that occurred at Habsheim & Bilbao, & maybe other landing accidents of A320's.

Originally Posted by zzuf View Post
QuagmireAirlines, I would be interested in how you would intend to wash out the phugoid damping term,....
That's easy, just ramp it out gradually from 200' to 50'. Routine. It won't excite anything, since its going away, and remember, speed protection has a valid presence above flare altitudes.
Originally Posted by zzuf View Post
You don't seem to have addressed if, at the speed available, there is any advantage in terms of manoeuvre in even attempting to increase alpha.
Yes, in flare, where Sully attempted to get more pitch, it would have increased lift, flaring harder, then it hits the water. Don't worry about "long term" speed effects since you hit the water quickly.
Originally Posted by zzuf View Post
Your stylized CL/Alpha curve may show the relationship of various alpha limits, but in no way does it represent the A320 CL/Alpha curve, nor does show just how much usable CL was left available.
Those are Airbus's "stylized" CL/Alpha curves. And its reality. It shows there's MORE CL available, which is what counts.

All this is probably a lower-quality version of the arguments at the NTSB just before they caved to political pressure to back away from their original correct position. On this forum, we do need the Airbus/BEA apologists to present better arguments, if possible. Too weak.
QuagmireAirlines is offline  
Old 23rd May 2017, 18:49
  #82 (permalink)  
 
Join Date: Apr 2003
Location: Europe
Posts: 2,974
Quagmire, would you do that in a 737? Fly through the stick shaker at 150', mushing it in close to CLmax as you say?
PENKO is offline  
Old 23rd May 2017, 19:03
  #83 (permalink)  
 
Join Date: Dec 2000
Location: very close to STN!!
Posts: 523
I have asked myself how that event would have turned out, all things equal, except it being a b737-800. Would you fly at flaps up manoeuvre speed bug initially? Is that as "informative" as the airbus target bug that they had?

Boeing FCTM says

"flap retraction and extension schedules provide speeds that are close to minimum drag"

"Dual engine failure is a situation that demands prompt action regardless of altitude or airspeed. Accomplish memory items and establish the appropriate airspeed to immediately attempt a windmill restart." Not so helpful in above situation.

He(and all aboard) was one lucky dude! Daylight, cavok, great FO, ferry boats already had their engines on to make a crossing, nicely spaced bridges, no boats in the way...if birds hit a little sooner or a little later...
stator vane is offline  
Old 23rd May 2017, 20:08
  #84 (permalink)  
 
Join Date: Apr 2003
Location: Europe
Posts: 2,974
Who knows. The stick shaker would have gone off for a start. At 150' that is not something you recover from easily.
PENKO is offline  
Old 23rd May 2017, 21:06
  #85 (permalink)  
 
Join Date: Jun 2014
Location: Village of Santo Poco
Posts: 794
Originally Posted by QuagmireAirlines View Post
---- This a red herring argument by Airbus. It is incredibly important to say that impact is imminent, and no phugoid ever has time to develop. Airbus loyalists, apologists, and lawyers love that statement. Truthy folks don't.
Not for nothing, Quagmire, but you could have saved yourself a whole lot of breath by just stating right off the bat that you don't like Airbus period. You're not the first or the last. No need to waste five pages blabbering on about the phugoid.
Amadis of Gaul is offline  
Old 24th May 2017, 01:14
  #86 (permalink)  
 
Join Date: Oct 2004
Location: australia
Posts: 173
Armadis of Gaul, not only as you say, but this guy obviously has no engineering background, despite his claim of being a flight control engineer.
I was going to write a detailed post on how he, on the basis of one data point, would rewrite the longitudinal control laws of a very success airline aircraft.
I happened to spend time, as a certification TP, at Airbus flying the experimental sidestick A300 with Gordon Corps and the early A320 with Nick Warner.
For this guy to say "a flare is a flare" shows he has zero knowledge of the months of engineering and flight test work which went into developing the laws to get the aircraft from 100 feet to nose wheel firmly on the ground, easily, consistently, and safely in all configurations and weather conditions.
He didn't think far enough ahead to realize this highly unlikely event could be repeated but with power available on at least one engine. His new control law will face a savage reality check when power is chopped and the standard strong nose down pitching moment occurs.
I could say he has the supreme confidence of the totally ignorant, but prefer to believe he is a fraud.
Unfortunately he will probably reappear with a different id.
zzuf is offline  
Old 24th May 2017, 04:14
  #87 (permalink)  
 
Join Date: Jun 2014
Location: Village of Santo Poco
Posts: 794
Originally Posted by zzuf View Post
Armadis of Gaul, not only as you say, but this guy obviously has no engineering background, despite his claim of being a flight control engineer.
I, too, found it somewhat far-fetched that a bona-fide engineer would open with "I just saw the movie Sully, and...", but who knows these days?
Amadis of Gaul is offline  
Old 24th May 2017, 04:32
  #88 (permalink)  
 
