From the MOM
The two LRRA systems provide height above ground readings to several
aircraft systems including the instrument displays, autothrottle, autopilots
and configuration/ground proximity warning. If one LRRA provides erroneous
altitude readings, typical flight deck effects, which require flight crew
intervention whether or not accompanied by an LRRA fault flag, include:
- Large differences between displayed radio altitudes, including radio
altitude readings of -8 feet in flight.
- Inability to engage both autopilots in dual channel APP (Approach)
mode
- Unexpected removal of the Flight Director Command Bars during
approach
- Unexpected Configuration Warnings during approach, go-around and
initial climb after takeoff
- Premature FMA (Flight Mode Annunciation) indicating autothrottle
RETARD mode during approach phase with the airplane above 27 feet AGL. There
will also be corresponding throttle movement towards the idle stop.
Additionally, the FMA will continue to indicate RETARD after the throttles
have reached the idle stop