Sully's Flare on the Hudson: Airbus Phugoid Feedback

[QuagmireAirlines]

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?:ugh:
(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.):rolleyes: 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/AccidentReports/Reports/AAR1003.pdf
http://www.exosphere3d.com/pubwww/pdf/flight_1549/ntsb_docket/431658.pdf

[misd-agin]

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 (http://www.cbsnews.com/news/ntsb-sully-could-have-made-it-back-to-laguardia/)

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.

[aterpster]

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?

[megan]

Why was Airbus not admonished for their flight control laws fighting Sully's pitch up flare commandsNot a unique event. A-320-214, EC-HKJ, Bilbao Airport, 7 February 2001

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/rdonlyres/8B514392-B79A-46DC-A7C8-DC1BA137D076/23171/2001_006_A_ENG1.pdf

[45989]

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

[Denti]

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.

[vilas]

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.

[Goldenrivett]

Hi vilas,
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 (http://www.aviation-accidents.net/garuda-indonesia-boeing-b737-300-pk-gwa-flight-ga421/)

[CONF iture]

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.

[vilas]

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.

[CONF iture]

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.

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.

[vilas]

CONF
If you hear the video recording at 3:14 you will notice NTSB is not laughing with you, may be at you.
What a joke ... Sully did not start the APU to remain in normal law


https://youtu.be/ZaV0godxu3w

[vilas]

CONF
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?

[misd-agin]

How do you know that to be so?

From direct conversation. They start quoting the movie 'facts' versus reading the official investigation.

[misd-agin]

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.

[QuagmireAirlines]

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.

Not a unique event. A-320-214, EC-HKJ, Bilbao Airport, 7 February 2001 https://www.fomento.gob.es/NR/rdonlyres/8B514392-B79A-46DC-A7C8-DC1BA137D076/23171/2001_006_A_ENG1.pdf
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.

[Amadis of Gaul]

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?:ugh:


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...

[QuagmireAirlines]

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.

[CONF iture]

So, vilas, where is it that quote from Sully :
"I choose to start the APU to remain in Norma law" ?

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 ...

[misd-agin]

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

[albatross]

Probably not relevant here but
Years ago 1988 there a crash of an airbus at an airshow in France (flew into trees off the end of the runway) where the automation was called into question.

https://youtu.be/I9gELPxPG8Q

https://en.m.wikipedia.org/wiki/Air_France_Flight_296

[Amadis of Gaul]

You don't say?

[albatross]

Well I only mentioned it because as I recall the Captain maintained at one point that the aircraft automation would not let him pull the nose up any further ..

[vilas]

CONF iture
where is it that quote from Sully :
"I choose to start the APU to remain in Norma law" ?Now that I provided NTSB statement in this regard how about you providing Sully's statement, not your opinion about 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.?I suggest you read about G-YMMM B777 London crash.The last moments of Hudson landing are very similar to this B777 crash where ice crystals restricted thrust in last 500ft of the approach. There speed dropped to stretch the glide. As the B777 reached the stall warning speed when the pilot pushed the control forward it landed short at VS of 1400ft/mt. As compared to this at Hudson airbus touched down at 750ft/mt. because the normal law alpha prot prevented further drop in speed as would have happened in direct law.

[vmandr]

@QuagmireAirlines

why the NTSB didn't cite ...

I take what government agencies publish with a pinch of salt.
Their Legal affairs ensure (most of the time) there is no liabilities,
conflict of interests etc. from those publications.
after all, you would not bite the hand that feeds you right?

@albatros

the aircraft automation would not let him

Yes. They train you to fly the plane and do not train you to fly ... the software :p flight=dynamic, software=static/canned, go figure...
(I am sure some 'tourist', will hop-in shortly to correct me :p )

[KayPam]

Well I only mentioned it because as I recall the Captain maintained at one point that the aircraft automation would not let him pull the nose up any further ..

This is very relevant.
If one reads the complete report of the french BEA, there are clear inconsistencies.

Inconsistencies that are very clear, even for a non-specialist. I just had to put together the inconsistent part, found within the whole document.

I pointed them out in a document that I posted on a french forum.
I could provide a quick translation.
There are strong suspicions of the French authorities camouflaging a flight control law problem on the A320, in order not to compromise its commercial carreer. In which they succeeded.

Is anyone interested in looking into my findings ?


What is the matter at hand with this Sully flare ?
Was the AOA (about 14°) at touchdown inferior to what it should have been (alpha max for CONF 2, whose value I don't know by heart) ??

[vmandr]

@Κaypam

the complete report of the french BEA, there are clear inconsistencies.


thanks for meeting my point and I do not doubt your findings.
as they say 'you don't bite the hand that feeds you' :p
meaning, if they want they can really be precise and consistent
but in doing so, there will be other - political / commercial - consequences
regarding certification, oversight etc. So the lesser of two evils....

as for 'the matter at hand' to me is clear some are trying to discredit the man. a shame !

[QuagmireAirlines]

What is the matter at hand with this Sully flare ?
Was the AOA (about 14°) at touchdown inferior to what it should have been (alpha max for CONF 2, whose value I don't know by heart) ??
I think Sully should have been able to get that very last 3.5 degrees (stall margin) of pitch he was asking for (by holding full aft stick).
I have no problem with the concept of alpha-protection mode, yet the execution of it below 100' AGL (radar altimeter) should not have included phugoid-dampening feedback terms, the items to blame here, and is what kept Sully from pitching a tad more in flare.
Keep in mind Sully hit the water at 9.5 degrees pitch, and 11 would have been considered ideal, so he was only off by a little, and that last 2 or 3 degrees of pitch denied him would have lowered his touchdown impact (feet/min vertical speed when hitting).

