A320 and the Miracle on the Hudson

[RtrHead]

I'm sitting here watching a Discovery program on the Miracle on the Hudson. The program seemed to hint at something I found very interesting, and I wanted to turn to the experts.

The program suggests that during the final flare, before touchdown in the water, that the A320 actually controls the pitch attitude of the aircraft - not the pilot. That not matter what input the pilot gives to the side stick, the aircraft will deliver a perfect touchdown attitude (approx 7 degrees.)

As I am not familiar with Airbus, fly by wire, etc, I wonder...is this true?

[Ka8 Flyer]

Hi Rtrhead,

short version: no way
long version:
In normal circumstances, the FBW will switch into a so-called flare mode as you approach the ground (or water...) and will introduce a nose-down elevator input, forcing the pilot to actively input up-elevator to maintain attitude. So without pilot input, the aircraft would actually lower the nose and make contact at a higher rate of descent.
But this is only for normal OPS. If the FBW system switches into an alternative mode due to power loss, the handling changes (to a point where the sidestick deflection is proportional to evelator deflection) - but at no point will the FBW system automatically pitch up to a specific attitude in either modes.
I'm not sure in which mode the hudson bus was in, shortly before contact. Even if both generators tripped offline (due to low engine RPM), there is still the RAT (a small turbine which drops into the airstream to deliver electrical/hydraulic power to key systems) plus I think the FO started the APU as part of the drill - but not sure if it was online at that point. If so, the FBW system would have been in normal law the entire way down.

[hetfield]

@RtrHead

Yeah, the busses are fully automatic. Even with all engines out. Just press the "ditching" button and get a cup of coffee.

If you are in a deep stall, don't worry. You and your fellow will not know anyway and the BEA or whoever will take care to cover AB asses.

[Wizofoz]

In Flare Mode, is Alpha Floor still active?

I'm wondering if what the program is alluding to is Sully going for minimum speed, holding the stick full back and letting the FBW maintain attitude for Maximum Alpha?

[Clandestino]

The program suggests that during the final flare, before touchdown in the water, that the A320 actually controls the pitch attitude of the aircraft - not the pilot. That not matter what input the pilot gives to the side stick, the aircraft will deliver a perfect touchdown attitude (approx 7 degrees.)

Absofreakingly not true.

The Hudson ditching was made in normal law, speed got so low that alpha protection was activated. It's not active during normal landing, not because it's deactivated but because excessive AoA is not approached.

Before this thread gets clogged by the contributions of "FBW is dangerous!" brigade, I would suggest that we use NTSB report (http://www.ntsb.gov/doclib/reports/2010/AAR1003.pdf) as the beginning point of our discussion.

[Wizofoz]

On the contrary, it displayed all the best aspects of FBW.

BUT, if Alpha Floor WAS active, it is KINDA right that at that stage the FBW was controlling the attitude, as the pilot was (probably deliberatly and with good reason) holding back stick in order for the FBW to deliver minimum speed at touch (splash?) down.

[hetfield]

Do you really think, Chesley Sullenberger and his F/O could it manage only by FBW?

On a 737 they wouldn't?
:ugh:

[Clandestino]

NTSB already answered that, far, far, far better than I ever could.

[Kit d'Rection KG]

Did they?

And did they leak it to the press first?

:cool:

[CONF iture]

if Alpha Floor WAS active

I would also suggest you to read the report first.
Do you really know what is Alpha Floor ?

[bubbers44]

Sully could have landed any Boeing in the Hudson as well as the Airbus. Don't give credit for his skillfull landing to a machine because Sully did it with his skill and nothing else. Airbus has a lot of automatic features but none of those helped him that day. His talent as a pilot saved all of those people, nothing else. We are all very proud of him.

[whenrealityhurts]

Just curious, full electrical failure, no batt or gen power...can the Airbus be flown?

[Last Ditch]

Just curious, full electrical failure, no batt or gen power...can the Airbus be flown?

Nope. Ditto with all Hyds gone.

[galaxy flyer]

Neither can the B777 or B787, for that matter. All hydraulics out, just about everything in the last 30 years for transport category planes.

GF

[I-2021]

Just curious, full electrical failure, no batt or gen power...can the Airbus be flown?

Yes, the airplane will fly in mechanical backup, that is mechanical control of THS via the trim wheel, mechanical control of the rudder and finally the thrust via the normal thrust lever control. It is a very very unlikely situation, because you need so many failures to end up in this situation that it is really unlikely to happen, but still you can get yourself out of it.

Cheers.

[FatFlyer]

Hi,

Just to be pedantic, alpha Floor protection is an autothrust function which adds TOGA(full) power and locks the thrust at this when the angle of attack reaches a certain figure. It is not a flight control function. They would not have had this as the engines had failed! it is also not available below a certain rad alt height.
The flight control protections, alpha prot and alpha max may have been available( if still in normal law) This would have allowed the aircraft to sink at just above the stall speed (as in habsheim crash) but would not maintain a pitch angle at touchdown(splashdown) it would however maintain an angle of attack if the stick was held full aft.

[john_tullamarine]

I think that "miracle on the Hudson" needs to be left where it belongs .. in typical media gross hyperbole.

The crew did a fine job - no-one doubts that - and underlying that was their ability to assess the situation and make a workable decision in a short timeframe.

Without taking away from their performance on the day, is it not reasonable to presume that a majority of experienced, competent crews probably would have gone for the same gameplan ?

What impressed me - from an NTSB interview transcript (which I linked in another thread) .. it was the F/O's first flight post endorsement training.

The crew necessarily had to ad lib a bit and, with the benefit of hindsight, possibly would have done the odd thing a little differently .. but a mighty fine performance overall in this observer's view.

[Safety Concerns]

I would also dispute the title. This was not a miracle. It was professional pilots doing a professional job. That is not to be seen as taking anything away from their achievement on that day.

It also clearly highlights the difference in outcomes when you have a crew in tune with their aircraft.

FULL RESPECT TO THEM

[Last Ditch]

Just curious, full electrical failure, no batt or gen power...can the Airbus be flown?


the airplane will fly in mechanical backup, that is mechanical control of THS via the trim wheel, mechanical control of the rudder


@ I-2021

We still need Hyd actuators to move the surfaces (G/Y for THS and G/Y/B for Rudders). This is unlikely in this scenario. However, if you have Hyds going, the Bus can be flown. I actually managed to "get her down" on one of the two occasions attempted during sims.

[LetsFlyAway]

Read the book.... Fly By Wire, The Geese, The Glide, The Miracle on the Hudson by William Langewiesche.

It explains in detail the capabilities of the aircraft and the events which occurred. It by no means expresses that either Sully or FBW were the dominant contributing factor to being "the miracle" that we all know of. It does however allow you to decide on your own what was happening that day.

Also a very intersting read into FBW systems.

[I-2021]

@ I-2021

We still need Hyd actuators to move the surfaces (G/Y for THS and G/Y/B for Rudders). This is unlikely in this scenario. However, if you have Hyds going, the Bus can be flown. I actually managed to "get her down" on one of the two occasions attempted during sims.

@ Last Ditch,

the question raised was about a total loss of electrical power, not hydraulic power.

[Last Ditch]

@I-2021
Ooops sorry.
For some reason I was assuming we had lost both engines as well, as at Hudson.

[Lookleft]

What in my mind demonstrates Captain Sullenberger's professionalism aside from his flying of the aircraft is that he knew, from studying and knowing his aircraft, that the correct ditching attitude was 11 degrees. A very small note in the QRH states this. Any more than that and the aircraft would have slammed down on the water, any less and the engines would have "bit" into the water and caused a break-up.

For those who start waffling on about Alpha floor look at Fatflyers post.

[CONF iture]

What in my mind demonstrates Captain Sullenberger's professionalism aside from his flying of the aircraft is that he knew, from studying and knowing his aircraft, that the correct ditching attitude was 11 degrees. A very small note in the QRH states this. Any more than that and the aircraft would have slammed down on the water, any less and the engines would have "bit" into the water and caused a break-up.
You may read the report first as well ...
The 11 degrees attitude was refused to the pilot even if adequate margin in the AoA before attaining alpha max was available ...
He could not pull more than 9.5 degrees attitude in the flare due to obscure system restriction.

[IFLY_INDIGO]

You may read the report first as well ...
The 11 degrees attitude was refused to the pilot even if adequate margin in the AoA before attaining alpha max was available ...
He could not pull more than 9.5 degrees attitude in the flare due to obscure system restriction.

Dear conf iture, where can I find the full report and all the actions the pilot did during their descent and ditching?

thanks in advance

[bubbers44]

Sully could have landed any airliner in the Hudson that day. He was an excellent pilot. He was restricted on how much he could pull up but he still planned the glide to a picture perfect ditching. His speed control kept the computer from screwing up his flare otherwise the computer would have put him in the Hudson at a high sink rate breaking up the airplane and costing many lives. If you can not flare beyond a certain AOA you can not break your descent rate.

[CONF iture]

Just here (http://www.pprune.org/6679423-post4.html), earlier in the thread.

[Lookleft]

Thanks for the correction CONF but my point was he knew what body angle he was after. Not sure if you are familiar with the Airbus QRH but the 11 degrees is a very small note in the ditching checklist. The main reason all survived was because of the experience and professionalism of the crew.

[infrequentflyer789]

Sully could have landed any airliner in the Hudson that day. He was an excellent pilot. He was restricted on how much he could pull up but he still planned the glide to a picture perfect ditching. His speed control kept the computer from screwing up his flare otherwise the computer would have put him in the Hudson at a high sink rate breaking up the airplane and costing many lives. If you can not flare beyond a certain AOA you can not break your descent rate.

Are we reading the same report ???


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.)


The computer didn't stop him flaring, or restrict him - his difficulty maintaining airspeed did that. He didn't have enough enegry to flare when he needed it.


Note: before deciding I or NTSB are criticising the crew: Could it have been done better ? Sure - if flying an armchair with plenty of time to read up on it and a few goes to practice (and BA38 could have made the runway too...). On the day - excellent decision making, CRM and airmanship and the right result. Hampered by a checklist designed for use at 20000ft, he got the speeds a bit wrong and bent the airframe a bit more than he might have. I'll take that any day given those circumstances.

[HarryMann]

Sully could have landed any Boeing in the Hudson as well as the Airbus. Don't give credit for his skillfull landing to a machine because Sully did it with his skill and nothing else. Airbus has a lot of automatic features but none of those helped him that day. His talent as a pilot saved all of those people, nothing else. We are all very proud of him.Lets stop eulogising shall we, and call it 50/50 ... sure the crew did well, particularly making the right decision very quickly and not losing focus.

[whenrealityhurts]

I hate to break up the mutual back slapping party, but just what were Sully's options?

- Slam into a skyscraper
- Do a 180 Glide back to the airport
- Make a left turn and land on miles of calm water.

He's not the first guy to ditch a plane, nor will he be the last, nor is he the first guy to have only one option and not be so stupid as to not take it.

[ReverseFlight]

Sure, Sully did not go by the book when he speed was below Vls. However, he was smart enough to start the APU out of sequence. I think that was the single key event which most influenced the outcome as it kept the aircraft in normal law for the flare. (The NTSB has since recommended revamping the checklists and introducing ditching training with dual-engine failure.)

[CONF iture]

The computer didn't stop him flaring, or restrict him - his difficulty maintaining airspeed did that. He didn't have enough enegry to flare when he needed it.
The computer did restrict him, absolutely.
He did have enough energy, not a ton, but enough to improve the touchdown. The system had decided otherwise.

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°).

The 11 degrees attitude as mentioned by Lookleft was refused to the pilot.

[Ashling]

From the NTSB report pages 97/98

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.

So the aircraft gave Sully the max performance that it could. A very different picture to the one presented if you only refer to the aircraft limiting AOA.

F speed for Config 2 was around 150kts and IAS was 125kts when he ditched (NTSB). So he was very close to the stall and was in fact in Alpha Protection from 150' rad alt. So far from the aircraft being at fault I would say that it perhaps saved them at this stage. Sully pulled full aft stick (in order to flare) close to the stall, had it been a Boeing it may well have stalled with disasterous consequences. Of course had it been a Boeing Sully may well have chosen not to pull full aft stick.

The key was his decision to start the APU which kept the aircraft in Normal Law. The aircraft looked after him and gave him the maximum performance it could while preventing a stall. Well done Sully and well done Airbus in my view.

So we can say Sully could have done better with his airspeed and that Airbus could have done better with their checklist but the end result was the best that could be hoped for. Sully looked after the jet and it looked after him.

[CONF iture]

From the NTSB report pages 97/98
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.

Absolutely.
It did performed as designed, but that’s not necessarily the performance the pilot was after for working the flare.

So the aircraft gave Sully the max performance that it could.
... as per its design … but that’s not necessarily the performance the pilot was after for improving the touchdown.

What is a flare Ashling ?


So far from the aircraft being at fault I would say that it perhaps saved them at this stage. Sully pulled full aft stick (in order to flare) close to the stall, had it been a Boeing it may well have stalled with disastrous consequences.
Oh such comment rings a bell, doesn't it ?
Please, would you address it directly to Sully, I think he might put its politically correct discourse aside for the reply …

The key was his decision to start the APU which kept the aircraft in Normal Law. Beyond that the aircraft looked after him and gave him the maximum performance it could while preventing a stall.
The good for starting the APU was to assure full hydraulic + electric but certainly not to maintain Normal Law. Direct Law was all what needed to FLARE properly ... without interference.

Where published figures say otherwise … ?

[Ashling]

Conf

Sully, and his crew, did a great job on the day. No one, least of all me, wants to take away from that. However that does not mean they did everything perfectly and so it is perfectly reasonable to look at the report and draw lessons from it. That does not amount to a personal attack on Sully. It is the NTSB that draw our attention to the issue of speed control and the effect it had. They also qualify their comments by explaining the effects of task saturation etc.

You quoted my comments re Sully rather selectively. Here is what I said

"So far from the aircraft being at fault I would say that it perhaps saved them at this stage. Sully pulled full aft stick (in order to flare) close to the stall, had it been a Boeing it may well have stalled with disasterous consequences. Of course had it been a Boeing Sully may well have chosen not to pull full aft stick."

I said "perhaps saved them" which is very different to saying did save them. It is fact that Sully pulled full aft stick close to the stall. He was trying to cushion the impact perhaps in the knowledge that he could not stall the aircraft in Normal Law. I went on to say that had it been in a Boeing he might have flown it differently.

Sully's 2 key decisions, to start the APU and to Ditch allied to his leadership and the teamwork of his crew is why they all survived. No one can take that away. However there were issues. A few quotes from the NTSB report.

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.)

The NTSB concludes the captain’s difficulty maintaining his intended airspeed during the final approach resulted, in part, from high workload, stress, and task saturation.

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.

14. Despite being unable to complete the Engine Dual Failure checklist, the captain started the auxiliary power unit, which improved the outcome of the ditching by ensuring that a primary source of electrical power was available to the airplane and that the airplane remained in normal law and maintained the flight envelope protections, one of which protects against a stall.

16. The captain’s difficulty maintaining his intended airspeed during the final approach resulted in high angles-of-attack, which contributed to the difficulties in flaring the airplane, the high descent rate at touchdown, and the fuselage damage.

17. The captain’s difficulty maintaining his intended airspeed during the final approach resulted, in part, from high workload, stress, and task saturation.

So airspeed control was an issue.

Starting the APU played a key part in the successfull outcome

The aircraft Max Performed when asked to. Had it given him more alpha the performance wouldn't have changed and they would have been closer to the stall. Note that Airbus are only told to highlight this more in training. No where does it say that they need to change the control logic or that the aircraft failed to give maximum performance when asked. You should know that you can raise the nose more to no effect.

Had Sully been able to carry more speed into the flare, F speed or at least VLS he would have found the flare much easier in any control law.

Sorry for all the quotes but you did ask.

[CONF iture]

The aircraft Max Performed when asked to. Had it given him more alpha the performance wouldn't have changed and they would have been closer to the stall. Note that Airbus are only told to highlight this more in training. No where does it say that they need to change the control logic or that the aircraft failed to give maximum performance when asked. You should know that you can raise the nose more to no effect.
Once again, what is a flare, Ashling ?
It is not something you sustain in the time.
It is only a transitory phase very limited in the time.

The potential to improve the touchdown was in the aerodynamics.
The electronic prevented the pilot to get it.

Where published figures say otherwise ?

[Ashling]

Conf

I've in excess of 12,000 hrs, flown Mil Fast Jet, instructed, and flown civil Boeing and Airbus. I think I know what a flare is by now. Thank you very much.

The aircraft max performed, there was nothing more it could do aerodynamically. You need to appreciate that more backstick does not always mean less rate of descent or a gentler touchdown. In fact it may mean the opposite. The aircraft certainly knew this hence why it attentuated the inputs to give max performance.

We won't agree so I'll leave it there except to say that Sully, his crew and the Jet did a fine job that day.

[Lyman]

Having viewed the landing on the water many times, I can say that the AoA looked highly appropriate, and the "Rollout" proceeded quite well. Again, in viewing the Ditch, I couldn't see much to criticize.

Absent deployed landing gear, she flew somewhat "different", I would say, than in an ordinary TouchDown.

Nose High, what if the Tail Holed? Nose High, what if the Nose planted, instead of skipped? Fatalities, perhaps many. Perhaps all.

You know, data can be a fluffy pillow for those who weren't there, but make a living crunching others' work.

Flare? On a water Landing? Hmm......

[Ashling]

In 11 of the 12 runs, the touchdown flightpath angle ranged between -1.5° and -3.6° (the touchdown flightpath angle achieved on the accident flight was -3.4°). In 1 of these 12 runs, a -0.2° touchdown flightpath angle was achieved by an Airbus test pilot who used a technique that involved approaching the water at a high speed, leveling the airplane a few feet above the water with the help of the radar altimeter, and then bleeding off airspeed in ground effect until the airplane settled into the water.

Interesting technique and not one I would have thought of before reading it.

[Lyman]

I assumed the first time I saw the Ditching that Flight path was in control, and selected. I thought a bit fast, but have rethought that as well. No one wants to Flare into the water, the resulting Nose drop onto the surface gives one the shivers. Better to skip than to plant.

Until I found out the R/E was lost, I assumed the "Ground Loop" was also "selected".

:ok:

[CONF iture]

You should know that you can raise the nose more to no effect.
...
You need to appreciate that more backstick does not always mean less rate of descent or a gentler touchdown.
Surprising comments from someone with your experience as aircraft was NOT stalled :
3.5 more degrees before reaching Alpha Max (P98), and probably another 3 additional degrees to reach Alpha Stall … Raise the nose is effective : Less rate of descent + gentler touchdown … That’s a flare.

But if Aircraft knows better …

Anyway, thanks for the exchange. I did appreciate.

[cwatters]

Surprising comments from someone with your experience as aircraft was NOT stalled :
3.5 more degrees before reaching Alpha Max (P98), and probably another 3 additional degrees to reach Alpha Stall … Raise the nose is effective : Less rate of descent + gentler touchdown … That’s a flare

I don't know about this aircraft but flying as close as possible to the stall doesn't allways produce the lowest rate of descent. For many gliders the lowest rate of descent occurs at a slightly lower AOA/higher speed. Pull up, slow down and you sink faster.

http://en.wikipedia.org/wiki/Polar_curve_(aviation)

The report says..

however, the airplane did provide maximum performance for the weight and configuration at that time.

But does it explain what "maximium performance" means in this context? Lowest rate of descent? (Who said minimum fuel consumption :) )

[CONF iture]

I don't know about this aircraft but flying as close as possible to the stall doesn't allways produce the lowest rate of descent. For many gliders the lowest rate of descent occurs at a slightly lower AOA/higher speed. Pull up, slow down and you sink faster.
Like sustaining Alpha Max with minimum thrust on a 320 would also produce an impressive rate of descent.

But for the period AoA is increasing as long as not stalled, the 320 will behave like the glider : rate of descent will diminish.

That’s the desirable effect of a very temporary situation known as the flare. I believe you, me, or any pilot on this world use more or less happily that very technique for every landing.

[Lyman]

Perhaps not every landing. Deck angle is the key to a survivable Ditching.

And there was no thrust to arrest sink.

Flare is a trap, here, you will find yourself with high deck angle and no energy, any further pull causes an immediate STALL, and the NOSE contents will spill out the fractures in the forward Hull on impact.

[CONF iture]

Flare is a trap, here, you will find yourself with high deck angle and no energy, any further pull causes an immediate STALL, and the NOSE contents will spill out the fractures in the forward Hull on impact.
That's something the Sully will judge : how and when to pull for the best.
For the occasion, the recommended deck angle has been refused by the system.

[ReverseFlight]

how and when to pull for the bestWhen the a/c is clean, green dot gives best L/D ratio for the furthest glide and min RoD. One thing I don't understand is once slats/flaps are pulled, how do you judge your best glide speed without the green dot ?

QRH just says "MIN APPR SPEED ... 150 kts".

[bubbers44]

Sully could have probably used another few degrees of flare that Airbus denied him but he didn't need it if you look at the video. More flare might have caused the tail to hit earlier and not let the nose come down as gently. He didn't need Airbus technology to ditch it but ground effect took care of any descent rate he had approaching the Hudson. The results speak for them self.

[Clandestino]

the airplane did provide maximum performance for the weight and configuration at that time.


3.5 more degrees before reaching Alpha Max (P98), and probably another 3 additional degrees to reach Alpha Stall … Raise the nose is effective : Less rate of descent + gentler touchdown … That’s a flare.
(...)
But for the period AoA is increasing as long as not stalled, the 320 will behave like the glider : rate of descent will diminish.

That’s the desirable effect of a very temporary situation known as the flare. I believe you, me, or any pilot on this world use more or less happily that very technique for every landing.
Now, if I knew nothing about aerodynamics and especially about drag curves, I would have a difficult time when deciding which version to believe; the one written and signed by the government appointed panel of experts or the one by the anonymous PPRuNe contributor. Tough one, eh?

Still I wonder whether people expressing their dismay about the performance of various accident investigation boards take themselves seriously enough to make their views clear to those who appoint such boards or are they just pulling our legs. Transport safety boards and governments don't read PPRuNe.

[CONF iture]

Now, if I knew nothing about aerodynamics and especially about drag curves, I would have a difficult time when deciding which version to believe; the one written and signed by the government appointed panel of experts or the one by the anonymous PPRuNe contributor. Tough one, eh?
Just believe the report then :
NTSB P98
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°).

And for the "maximum performance" of P97 :
But does it explain what "maximium performance" means in this context? Lowest rate of descent? (Who said minimum fuel consumption :) )

[IFLY_INDIGO]

how come the airplane was in normal law after dual engine failure? FCOM says that if flight control computers detect G+Y hyd failure, it goes into alternate law. isn't it?

were the engines windmilling enough to keep the hydraulics system pressure above 1450psi?

or did the flight crew selected yellow pump on after turning the APU on?

NTSB report is not clear about it..

[ReverseFlight]

NTSB report is not clear about it.. As I said in post #29 (and the NTSB report states in several places), Capt Sully switched on the APU and this prevented EMER ELEC CONFIG kicking in, which would have brought him into alternate law.

[IFLY_INDIGO]

As I said in post #29 (and the NTSB report states in several places), Capt Sully switched on the APU and this prevented EMER ELEC CONFIG kicking in, which would have brought him into alternate law.

that is just one condition of 'flight control law reconfiguration'.. kindly refer to the FCOM again... there are several conditions which may lead to flight control law reconfiguration. dual hydraulic failure is one of them. my question pertains to the condition of dual hydraulic failure leading to law reconfiguration.

[CONF iture]

how come the airplane was in normal law after dual engine failure?
For the ditching, ENG 1 was running, badly, but running.

[ReverseFlight]

my question pertains to the condition of dual hydraulic failure leading to law reconfiguration.The NTSB report (eg page 90) states that there was hydraulic pressure despite the dual engine failure and so it wasn't an issue.
FDR data indicated that, during the accident event, all three (green, blue, and yellow) hydraulic systems were available ...I know what the FCOM says otherwise but maybe Airbus knows more than we do. I wouldn't have turned down the offer of full hydraulics if were in Sully's shoes. Would you ?

[IFLY_INDIGO]

In case if we lose both the engines and EDP is not generating sufficient pressure, we can turn on the APU and then yellow pump. yellow system would come alive and with that PTU would also function to bring green system alive.. airplane may go into normal law again.. is it possible for the airplane in alternate law to move back into normal law once the failure is gone?

[twistedenginestarter]

He didn't need Airbus technology to ditch it but ground effect took care of any descent rate he had approaching the Hudson. The results speak for them self.

I've just read the report. In no way does it criticise the pilots.

However the fact is Sully flew the plane slower than he should and slower than he meant to - that's what the report says.

The results were ... damage to the rear of the plane that might have been avoided if he had been at a higher speed into the flare, and thus with more energy to arrest the rate of descent.

His actions in the final moments were being mediated by envelope protection. It's not entirely clear whether that was benign or counter-productive but I still think there is an argument that a comparable B737 could have led to a more serious outcome. You can't say it wouldn't have made any difference because there clearly was computer intervention.

