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Old 17th May 2017, 16:31   #1 (permalink)


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Sully's Flare on the Hudson: Airbus Phugoid Feedback

I just saw the Sully movie, and it reminded me to dig into the NTSB Accident Report about how the ditching was done.

Why was Airbus not admonished for their flight control laws fighting Sully's pitch up flare commands?
(I'm thinking Airbus should have been told to phase out the phugoid damping at low radar altimeter near and at flare. Phugoid damping is fine at higher altitudes of course.) Seems obvious to me. Can you shed some light on this?

The fly-by-wire control law software injected phugoid damping at very low AGL (100' and below), resulting in a slightly lower pitch angle (9.5 deg) at touchdown than actually recommended by Airbus and other studies (10 to 12 degrees; 11 is ideal).

Sully had full aft stick in the last 2 seconds before impact, trying to get a couple more degrees of pitch, which would have given him about a 12 degree pitch angle at touchdown. Skiles remarked the airplane submarined a bit on impact, consistent with ditching studies and the 9.5 degree pitch angle they struck the water with. Any less pitch and the nose might have cracked open.

The airplane had about 4 degrees of alpha stall margin remaining at touchdown which was never used. Important fact.

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?

References:

From the Aircraft Performance Study:

"Figure 7 shows that between 15:30:36 and the touchdown at 15:30:43, the pitch angle increases from 9.5° to 11° and then settles back to 9.5°, even though in the last two seconds the left longitudinal side stick is at its aft limit, and α is below αmax. "

" A phugoid damping feedback term in the flight control laws, that is active in α−protection mode, attenuated the airplane’s nose-up pitch response to progressively larger aft side stick inputs made below 100 ft radio altitude. "

The airplane touched down on the Hudson at 15:30:43, under the following conditions:


Airspeed (FDR): 125 KCAS
Groundspeed (computed): 126 knots
Rate of descent (computed): -750 ft/min
γ (relative to Earth, computed): -3.4°
γ (relative to airmass, computed): -3.5°
θ (FDR): 9.5°
φ (FDR): 0.4°
α (computed): 12.9° α (measured): 13.7° (left vane), 14.5° (right vane)
β (computed): 2.2°
Drift angle (computed): -1.4°
Track angle (computed): 210.7°
true Heading angle (FDR): 209.9° true

From the NTSB Accident Report:

"The Airbus simulation indicated that the captain’s aft sidestick inputs in the last 50 feet of the flight were attenuated, limiting the ANU response of the airplane even though about 3.5° of margin existed between the airplane’s AOA at touchdown (between 13° and 14°) and the maximum AOA for this airplane weight and configuration (17.5°). Airbus’ training curricula does not contain information on the effects of alpha-protection mode features that might affect the airplane’s response to pilot sidestick pitch inputs. The flight envelope protections allowed the captain to pull full aft on the sidestick without the risk of stalling the airplane.
The NTSB concludes that training pilots that sidestick inputs may be attenuated when the airplane is in the alpha-protection mode would provide them with a better understanding of how entering the alpha-protection mode may affect the pitch response of the airplane. The NTSB recommends that the FAA require Airbus operators to expand the AOA-protection envelope limitations ground-school training to inform pilots about alpha-protection mode features while in normal law that can affect the pitch response of the airplane. "

https://www.ntsb.gov/investigations/...ts/AAR1003.pdf
http://www.exosphere3d.com/pubwww/pd...ket/431658.pdf
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Old 17th May 2017, 22:08   #2 (permalink)
 
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Section 2.3.3

Despite not reaching this portion of the Engine Dual Failure checklist, the captain stated during postaccident interviews that he thought that he had obtained green dot speed immediately after the bird strike, maintained that speed until the airplane was configured for landing, and, after deploying the flaps, maintained a speed “safely above VLS,” which is the lowest selectable
airspeed providing an appropriate margin to the stall speed. However, FDR data indicated that the airplane was below green dot speed and at VLS or slightly less for most of the descent, and
about 15 to 19 knots below VLS during the last 200 feet.
The NTSB concludes that the captain’s difficulty maintaining his intended airspeed during the final approach resulted in high AOAs, which contributed to the difficulties in flaring the airplane, the high descent rate at touchdown, and the fuselage damage. (See additional discussion in section 2.7.1.)

