Note: Jeppessen approach charts use the abbreviation DA (H). The decision altitude “DA” is referenced to mean sea level (MSL) and the parenthetical decision height (DH) is referenced to the TDZE or threshold elevation.
3.5. Obstacle Clearance Altitude (OCA) / Height (OCH):
The lowest altitude (OCA), or alternatively the lowest height above the elevation of the relevant runway thresholds or above the aerodrome elevation as applicable (OCH), used in establishing compliance with appropriate obstacle clearance criteria. Sometimes, the term OCL (obstacle clearance limit) can be found in the documentation, but this term is being phased out. When an operator establishes his operating aerodrome minima, he must take into account the OCH only for CAT II. The minimum DH for CAT II is always equal or higher than any OCH mentioned in the aerodrome chart. OCH is a function of the approach category of aircraft (A to E):
3.6. Obstacle Free Zone : The airspace above the inner approach surface, inner transitional surfaces, and balked landing surfaces and that portion of the strip bounded by these surfaces, which is not penetrated by any fixed obstacle other than a low-mass and frangible mounted and required for air transportation purposes.
3.7. Minimum Approach Break-off Height (MABH).
The lowest height above the ground, measured by radio altimeter, such that if a missed approach is initiated without external references.
Chapter -3 DEFINITIONS Page – 13
:
-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. 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/FCOM 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-IIIA Ops. For A319/A320/A321/A330 aircraft the AFM specifies MABH as 15 ft. Note: (In India MABH is not in use).
3.8. Alert Height (AH)
The alert height is the height above touch down, above which a CAT3 autoland would be discontinued and a missed approach executed, if a failure occurred in either the airplane systems or the relevant ground equipments.
AH defined for Cat-III Ops with fail-operational automatic landing system.
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Above AH, a missed approach must be initiated, if a failure (*) affects the fail operational automatic landing system. (*) The list of these failures is mentioned in AFM
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Below AH, if such a failure occurs, the flare, touchdown and roll out may be accomplished using the remaining automatic system. . (Except when AUTOLAND Warning is triggered).
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