[rudderrudderrat]

You can't say it wouldn't have made any difference because there clearly was computer intervention.

True - but I'm sure Captain Sullenberger would have held a conventional aircraft off the water for as long as possible until he felt the stick shaker. One flew to Alpha Max using FBW, the other would have been to stick shaker onset using pilot skill.

[Lyman]

I suppose the important thing is deck angle at water impact. Slow and steep does not seem like a good solution to me. Especially at the pointed end.

[CONF iture]

One flew to Alpha Max using FBW
Issue is, Normal Law refused to deliver Alpha Max.


The results were ... damage to the rear of the plane that might have been avoided if he had been at a higher speed into the flare
The results were ... damage to the rear of the plane that might have been avoided if the airplane did not refuse the pilot's inputs for the flare.

His actions in the final moments were being mediated by envelope protection.
Not envelope protection.
We could say so only if the aircraft had first delivered Alpha Max.

[Ashling]

The issue with the flare was the speed control prior to it that led to the aircraft entering Alpha Prot at 150ft. At impact the aircraft was at 125kts config 2 (F speed would have been about 150kts) at a weight of @150,000lbs (68k) doing 750 fpm. It had very low surplus energy. Certainly not enough to significantly reduce the rate of descent but it was at significant risk of stall or would have been had it not been for the fly by wire protections that did their job. The reason the aircraft refused Sully Alpha Max was that at that point it was unsafe to go there at the rate he requested but it did Max Perform for him.

Confiture will continue to insist that the aircraft failed and that trying to trade a couple of knots close to the stall is a smart idea. He fails to understand the speed and energy regime they were in and is wrong. If they had stalled they would have died. The aircraft prevented this as Sully knew it would hence his abrupt full aft stick. The lesson is that if you have to ditch keep your speed up prior to impact so that you have energy to trade.

The NTSB said that the aircraft max performed but did not explain why Alpha Prot did not give Alpha Max. Below are some snippets from Airbus's submission to the NTSB which explain why the aircraft did what it did. The BEA in their comments at the end of the NTSB report highlight the same things in more detail but I cannot paste them for some reason.



During the remaining portion of the flight, the Aircraft remained in Normal
Law, and on occasion was flown within the alpha protection range. Notably
from approximately 150 ft down to the water impact the Aircraft was in
slats/flaps configuration 2. During this time period the Aircraft was in the
alpha protection mode which allowed the flight crew to remain focused on
their priorities, conversely if the Aircraft had been a non fly-by-wire aircraft,
the flight crew would have had to fly in and out of the stick shaker to
maintain the desired descent profile.

All water entries studies performed either by NACA or later by IMFL lead
to the conclusion that for the A320 aircraft on shallow water, the optimum
configuration is to have:
- Landing gear retracted
- Full slats/flaps configuration for minimum speed
- Pitch around 11°
- Slope around -0.5°
These data translate into a 3.5 ft/sec vertical speed at water impact.
It also shows that in case of water impact with an aircraft pitch below ≈ 8°,
or above ≈15° major airframe structural breakage is expected.

In Flight 1549, the Aircraft’s energy just prior to water impact was
insufficient to significantly decrease the vertical speed during flare, leading
to a water impact at around 13ft/s. Despite this rate of descent at water
impact, the extent of aircraft damages did not prevent a safe evacuation for
all persons on board. Aircraft pitch at water impact was close to the
optimum recommended value.

During the flight time in between the birds and the water impacts, the
Aircraft was flown occasionally within the alpha protection range (around 1
minute 7s), notably from about 150 ft RA down to water impact.
As far as aircraft trajectory is concerned, it has to be noted that the flight
control laws in the alpha protection domain do include some additional
features. AoA protection takes also care of the aircraft trajectory and, thus,
looks after phugoid damping as well as AoA control. 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.

It is obvious that achieving the optimum water impact configuration when
engine thrust is available (actually setting a Flight Path Angle of -0.5° on the
FCU), is more easily achievable.
However 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.

As an aside there is a reason the QRH and ECAM do not direct you to use the Yellow Electric Pump in a Green and Yellow Hyd failure. It will overheat and you will lose it very quickly.

[Ka8 Flyer]

For the sake of argument, let's assume you switch off the engines (fuel cut-off) and continue to glide at 150 KIAS. How long before your hyd pressure drops below say 2000 PSI (without extending the RAT and engines windmilling somewhere between idle and 0 RPM)?
I guess it would depend on the number and degree of flight control deflections, but could you glide for say a minute and have enough pressure to flare? Or even extend flaps a bit?
Just like in a car you have 4-5 full brake pedal deflections before accu pressure is gone once you turn off the engine...

Just wondering...

[CONF iture]

Aircraft did not fail … it just refused.
Certainly not enough to significantly reduce the rate of descent but it was at significant risk of stall or would have been had it not been for the fly by wire protections that did their job.
Their job is to deliver Alpha Max and certainly not to prevent to benefit from it.

Why would you stall at Alpha Max ... !?
Airbus would not be too happy to hear that one.

