What happens if, during an automatic approach and landing, the radio altimeter fails?
When performing an automatic landing, a radio altimeter is required because the decision heights below CAT I limits (200 ft) are based on radio height and because the autopilot moves from one mode to another, for example, from FLARE to ROLLOUT, using a variety of inputs, particularly radio height.
Should the radio altimeter fail during the autoland procedure, the height indication is removed and replaced by a warning flag. As the individual steps of the automatic landing procedure are triggered by radio heights, this situation requires the aircraft to go around and be landed manually.
As for the incorrect options:
- Glideslope relies upon the ILS and not the radio altimeter.
- The autobrake relies on weight-on-wheels, throttles retarded and wheels spun up to speed.
- The autoland will not be fully operational.
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