Probably not relevant here but
Years ago 1988 there a crash of an airbus at an airshow in France (flew into trees off the end of the runway) where the automation was called into question.
https://youtu.be/I9gELPxPG8Q ..https://en.m.wikipedia.org/wiki/Air_France_Flight_296

I thought of that one too this morning. I'd almost forgotten about it. I can remember the old 1988 video of it at the Paris Air Show. That should have been a warning to Airbus to examine what was going on at very very low altitudes (100 feet and below) to allow the pilot to really use every last stall margin possible.

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.

OK, agreeing he was too low on airspeed during the descent. I'm a flight control engineer, which makes me conclude I'm just glad he was flying and not stalling! Sure an optimal speed which happens to give max(L/D) at whatever weight they had would be great, giving more pitch room to flare too. Yet, getting airspeed as low as possible is laudable, which he did, and he did have 3.5 degrees of actual stall margin left, so did he really mess it up that bad????

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.
As I've said, alpha-protection is a good thing above 100 feet, just not at 100' AGL and below when the phugoid damping feedback was preventing full use of max lift from flare to impact.

This is very relevant.
I pointed them out in a document that I posted on a french forum.
I could provide a quick translation.
Is anyone interested in looking into my findings ?
I'd look into it, from a flight control engineer's point of view, not a pilot. Although I've flown gliders, and, coincidentally, that time Sully was a glider pilot too... :hmm:
Can you post an English translation in a pdf file or something?

[KayPam]

Having had a look at the flight data plots, it is quite obvious Sully wanted to flare more.

However, the flight control laws having obviously not designed to accomodate an engine-out, flaps 2, gear-up water landing, they were not optimized for this case.

So it is very possible that direct law in this kind of context would allow for better landing than normal law - that is if the pilot is competent, of course.

I don't see how you would discredit Sully. What more should he have done ? A smoother landing ? Let us laugh.

But hey guess what, it so happens that the Habsheim pilots who crashed their airplane in a forest were in conf 2 as well ! So we can just take the alpha max value from the report : 17.5°
But alpha in our case (Sully) was about 14°, whereas full back stick order had been maintained for 2 seconds.
Just as a reminder, 14.5° is alpha prot (still from the report) and with stick neutral in alpha protection, alpha prot is targeted.
So basically, Sully had been pulling for 8 seconds but that did not give him any more than alpha prot. An AOA that he had at the beginning of his nose up order.
An aircraft not responding to your flare order, even if it is at a low speed, is obviously a problem.*
I will investigate this a bit more, later.

However, it should be noted that had the aircraft responded to the nose up order, it would have impacted the water with a higher pitch up angle and a lower vertical speed. Nobody can say for sure if the damages would have been worse or not.

[Winnerhofer]

As the alpha-protection was triggered during this event, the system commanded a nose down signal, which was performed, even though both pilots had their sticks full backward, commanding a “climb”.

[QuagmireAirlines]

However, it should be noted that had the aircraft responded to the nose up order, it would have impacted the water with a higher pitch up angle and a lower vertical speed. Nobody can say for sure if the damages would have been worse or not.

There was an A320 water ditching study done (I lost the link for it, I'll see if I can find it again later) that said 10 to 12 degrees pitch angle is OK. (Sully hit at 9.5 deg pitch, so not bad, only a tad low.) Airbus also recommended strongly that you hit the water with a pitch angle of 11 degrees. This has been looked at, and I think you essentially want to do a 3-point landing on the 2 engine nacelles and tip of the tail at the same time.

If Sully could have used that last bit of 3.5 deg stall margin (pitch up) that the Airbus control laws denied him during the last 50 feet or so, he would have hit at about 12 degrees, with a lower vertical speed, all better.

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.

[QuagmireAirlines]

Well I only mentioned it because as I recall the Captain maintained at one point that the aircraft automation would not let him pull the nose up any further ..
About that 1988 Paris Air Show accident, did the pilot have any stall margin remaining? I haven't read anything about that accident in a long while. I was in the middle of control law development for some MD-11 modes at the time it happened.

[CONF iture]

Now that I provided NTSB statement in this regard how about you providing Sully's statement, not your opinion about
My opinion is not different from any pilot who lose one or both engines on a twin, starting the APU to ensure a primary source for electrical power - Not only it is a request from the checklist, but it is just common sense. Don't tell me now that a guy a 737 does it to maintain the flight envelope protections ...

[vilas]

CONF iture
You Herd the NTSB statement so it's not a joke. It was NTSB"s opinion based on surely some corroborating evidence. I didn't say that was the only purpose. Your opinions generally contain ample amount of visceral hatred of some aircraft or equipment so they are not necessarily like any other pilot. So we leave it at that. Now tell me your opinion about the second part, the comparison of dropping speed in the London B777 accident without protection and the result.

[KayPam]

There was an A320 water ditching study done (I lost the link for it, I'll see if I can find it again later) that said 10 to 12 degrees pitch angle is OK. (Sully hit at 9.5 deg pitch, so not bad, only a tad low.) Airbus also recommended strongly that you hit the water with a pitch angle of 11 degrees. This has been looked at, and I think you essentially want to do a 3-point landing on the 2 engine nacelles and tip of the tail at the same time.

If Sully could have used that last bit of 3.5 deg stall margin (pitch up) that the Airbus control laws denied him during the last 50 feet or so, he would have hit at about 12 degrees, with a lower vertical speed, all better.

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.
Okay there has been some studying and simulation after the cactus 1549 accident-landing.

(But at the time of this water landing, these studies had not been done)

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)

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.

So you can see that the situation is not very straightforward and yes, the optimum pitch angle for ditching has been studied but it's very hard to say what the situation would have been with a direct law instead of a normal law.
About that 1988 Paris Air Show accident, did the pilot have any stall margin remaining? I haven't read anything about that accident in a long while. I was in the middle of control law development for some MD-11 modes at the time it happened.

