From the NTSB investigation of the A320 ditchng in the Hudson river on 15 January 2009:
NTSB Accident Number DCA09MAS026, Docket Item 86
Aircraft Performance 13 - Factual Report of Group Chairman
url=http://dms.ntsb.gov/pubdms/search/document.cfm?docID=322563&docketID=47230&mkey
Reference 8 documents an Airbus simulation of the last 300 ft of the flight, and indicates that the airplane was performing as designed as was in α-protection mode from 150 ft to touchdown. Per Reference 6 (quoted above), in α-protection mode, “the angle of attack is proportional to side stick deflection. That is, in the αprot range, from αprot to αmax the side stick commands α directly” while keeping α < αmax. However, in α-protection mode, the flight control system incorporates a phugoid-damping feedback term in addition to side stick commands when computing the commanded elevator position (which in turn determines the pitch angle response). As described by Airbus,
… the aircraft was in angle-of attack (AoA) protection from about 150 ft RA.
When in AoA protection law, stick command is AoA objective. Stick neutral commands alpha-prot and
full back stick commands alpha-max.
However, AoA protection shall take care of the A/C trajectory and, thus, looks after phugoid damping
as well as AoA control: there are feedbacks within the AoA protection law aiming at damping the
phugoid mode (low frequency mode). The feedbacks are CAS and pitch attitude variations. Without
these feedbacks, an aircraft upset from its stabilized flight point up to constant high AoA would enter
a phugoid (which is, by definition, a constant AoA oscillation) without possibility to stabilize the
trajectory. As a consequence, commanded AoA is modulated as a function of speed and attitude
variations: for instance, if A/C speed is decreasing and/or pitch attitude is increasing, pilot's
commanded AoA is lowered in order to avoid such a situation to degrade.
On the last 10 sec of the "Hudson" event, it is confirmed that pitch attitude is increasing and CAS
decreasing. Then, the phugoid damping terms are non nul and are acting in the sense to decrease
the finally commanded AoA vs. the stick command, in order to prevent the aircraft from increasing the
phugoid features.
Based on this explanation, it appears that on the accident flight, the nose-up side stick commands from 15:30:36 to 15:30:43 were offset somewhat by the phugoid-damping feedback term, thereby limiting the pitch angle and α increase below 150 ft radio altitude.
E. Conclusions
(…) 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.