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If the minimum DH is not specified in the AFM, the MABH or the information on the height loss after go around can be used to determine the minimum DH. The obstacle environment in the precision segment of the approach must permit an aeroplane coupled to the ILS by an automatic flight control system to fly safely without visual reference to the ground down to the TDZ and carry out a missed approach. In Category III operations as in other operations, the aeroplane should be capable of executing a missed approach from any height prior to touchdown. The height loss allowance used in the determination of the decision height for a Category III operation using a fail-operational automatic or hybrid system because the fail-operational characteristics assure that the landing flare will occur. Moreover the missed approach height loss will become less as the height of missed approach initiation decreases. For Category III operations with landing systems which are less than fail operational (e.g. a fail-passive system), the flare is not assured and consequently a height loss allowance may be used in the determination of the decision height.
In those Category III operations where DH are used, specific DH are associated with RVRs. They are generally specified at or below 50ft. Their purpose is to specify the lowest height at which a pilot must be assured that an aeroplane is being satisfactorily delivered to the runway and that adequate visual reference is available for control of the initial part of the landing roll.
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i) For Category III fail-passive operations, a DH is used. ii) For Category III fail-operational operations either a DH or
an Alert Height may be used. iii) If a DH is used, any necessary visual reference is specified. iv) Where necessary the published DH takes into account the
terrain profile before the runway threshold, e.g. Delhi RWY 28 with terrain before the runway sloping down to the threshold, DH may be indicated as:
Radio Altimeter (RA) 95 ft for DH 100 ft
4.3 Visual Reference at DH.
The required visual reference means the visual aids of that section of the runway or of approach area which should have been in view for sufficient time for the pilot to have made an assessment of the aircraft position and rate of change of position in relation to the desired flight path.
4.4 Visual Reference for CAT II Ops with DH
In CAT II operations the conditions required at DH to continue the approach is that the visual references should be adequate to monitor the continued approach and landing, and that the flight path should be acceptable. If both these conditions are not satisfied, it is mandatory to initiate a go around. For CAT II approaches, having visual references means being able to see to land (i.e. being able to land manually) It is generally accepted that visual segment requires is 120 m. for CAT-II Ops (90 m. with automatic approach till touchdown)
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