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:

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.

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..

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

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.

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.

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.

When 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".

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.

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.

Just believe the report then :
And for the "maximum performance" of P97 :

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..

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.

For the ditching, ENG 1 was running, badly, but running.

The NTSB report (eg page 90) states that there was hydraulic pressure despite the dual engine failure and so it wasn't an issue.
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 ?

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?

Did Airbus save the day?
 

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.

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.

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.

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 the airplane did not refuse the pilot's inputs for the flare.

Not envelope protection.
We could say so only if the aircraft had first delivered Alpha Max.

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.

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...

Aircraft did not fail … it just refused.
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

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

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.

APU
 

The report gives a clear answer:

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.
I would not have missed it for anything :
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.

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 ?

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.

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.

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 … ?

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.

fly-by-wire
 

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

Check your PM's

• 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).

That reason is to be able to comfortably reach Alpha Max and not to have to stop already 4 degrees before Alpha Max.


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 ?