Originally Posted by A4
I'd be interested in your source for this. Are you referring to Airbus narrow body or 330/340?
It is the same for the narrow bodies and wide bodies. You may have a reference to flare law on 1.27.30 depending if your manuals are stock standard or company tailored. It is irrelevant to the crew face with such a situation, telling them direct law will put them in the correct frame of mind.
Your reference to FCOM 3 does not show direct law in all 3 axis (like triple IR), and in pitch you still have pitch rate feedback.
This is an excerpt from the Airbus A320 instructors manual.
9 - DOUBLE RADIO ALTIMETER FAILURE
The double Radio Altimeter (R/A) failure has various consequences on many systems of the a/c, since the R/A information is used to switch flight control laws or AP - ATHR modes etc. or in auto call outs and warnings. It is clearly materialized by RA red flag on PFD (just underneath the horizon).
The procedure is simple, since the ECAM provides all required information. However, the consequences of such malfunction affect the a/c normal operations in many directions.
Consequences upon Fly by Wire systems:
Rather than using R/A information, the FBW systems use the LGCIU outputs for most of the logics.
a. APPROACH
- The flare law (usually blended in at 50 ft) engages when the landing gear is extended.
- The warning USE MANUAL PITCH TRIM comes up few seconds later on PFD, in order to remind the crew that the remainder of the approach is to be flown in a comfortable direct law (with pitch rate feed back).
b. LANDING
- The ground law engages when MLG is compressed and pitch attitude is less than 2.5°.
- The ground spoilers extend using wheel speed information.
c. WARNINGs
- The low energy warning is lost in case of double R/A malfunction.
Consequences for FMGC:
Basically LAND modes do not engage, since it ensures that (for autoland) subsequently FLARE and ROLLOUT will engage which is not possible as the R/A information is missing.
Consequently (landing gear extended):
- FD basic modes available only,
- approach modes are not available (no LOC, no G/S),
- autoland modes are not available (no LAND, no FLARE, no ROLLOUT),
- ATHR remains in speed mode, in approach (no RETARD),
- APs are lost.
Consequences for GPWS / EGPWS:
Most functions require R/A signal information. Therefore GPWS and EGPWS, if applicable, are lost.
Originally Posted by CONF iture
His speed was low, but as proved by AoA + attitude data, there was ample space for touchdown improvement as requested by the pilot.
There was still a margin of 3.5 degrees to reach alpha max, and probably 3 additional degrees to reach the AoA for a stall.
The features prevented the pilot to flare the airplane as he would have loved.
For a ditching, Airbus recommends a touchdown with approximately 11 degrees of pitch and minimum aircraft vertical speed, but despite the full backstick, the protections did not allow more than 9.5 degrees and the pilots could do nothing to decrease that impressive vertical speed of 750 feet a minute.
You are mixing up pitch attitude with angle of attack, a common mistake made by amatures.
You assertions that a higher attitude or AoA would have resulted in a better outcome are not based upon any fact. Vertical speed is the killer, and I have yet to see a single aerodynamics text that states that minimum vertical speed is only achieved at maximum AoA or maximum pitch angle.
Even on the 777/787 (QRH Ditching checklist) they say to maintain VREF+30 until touchdown and flare to achieve the minimum rate of descent. Even Boeing does not advocate maximum AoA or pitch attitudes, they advocate minimum ROD.
Fact is the aircraft landed intact, and all passengers and crew walked away without a single loss of life, something that is very rare with any forced landing on water, regardless of the aircraft size.