OCA is referenced to:

• A. the Aerodrome Reference Point
• B. the relevant Runway Threshold
• C. Mean Sea Level
• D. an Aerodrome Elevation

Answer: (C)

OCA stands for Obstacle Clearance Altitude, and it is published relative to Mean Sea Level. The reason this is the reference is to keep obstacle clearance consistent regardless of how high or low the airport sits above sea level. Publishing OCA in feet above MSL lets pilots determine the exact altitude to maintain along an approach path in relation to sea level, so obstacle clearance is comparable from one aerodrome to another.

The field elevation (the airport’s elevation above MSL) or the runway threshold are not the reference for OCA. Those numbers describe local height relative to MSL or a physical location, but they do not define the minimum safe altitude along the approach. For example, if the aerodrome sits at 700 ft MSL and the OCA is 1,200 ft MSL, the aircraft must be at 1,200 ft MSL to ensure obstacle clearance, not simply at a height relative to the runway or a fixed point on the field.

That’s why the correct reference for OCA is Mean Sea Level.