Obstacle Clearance Altitude (OCA)

Obstacle Clearance Altitude (OCA) is the lowest altitude or height above the relevant runway threshold or aerodrome elevation, used to ensure an aircraft avoids obstacles during flight, providing a safety margin.

Purpose

OCA/Height (OCH) ensures that aircraft maintain a safe vertical distance from obstacles during takeoff, in-flight maneuvers, and landing, especially in areas with numerous ground or aerial obstacles.

Historical Context

• The concept of obstacle clearance evolved alongside the development of aviation and instrument flight procedures.
• Early flight procedures focused on visual flight rules (VFR), but as aviation technology advanced, the need for instrument flight rules (IFR) and procedures to navigate in low visibility conditions became crucial.
• The International Civil Aviation Organization (ICAO) played a significant role in standardizing these procedures, including the development of OCA/H.
• ICAO PANS-OPS (Procedures for Air Navigation Services - Obstacle Clearance) is a key document that outlines the principles and criteria for designing instrument approach procedures, including the calculation of OCA/H.

ICAO PANS-OPS and OCA/H

• PANS-OPS specifies the minimum obstacle clearance criteria, including the required vertical and horizontal clearances from obstacles.
• OCA/H is calculated by adding the minimum obstacle clearance (MOC) allowance to the height of the highest obstacle within a specified area.
• OCA/H is published on the instrument procedure plate and is aircraft category dependent.

OCA vs. OCH

• OCA (Obstacle Clearance Altitude): Refers to the lowest altitude above mean sea level (AMSL) that an aircraft must maintain to ensure obstacle clearance.
• OCH (Obstacle Clearance Height): Refers to the lowest height above the elevation of the relevant runway threshold or aerodrome elevation, if the threshold is more than 2 meters (7 feet) below aerodrome elevation.

Importance of OCA/H

• OCA/H is a critical safety parameter in instrument approach procedures.
• Pilots must adhere to the published OCA/H to ensure that they maintain a safe vertical clearance from obstacles during the approach and landing.
• Understanding the basis for the calculation of minima (OCA/H) is essential for pilots, as it can vary depending on the approach procedure and the applicable regulations.

Example

If the highest obstacle within a specified area is 100 feet above the aerodrome elevation and the MOC allowance is 50 feet, the OCA/H would be 150 feet above the aerodrome elevation.

Related Terms

• MOC (Minimum Obstacle Clearance): The vertical clearance required to ensure obstacle clearance.
• MDA/H (Minimum Descent Altitude/Height): The lowest altitude/height to which an aircraft may be descended during an instrument approach.
• TERPS (US Terrain Procedures): The US system for calculating obstacle clearance during instrument approaches.

Historical Evolution and Current Standards

• The concept of obstacle clearance has evolved significantly over time, with advancements in navigation technology and flight procedures.
• Modern instrument approach procedures rely on precise navigation systems and sophisticated obstacle clearance calculations to ensure safe and efficient operations.
• ICAO PANS-OPS continues to be the primary standard for obstacle clearance criteria, with national aviation authorities implementing and adapting these standards to their specific needs.