Yes, that's an inconsistency of the report itself !
Alpha max is quoted at -17.5°. The report itself says that if the pilot pulls full back then they will get alpha max. The report says that the pilot pulled full back.
But the report also shows, in the flight data tables, that aoa never went above 15°.

So there was a +2.5° margin relative to alpha max !
That's a problem in itself.

I believe that had the aircraft responded correctly to the full back stick input, maybe the gears would have caught some leaves but the aircraft would have had a much better chance of going around "correctly" and remain in flight.
Because there was a slight reserve of speed but that could have been enough to go above the trees, and once the aircraft was above the trees, the engines had spooled up.

Any one with a bit of physics knowledge could go into the flight data tables and read the values. Then :
- compute V alpha max
- compare V alpha max with the speed before impact
- conclude if this speed difference converted into height could have been enough to go around the trees. (based on approximate estimation of the fuselage's height under the tree line)

These reports are very carefully written, toughly proofread and thoroughly revised. So any inconsistency can be regarded as highly suspicious.
Best case scenario explanation : it was written and proofread in a hurry.

[Amadis of Gaul]

I'm starting to get the sense that the whole purpose of this thread is to make sure that as many people as possible are aware that the OP knows what a phugoid is.

[PDR1]

...or perhaps doesn't.

But in relating the two accidents I'd suggest people are conflating two unrelated issues.

In the Sully case we're talking about a flare - converting airspeed into a brief burst of lift to reduce the vertical and horizontal speed on "touchdown". It may well be true that taking a further couple of degrees of AoA could have produced a different combination of horizontal/vertical velocity and attitude at touchdown. It is by no means certain that this combination would have been any better or worse than the combination that he actually used. Given that the combination used resulted in everyone getting off the aeroplane it's hard to make a case that a higher AoA would have been "better". It's quite possible that the higher AoA would have dragged the tail in the water and ruptured the fuselage, allowing it to flood MUCH quicker. But It digress - that's the Sully case.

In the OTHER case the aeroplane had entered the manoeuvre with excess speed and then slowed by retarding the power, so when the alpha-floor-limit manoeuvre was initiated the engines were running at a much lower power level, and so took more time to spool up than was available. The insufficient climb rate was due to a lack of THRUST, not a lack of LIFT. Further increases in AoA would almost certainly have had no effect on the rate of climb because the associated increase in drag would soak up the increasing thrust and just delay the achievement of PRoC. It might change the fuselage attitude, but it would be unlikely to change the flightpath.

The two situations are completely different.

[KayPam]

The common point between those two accidents is that alpha max was not reached, even with full back stick, despite airbus claiming that alpha max will be reached if full back stick is applied.

The airplane should react like it is supposed to.

So that aircrews can know what to expect.

[CONF iture]

You Herd the NTSB statement so it's not a joke. It was NTSB"s opinion based on surely some corroborating evidence.
Where is that corroborating evidence then ... ?
There is a hype put on those protections in this case that is simply not necessary.
And what allows you to state that Sully would have stalled without protection ... ?
Truth is that Sully would have most probably obtained a better touchdown with direct law.

The threshold at LHR is not much moveable, on the Hudson Sully could place it at his willing which helps a lot when it's time to flare, as long as the protections don't interfere ...

[misd-agin]

It's unknown how Sully would have performed without FBW protection. He significantly misread his airspeed. Why are we to believe that if he wasn't in a FBW a/c that he'd have read his airspeed better? If he'd flown a non FBW a/c, that didn't have AOA protection, he'd have triggered he stick shaker, and perhaps pusher, unless he'd lowered he nose and rudeced the AOA. At 150', or less, that would have generated an increased rate of descent that we can only speculate as to if he would have had enough airspeed, and AOA available, to reduce the increased sink rate prior to touchdown. IMO it would have been a much more difficult situation to handle.

Touchdown was at less than the desired pitch attitude of 11 degrees. But Airbus also didn't recommend reaching the pitch limited AOA at 150' either. If normal speeds had been maintained during the final segment of the glide there would have been enough airspeed to raise the nose to achieve more than 9.5% NU.

Learning point - slight excess airspeed might be a goal for a water ditching. It gives you the opportunity to choose to touchdown point with a slight degree of variability due to wave patterns or sea state. Wiith low airspeed/high AOA you basically have only one touchdown point.

[vilas]

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.

[KayPam]

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.

[tubby linton]

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?

[Capn Bloggs]

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)...

[Goldenrivett]

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.

[CL300]

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)

[KayPam]

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.

[QuagmireAirlines]

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.

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.

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?

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?).
http://i.imgur.com/0HfyFAg.jpg

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.)

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:
http://i.imgur.com/ele8QyS.jpg

[misd-agin]

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.

[_Phoenix]

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]

[vilas]

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.)

[KayPam]

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.

[vilas]

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.

[vilas]

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).

[Goldenrivett]

Hi vilas,

Thanks for the correction.

[tubby linton]

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?

[FlightDetent]

I am inclined to believe that RA's - if functional - are as effective over water as over any other surface.

[megan]

I am inclined to believe that RA's - if functional - are as effective over water as over any other surfaceThey are, as use in the offshore oil helo industry proves.

[zzuf]

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.

[PENKO]

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.

[PENKO]

What some people probably don't understand is that alpha-protect is not a normal flying state to be in. It is unthinkable to intentionally enter alpha-protect during an approach to land or ditch. Alpha-protect is a last ditch effort (by the aircraft and no pun intended!) to prevent a stall.

KayPam, Sully did not 'exactly know what he was doing'. He thought he was flying above VLS, i.e. above normal safe approach speed. But in fact he was far, far, very unsafely far below. Read the report. Again, no criticism, just facts.

