*From*: pdodds@xxxxxxxxxxxx (Peter Dodds)*To*: jhb_airlines@xxxxxxxxxxxxx*Date*: Thu, 27 Oct 2005 16:06 +0100 (BST)

The Decision Height DH or Minimum Descent Height MDH isn't on the charts because it is a calculated value for different conditions and different airfields. For a precision approach, the DH is defined For a non-precision approach, the MDH is defined. The definitions differ according to the pilot's qualifications. For a precision approach, the minimum height to which a pilot must descend before executing a missed approach is referred to as the Decision Height. (For a non-precision approach, it is referred to as Minimum Descent Height). The DH depends on a number of factors. Using Hawarden Runway 23 ILS Approach Plate for example: - a) Obstacle clearance height (OCH) for a category A aircraft with a 3% climb grade performance capability is 300 feet above aerodrome level (QFE). b) Add the aerodrome threshold elevation (from the approach plate) (17 feet) to obtain the OCH above sea level of 317 feet (QNH). c) The minimum descent height for an ILS approach is 200 feet aal. Check which of a) or b) is the greater. In this case it is a). d) Is an altimeter correction necessary? For a precision approach, an altimeter correction of +50 feet is required. e) IMC rated pilots must add 200 feet to the minimum descent height thus far calculated. The minimum is therefore increased to 550 feet aal. f) The absolute minimum descent height for an IMC rated pilot on an ILS approach is 500 feet aal. Check if d) is greater than 500. g) In this case it is, so the Decision Height (DH) is 550 feet, and the Decision Altitude (DA) is 567 feet. Bones will no doubt be able to add to this in respect of CAT I, CAT II and CAT III ILS approaches. Petr

**Follow-Ups**:**[jhb_airlines] Re: Decision height***From:*Bones

**References**:**[jhb_airlines] Decision height***From:*Phil Reynolds

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