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Sully's Flare on the Hudson: Airbus Phugoid Feedback

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Sully's Flare on the Hudson: Airbus Phugoid Feedback

Old 20th May 2017, 17:00
  #41 (permalink)  
 
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misd-agin, this is exactly what I am saying.
Truth is that Sully would have most probably obtained a better touchdown with direct law.
It is your opinion and not the truth. Sully dropped speed not because of any law but situational pressures and those would not change with protection, without protection, in 777 or 737. Since there was no precise touch down point in Hudson unlike at Heathrow he could have maintain speed till flare but he could not and when speed reaches near stall pushing the nose down has to result in increased ROD. You seem to have some attraction to direct law and you recommend it almost as a panacea. Without protections as the event unfolded all Sully's good work may have come to a nought. At touchdown of 750ft/mt and the aircraft was a right off, a ROD of 1400 could have broken up the aircraft and caused casualties.
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Old 20th May 2017, 21:44
  #42 (permalink)  
 
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Like I said, no one knows what sully would have done with direct law on that day.

He was applying full back stick because he tought 9.5° of pitch angle with no increasing tendency was not enough.
But how much did he want ???

With direct law, he would have gotten exactly what he wanted, within the limits of the airplane and physics.
But he got limited by normal law.

That is for sure.


For the rest, you probably know it is possible to dive in order to get some speed and then to pull up to perform flare, many pilots do it on a light aircraft on a regular basis, there is no reason why it would be impossible on a large aircraft.

Even without going that far, most flares are transitory situations in which the runway height has a critical importance.
To be clearer, I mean that in a flare, speed is traded for a rate of descent reduction. So if the runway was 50 meters below, flaring at the orginal height would be.. 50 meters too high.

So it is clear that light or large aircraft, pilots have to fly transitory situations in which they have to precisely assess their height above the runway, so there is no reason why it would be impossible to land at any combination of pitch angle/vertical speed desired, on any aircraft. The only variable will be change in speed, but since the situation is transitory, it is not a problem to see one's speed change, as long as it doesn't do so for too long a time.
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Old 20th May 2017, 23:25
  #43 (permalink)  
 
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Looking at some old training material high AoA protection used to be deactivated below 100ftRA but a diagram in the FCOM dated 2012 shows that is now also available in flare mode. Flare mode becomes active 1second after 50ft Ra. I wonder what the bus would have done if he had used a handful of nose up trim to get the nose up?
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Old 21st May 2017, 00:52
  #44 (permalink)  
 
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For the rest, you probably know it is possible to dive in order to get some speed and then to pull up to perform flare, many pilots do it on a light aircraft on a regular basis, there is no reason why it would be impossible on a large aircraft.
Err no, all you do is end up going down faster for a minor increase in speed that won't help in the slightest during the recovery from the ensuing even steeper g-ravity descent towards earth (or water, as the case may be)...
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Old 21st May 2017, 09:06
  #45 (permalink)  
 
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Hi tubby linton,
I wonder what the bus would have done if he had used a handful of nose up trim to get the nose up?
In Normal or ALT Law, the elevator position is controlled by the FBW computers. I think if Sully had added stabiliser trim, then the elevators would have moved in the opposite sense in order to maintain the same Alpha.

You'd have to apply a lot of nose up trim to before you ran out of elevator authority.

Last edited by Goldenrivett; 21st May 2017 at 12:31. Reason: typo
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Old 21st May 2017, 10:29
  #46 (permalink)  
 
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Sully made it because the aircraft helped him to do so. Whatever the protections build-in the plane, there is nowhere in the design criterion to flight test the plane in ditching. Barely recommendations.
There was no casualties that day, not ONLY because of Sully, but because some actions were not accomplished and some decision were made. You are talking about 1° or 2 seconds of aft stick... When I see the videos about XWind landing in a FBW airplane, I am just thinking about the amount of luck ( a big bucket) these drivers are carrying.
I still think that if he had flown a conventional aircraft they would have ended in multiple parts in the Hudson.
But this is history, he made it, within some minutes. He is an history ( deserved or not)

Last edited by CL300; 21st May 2017 at 10:43.
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Old 21st May 2017, 12:05
  #47 (permalink)  
 