[zzuf]

Penko, I guess I am with you.
Lack of good accurate speed control suggests to me that the crew were close to overloaded by the circumstances.
Those who suggest that reversion to direct law may have produced a better outcome are really suggesting that, after configuring to direct law the pilot is now capable of flying the aircraft beyond the buffet boundary, at the lift boundary, at the same time avoiding the stall boundary. Something he would have to learn in a one off situation at 150 feet on a power off ditching approach.
I would be somewhat surprised if any pilots, other that Airbus test pilots, would have a clue how close to the stall boundary they are flying, having penetrated the buffet boundary. It is not in the airline pilots job description - nor should it be.
I can make no comment on whether there was sufficient energy remaining for any significant ROD reduction following the direct law scenario. Without knowledge of the characteristics of the CL/Alpha curve it is a big ask to assume any significant amount of extra lift was available by increasing Alpha by a couple of degrees.
Full marks to Sully for making a wise decision to ditch, he was ably supported by Airbus and those who certificated the product, a great result from a most unenviable situation.

[Smilin_Ed]

All of this seems to point to using Angle of Attack in this situation.

[QuagmireAirlines]

No question that Sully was wrong to fly the approach way to slow. The NTSB report and pilots here know that. That said, he had a little pitch-up room i flare which was denied. Next time, or when other landing events trigger alpha-protect as we've seen in the past, is when changes will benefit passengers.

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.
Sully still had 3.5 degrees of pitch before alpha-max, and actual stall doesn't even happen until CL-max at around 5 degrees more than his A320 got. Remember, while close to the ground, with a descent rate, going right to stall in those final 2 seconds would be fine right before impact. Right to the edge, maybe even hitting CL-max and ground effect at the same time, knowing the water was very close. IF Sully would have been able to get 11 or 12 degrees pitch in flare like he was asking for, his descent rate would have been less, and his deck angle would be right at ideal for water impact.
From the Aircraft Performance Study document:
---"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. "
http://i.imgur.com/MsWRf0u.jpg

This thread started with the claim being made that it was all caused by the phugoid damping term in the flight control laws. Straight from the Aircraft Performance Study:
----"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. "

Perhaps some one can tell me how phugoid damping is important in this accident but it is not during general handling of the A320? That damping doesn't belong there so close to the ground in flare. It lowered the nose a bit when he needed a bit of nose-up. Above about 100 ft AGL (RA), its good to have the damping. The damping is there in alpha-protection "general handling" and usually pilots aren't aware of it or almost never use it, which is OK. Its only there in general handling to help out in low airspeed situations.

[vilas]

The aircraft was flown outside the flight envelope because of human limitations under the circumstances and the aircraft behaved as designed. So where is the problem? The speed was dropping all along and without thrust that increases ROD because higher AOA causes more drag. Were it not so the VLS will have no meaning. Trying to pick and choose to connect dots is a waste of time. Another day he may have deactivated the GPWS and would have got speed, speed warning which would have helped him maintain the speed, yet another day he may not have started the APU and reached stall warning at 100ft and pushing the stick forward would have been worse. You try it in SIM in alternate law at 150ft get your speed to stall warning, push the stick and land you will need to stand more than a drink to your buddies.

[FCeng84]

There seems to be quite a bit of discussion here about where this airplane was with respect to lower speed limits. I wonder what is known about the relationship between speed across the water and chances of surviving a water landing? When landing on a paved runway (with or without gear down) touchdown sink rate is more important than ground speed. When landing on water, speed across the water might be more important. I would have thought Sully's desire would have been to get as slow as possible short of stalling to reduce the kinetic energy that would be released in short order once on the water. To that extent, Sully might have wanted to be able to command a few more degrees nose up pitch attitude to further reduce speed at touchdown.

[QuagmireAirlines]

As a flight control engineer, what do you think about the phugoid damping terms active below 100' AGL radar altimeter? That left Sully with not enough pitch angle, a higher rate of descent, and a higher speed.
A ditching study on the A320 said 11 or 12 theta is fine, a 3-point landing on the water. Of course, too much speed can rip the nacelles off. Too much rate of descent can crack the fuselage.

[PENKO]

Read the report, Sully did not want to be as slow as possible, he did not want any of the things you try to ascribe him with.
(...) 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.

Sully thought he was above VLS. 'Safely' above VLS. Sully even declined the FO's offer for more flaps (lower stall speed!) arguing it won't make much difference.

During postaccident interviews, the captain stated that he used flaps 2 because there were “operational advantages to using flaps 2.” He stated that using flaps 3 would not have lowered the stall speed significantly and would have increased the drag. He stated that he was concerned about having enough energy to successfully flare the airplane and reduce the descent ratesufficiently. He stated that, from his experience, using flaps 2 provides a slightly higher nose attitude and that he felt that, in the accident situation, flaps 2 was the optimum setting.

Again, I am not criticizing Sully at all, just disputing some of the more miraculous claims on this thread.

[PENKO]

Quagmire, again you are putting the cart in front of the horse. Sully did not get those few extra degrees from his Airbus because the Airbus was doing its best not to stall due to Sully's control inputs. You can't first run out of speed, forcing the aircraft in to protect mode and then still expect the aircraft to give you a few extra degrees of pitch for free. A 737 would stall and crash.

The only interesting part in this debate/investigation is that Sully was not aware of the fact that he was in alpha protect. He was not aware how critically low his speed was, probably due to the human factor issues of Ditching in the Hudson. That is something to take away with from this ditching! It could happen to anyone.

[QuagmireAirlines]

You keep missing the fact that he still had 3.5 degrees of stall margin remaining at the same time he held full aft stick. And CLmax is at a little higher alpha than AlphaMax.

