Obstacle Clearance Areas (OCA)
Definition
An Obstacle Clearance Area (OCA) is a designated volume of airspace within which aircraft are guaranteed a minimum vertical or lateral separation from terrain and obstacles. These areas are part of the design of instrument flight procedures (IFPs) and ensure that aircraft operating under IFR remain safe from terrain or obstacles, even during low visibility or non-precision approaches.
OCAs are calculated and protected by procedure designers following ICAO PANS-OPS or TERPS criteria, depending on the region.
Purpose
Where OCAs Are Applied
| Flight Phase | OCA Purpose |
|---|---|
| Initial/Intermediate segments | Guarantee terrain clearance during transitions from enroute to approach |
| Final approach | Provide protection from obstacles along the descent path |
| Circling | Define a protected area around the runway where circling is permitted |
| Missed approach | Ensure aircraft can climb safely if a missed approach is initiated |
| Departure (SID) | Provide obstacle clearance until reaching enroute structure |
Standard Vertical Obstacle Clearance Values (ICAO)
| Segment | Minimum Clearance |
|---|---|
| Enroute | 1,000 ft (2,000 ft in mountainous areas) |
| Initial Approach | 300 ft |
| Intermediate Segment | 150 ft |
| Final Segment | Varies by procedure type (often 75 ft for precision, 295 ft for non-precision) |
| Missed Approach | 30 ft above obstacles within protected area |
Obstacle Clearance Height (OCH) vs. Minimum Descent Altitude (MDA)
Circling Approach Obstacle Clearance Areas
| Aircraft Category | Radius (based on speed) |
|---|---|
| A (≤ 91 KIAS) | ~1.68 NM |
| B (≤ 120 KIAS) | ~2.66 NM |
| C (≤ 140 KIAS) | ~3.28 NM |
| D (≤ 165 KIAS) | ~4.20 NM |
These areas are centered on the runway threshold and assume a bank angle and maneuvering speed.
Depiction on Charts
OCAs themselves are not shown directly, but their limits determine: