Obstacle Clearance Height (OCH) is the lowest height above the airport elevation or runway threshold at which an aircraft can safely descend on an instrument approach, ensuring required clearance over terrain and obstacles. It forms the foundation for calculating approach minima, such as Minimum Descent Altitude (MDA) or Decision Altitude (DA).
In ICAO terminology:

• OCH is used when the altitude is measured above airport elevation (height).
• The corresponding Obstacle Clearance Altitude (OCA) is expressed in MSL (altitude).

• Guarantee a minimum vertical buffer between the aircraft and terrain/obstacles
• Establish the minimum safe height at which an aircraft may descend on an instrument approach
• Provide the basis for MDA/DA minima by aircraft category and procedure type
• Ensure consistency and safety in procedure design globally

These values may vary slightly depending on national regulations or procedure design criteria (e.g., PANS-OPS vs. TERPS).

• During procedure design to determine safe descent limits
• By pilots to understand how low they can safely descend on approach
• In training and checkride scenarios to evaluate approach performance

• ✅ Always compare OCH to airport elevation for situational awareness
• ✅ Understand the difference between OCH and MDA — MDA includes operational buffer
• ✅ Refer to OCH values when reviewing approach chart minima
• ✅ Use LearnATC’s approach chart decoder tools to visualize protected areas and obstacle clearance profiles