Nobody disputes that Sully should have had more airspeed. The fact is he didn't, OK, pilot error, we got it. Enough already: Not the topic here. ... The fact is automation saw fit to deny just enough remaining alpha to get the plane to a better deck angle for ditching. That also would mean a lower impact speed could have been obtained. Look into the other several A320 accidents in the past where alpha protection in flare occurred (mentioned in this thread to show the history).

I'd like to find out who thinks its a good idea (like Airbus does) to allow the fly-by-wire to give nose down commands when there is still margin to stall in flare? And what possible reason would there be to limit max lift that way down low in flare?

Again, I do think Airbus's alpha protection is fine above flare altitudes, just not below around 100 feet.

[error_401]

I've not been able to find the FDR diagrams.

I would like to see the RA vs. Speed vs. Sidestick input.

Just a thought...

[KayPam]

Quagmire, again you are putting the cart in front of the horse. Sully did not get those few extra degrees from his Airbus because the Airbus was doing its best not to stall due to Sully's control inputs. You can't first run out of speed, forcing the aircraft in to protect mode and then still expect the aircraft to give you a few extra degrees of pitch for free. A 737 would stall and crash.

The only interesting part in this debate/investigation is that Sully was not aware of the fact that he was in alpha protect. He was not aware how critically low his speed was, probably due to the human factor issues of Ditching in the Hudson. That is something to take away with from this ditching! It could happen to anyone.

That's just not true.
alpha max is 17.5 in conf 2.
The max angle of attack that was reached was 15°.
Alpha max in itself includes a margin as well.

So there was more than enough margin.

Sully was probably aware of how low his speed was, thanks to the appropriate information on the PFD (speed trip with orange/red colouring), but maybe he actually wanted to have a forward speed as low as possible.

[Vessbot]

Sully was probably aware of how low his speed was, thanks to the appropriate information on the PFD (speed trip with orange/red colouring), but maybe he actually wanted to have a forward speed as low as possible.

Christ. This was addressed in the first reply in this thread, with a quote from the official accident report, and then referred to a few more times.

He was not aware how low his speed was, and he did not want what he had. (He wanted what he erroneously thought he had)

"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."

[QuagmireAirlines]

Wait a second. I just found something buried on page 194 of the final NTSB Accident Report appendix. Mini-scandal. Yellow alert. Call the Lone Gunmen of the X-Files.
http://cdn.collider.com/wp-content/uploads/2015/07/the-x-files-david-duchovny-lone-gunmen.jpg

Starting on Page 194 (appendix area) of the NTSB Report, it shows the BEA (Bureau d’Enquetes et d’Analyses’) requested the NTSB reverse their initial indictment of the phugoid damping in flare, saving Airbus from criticism. And the NTSB complied. Not if I was there. Must have upset a least a couple of NTSB pilots or flight control engineers on the team.

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.

Then the BEA makes a strange remark: "In reality, phugoid oscillations induce pitch variations that can have more severe consequences than a high vertical speed when entering the water."
Except for the fact that the flight is over way, way, before any phugoid oscillation can develop. Its flare for pete's sake. The BEA made a valid case for higher AGL use of the damping terms, but not for close to the water or ground during descent at flare.

[FlightDetent]

The captain declared he was well above the speed. The data trace indicate the speed was significantly below any meaningful target. (more than 15 knots, that is a LONG distance on the speedtape)

KayPam, how do you condense the two statements above into "was probably aware"???

[zzuf]

All this chat about alpha margin is totally irrelevant without knowledge of the extra manoeuvre capability to be gained by an alpha increase. We all know that the speed was way below a safe speed and I would guess that all the swept wing jet pilots here would know that any similar aircraft would have extremely limited manoeuvre capability at this speed.
Without a total CL/Alpha curve for the aircraft on the day (and total drag curve) most of this discussion is speculation. The only thing that really matters to the pilot is how much extra G he can pull without making the situation even worse.

With reference to the phugoid damping:
1. In alpha protect Airbus found that that phugoid could be excited, at an alpha=alpha protect (phugoid is essentially a constant alpha oscillation).
2. For whatever reason, Airbus decided this characteristic was unacceptable and included a damping term in the flight control laws.
3. While, generally speaking, the phugoid is a non event because there wlll be adequate longitudinal control authority to stop the oscillation during either the pitch up or pitch down phase.
4. However, if you put in a hard controllability limit (like alpha protect) the pilot will be unable to stop the oscillation during the pitch down phase - he is denied the ability to increase alpha by the control laws.
5. Best to allow the flight control protection laws (but not the pilot) a little extra authority in terms of alpha capabilty so this oscillation can be prevented.
6. It has been suggested to give the pilot a "little more authority" to be able to demand a slightly higher alpha by removing the phugoid damping term. Seem to me this just moves the phugoid to a higher alpha and in no way guarantees that the pilot will be able to stop the phugoid if it triggers in the nose down mode, at the new alpha protect, when on final approach to his ditching.
7. Seems to me that if you think alpha protect is a good idea, but you don't wish to damp the phugoid you must accept that the nose down pitch phase will not be controllable.
8. If you give the pilot sufficient authority to stop the phugoid at any time, by definition you no longer have alpha protect.

[QuagmireAirlines]

4. However, if you put in a hard controllability limit (like alpha protect) the pilot will be unable to stop the oscillation during the pitch down phase - he is denied the ability to increase alpha by the control laws.
Again, ground or water impact occurs before any oscillations can even begin to develop. Thats why its all good to have Airbus's alpha protection ABOVE flare AGL. Not below. The NTSB initially said it.

I've not been able to find the FDR diagrams.
I would like to see the RA vs. Speed vs. Sidestick input.
Just a thought...http://www.exosphere3d.com/pubwww/pd...ket/431658.pdf has them at the end. See Figure 14b, and then altitude plots are before that.

