Alert Height and MABH

Alert Height

An alert height is a predetermined altitude above the threshold of the runway, calculated based on the aircraft’s characteristics and the reliability of its fail-operational landing system. It represents the point at which a Category III operation would be terminated, and a missed approach initiated, should a failure occur in any redundant component of the landing system or the associated ground equipment.

MABH – Minimum Approach Break-off Height

The Minimum Approach Break-off Height (MABH) is the lowest height above the ground, measured by radio-altimeter, such that if a missed approach is initiated without external references:
− in normal operation, the aircraft does not touch the ground during the procedure;
− with an engine failure during a missed approach, it can be demonstrated that taking this failure probability, an accident is extremely improbable.

The MABH will remain indicated in some Airbus AFM (except FAA AFM). For some Airbus type the MABH is replaced by an indication of the minimum DH and for more recent certifications the MABH will be replaced in the procedure section of the AFM by an indication of the altitude loss during automatic go around.

The MABH or the altitude loss during automatic go around can be used by the airlines to determine the minimum DH in CAT III operation

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