Join Date: May 2017
Location: Hk
Posts: 3
Little Thread Drift

I have been interested to read this thread and like many of the thoughts, I will not add to it as feel I'm not qualified to comment any more than has already been said. But my question leads to something in that vain that you all maybe able to shed light on. I know the 330 not 320. After the Hudson incident a new simplified checklist appeared in part thanks to Sully and obviously findings. But on close examination between old qrh checklist and new abbreviated one. The old suggests double engine flameout land/ditch in Conf1. Abbreviated recommends Conf2. I can not understand why that would differ. Your brain power would be helpful . Sorry for drifting a little
Whoo is offline  
Old 24th May 2017, 09:03
  #89 (permalink)  
 
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 84
Posts: 474
@Quagmire

There are many points in your postings with which I disagree, but here are just a few:
As a general point, making fundamental system changes on the basis of a single scenario is poor airworthiness. If you want your proposals to be considered seriously you must discuss their effects in a variety of possible situations.

“Since the phugoid is a roller coaster up-down slow cycle, why are phugoid damping terms allowed to pitch the aircraft down close to the ground when the ground is right there, close up, and the next event is touchdown, so the phugoid can't be excited anyway?”
You are missing the point that although the pitch rate and speed variation feedbacks stabilise the phugoid they also have an effect on the short period motion.
As a control engineer you will appreciate that the more rapidly you approach a system limit the more likely you are to overshoot it and the bigger the probable overshoot dimension. If the system limit is a critical safety boundary like a stall you would take steps to ensure that you did not overshoot. This is a short period phenomenon. The pitch rate feedback in particular does this job by limiting the rate at which you can approach the limit (but does not prevent you getting to the limit slowly).
“I think the solution would be to ONLY have a little pitch rate damping, and do away with any other anti-phugoid mode feedback in alpha-protect below 100' AGL near flare”
In effect your proposal would remove dynamic stall protection below 100 ft or thereabout. One can argue whether the feedback gains are set too high and unduly limit the rate at which pitch can be increased, but not I think concede the principle that dynamic stall protection must be retained even at low altitudes.
“To clarify, you can't go into a roller coaster phugoid excitation since your altitude apogee is already close to the ground. Enter the edge of stall at 30 feet off the ground while descending, no problem”.
WOW! – go to the edge of a stall at 30 ft RA with no stall protection – I don’t think you will find many takers on this forum.
You are assuming that pitch attitude is the only important parameter in water landings, which is not correct. The probability of structural damage depends on the overpressure generated on the fuselage undersurface by the impact (and let us not forget that despite the fact that Flight 1549 ended well, the pressures were twice the design value).
The pressure generated depends on the square of the total (forward plus downward) velocity times the sine of the angle between water surface and fuselage at impact. This latter is the sum of pitch attitude and flight path angle with a correction for the shape of the fuselage at the point of impact. Pitch plus FPA of course equals angle of attack; angle of attack dictates lift coefficient and lift coefficient times speed squared equals lift. Flaring takes time! So if you flare late and hit the water before the flight path has had time to change and the speed to bleed off you risk generating more impact pressure not less because you have increased the impact angle. Piloting technique matters!
“To me the perfect landing would have been 11 degrees pitch, 1 degree stall margin remaining, and the slow airspeed that corresponds to, while hitting the water at 500 ft/min. Let's call perfect an "11/1/500". Instead, Sully did it as an "9.5/3.5/750" and I blame that on some unwanted feedback terms in alpha-protect below 50' AGL.”
Let’s dissect that proposal:- alphamax was 17.5 deg, so with 1 degree stall margin alpha would be 16.5 deg, and with 11 degree pitch that means a flight path angle of -5.5 degrees. The speed would drop to 115 kts, making the rate of descent 1100 fpm not 500 – far from perfect! If you want to be taken seriously you need to get your facts right.
Airbus have said that below 8 degrees pitch and above 15 degrees there is a much increased risk of major damage. They have also said that the optimum attitude for ditching varies with circumstances such as sea state. 11 degrees, sitting just about the middle of the usable range should I think be considered as an acceptable compromise rather than an ideal. We do know, do we not, that 9.8 degrees at 128 kts onto calm water is a viable combination.