[zzuf]

QuagmireAirlines
I would be interested in how you would intend to wash out the phugoid damping term, from the alpha protect law, at low altitude, over a very short time interval, while there is a good chance that the side stick is being held fully aft, and not create even worse handling problems. Seems like it could be a fun day in the engineering sim.
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.
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.

[zzuf]

Quote Airbus response to NTSB

There are feedbacks within the AoA protection law aiming at damping the phugoid mode (low frequency mode). Without these feedbacks, an aircraft upset from its stabilized flight point up to constant high AoA would enter a phugoid (which is, by definition, a constant AoA oscillation) without possibility to stabilize the trajectory. As a consequence, commanded AoA is modulated: for instance, if aircraft speed is decreasing and/or pitch attitude is increasing, pilot's commanded AoA is lowered in order to avoid such a situation to degrade.
Trying to run simulation without such damping features on the very last seconds of the flight, without considering what could have been the effect such features brought upstream during the flight on the overall Aircraft trajectory and management by the crew would be pure speculation, as not supported by technical facts.
On the last 10 sec in the air of Flight 1549 , DFDR data show that pitch attitude is increasing and CAS decreasing. 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.
......
..... with a loss of engine thrust, as in Flight 1549, the aircraft energy management significantly increases the pilot workload. Under these circumstances, aircraft is still able to reach the optimum water impact configuration, but this is a demanding task which requires time and significant pilot focus. Typically, the flare initiation height will be critical to the achievement of the optimum water entry conditions.

Would appear that Airbus have no enthusiasm for the "turn off" phugoid damping just prior to the flare scenario.

[vilas]

Phugoid basically are pitch oscillations at high AOA. Manually trying to control that when they are rapid usually will lead to being out phase and aggravating it. That is why they are done automatically something like yaw damping. Pilot has no business to enter that regime willingly. In 447 the aircraft was flown into flight envelope regime never experienced even by a test pilot and normal pilots and non pilots kept discussing/are still discussing modifications to recover from that regime without any data assigning blame to the design. AF447, Hudson is done and dusted no one in authority is going take any notice about the same issues over and over again.

[QuagmireAirlines]

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.
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.


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.

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.
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.
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. :p

[PENKO]

Quagmire, would you do that in a 737? Fly through the stick shaker at 150', mushing it in close to CLmax as you say?

[stator vane]

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...

[PENKO]

Who knows. The stick shaker would have gone off for a start. At 150' that is not something you recover from easily.

[Amadis of Gaul]

---- 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.

[zzuf]

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.

[Amadis of Gaul]

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?

[Whoo]

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

[Owain Glyndwr]

@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!

[zzuf]

@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.

[QuagmireAirlines]

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.

@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!!!!:cool:

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? :p

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.

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.

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.

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.

[CONF iture]

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

[vilas]

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.

[misd-agin]

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]

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.

[vilas]

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.

[T28B]

If I may encourage all involved:
Play the ball, not the man
We can all disagree without being disagreeable.

[Owain Glyndwr]

@T29B
OK, accepted,, I have deleted the offending post

[megan]

Everything would really have been in place if he had maintained more airspeedNot 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?

[CONF iture]

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.

[Owain Glyndwr]

@Quagmire

You will note that I have removed the post that offended you, but I would say that the private comments I received were supportive.
At various times in your campaign you invited alternative views from those who you described as "Airbus apologists". Apart from the fact that I feel no need to aplogise for a very successful aircraft with one of the best safety records in the world, I am disappointed and illuminated by the fact that when such views are posted they are airily dismissed as "nonsense" or "boring"
Disappointed because on Tech Log one expects, or at least hopes, to read reasoned technical argument and counter-argument; illuminated because arbitrary rejection of opposing views without discussion is often the refuge of those who have no viable arguments of rebuttal.
But I long ago recognised the futility of arguing with closed minds, so for me the best option will be to let this thread die from neglect.

[vilas]

Objections many a times are visceral not technical making any discussion waste of time.

[john_tullamarine]

You put a catastrophic scenarios where there is none.

Oh ? I might have thought that passing 150ft at 1200ft/min approximated something a lot closer to mischief than most airline pilots routinely have to manage .. and, this, without any thrust to help out ?

By all accounts, declarer appears to have made a pretty reasonable job of the bad hands dealt to them on the day.

[PENKO]

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.
Are you absolutely sure? From the report: 'Airbus performed a simulation of the last 300 feet of the accident flight, which indicated that the airplane was performing as designed and was in alpha-protection mode from 150 feet to touchdown.'



Honestly answer us please CONFiture, what result do you expect in a 737 with an average pilot:
-Rate of descent 1200 fpm, double the normal.
-SpeedVLS -17, that's more or less Vapp-17 in Boeing terms, leagues below the safe speed, probably approaching stall
-At or near stick shaker
-150'
-no power

Recover.
How??

[john_tullamarine]

As most are aware, we tolerate quite a range of robust discussion on Tech Log - barring anything which is over the top.

However, I think it appropriate to note, for those who don't know them, that OG and zzuf are, in their respective fields, peer-acknowledged technical experts.

Do this in a 737:

Thanks all the same but I think one should pass.

[PENKO]

Apha-prot warning.

Whilst the experts debate each other wether you can have stall protection without phugoid damping, maybe Airbus should find a way of letting unfortunate pilots know that they have entered alpha protection?

Ultimately the only reason we are having this discussion now is the fact that Sully had no idea whatsoever that his airplane was in alpha protect mode. Maybe we do need a sidestick shaker?

[Owain Glyndwr]

Maybe we do need a sidestick shaker?

Actually, I think that would be a good idea!

[vilas]

PENKO
There is already a warning in normal law which calls "speed..speed...speed" but got suppressed by priority of GPWS. Although Sully's hands were full but just like he switched on the APU out of turn had he switched off the nuisance creating GPWS he would have got the warning. May be airbus needs to address that by installing a parallel channel with another loud speaker to create safer priority for warnings.