Do you remember what is Alpha Max ?

http://i45.servimg.com/u/f45/11/75/17/84/hud_0210.png (http://www.servimg.com/image_preview.php?i=115&u=11751784)

See how you can benefit from it on the LIFT side.
Just what you need, WHEN you need it : Flare time !

[Ashling]

There is a difference between theory and practise which is why Alpha Prot damps the input, if required, to prevent overshooting the max value.

You cannot flare an aircraft with the energy levels they had, at least not in any meaningfull way, but you can most definately stall it. That would be catastrophic in this instance. It is not worth the risk which is why the aircraft protected them but at the same time gave them the maximum performance it could.

You asked for an explanation, its all in Airbus's submission and that of the BEA at the end of the NSTB report. You clearly could not be bothered to read those prior to your last post or you refuse to accept that phugoids exist and need to be mitigated against. You do not know better thsn the NTSB (who did not criticse the aircraft or its FBW) or the BEA and you definately do not know the aerodynamic qualities of the aircraft better than the manufacturer.

[Caygill]

IFLY_INDIGO how come the airplane was in normal law after dual engine failure? FCOM says that if flight control computers detect G+Y hyd failure, it goes into alternate law. isn't it? The report gives a clear answer:


The NTSB concludes that, despite being unable to complete the Engine Dual Failure checklist, the captain started the APU, which improved the outcome of the ditching by ensuring that a primary source of electrical power was available to the airplane and that the airplane remained in normal law and maintained the flight envelope protections, one of which protects against a stall.

[CONF iture]

There is a difference between theory and practise which is why Alpha Prot damps the input, if required, to prevent overshooting the max value.
Ashling,

How many more protections are you ready to make up on this aircraft ?

Alpha Max is not already enough 3 degrees short of Alpha Stall. Do you need another Alpha to protect you from approaching Alpha Max. What is the code name for such Alpha ?
Does Alpha Phugoid look nice ?

Appendix D had been read for a long time (http://www.pprune.org/5749094-post35.html).
I would not have missed it for anything :
Despite what pretend Mister Langewiesche or the BEA in the Appendix D, I believe Sully would have obtained a better touchdown on his own without the protections interfering with his experience.

By the way, take that Appendix D and replace the BEA logo by the Airbus one and you will notice how much more natural the reading proceeds ...

Sully would not have got a single of those phugoid oscillations by being allowed to raise the nose on the flare.
Not at that altitude.
Not in that time frame.
Better attitude Lower RoD just positive stuff.

Furthermore, it is not certain that, without the phugoid oscillation damping function, it would have been possible to meet all the recommended parameters.
How hypocrite is it.
Do you think they would only have tested without that POD function ?
Not a chance, too proud to have to admit that flaring in Direct Law may have been as good as flaring in their complex Normal Law with the indispensable magical Phugoid Oscillation Damping function.

Some pretend that computerized aircrafts are less complex ...
Ashling, would you provide the FCOM's references about that Phugoid Oscillation Damping function. I must admit I have probably missed it ... or not ?

[Ashling]

Phugoid oscillations are to do with short term longnitudinal stability in this case. Its another way of explaining why its easy to over control close to the stall and end up in the stall. Ever flown a low speed scissors in air combat ?

There is no Alpha Phugoid, I did not say there was and neither do the BEA or Airbus. Kindly refrain from misrepresenting what others say.

You very sarcasticaly say "indepensable" "magical" when referring to the aircrafts protections. Did you design it?, did you test it? No, thought not yet you suggest you know better than those who did.

The NTSB criticised Airbus for not fully explaining the Alpha Prot function in the FCOM and in training BUT THEY DID NOT CRITICISE THE PERFORMANCE IT GAVE ON THE DAY.

The jet was at 150,000 lbs (68k), 750 Fpm Rod and almost 25 kts !! below the speed for the configuration they were in. That is a bad bad place to try to flare from. To put that in context its the equivelant of trying to flare a jet close to max weight from 20 - 25 kts below Vref without power. You seem to say that you can flare effectively from this position and then accuse me of not understanding what a flare means. Unbelievable. Makes me wonder if you are even a pilot.

Your contention seems to be to free the pilot up to fly right to the stall, for what benefit ? A small reduction in rate of descent at best. At what risk ? A stall leading to catastrophy. The risk reward trade is all wrong and obviously so. You cannot risk a stall in a ditching and if you think commercial pilots are able to finnesse their handling, without prior practise, to do this then you are a bigger fool than I take you for.

You are hell bent on denegrating Airbus when it was Airbus that gave the Captain the care free handling he needed to protect him in those last few crucial moments before impact. You refuse to accept that it was the speed control that prevented the aircraft from being able to flare effectively not the FBW. That protected him.

You have got your focus all wrong.

[CONF iture]

Make up your mind Ashling, is it a protection or it is not ?
If it prevents the aircraft to approach Alpha Max it has to be a limitation ...

Now a question for you :
If High AoA protection when activated prevents an aircraft to approach Alpha Max (in this case by 4 degrees), when will we ever reach Alpha Max that Airbus is so proud of ?

NTSB did not criticize Airbus – They just mention that the flight envelope protections did not allow an increase in AoA despite the fact that a margin of 4 degrees existed before reaching Alpha Max, not Alpha Stall, Alpha Max.

