Originally Posted by
KayPam
Based on your info that I was not aware of :
Alpha max is 17.5°, Sully touched with 9.5° of pitch and 750fpm which gives -3.5° of flight path angle (this gives an AOA of 13° instead of 14°, so this calculation is precise within 1°)
If there had been an AoA increase as demanded by Sully : full back stick and 17.5° of AoA : we would have had something between -2 and 0° of flight path angle, so pitch angle would have been somewhere between 15.5 and 17.5° : way too much (even with the 1° error margin, 14.5 is still 2.5° above the 12° limit you quoted)
Alpha doesn't stay constant during flare. Yes, in this case, he wanted to get closer to stall (not a stall, just closer). Flare consists of raising the nose and stopping at a pitch angle, in this case it would have been 11 or 12 degrees pitch. This is the ditching deck angle. Then holding it until impact.
Originally Posted by
KayPam
But on the other hand, maybe Sully was instinctively targeting 11-12° of pitch, and was pulling full back only because the aircraft was not responding.
Yes, thats clear from seeing him at full aft stick trying to get more pitch.
Originally Posted by
tubby linton
Looking at some old training material high AoA protection used to be deactivated below 100ftRA but a diagram in the FCOM dated 2012 shows that is now also available in flare mode. Flare mode becomes active 1second after 50ft Ra. I wonder what the bus would have done if he had used a handful of nose up trim to get the nose up?
Can you elaborate on that? It gets to the heart of this thread topic. Appreciate your succinct response, thanks. It makes sense to let alpha get all the way to stall, if necessary, at the start of the flare, since touchdown happens very soon after.
In the other A320 accidents people have brought up in this thread, it does appear alpha-protection was on during flare though.
Still, getting rid of alpha-protection isn't the way I'd do it as a flight control system engineer. I'd keep the avoidance of a deep stall, but simply allow a near-stall when close to the ground with a descent rate happening.
On a related topic, why didn't the copilot say "We are flying too slow." 3 minutes to touchdown, maybe even repeat that? Both guys freaking out?
Originally Posted by
tubby linton
Looking at some old training material high AoA protection used to be deactivated below 100ftRA but a diagram in the FCOM dated 2012 shows that is now also available in flare mode. Flare mode becomes active 1second after 50ft Ra.
I found the 2012 FCOM you mentioned. Alpha Protection is there for flare.
Now to find the older material you mentioned that omitted AoA protection below 100' (maybe an old manual error?).
The damping feedback nosed the aircraft down in flare, that is where it "popped up". The NTSB never faulted the damping term problems and I still don't agree the nose-lowering should have occurred so
close to the ground. (We all can agree with the NTSB findings regarding low airspeeds, which caused flare to be flown in AoA Protection, yet the concept of AoA Protect isn't bad, its the additional phugoid feedback terms in flare that are puzzling.)
Originally Posted by
misd-agin
Vilas - we haven't been given exact airspeed but they might have been 5-6 kts above stall while misreading their airspeed. This occurred at 150'. At 100', while still unaware of how close to a stall they were and that the AOA protection was actively proteciting them from a stall, he attempts to raise the nose. I don't know of any pilot, knowing that they're perhaps 5-6 kts above stall speed, that would knowingly raise the nose at 100'. It might be a life altering experience. While FBW might not be perfect it was a significant factor in the survival rate.
I agree AoA Protect is a good thing. It can prevent disaster, no doubt.
In this pre-flare and flare case, Sully got it to the correct ditching pitch angle (11 degrees) 4 seconds before impact, and then the AoA Protection unwanted damping terms lowered the nose to 9.5 degrees. Survivable, but not best. Holding the 11 degrees would not have stalled the aircraft since Alpha Max was still high enough. See the actual trace:
