Cactus 1549

[Pin Head]

Hi

Bored on a saturday and watching the movie.

They go on about starting the APU helped them.

My NNC for ditching says APU off. Why and then If it helped them so much why can't Boeing follow Airbus example?

Try and leave the hollywoood spin out of it.

Thank you

[Le Flaneur]

By starting the APU and bringing the APU generator on line, the crew of 1549, kept the normal flight control laws (more importantly, the flight envelope protections) available.

This was important, as without auto throttle available (engines flameout) and the neutral speed stability of the C* pitch law, it is not possible to trim to an airspeed. Sully was attempting to maintain green dot (approx max L/D airspeed), but because of the workload and distractions, got slow.

As a result, the last ~200 feet of the flight was on the AOA limiter, which prevented a stall/departure.

However, because he was on the AOA limiter, there wasn't enough energy to flare, which resulted in a hard touchdown and damage to the lower fuselage.

For details, see the NTSB report NTSB/AAR-10/03, specifically the analysis part starting with Section 2.3, Flight Crew Performance.

[KayPam]

I concur with everything but your second sentence :
He got slow because he pulled some pitch up and even converted some of its kinetic energy into height.
Trimming into an airspeed with a gliding airbus is simple : just do nothing and the aircraft will maintain its pitch angle, speed will then progressively stabilize at the speed which corresponds to this pitch angle.
If this chosen pitch angle is too high then yes, the speed protection will come into play.

The APU is useful in maintaining alive all the electrics and hydraulics. Guaranteeing optimum manoeuvrability.

[Pin Head]

Ok great stuff.

737 NG NNC for dual engine failure has start APU at item 8, and then off in the ditiching drill.

What does the Airbus say about APU in a ditching situation?

Thanks

[pineteam]

Hi,

A320 QRH: First item for ditching in case of all engines failure is to start the APU. Then at Touchdown: Engines and APU off.

[RUMBEAR]

How did starting the APU retain normal flight control law? I haven't read the report but loss of G & Y HYD system will result in ALT LAW.

[Fursty Ferret]

Trimming into an airspeed with a gliding airbus is simple : just do nothing and the aircraft will maintain its pitch angle, speed will then progressively stabilize at the speed which corresponds to this pitch angle.

No, it won't. Hands off, it'll maintain its current flight path until the speed reaches Valphaprot, then a nose down term will be introduced.

How did starting the APU retain normal flight control law? I haven't read the report but loss of G & Y HYD system will result in ALT LAW.

I believe at least one engine continued to run sub-idle, but sufficient to provide hydraulic pressure.

[KayPam]

Oh yes I made a mistake about the airplane maintaining it's pitch angle, it's rather maintaining a flight path since the rule is to maintain a 1G load factor.
If the speed varies, the pich angle will also vary to maintain a 1G load factor.


But the conclusion remains the same :
If your chosen flight path is sufficiently downwards, speed will stabilize around a particular value.
If your chosen flight path is too optimistic relative to the gliding performance.. You will get a speed around Valphaprot or more (but not so stable..)

In the case of cactus1549 I really don't understand why he reduced his speed so much by climbing.

[Jonty]

Give it a go in the sim and see if you do any better. Plenty have tried and failed.

[pattern_is_full]

Originally Posted by KayPam

In the case of cactus1549 I really don't understand why he reduced his speed so much by climbing.

Probably an early flare due to the optical illusions of landing on smooth, unmarked water (well-known issue for float-plane pilots). It is very hard to judge height in such conditions.

Note that due to the gear being up, there were no audible altitude call-outs, just "Pull Up! Terrain! Pull Up, Terrain!" Visual judgement was all that was available.

[FlightDetent]

Originally Posted by KayPam

Oh yes I made a mistake about the airplane maintaining it's pitch angle, it's rather maintaining a flight path since the rule is to maintain a 1G load factor.

If the speed varies, the pitch angle will also vary to maintain a 1G load factor.

KP, you venture too far based on FCOM wording. Again. A descending trajectory is not a constant 1g one. Even level flight based on 1g is a problematic achievement because the idea that the FCS matches actual n to the 1g value would only work if that 1g value had been corrected for latitude, geopotential altitude and speed vector.

If anyone's interested let's start a separate thread. I'd happily participate, last time I flexed my brain on this many ends were still loose by the time I had reached my limits. "Why physics does not support Airbus claim to fly level on 1g, and the implications". Or "Why does my Airbus start to climb like crazy with AP OFF on an easterly track south of Gibraltar".

I wish not to hijack this one, just that Fusty's "maintain its current flight path" is as accurate as it gets.

[misd-agin]

Originally Posted by Jonty

Give it a go in the sim and see if you do any better. Plenty have tried and failed.

Try what? Ditch? Return to LGA? The majority of RTB's to LGA were successful. I think it was 7/8 to rwy 13, 2/4 to rwy 22, and 0/1 to rwy 31.


What's disappointing is the NTSB and Airbus havn't explained to pilots what steps helped, or hindered, the ability to recover the aircraft at LGA.


The NTSB report has the FDR readout. They were below green dot when the bird strikes occurred, got slower, Sully took over and accelerated, and then ultimately slowed to slightly above stall speed despite thinking he was comfortably above Vls. Stress will do that. The FBW protections were a huge part of the successful outcome.


Starting the APU kept them in Normal Law. That kept the AOA protection that was key to a successful outcome. Are you flying a FBW Boeing? Does starting the APU keep your FBW Boeing in Normal Law?

[Bull at a Gate]

Misd-agin, not only are your figures wrong, but as the NTSB noted, "The immediate turn made by the pilots during the simulations did not reflect or account for real-world considerations."

[pattern_is_full]

Originally Posted by misd-agin

What's disappointing is the NTSB and Airbus havn't explained to pilots what steps helped, or hindered, the ability to recover the aircraft at LGA.

The sim pilots knowing ahead of time that they would experience a dual engine failure was a huge factor - but not much help in the real world!