Decision Height (DH)
Decision Height (DH) is the height above touchdown zone elevation (in feet) at which, during a precision approach, a missed approach must be initiated if the required visual reference has not been established. DH is published for ILS, LPV, and other vertically-guided approaches.
DH vs. MDA
Decision Height (DH) applies to precision approaches (ILS, LPV). The aircraft is descending on a glidepath; DH is the height at which a decision must be made immediately. Minimum Descent Altitude (MDA) applies to non-precision approaches (VOR, NDB, LNAV). The aircraft levels off at MDA and can continue to fly level in the hope of acquiring the required visual reference.
Required Visual Reference
To continue below DH and land, the pilot must have a “required visual reference” — defined as sufficient visual contact with the runway environment (threshold, touchdown zone, approach lights, or other specified visual cues) to be able to continue safely to touchdown. If this reference is not acquired at DH, a missed approach must be immediately initiated.
CAT I, II, III
- CAT I ILS: DH typically 200 ft, visibility 1/2 SM or RVR 1,800 ft
- CAT II ILS: DH typically 100 ft, RVR 1,200 ft
- CAT III ILS: DH from 50 ft to no DH, RVR as low as 75 ft (CAT IIIc)
On the TC Written Exam
Know the difference between DH (precision, descending) and MDA (non-precision, level-off). Know that at DH, if visual reference is not established, immediate missed approach is required — not a decision to try a bit harder.
Related Terms
See also: MDA, ILS, Missed Approach, Required Visual Reference.