A320 and the Miracle on the Hudson

[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".

[CONF iture]

Originally Posted by Ashling

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]

Originally Posted by cwatters

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]

Originally Posted by Lyman

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 best

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".

[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]

Originally Posted by National Transportation Safety Board

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

Originally Posted by CONF iture

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]

Originally Posted by Clandestino

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 :

Originally Posted by cwatters

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]

Originally Posted by ReverseFlight

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]

Originally Posted by IFLY_INDIGO

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]

Did Airbus save the day?

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]

Originally Posted by rudderrudderrat

One flew to Alpha Max using FBW

Issue is, Normal Law refused to deliver Alpha Max.

Originally Posted by twistedenginestarter

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.