This is a cut and paste from the Airbus 330/340 FCTM (Flight Crew Training Manual)
"DUAL RADIO ALTIMETER FAILURE
The Radio Altimeters (RAs) provide inputs to a number of systems, including the GPWS and FWC for auto-callouts. They also supply information to the AP and A/THR modes, plus inputs to switch flight control laws at various stages.
Although the ECAM procedure for a RA 1 + 2 FAULT is straightforward, the consequences of the failure on the aircraft operation require consideration.
Instead of using RA information, the flight control system uses inputs from the LGCIU to determine mode switching. Consequently, mode switching is as follows:
·
At take-off, normal law becomes active when the MLG is no longer
compressed and pitch attitude becomes greater than 8°
· On approach, flare law becomes active when the L/G is selected down and
provided AP is disconnected. At this point, “USE MAN PITCH TRIM” is
displayed on the PFD.
·
After landing, ground law becomes active when the MLG is compressed and
the pitch attitude becomes less than 2.5°
It is not possible to capture the ILS using the APPR pb and the approach must be flown to CAT 1 limits only. However, it is possible to capture the localiser using the LOC pb.
Furthermore, the final stages of the approach should be flown using raw data in order to avoid possible excessive roll rates if LOC is still engaged. Indeed, as the autopilot gains are no longer updated with the radio altitude signal, the AP/FD behaviour may be unsatisfactory when approaching the ground.
There will be no auto-callouts on approach, and no “RETARD” call in the flare
The GPWS/EGPWS will be inoperative; therefore terrain awareness becomes very important. Similarly, the “SPEED, SPEED, SPEED” low energy warning is also inoperative, again requiring increased awareness."
Nice document the Airbus FCTM
. LGCIU = Landing Gear Control Interface Unit.