FAA Instrument Rating Airplane (IRA)IFR Procedures and ChartsMedium
A pilot is flying an ILS approach to Runway 36. The approach chart shows a Decision Altitude (DA) of 480 feet MSL. The airport elevation is 120 feet MSL. At what altitude above the runway threshold should the pilot make the decision to continue or execute a missed approach?
- A120 feet above runway threshold.
- B480 feet above runway threshold.
- C360 feet above runway threshold.
- DThe DA is an MSL altitude, not an AGL altitude, so the question is invalid.
Show answer & explanationAnswer & explanation
Correct answer: C. 360 feet above runway threshold.
Decision Altitude (DA) is expressed in feet MSL. To find the height above the runway threshold (which is essentially AGL relative to the airport elevation, assuming the threshold is near the airport elevation), subtract the airport elevation from the DA. 480 feet MSL (DA) - 120 feet MSL (Airport Elevation) = 360 feet AGL.
Why the other options are wrong
- A. This is the airport elevation, not the decision height.
- B. DA is MSL, not AGL relative to the threshold.
- D. While DA is MSL, the question asks for the equivalent AGL height above the threshold, which is a valid and common calculation for pilots.
Decision Altitude (DA) vs. Decision Height (DH)
Decision Altitude (DA) is a specified altitude in MSL on a precision approach at which a missed approach must be initiated if the required visual reference to the runway environment has not been established. Decision Height (DH) is the equivalent height above the runway threshold (AGL).
- DA is MSL; DH is AGL.
- DA - Airport Elevation ≈ DH.
- Used for precision approaches (ILS, PAR, GLS, LPV, LNAV/VNAV).
- Initiate missed approach if visual reference is not met at DA/DH.
Memory trick: DA: 'Datum Absolute' (MSL). DH: 'Datum Height' (AGL).