2.3.2

Simulation flights were run to determine whether the accident flight could have landed successfully at LGA or TEB following the bird strike. The simulations demonstrated that, to accomplish a successful flight to either airport, the airplane would have to have been turned toward the airport immediately after the bird strike. The immediate turn did not reflect or account for real-world considerations, such as the time delay required to recognize the extent of the engine thrust loss and decide on a course of action. The one simulator flight that took into account real-world considerations (a return to LGA runway 13 was attempted after a 35-second delay) was not successful. Therefore, the NTSB concludes that the captain’s decision to ditch on the Hudson River rather than attempting to land at an airport provided the highest probability that the accident would be survivable.

NTSB: Sully Could Have Made it Back to LaGuardia - CBS News

If you're prepared, and have practiced dual engine failures, an immediate turn would have made the airport. But it's a high risk event especially if you're not prepared or haven't practiced it. I think the success rate to RTB to rwy 13 was 6/7 or 7/8. Rwy 22 about 50/50 and 31 was 0/1 or 0/2.

Unfortunately even professional pilots have chosen the movie as their base for information instead of the NTSB report.
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Old 18th May 2017, 00:31   #3 (permalink)
 
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Quote:
Originally Posted by misd-agin View Post

Unfortunately even professional pilots have chosen the movie as their base for information instead of the NTSB report.
How do you know that to be so?
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Old 18th May 2017, 01:08   #4 (permalink)
 
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Quote:
Why was Airbus not admonished for their flight control laws fighting Sully's pitch up flare commands
Not a unique event. A-320-214, EC-HKJ, Bilbao Airport, 7 February 2001

Quote:
The aeroplane did not respond to the pilots’ commands on the controls to pitch up the aircraft and to reduce the vertical speed on the flare, causing the aircraft to impact against the threshold of the runway in a slight nose-down attitude.

The cause of the accident was the activation of the angle of attack protection system which, under a particular combination of vertical gusts and windshear and the simultaneous actions of both crew members on the sidesticks not accounted for in the design, prevented the aeroplane from pitching up and flaring during the landing.
https://www.fomento.gob.es/NR/rdonly...006_A_ENG1.pdf
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Old 18th May 2017, 01:21   #5 (permalink)
 
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Seen so much written here by armchair experts.
Question. How many of you have flown an Airbus 320/330 (not qualified on the other types) at or close to Vls or Alpha Prot or Alpha floor ?
Stick with the Fox news version. That way you won' sound so silly. He did a great job that in the same circumstances 99.99% of us would'nt have pulled it off
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Old 18th May 2017, 03:42   #6 (permalink)
 
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Not a unique event. A-320-214, EC-HKJ, Bilbao Airport, 7 February 2001
However, they did change the control laws after that incident, did they after the hudson river landing? I don't think so.
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Old 18th May 2017, 07:01   #7 (permalink)
 
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The 19kt deficiency triggered alpha protection mode which pitches the nose down. Isn't that's what it is supposed to do?If aircraft would have been in alternate law it would have stalled. Bilbao was different where alpha protection was not pilot induced but environmental conditions caused the alpha to reach alpha prot.
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Old 18th May 2017, 08:12   #8 (permalink)
 
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Hi vilas,
Quote:
If aircraft would have been in alternate law it would have stalled.
Not necessarily. I'm sure Sully's flying skills are as good as this B737-300 crew and he would have "respected the stall warning" if he was in ALT LAW.

Garuda Indonesia flight GA421 - Aviation Accidents Database

Last edited by Goldenrivett; 18th May 2017 at 12:12.
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Old 18th May 2017, 13:19   #9 (permalink)
 
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Quote:
Originally Posted by vilas
If aircraft would have been in alternate law it would have stalled.
... Another of those unfounded statements.

Sully would have been better served with direct law to manage the touchdown as its liking.
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Old 18th May 2017, 13:42   #10 (permalink)
 
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GR


We are not doubting Sully's skills. He surely had it to manage also in normal law but the tremendous time pressure and doing something for which he wasn't trained distracted him from maintaining VLS in normal law so how can you rule out similar thing in alternate law. And saying direct law would have served him better is not my indictment but Sully's because he chose to put the APU on out of turn to remain in normal law. If it was in alternate or direct law with stall warning he would have pushed the nose down with no different results.The aircraft would have landed better in any law if speed was VLS. I am talking of the airplane behaviour not anybody's skills.
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Old 18th May 2017, 15:04   #11 (permalink)
 
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Quote:
Originally Posted by vilas
And saying direct law would have served him better is not my indictment but Sully's because he chose to put the APU on out of turn to remain in normal law.
What a joke ... Sully did not start the APU to remain in normal law, but to secure some redundancy for electric and hydraulic, and have a chance to achieve an engine restart.