[Amadis of Gaul]

So if you can't win an argument against me and the original NTSB findings, just insult your way to victory. Yeah, that works.

I guess that's one way to look at it. Another one might be that some people take any disagreement as a personal insult.

[PENKO]

Exactly what I was thinking Vilas. As you say, the SpeedSpeedSpeed got suppressed due to out of sequence actions. But we can't blame Sully and Skyles for that, the situation was hectic, there was no time for a flame-out checklist designed to be read from the relative calm of level 380.

Hence the thought of an insupressable sidestick shaker/vibrator/buzzzer that unambiguously tells the pilot he's in alpha prot.

[CONF iture]

John,
The catastrophic scenarios was to face an all engines failure 3000 ft in the climb over a big city.
The catastrophic scenarios would have been to opt for another option ... but in the end, find yourself unable to make the desired runway ...

Are you absolutely sure?
Absolutely, as stated in the report.

Above 150ft the airplane was not in alpha prot.
Below 150ft it was.
And additionally passing 50ft it was not anymore.


Honestly answer us please CONFiture, what result do you expect in a 737 with an average pilot
I won't tell for the 737, not to exacerbate any passion, but we can take a 320 with a direct relation between stick and elevators displacements.
Data show that there were margin for improvements to be made at attitude and ROD levels for touchdown.
Airbus, for good reasons, put a margin between a-prot and a-max, and they did also between a-max and a-stall.
When Sully touchdowned there were still 3.5 deg to even reach a-max ...

[misd-agin]

Megan - "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?"


Sully had 4,765 hrs in the AB. That's years of flying. I wasn't a fan of tape displays, especially for altimeters(!!), but after 4-5K+ hrs flying them I think the modern tape displays are better than round dial airspeed tapes.

I'd still might prefer round altimeters, needle straight up is also a thousand foot mark. Perhaps easier to not exceed your cleared altitude with an obvious readout (needle at 12 o'clock) vs the tape altimeter readouts.

[misd-agin]

Vilas - "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."

A post stated that AB had an ideal pitch attitude range for ditching. The range is in a previous post in this thread. The higher pitch attitude range would probably result in a tail first landing considering the tail strike regime of 11.7-13.5 degrees for a 320. So apparently a slight tail first landing, for an aircraft about to be written off, is in the ideal range.

[Vessbot]

Apha-prot warning.

Whilst the experts debate each other wether you can have stall protection without phugoid damping, maybe Airbus should find a way of letting unfortunate pilots know that they have entered alpha protection?

Ultimately the only reason we are having this discussion now is the fact that Sully had no idea whatsoever that his airplane was in alpha protect mode. Maybe we do need a sidestick shaker?


..

I'm not an Airbus guy, but isn't this signified by being in the black/yellow checker range of the airspeed?

[misd-agin]

Page 11 of the report, 28/213 of the PDF, goes over the airspeed display and the display symbology.

https://www.ntsb.gov/investigations/AccidentReports/Reports/AAR1003.pdf


alpha protect and alpha max are represented by the top of the black/yellow checkerboard and the red bands.

[vilas]

During the flight time in between the birds and the water impacts, the
Aircraft was flown occasionally within the alpha protection range (around 1minute 7s), notably from about 150 ft RA down to water impact.

[PENKO]

I'm not an Airbus guy, but isn't this signified by being in the black/yellow checker range of the airspeed?

Yes, yet that is not clear enough apparently. Arguably the same can be said for the barberpole in a 737, hence the stick shaker/clacker...

[vilas]

Above 150ft the airplane was not in alpha prot.
Below 150ft it was.
And additionally passing 50ft it was not anymore.
This cannot be true because once it is activated, to deactivate: (from FCOM)
1. The pilot must push the stick greater than 8 degrees forward or
greater than .5 degrees for at least for .5 seconds when alpha is less than alpha max.
2. In addition below 200ft, the angle of attack protection is also deactivated when,
-the side stick deflection is less than half nose up and
-actual alpha is less than alpha prot-2 degrees.
When Sully was pulling full back stick to get flare how can he be out of alpha prot?

[QuagmireAirlines]

Had the elevators collaborated to Sully request anything better, and certainly not worse, was achievable.
When Sully touchdowned there were still 3.5 deg to even reach a-max . Then you too agree with the original NTSB findings (and the FAA, see below) where they said flare was interfered with by the pitch-down terms by Airbus. I think its obvious. Also, note the CLmax was probably still ~5 degrees from where Sully pitched, in this case (above even the 3.5 deg margin to alphaMax). If you agree with the facts in the NTSB report such as flight data and statements they made about alpha-prot during the flare phase, then its hard to believe some others on this forum still can't see it. Its plain as day.

In summary, there is also: http://www.tc.faa.gov/its/worldpac/techrpt/ar11-28.pdf (Bilbao, 2001) "In some other incidents, crews wanted to land in a nose-high attitude (e.g., because of suspected nose wheel problems) but it was prevented by the alpha-prot feature, and the aircraft sustained preventable damage. The Mode cause code was assigned because of unsuitable control limitations imposed by the alpha-prot function."--- Although the FAA's statement about "Mode cause" isn't quite right, since its not the Mode itself, its the way the mode is constructed, which can be fixed easily in flare situations.

We've mentioned the Habsheim, Bilbao, & Hudson River A320 accidents already above, yet the recent Halifax, 2015 accident is very suspicious as well. The crew tried to pull up in time, but couldn't get the aircraft up in time. There is an accident report, yet I'm still looking for flight data plots to see if alpha-prot interfered with using the maximum amount of lift before impact close to the ground. Anybody know about that one? I do know there is a lawsuit being prepared now, so I might contact them to find out what they know about the pilot/aircraft dynamics in the moments of attempted pull-up before impact.

