Originally Posted by
QuagmireAirlines
There was an A320 water ditching study done (I lost the link for it, I'll see if I can find it again later) that said 10 to 12 degrees pitch angle is OK. (Sully hit at 9.5 deg pitch, so not bad, only a tad low.) Airbus also recommended strongly that you hit the water with a pitch angle of 11 degrees. This has been looked at, and I think you essentially want to do a 3-point landing on the 2 engine nacelles and tip of the tail at the same time.
If Sully could have used that last bit of 3.5 deg stall margin (pitch up) that the Airbus control laws denied him during the last 50 feet or so, he would have hit at about 12 degrees, with a lower vertical speed, all better.
To me the perfect landing would have been 11 degrees pitch, 1 degree stall margin remaining, and the slow airspeed that corresponds to, while hitting the water at 500 ft/min. Let's call perfect an "11/1/500". Instead, Sully did it as an "9.5/3.5/750" and I blame that on some unwanted feedback terms in alpha-protect below 50' AGL.
Okay there has been some studying and simulation after the cactus 1549 accident-landing.
(But at the time of this water landing, these studies had not been done)
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)
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.
So you can see that the situation is not very straightforward and yes, the optimum pitch angle for ditching has been studied but it's very hard to say what the situation would have been with a direct law instead of a normal law.
Originally Posted by
QuagmireAirlines
About that 1988 Paris Air Show accident, did the pilot have any stall margin remaining? I haven't read anything about that accident in a long while. I was in the middle of control law development for some MD-11 modes at the time it happened.
Yes, that's an inconsistency of the report itself !
Alpha max is quoted at -17.5°. The report itself says that if the pilot pulls full back then they will get alpha max. The report says that the pilot pulled full back.
But the report also shows, in the flight data tables, that aoa never went above 15°.
So there was a +2.5° margin relative to alpha max !
That's a problem in itself.
I believe that had the aircraft responded correctly to the full back stick input, maybe the gears would have caught some leaves but the aircraft would have had a much better chance of going around "correctly" and remain in flight.
Because there was a slight reserve of speed but that could have been enough to go above the trees, and once the aircraft was above the trees, the engines had spooled up.
Any one with a bit of physics knowledge could go into the flight data tables and read the values. Then :
- compute V alpha max
- compare V alpha max with the speed before impact
- conclude if this speed difference converted into height could have been enough to go around the trees. (based on approximate estimation of the fuselage's height under the tree line)
These reports are very carefully written, toughly proofread and thoroughly revised. So any inconsistency can be regarded as highly suspicious.
Best case scenario explanation : it was written and proofread in a hurry.