Obstacle Clearance Height (OCH)
Definition
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).
Purpose
- 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
Standard Obstacle Clearance Values
| Approach Segment | Minimum Clearance |
|---|---|
| Final segment (non-precision) | 295 ft above highest obstacle in the protected area |
| Final segment (precision CAT I) | 250 ft |
| Missed approach segment | 30 ft minimum margin above obstacles |
| Circling approach | Varies by aircraft category (see circling minima) |
These values may vary slightly depending on national regulations or procedure design criteria (e.g., PANS-OPS vs. TERPS).
How OCH Relates to Approach Minima
Example on Approach Chart
When OCH Is Used
- 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
Tips for Pilots
- ✅ 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