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Originally Posted by Capn Bloggs View Post
Err no, all you do is end up going down faster for a minor increase in speed that won't help in the slightest during the recovery from the ensuing even steeper g-ravity descent towards earth (or water, as the case may be)...
Get in an airplane before saying things like that.
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Old 21st May 2017, 14:41
  #48 (permalink)  
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Originally Posted by KayPam View Post
Based on your info that I was not aware of :
Alpha max is 17.5°, Sully touched with 9.5° of pitch and 750fpm which gives -3.5° of flight path angle (this gives an AOA of 13° instead of 14°, so this calculation is precise within 1°)
If there had been an AoA increase as demanded by Sully : full back stick and 17.5° of AoA : we would have had something between -2 and 0° of flight path angle, so pitch angle would have been somewhere between 15.5 and 17.5° : way too much (even with the 1° error margin, 14.5 is still 2.5° above the 12° limit you quoted)
Alpha doesn't stay constant during flare. Yes, in this case, he wanted to get closer to stall (not a stall, just closer). Flare consists of raising the nose and stopping at a pitch angle, in this case it would have been 11 or 12 degrees pitch. This is the ditching deck angle. Then holding it until impact.

Originally Posted by KayPam View Post
But on the other hand, maybe Sully was instinctively targeting 11-12° of pitch, and was pulling full back only because the aircraft was not responding.
Yes, thats clear from seeing him at full aft stick trying to get more pitch.

Originally Posted by tubby linton View Post
Looking at some old training material high AoA protection used to be deactivated below 100ftRA but a diagram in the FCOM dated 2012 shows that is now also available in flare mode. Flare mode becomes active 1second after 50ft Ra. I wonder what the bus would have done if he had used a handful of nose up trim to get the nose up?
Can you elaborate on that? It gets to the heart of this thread topic. Appreciate your succinct response, thanks. It makes sense to let alpha get all the way to stall, if necessary, at the start of the flare, since touchdown happens very soon after.

In the other A320 accidents people have brought up in this thread, it does appear alpha-protection was on during flare though.
Still, getting rid of alpha-protection isn't the way I'd do it as a flight control system engineer. I'd keep the avoidance of a deep stall, but simply allow a near-stall when close to the ground with a descent rate happening.


On a related topic, why didn't the copilot say "We are flying too slow." 3 minutes to touchdown, maybe even repeat that? Both guys freaking out?

Originally Posted by tubby linton View Post
Looking at some old training material high AoA protection used to be deactivated below 100ftRA but a diagram in the FCOM dated 2012 shows that is now also available in flare mode. Flare mode becomes active 1second after 50ft Ra.
I found the 2012 FCOM you mentioned. Alpha Protection is there for flare.
Now to find the older material you mentioned that omitted AoA protection below 100' (maybe an old manual error?).


The damping feedback nosed the aircraft down in flare, that is where it "popped up". The NTSB never faulted the damping term problems and I still don't agree the nose-lowering should have occurred so close to the ground. (We all can agree with the NTSB findings regarding low airspeeds, which caused flare to be flown in AoA Protection, yet the concept of AoA Protect isn't bad, its the additional phugoid feedback terms in flare that are puzzling.)

Originally Posted by misd-agin View Post
Vilas - we haven't been given exact airspeed but they might have been 5-6 kts above stall while misreading their airspeed. This occurred at 150'. At 100', while still unaware of how close to a stall they were and that the AOA protection was actively proteciting them from a stall, he attempts to raise the nose. I don't know of any pilot, knowing that they're perhaps 5-6 kts above stall speed, that would knowingly raise the nose at 100'. It might be a life altering experience. While FBW might not be perfect it was a significant factor in the survival rate.
I agree AoA Protect is a good thing. It can prevent disaster, no doubt.
In this pre-flare and flare case, Sully got it to the correct ditching pitch angle (11 degrees) 4 seconds before impact, and then the AoA Protection unwanted damping terms lowered the nose to 9.5 degrees. Survivable, but not best. Holding the 11 degrees would not have stalled the aircraft since Alpha Max was still high enough. See the actual trace:
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Old 21st May 2017, 15:56
  #49 (permalink)  
 
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Vilas - we haven't been given exact airspeed but they might have been 5-6 kts above stall while misreading their airspeed. This occurred at 150'. At 100', while still unaware of how close to a stall they were and that the AOA protection was actively proteciting them from a stall, he attempts to raise the nose. I don't know of any pilot, knowing that they're perhaps 5-6 kts above stall speed, that would knowingly raise the nose at 100'. It might be a life altering experience. While FBW might not be perfect it was a significant factor in the survival rate.
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Old 21st May 2017, 16:05
  #50 (permalink)  
 
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Retard

The flight computer will always work as programmed, but in some cases it’s just a bitch that have to have the last word.
The last communications recorded show the differences about situational awareness of the pilot vs the flight computer… the thrust was retarded already, since the engines ingested the birds.
HOT-1 got any ideas?
EGPWS caution terrain.
CAM-2 actually not.
EGPWS terrain terrain. pull up. pull up.
HOT-1 we're gonna brace.
HOT-2 * * switch?
HOT-1 yes.
GPWS (fifty or thirty)
FWC - RETARD
[End of Recording]
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Old 21st May 2017, 16:21
  #51 (permalink)  
 
