I just saw the Sully movie, and it reminded me to dig into the NTSB Accident Report about how the ditching was done.
Why was Airbus not admonished for their flight control laws fighting Sully's pitch up flare commands?
(I'm thinking Airbus should have been told to phase out the phugoid damping at low radar altimeter near and at flare. Phugoid damping is fine at higher altitudes of course.)
Seems obvious to me. Can you shed some light on this?
The fly-by-wire control law software injected phugoid damping at very low AGL (100' and below), resulting in a slightly lower pitch angle (9.5 deg) at touchdown than actually recommended by Airbus and other studies (10 to 12 degrees; 11 is ideal).
Sully had full aft stick in the last 2 seconds before impact, trying to get a couple more degrees of pitch, which would have given him about a 12 degree pitch angle at touchdown. Skiles remarked the airplane submarined a bit on impact, consistent with ditching studies and the 9.5 degree pitch angle they struck the water with. Any less pitch and the nose might have cracked open.
The airplane had about 4 degrees of alpha stall margin remaining at touchdown which was never used. Important fact.
Since the phugoid is a roller coaster up-down slow cycle, why are phugoid damping terms allowed to pitch the aircraft down close to the ground when the ground is right there, close up, and the next event is touchdown, so the phugoid can't be excited anyway?
References:
From the Aircraft Performance Study:
"Figure 7 shows that between 15:30:36 and the touchdown at 15:30:43, the pitch angle increases from 9.5° to 11° and then settles back to 9.5°, even though in the last two seconds the left longitudinal side stick is at its aft limit, and α is below αmax. "
" A phugoid damping feedback term in the flight control laws, that is active in α−protection mode, attenuated the airplane’s nose-up pitch response to progressively larger aft side stick inputs made below 100 ft radio altitude. "
The airplane touched down on the Hudson at 15:30:43, under the following conditions:
Airspeed (FDR): 125 KCAS
Groundspeed (computed): 126 knots
Rate of descent (computed): -750 ft/min
γ (relative to Earth, computed): -3.4°
γ (relative to airmass, computed): -3.5°
θ (FDR): 9.5°
φ (FDR): 0.4°
α (computed): 12.9° α (measured): 13.7° (left vane), 14.5° (right vane)
β (computed): 2.2°
Drift angle (computed): -1.4°
Track angle (computed): 210.7°
true Heading angle (FDR): 209.9° true
From the NTSB Accident Report:
"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°). Airbus’ training curricula does not contain information on the effects of alpha-protection mode features that might affect the airplane’s response to pilot sidestick pitch inputs. The flight envelope protections allowed the captain to pull full aft on the sidestick without the risk of stalling the airplane.
The NTSB concludes that training pilots that sidestick inputs may be attenuated when the airplane is in the alpha-protection mode would provide them with a better understanding of how entering the alpha-protection mode may affect the pitch response of the airplane. The NTSB recommends that the FAA require Airbus operators to expand the AOA-protection envelope limitations ground-school training to inform pilots about alpha-protection mode features while in normal law that can affect the pitch response of the airplane. "
https://www.ntsb.gov/investigations/...ts/AAR1003.pdf
http://www.exosphere3d.com/pubwww/pd...ket/431658.pdf