Quote:
If it was in alternate or direct law with stall warning he would have pushed the nose down with no different results.
To the contrary, an earlier warning would have prompted Sully to give some slack to the stick and keep that energy for later when it all matters : flare and touchdown.
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Old 18th May 2017, 15:57   #12 (permalink)
 
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CONF
If you hear the video recording at 3:14 you will notice NTSB is not laughing with you, may be at you.
Quote:
What a joke ... Sully did not start the APU to remain in normal law

https://youtu.be/ZaV0godxu3w
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Old 18th May 2017, 16:18   #13 (permalink)
 
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Quote:
To the contrary, an earlier warning would have prompted Sully to give some slack to the stick and keep that energy for later when it all matters : flare and touchdown.
Care to explain what is earlier stall warning or alpha MAX? Also if you push the stick forward from stall warning how do you gain speed without increased rate of descent?
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Old 18th May 2017, 20:11   #14 (permalink)
 
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Originally Posted by aterpster View Post
How do you know that to be so?
From direct conversation. They start quoting the movie 'facts' versus reading the official investigation.
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Old 18th May 2017, 20:21   #15 (permalink)
 
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Once you reach AOA limits you're not managing energy, the a/c is managing itself.


As far as I know every AB qualified pilot gets to experience the AOA protection in the simulator during the type rating course.

Most approaches are flown near Vls. It's about 1.23 Vs. Page 28/213 of the ntsb report linked in the first post shows the airspeed display.
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Old 18th May 2017, 23:36   #16 (permalink)


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Great discussion, thanks to all. I've always thought Sully did it about right. Not perfect, yet very good. My background is flight control laws (& physics), so I see this as a potential control law problem, not a piloting problem in this case.

Quote:
Originally Posted by megan View Post
Not a unique event. A-320-214, EC-HKJ, Bilbao Airport, 7 February 2001 https://www.fomento.gob.es/NR/rdonly...006_A_ENG1.pdf
Quote:
Originally Posted by Denti View Post
However, they did change the control laws after that incident, did they after the hudson river landing? I don't think so.
OK, I need to read about that 2001 A320 problem. I guess Airbus and others continue to believe phugoid damping close to the ground near flare is a good idea. I'm still puzzled why the NTSB didn't cite the fly-by-wire control laws keeping the aircraft from achieving 11 or 12 degrees pitch at touchdown, a stronger flare that Sully was trying to get with full aft stick.

I see Sully flew slightly too slow during those last 3 minutes before the Hudson landing. Still, it looks to me like he had enough aero stall margin when it counted near the water, but was prevented from using it for reasons I don't get.

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. Ground effect aero lift helps some, mushing it in.

Sully hit the water at 750 ft/min when I think he could have got it to about 600 ft/min if he would have been allowed to use that last remaining 3 degrees of pitch angle. I'd like to fly that water landing WITHOUT the Airbus feedback terms taking some pitch away to find out. I'll try to find out if anybody out there has done this on the sim with the alpha-protect mode feedback damping (interference) removed.
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Old 19th May 2017, 01:23   #17 (permalink)
 
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Quote:
Originally Posted by QuagmireAirlines View Post
I just saw the Sully movie, and it reminded me to dig into the NTSB Accident Report about how the ditching was done.

Why was not admonished for their laws fighting Sully's pitch up flare commands?
I recently saw the movie "Airplane" again and was wondering why Boeing was not admonished for using a warning light somewhat more definitive than "A little hot" for engine temp indication.

I guess we each have our own questions about things...
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Old 19th May 2017, 02:44   #18 (permalink)


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Quote:
Originally Posted by vilas View Post
The 19kt deficiency triggered alpha protection mode which pitches the nose down. Isn't that's what it is supposed to do?If aircraft would have been in alternate law it would have stalled. Bilbao was different where alpha protection was not pilot induced but environmental conditions caused the alpha to reach alpha prot.
Going into alpha protection mode is fine, at any altitude, even in flare. I don't think thats the problem here. Also, Sully wouldn't have stalled it since he saw pitch & speed just fine, and could feel the limits, basic skills. Bilboa also was a case where the pilots should have been able to get a few more degrees of pitch to soften the ground impact, but alpha-protect damping terms prevented pitch up in flare. (There was another A320 problem like Bilboa that happened in Canada in 1999.)

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. That way, the pilot isn't allowed to stall while retaining the capability to use all the pitch angle available in flare if the pilot deems it necessary, as Sully requested by full aft stick.

But the greater minds at the NTSB know something I don't since they missed all this stuff that seems obvious.
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Old 19th May 2017, 02:48   #19 (permalink)
 
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So, vilas, where is it that quote from Sully :
"I choose to start the APU to remain in Norma law" ?

Quote:
Originally Posted by QA
I'm still puzzled why the NTSB didn't cite the fly-by-wire control laws keeping the aircraft from achieving 11 or 12 degrees pitch at touchdown, a stronger flare that Sully was trying to get with full aft stick.
I find the report a lot on how the airplane is good, but I am also surprised why they avoided to run a similar scenario but this time with the airplane in direct law to know how getting rid of the protections could have actually helped Sully to obtain the touchdown he was after ...
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Old 19th May 2017, 13:28   #20 (permalink)
 
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He didn't read the airspeed "just fine." He misread it by more than 15-19 kts ("safely above Vls" vs 15-19 kts below Vls) and didn't process that his speed was barely above the red band which is an image we never see in normal approaches. That is what extreme stress will do to you and is understandable. The NTSB reports covers the stress impact on human performance.

Without the AOA protection AB built into the FBW we can only speculate what would have happened without AOA protection. At a high AOA, reached at 150', he then attempted to raise the nose at 100'. That attempt, with only a couple of knots above stall speed, might have lead to a situation where the vertical descent rate would have been even higher at water entry. That's if he realized it and didn't stall the a/c. If he had stalled a non FBW protected a/c the outcome would have been worse.

From the NTSB report - 2.3.3 -

Despite not reaching this portion of the Engine Dual Failure checklist, the captain stated during postaccident interviews that he thought that he had obtained green dot speed immediately after the bird strike, maintained that speed until the airplane was configured for landing, and, after deploying the flaps, maintained a speed “safely above VLS,” which is the lowest selectable
airspeed providing an appropriate margin to the stall speed. However, FDR data indicated that the airplane was below green dot speed and at VLS or slightly less for most of the descent, and
about 15 to 19 knots below VLS during the last 200 feet.
The NTSB concludes that the captain’s difficulty maintaining his intended airspeed during the final approach resulted in high AOAs, which contributed to the difficulties in flaring the airplane, the high descent rate at touchdown, and the fuselage damage. (See additional discussion in section 2.7.1.)
During emergency situations, such as the accident event, pilots experience high levels of stress resulting from high workload, time pressure, and noise. Stress can distract pilots from cockpit duties and result in pilot errors or performance degradation.133 For example, stress can lead to a phenomenon known as “tunnel vision,” or the narrowing of attention in which simple things can be overlooked (for example, airspeed and descent rate (snip)



The airplane’s airspeed in the last 150 feet of the descent was low enough to activate the alpha-protection mode of the airplane’s fly-by-wire envelope protection features. The captain progressively pulled aft on the sidestick as the airplane descended below 100 feet, and he pulled the sidestick to its aft stop in the last 50 feet, indicating that he was attempting to raise the airplane nose to flare and soften the touchdown on the water. The A320 alpha-protection mode incorporates features that can attenuate pilot sidestick pitch inputs. Because of these features, the
airplane could not reach the maximum AOA attainable in pitch normal law for the airplane weight and configuration; however, the airplane did provide maximum performance for the weight and configuration at that time.
The Airbus simulation indicated that the captain’s aft sidestick inputs in the last 50 feet of the flight were attenuated, limiting the ANU response of the airplane even though about 3.5° of margin existed between the airplane’s AOA at touchdown (between 13° and 14°) and the maximum AOA for this airplane weight and configuration (17.5°). Airbus’ training curricula does not contain information on the effects of alpha-protection mode features that might affect the airplane’s response to pilot sidestick pitch inputs. The flight envelope protections allowed the captain to pull full aft on the sidestick without the risk of stalling the airplane.
The NTSB concludes that training pilots that sidestick inputs may be attenuated when the airplane is in the alpha-protection mode would provide them with a better understanding of how entering the alpha-protection mode may affect the pitch response of the airplane. The NTSB recommends that the FAA require Airbus operators to expand the AOA-protection envelope limitations ground-school training to inform pilots about alpha-protection mode features while in normal law that can affect the pitch response of the airplane

Last edited by misd-agin; 19th May 2017 at 13:29. Reason: Word change
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