“Originally, the early NTSB reports mentioned it all. Page 194 of the NTSB Report (the BEA French section) says:
"However, this accident demonstrates that, by offsetting the pilot's ANU sidestick inputs, the phugoid-damping feedback function of the alpha-protection mode could make flaring the airplane to attain the recommended ditching touchdown parameters more difficult." --- That statement didn't make it into the final report, but was in the appendix as a reported document change”
But the final report did say:-
“Deliberately or inadvertently slowing the airplane into the alpha-protection mode may result in an attenuation of pilot nose-up stick inputs, making it more difficult to flare the airplane, even if AOA margin to alpha maximum exists.”
“On this forum, we do need the Airbus/BEA apologists to present better arguments, if possible. Too weak.”
Sounds like a Trumpism!
Owain Glyndwr is offline  
Old 24th May 2017, 10:50
  #90 (permalink)  
 
Join Date: Oct 2004
Location: australia
Posts: 173
@Owain
Thanks for your response, in retrospect my comment was generated as a result of total frustration at the technical level of responses, from a "flight controls engineer", particularly the dismissive treatment of the Airbus to NTSB document, which I thought could encourage an interesting discourse about at least one aspect of Airbus FBW.
The A320 certification was a huge workload for the certification office staff as well as the specialist staff from the design office and flight test.
The A320 was my favorite of all the transport category jets of that generation that I flew. I often commented that, at medium altitude, in direct law it was a "nicer" handling aircraft than the B734 with all systems operating normally.
Nick and I were both keen sailors and planned to meet on a Fijian island after early retirement. The plan was to spend some serious time covering the really difficult areas of the certification and the behind the scenes horse trading over the many items that required at least tacit agreement before certification special condition were developed. All to be done over a bottle of red or two. Sadly it never happened.
zzuf is offline  
Old 24th May 2017, 14:45
  #91 (permalink)  
Thread Starter
 
Join Date: May 2017
Location: San Diego
Posts: 65
I side with the ORIGINAL NTSB finding. Its as simple as that.
People writing in here can deny actual flight physics, can insult me for no reason, can act as if they know everything, can make up "facts", etc. The worst part is (yawn) going on and on and on about off-topic A320 non-flare flight, as if that somehow erases Airbus's errors in flare.

And stop the "flare at 150'" nonsense. Also, going to the near stall doesn't mean exceeding stall alpha. Try again with your weird emotional grunting, cause its not working.

Originally, the early NTSB reports mentioned it all. Page 194 of the NTSB Report (the BEA French section) says:
"However, this accident demonstrates that, by offsetting the pilot's ANU sidestick inputs, the phugoid-damping feedback function of the alpha-protection mode could make flaring the airplane to attain the recommended ditching touchdown parameters more difficult." --- That statement didn't make it into the final report, but was in the appendix as a reported document change at the behest of BEA.

Originally Posted by Owain Glyndwr View Post
@Quagmire WOW! – go to the edge of a stall at 30 ft RA with no stall protection – I don’t think you will find many takers on this forum.
Except I never said remove stall protection. Its should be there, just not the phugoid feedback, as the NTSB originally noted. Get it right if you want to drone on incessantly! Too much of your nonsense to correct here.
I know I must be on the right track since weird arguments and insults are the best some are able to muster to counter me and the original NTSB findings!!!!

Originally Posted by Amadis of Gaul View Post
Not for nothing, Quagmire, but you could have saved yourself a whole lot of breath by just stating right off the bat that you don't like Airbus period. You're not the first or the last.
On the contrary, I do like Airbus for their other flight modes. Stop the emotional garbage and insults you spew. I can disagree, like the NTSB initially did, with how flare control laws on A320 work. Don't like it?

Originally Posted by zzuf View Post
2. For whatever reason, Airbus decided this characteristic was unacceptable and included a damping term in the flight control laws.
"For whatever reason", really? You pretend to understand flight, yet don't know? They don't have a valid reason. Fact is, as the NTSB noted originally, they made a mistake to put the damping in there during flare.

Originally Posted by zzuf View Post
8. If you give the pilot sufficient authority to stop the phugoid at any time, by definition you no longer have alpha protect.
Not at all true. Alpha protect remains intact, which is a good thing, even in flare, the regime this thread is about.

Originally Posted by Amadis of Gaul View Post
far-fetched that a bona-fide engineer would open with "I just saw the movie Sully, and...", but who knows these days?
I can't watch a movie? Everybody knows movies may not be accurate. Common sense. Not a difficult concept to master.

Originally Posted by Owain Glyndwr View Post
specification prepared by someone else for the MD11 - an aircraft whose landing record was nothing to write home about.
Wrong on many, many levels. ... You assume a lot, and very wrongly.
...Although you are right the MD-11 handling qualities in landing are not optimal. I didn't work on that part of the control laws when on the MD-11. Wish I had been assigned that. Very off-topic to this thread, yet I'll say I'm not happy with the decisions made on MD-11. Back then, I had just finished a lot of eigenvector handling qualities research, and management all came from DC-9 and 727 backgrounds, and were mostly electrical engineers with a lot of misconceptions about flight physics & human factors.
Its similar to some of the rude, insulting people on this forum like Owain who can't use logic properly, carry misconceptions, and like to insult.

