Obstacle Clearance Altitude (OCA) is the lowest altitude above mean sea level at which an aircraft must fly during an instrument approach to ensure safe vertical clearance above obstacles. It is a key safety element in instrument flight procedures, determined through rigorous regulatory standards.
Obstacle Clearance Altitude (OCA) is the lowest altitude above mean sea level (AMSL) that an aircraft must cross a specified point during an instrument approach procedure to ensure a safe vertical margin above any obstacles. OCA is a critical safety parameter, ensuring that even in poor visibility or instrument meteorological conditions (IMC), aircraft remain safely separated from terrain and structures.
OCA is determined for every approach procedure through thorough obstacle assessment in the protected area of each approach segment, following international standards like ICAO PANS-OPS or FAA TERPS. When expressed as a height above aerodrome or runway threshold elevation, it is called Obstacle Clearance Height (OCH). Often, charts publish OCA/H to denote both altitude (AMSL) and height (above threshold).
| Term | Reference Datum | Use |
|---|---|---|
| OCA | Mean Sea Level (AMSL) | Aircraft altimeters, charting |
| OCH | Runway threshold/Aerodrome elevation | Procedure design, obstacle margin |
OCA’s purpose is singular and vital: guarantee obstacle clearance during all phases of instrument approaches. It underpins approach minima—Minimum Descent Altitude (MDA) or Decision Altitude (DA)—by setting the lowest permissible flight level without visual contact with the runway. If the required visual cues are not acquired at OCA/H, a missed approach must be initiated.
OCA is used in:
Its role is foundational in preventing Controlled Flight Into Terrain (CFIT) and supporting safe IFR operations worldwide.
ICAO’s PANS-OPS (Procedures for Air Navigation Services – Aircraft Operations), documented in Doc 8168, is the global standard for procedure design and obstacle clearance. It prescribes:
FAA TERPS (Terminal Instrument Procedures), outlined in FAA Order 8260.3, governs U.S. procedure design. While sharing ICAO’s intent, TERPS uses slightly different calculations and MOC values tailored to U.S. airspace and infrastructure.
Both standards ensure OCA/H values reflect a rigorous, harmonized approach to obstacle clearance, balancing efficiency and safety.
Instrument approaches are divided into segments, each with a protected area and defined MOC:
| Segment | Purpose |
|---|---|
| Enroute | Safe cruise |
| Initial Approach | Transition from enroute |
| Intermediate Approach | Alignment with final approach |
| Final Approach | Descent to minima |
| Missed Approach | Safe climb-out after go-around |
Protected areas are shaped to account for navigation accuracy, wind, and aircraft category. The highest obstacle in each is identified.
Each segment applies a standard MOC above the highest obstacle:
| Segment | ICAO MOC | Typical TERPS MOC |
|---|---|---|
| Enroute | 1,000 ft (2,000 ft mountainous) | 1,000–2,000 ft |
| Initial Approach | 300 ft | 300 ft |
| Intermediate | 150 ft | 150 ft |
| Final (Precision) | 75 ft | 100 ft+ |
| Final (Non-Precision) | 295 ft | 250–350 ft |
| Missed Approach | 30 ft | 40 ft |
Example:
OCH = 120 + 295 = 415 ft above threshold
OCA = 150 + 415 = 565 ft AMSL
OCA’s origins date to early instrument flying, with conservative minima. Standardization began with ICAO in 1944 and matured with PANS-OPS (Doc 8168) and national systems like TERPS. GNSS, RNP, and PBN technologies have enabled more precise OCA/H determinations, supporting both safety and operational efficiency.
| Category | Circling Radius (NM) | MOC (ICAO) |
|---|---|---|
| A | 1.68 | 394 ft |
| B | 2.66 | 394 ft |
| C | 3.28 | 394 ft |
| D | 4.20 | 394 ft |
Straight-in Non-Precision Approach
Highest obstacle: 80 ft above threshold
MOC: 295 ft
Threshold elevation: 150 ft AMSL
OCH = 80 + 295 = 375 ft above threshold
OCA = 150 + 375 = 525 ft AMSL
Missed Approach
Highest obstacle: 50 ft above aerodrome
MOC: 30 ft
Aerodrome elevation: 110 ft AMSL
OCH = 50 + 30 = 80 ft above elevation
OCA = 110 + 80 = 190 ft AMSL
The Obstacle Clearance Altitude (OCA) is a cornerstone of instrument flight safety, ensuring that aircraft remain clear of obstacles during all phases of approach, missed approach, and circling, even in poor visibility. Its rigorous determination and universal application through international standards allow for safe, predictable, and efficient instrument operations worldwide.
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