You refuse to accept that it was the speed control that prevented the aircraft from being able to flare effectively not the FBW.
Where did I refuse that maintaining more speed to the water would not have helped for the flare ?
Please quote ?
FBW has nothing to do either, don’t be confused.

On your side you’re not ready to consider that a margin of 4 degrees existed in the aerodynamics (NTSB BEA Airbus do not refute this, do they ?) before reaching Alpha Max (which is not Alpha Stall Ashling … just not yet) and therefore the potential to improve the touchdown, as Sully was trying to, was a reality that the flight envelope protections refused to authorize.

You prefer to scare around with a catastrophe scenario … go ahead, but remember, Alpha Max is not Alpha Stall, and if Normal Law is afraid of the phugoid oscillations, just give Direct Law to Sully, I’m sure he will deal OK with them.

Still nothing in the FCOM on the Phugoid Oscillation Damping function … ?

[Ashling]

However, when in normal law, regardless of the pilot's input, the computers will prevent excessive maneuvers and exceedance of the safe envelope in pitch and roll axis.Taken from the flight control section of the 320 FCOM

No Flight Crew Manuals mention all the aerodynamic characteristics of an aircraft. In this instance Airbus elaborated by explaining that the FBW reduced the input due to Longnitudinal Damping (Phugoid). Its a technical way of saying that its easy to overcontrol close to the stall which we all should appreciate especially as the aircraft has a pitch up tendancy close to the stall as highlighted in Safety First which you referenced in another thread.

If you seriously think you can flare an aircraft effectively from 25 odd knots below the manoever speed for the weight and config they were in you are deluded.

Further if you think they would have been better off in Direct Law then you are mistaken. In Direct Law the aircraft is free to stall. At the speed they were at the Stall Warning would have been hammering away for quite some time and there is no AoA gauge and no way of knowing how close to the stall you are. Are you really saying that in Direct Law 25 knots slow on the manoever speed with the stall warning blaring away that the right thing to do is pull.

If it had been a Boeing the stall warning would have been blaring away too, allied to the stick shaker and again no AoA indication so no way to know how close you are to the stall and no feel due to the stick shaker.

Sully did not have to worry about that. He was in an Airbus in normal law, thanks to his actions, that allowed him care free handling and gave him maximum performance. He would emphaticaly have been considerably worse off if he had been in alternate or direct law whatever speed profile he flew.

I am well aware Alpha Max is not Alpha stall. There is a margin for a reason. I am also well aware that there was a gap of 3 AoA to Alpha Max when they impacted due to the FBW mitigating Sully's full aft input. Unlike you I am willing to accept that there was a reason that the FBW mitigated this input. Airbus and the BEA have explained that reason and you still insist you know best. I did not design or certify the aircraft and neither did you. You have no basis in fact for denegrating the design or performance of the aircraft because you do not have the data to base that judgement on. You should take note that the NTSB, who have no axe to grind in Airbus's favour, do not criticise the aircraft or its design.

I am not an aerodynamicist but my practical experience is that it is very very easy to overcontrol close to the stall. It takes finesse and current practise to operate in that regime. Any large aggressive inputs will lead to a stall. Thats why you pull to the buffet then squeeze into it in a Max Rate Turn. At low speed the buffet band is even easier to pull through. So if a pilot makes a full aft stick input in that regime then it is no surprise to me that the aircraft mitigates it. Its exactly what I would have expected.

I've contributed more than enough now so for the time being I'll leave it to others to keep this going if they so desire.

[sammymoyo]

hi there,
Need your help.i am a young commercial pilot and i just got my licence recently.i am preparing for an interview with a certain airline.from my understanding they will ask me a few questions about how "fly-by-wire" works and i havnt the slightest clue.if u could be so kind as to take a few minutes and just briefly explain to me how it works,i would be forever grateful to you.

thanks in advance

[Ashling]

Check your PM's

[CONF iture]

However, when in normal law, regardless of the pilot's input, the computers will prevent excessive maneuvers and exceedance of the safe envelope in pitch and roll axis.Taken from the flight control section of the 320 FCOM

Yes - Alpha Max is part of the safe envelope in pitch.
Yes - The computers will prevent exceedance of Alpha Max, not exceedance of (Alpha Max minus 4 degrees).


I am well aware Alpha Max is not Alpha stall. There is a margin for a reason.
That reason is to be able to comfortably reach Alpha Max and not to have to stop already 4 degrees before Alpha Max.


If it had been a Boeing the stall warning would have been blaring away too, allied to the stick shaker and again no AoA indication so no way to know how close you are to the stall and no feel due to the stick shaker.
My memories from Boeing are a bit far now so I could be wrong but except from the stick shaker I don't remember any other audio stall warning ?
Anyway IIRC a few procedures in the Boeing recommend to even go to the stick shaker if necessary. Is it because the Danger to stall is so great Ashling ?
Aslo, again IIRC, the Pitch Limit Indicator on Boeing shows the AoA margin to the stick shaker.


Question still stands Ashling :
If High AoA protection when activated prevents an aircraft to approach Alpha Max (in this case by 4 degrees), when will we ever reach Alpha Max that Airbus is so proud of ?