So if you can't win an argument against me and the original NTSB findings, just insult your way to victory. Yeah, that works.
QuagmireAirlines is offline  
Old 24th May 2017, 16:34
  #92 (permalink)  
 
Join Date: Jan 2005
Location: W of 30W
Posts: 1,939
Everything was in place to improve the touchdown when Sully decided to pull the sidestick, up to full deflection - Only computers prevented him to obtain better figures.

And for vilas, Skyles made a very informative public speech on the event and details many good reasons why the APU was started - To be protected was not one of them ...
https://youtu.be/BN7EdfJavco
CONF iture is offline  
Old 24th May 2017, 19:41
  #93 (permalink)  
 
Join Date: Jun 2007
Location: Wanderlust
Posts: 2,334
and details many good reasons why the APU was started

CONF iture
Thanks for the video but what I wrote as you know is NTSB's opinion. He is apparently talking to non professionals. You decide who knows better. Actually I don't hear him give any valid reason for starting the APU. In this particular accident as it turned out since it was three minutes flight turning on APU didn't matter at all. Because the left engine provided electrics and hydraulics till they shut it down for relight. Auto relight must have come on for the right engine and failed and there was not much time to attempt starter assisted start. Also, although there are not many takers for your theory, according to you remaining in normal law (because of the APU) actually spoiled the party by preventing proper flare (actually stall if you ask me). But on another day in similar situation APU would matter. Because without G+Y and in ELEC EMER you won't get flaps and force landing with that slat only high attitude without thrust is not happy situation. With APU on in normal electrics you can switch on yellow electric pump and get flaps to three which would reduce landing distance and thereby chances of survival considerably.

Last edited by vilas; 24th May 2017 at 20:01.
vilas is online now  
Old 24th May 2017, 20:09
  #94 (permalink)  
 
Join Date: Jan 2006
Location: US
Posts: 2,088
Everything was in place to improve the touchdown when Sully decided to pull the sidestick, up to full deflection - Only computers prevented him to obtain better figures.


Everything would really have been in place if he had maintained more airspeed.

The computers might have saved everyone's bacon by not allowing an excursion into an AOA regime that might have triggered an AOA that was too high, at too high an altitude, with resulting increased rate of descent at water impact.
misd-agin is offline  
Old 24th May 2017, 20:11
  #95 (permalink)  
 
Join Date: Jan 2006
Location: US
Posts: 2,088
Slats only is probably good enough. 50-70% of the stall speed reduction in airliners comes from slat extension. Flaps are much less effective in reducing stall speed.
misd-agin is offline  
Old 24th May 2017, 20:43
  #96 (permalink)  
 
Join Date: Jun 2007
Location: Wanderlust
Posts: 2,334
misd-agin
Only slat attitude is very high. Even in normal landing tail strike is possible if not flared carefully. Landing distance also almost 1000mtrs higher without flaps. And you won't convince CONF iture. He has good company in Quagmire.
vilas is online now  
Old 24th May 2017, 20:51
  #97 (permalink)  
Moderator
 
Join Date: Apr 2015
Location: The Gulf Coast
Posts: 649
If I may encourage all involved:
Play the ball, not the man
We can all disagree without being disagreeable.
T28B is offline  
Old 24th May 2017, 21:49
  #98 (permalink)  
 
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 84
Posts: 474
@T29B
OK, accepted,, I have deleted the offending post
Owain Glyndwr is offline  
Old 25th May 2017, 02:35
  #99 (permalink)  
 
Join Date: Mar 2005
Location: N/A
Posts: 2,917
Everything would really have been in place if he had maintained more airspeed
Not a FW guy, but never liked tapes. Always thought it took too much to interpret in comparison to a round dial and noting the o'clock position of the needle. Man/machine interface problem for Sully?
megan is offline  
Old 25th May 2017, 04:39
  #100 (permalink)  
 
Join Date: Jan 2005
Location: W of 30W
Posts: 1,939
Originally Posted by misd-agin
The computers might have saved everyone's bacon by not allowing an excursion into an AOA regime that might have triggered an AOA that was too high, at too high an altitude, with resulting increased rate of descent at water impact.
There was no such danger. Passing 150ft at 1200ft/min the airplane didn't reach yet alpha prot - Autotrim was still in action.
Had the elevators collaborated to Sully request anything better, and certainly not worse, was achievable.
You put a catastrophic scenarios where there is none.
CONF iture is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.