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Below is the clear cut conclusion by NTSB. I don't know from where suddenly the phugoid damping has popped up. Had the speed been correctly maintained the AOA would have remained outside the alpha prot and better flare would have resulted.
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.)
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Old 21st May 2017, 18:15
  #52 (permalink)  
 
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However, if the speed had been higher, that would have meant more energy to dissipate hence more damage.
Maybe Sully knew exactly what he was doing, and a very cautious computer did not allow him the 3.5° of AOA that he required.
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Old 21st May 2017, 19:33
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This is absurd. Your theories are opposite of NTSB. The damage was caused by the high rate of descent due to very low speed below 100ft which could not be arrested. If speed was correct proper flare was possible resulting in less ROD. If protection was not there speed would have continue to drop further and at stall warning the push to prevent stall would have increased the rate of descent even more causing more damage.
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Old 21st May 2017, 20:04
  #54 (permalink)  
 
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tubby
I wonder what the bus would have done if he had used a handful of nose up trim to get the nose up?
Golden
In Normal or ALT Law, the elevator position is controlled by the FBW computers. I think if Sully had added stabiliser trim, then the elevators would have moved in the opposite sense in order to maintain the same Alpha.
I am afraid none of that is going to happen. See below:
DSC-27-20-10-20 P 1/8


When angle-of-attack protection is active, the THS setting is limited between the setting at the aircraft’s entry into this protection and 3.5 ° nose down. (Neither the pilot nor the system can apply additional nose-up trim).
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Old 21st May 2017, 22:19
  #55 (permalink)  
 
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Hi vilas,

Thanks for the correction.
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Old 21st May 2017, 22:45
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An interesting point Vilas and thank you for providing the reference.
At 50ft RA the aircraft would have transitioned to flare mode but he was not over a hard. surface such as a runway but the river.
Did the radalt provide accurate data over the river?
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Old 22nd May 2017, 00:21
  #57 (permalink)  

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I am inclined to believe that RA's - if functional - are as effective over water as over any other surface.
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Old 22nd May 2017, 00:23
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I am inclined to believe that RA's - if functional - are as effective over water as over any other surface
They are, as use in the offshore oil helo industry proves.
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Old 22nd May 2017, 05:23
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It seems to me that because of the low speed Sully was lucky to have been able to achieve the flare that he did - it was at the edge of having enough manoeuvre capability.
This thread started with the claim being made that it was all caused by the phugoid damping term in the flight control laws. I have no idea of the laws and transfer functions involved in the alphprot law, but would guess that phugoid damping would be a very low order input compared with, say, alpha limit, alpha dot, pitch rate and pitch rate dot for example.
Bearing in mind that the phugoid is a very long period oscillation that is so insignificant to the handling of most aircraft types it is not specifically tested during aircraft certification, unlike the dutch roll.
It is probably fair to say that the vast majority of pilots would have no idea of the phugoid modes of the aircraft they fly, particularly the period and damping ratio of the oscillation.
Perhaps some one can tell me how phugoid damping is important in this accident but it is not during general handling of the A320? Try to provoke the phugoid in an A320 in normal law - it is not possible - I don't know if a phugiod damping term is responsible for this or things like G command, pitch rate demand, SPO damping, and probably a myriad of other inputs to the normal law are.
I don't hear too many A320 pilots complaining about poor handling characteristics in pitch, normal law, because of the very heavily damped phugoid.
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Old 22nd May 2017, 06:05
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Agree zzuf. This is a weird thread. I am not going to criticize Sully, he did what he did and he and his pax swam away. Yes his airspeed was way, way, way too slow the last 150 feet. But, as the accident report explains, there are human factor explanations for this critically low speed. So no criticism. But you cannot defend flying 17 knots below VLS at 150'. Not in any airliner.

So those here who argue that flying way below VLS (or way below Vapp for that matter) and still expect a normal flare have not understood anything about flying ANY airliner. Do that at 150' on any other airliner and you will stall it in the moment you try a flare. If you want minimum speed on impact, there are other ways to achieve that, not by stalling at 150'.

So Airbus saves the day by preventing a stall at 150', yet gets criticized for not making an exception for Sully and giving him an extra degree or two of flare. In a car that's arguing that the airbag obstructs your vision during a head on collision.

Last edited by PENKO; 22nd May 2017 at 06:51.
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