And I love the title of this paper, which gets to the heart of all this flare-Airbus "surprise!!!!" :eek: stuff: Sarter, N. B., & Woods, D. D. (1997). Teamplay with a powerful and independent agent: A corpus of operational experiences and automation surprises on the Airbus A320. Human Factors, 39, 553-569. Available from:
https://www.researchgate.net/publication/48381508_Pilots_Controllers_and_Mechanics_on_Trial_Cases_Con cerns_and_Countermeasures

[tubby linton]

Quagmire, Halifax was due to pilots forgetting to fly the aircraft, rather than just watching the automation. It had nothing to do with the aircraft protections..If they did pull up it would have been load factor protection to +2g as the flaps were out, and this protection is using the aircraft certified limits as a target. I do wonder what you are trying to prove with your discussion of the protections, or is it data gathering for a law suit?

[CONF iture]

The old suggests double engine flameout land/ditch in Conf1. Abbreviated recommends Conf2.
I don't see the logic behind the difference neither, but I would think Airbus soon will also modify the ALL ENG FLAMEOUT to CONF2 to match the new EMERGENCY LANDING ALL ENG FAILURE ...

When Sully was pulling full back stick to get flare how can he be out of alpha prot?
You are correct, he was still under alpha protection mode below 50ft, despite the fact he was not even in the alpha protection range. So margin there was to improve touchdown.

I do wonder what you are trying to prove with your discussion of the protections
I won't tell for QA, but my point is merely to counteract unsubstantiated comments such as :

Sully would have stalled if not protected
Sully started the APU to remain protected

[Once_a_nomad]

The A vs B discussion is usually a robust sign that a thread is pretty dead. Anyway I find the airspeed discussion more interesting than phugoid damping...but I for the record I felt compelled to add that I believe their airspeed, under "just culture", was an honest mistake. (one never knows who else might read these threads). Nothing was normal that day. Pitch wasn't "normal". ECAM (a lot, despite some inhibited warnings). TCAS. Windshear. Priority Left. GPWS and EGPWS. There's too much noise. QRH is either too long and/or irrelevant (like evacuation - select parking brake on).

The flight envelope protection debate...Sullenberger flew the aircraft and everybody walked away. I briefly wondered if he was actually aware of what mode/law it was in (was he thinking "what's it doing now" when he pulled the stick back full?), but for myself I came to the conclusion it did not matter there in that particular case.

Sullenberger and Skiles were prepared for the unexpected. They managed the unexpected. For me that's a key takeaway.

[john_tullamarine]

The A vs B discussion is usually a robust sign that a thread is pretty dead.

I wouldn't have thought that there has been other than sideline A vs B in the thread ? It is difficult for those of us who have only flown the one, to appreciate the subtleties of the other .. hence some comparison in discussion probably isn't a bad thing ?

The thread is going around in circles a little but, overall, I think it has value yet to be explored.

I believe their airspeed, under "just culture", was an honest mistake.

It is evident that no-one is suggesting otherwise .. indeed, I have no doubt that all here are quite fervent in their .. "rather them than me" quiet thoughts.

Sullenberger and Skiles were prepared for the unexpected. They managed the unexpected. For me that's a key takeaway.

Indeed.

[vilas]

I won't tell for QA, but my point is merely to counteract unsubstantiated comments such as
I clarified that it was NTSB which thinks switching on APU was smart thing because the aircraft remained in normal law, I produced the clip to substantiate it. I didn't say that was the only purpose. Also I explained that in this particular accident APU didn't matter. As against this you make statements like Truth is that Sully would have most probably obtained a better touchdown with direct law. Only your opinion without any substantiation. It is possible he probably would have stalled.
Below 150ft it was.
And additionally passing 50ft it was not anymore. Again opinion without any evidence, even erroneous.
The truth is it is not possible to enter into alpha prot below 200ft if correct speed is maintained.

[Amadis of Gaul]

I do wonder what you are trying to prove with your discussion of the protections, or is it data gathering for a law suit?

I believe he wants Airbus to be "admonished" more sternly. I'm wondering why he can't just write a strongly-worded letter to them. I'm sure that will instill the fear of Krishna into them lickety-split.

[CONF iture]

Again opinion without any evidence, even erroneous.
Below 50ft a-prot was at 15,5 deg and a-max 17,5
The airplane was 1,5deg short of a-prot and 3,5 from a-max
Are you pretending the data are erroneous ?

If aircraft would have been in alternate law it would have stalled.
It is possible he probably would have stalled.
Finally you are getting more cautious.
Would you meet Sully and Skyles :
"Congratulations, but how lucky were you not to stall ... thanks to the a-protection"

[QuagmireAirlines]

Quagmire, Halifax was due to pilots forgetting to fly the aircraft, rather than just watching the automation. It had nothing to do with the aircraft protections..If they did pull up it would have been load factor protection to +2g as the flaps were out, and this protection is using the aircraft certified limits as a target. I do wonder what you are trying to prove with your discussion of the protections, or is it data gathering for a law suit? Its obvious the Halifax accident started from using flight path angle hold and drifting down. That part is clear, in only the first part of the scenario.
Understand when the pilots finally pulled up, it would be helpful to know if they were able to use all the alpha possible, up to stall, close to the ground where they tried to pull up. .... Why assume it took over 2g's to recover? Any evidence of that, like FDR plots? Or just a guess? We need FDR plots.

When I started this thread, I was exploring what happened between the Aircraft Performance Study statements for the Sully incident and the NTSB Accident report, and later found some other BEA involvment and earlier NTSB statements. Then some on here reminded me there have been other similar alpha-protect accidents in the past. Then the FAA statements were found. I'm not involved with lawyers or courts, and just want the NTSB and FAA to stand up for their own statements. The way the world works, governmental authorities like those are the way to get real change.