Clearance Altitude: Minimum Altitude Providing Obstacle Clearance

Clearance altitude is a cornerstone of aviation safety. It represents the lowest altitude an aircraft can safely fly, ensuring adequate separation from terrain and both natural and man-made obstacles. This concept is embedded in every phase of flight—planning, enroute navigation, and approach—and is enforced by both national and international regulations.

Definition

Clearance altitude is the established minimum altitude required for an aircraft to safely clear all obstacles and terrain within a specified area or route segment. Determined by regulatory authorities such as the FAA and ICAO, it incorporates detailed surveys of terrain, obstacles, and built-in safety buffers (typically 1,000 feet in non-mountainous regions, 2,000 feet in mountainous areas per ICAO Doc 8168). Clearance altitudes are always referenced to mean sea level (MSL) unless otherwise specified and are published on official aeronautical charts.

Clearance altitude forms the basis for several specialized minima used in aviation:

• Minimum Enroute Altitude (MEA)
• Minimum Obstruction Clearance Altitude (MOCA)
• Off-Route Obstruction Clearance Altitude (OROCA)
• Obstacle Clearance Altitude (OCA) / Obstacle Clearance Height (OCH)
• Maximum Elevation Figure (MEF)

These values support safe flight under both instrument (IFR) and visual (VFR) conditions.

Minimum Enroute Altitude (MEA)

What is MEA?

Minimum Enroute Altitude (MEA) is the lowest published altitude between fixed navigation points that:

• Provides adequate obstacle clearance (1,000 or 2,000 feet depending on terrain type)
• Assures reliable navigation reception throughout the route segment

Where is MEA Used?

MEA is exclusive to IFR operations on published airways (Victor airways, Jet routes, and RNAV routes). It is annotated on IFR enroute charts next to each airway segment.

How is MEA Determined?

Authorities perform detailed terrain and obstacle analysis within a corridor (typically 4 NM each side of the airway). The highest obstacle sets the baseline, to which the regulatory buffer is added. Navigation signal coverage is tested to ensure reliability at the MEA.

Application Example

If an airway segment has an MEA of 7,000 feet, IFR aircraft must remain at or above 7,000 feet, ensuring both obstacle clearance and navigation aid reception.

Minimum Obstruction Clearance Altitude (MOCA)

What is MOCA?

Minimum Obstruction Clearance Altitude (MOCA) is the lowest published altitude between navigation fixes on an airway segment that:

• Provides required obstacle clearance
• Guarantees navigation signal coverage only within 22 NM of a VOR

MOCA enables lower flight where possible, particularly for weather avoidance or operational necessity, but with limited navigation assurance.

Where is MOCA Used?

MOCA is charted for IFR airways and is depicted with an asterisk (e.g., *3400). It is especially valuable in mountainous or variable terrain.

How is MOCA Determined?

MOCA uses the same corridor and buffer as MEA but only assures navigation signal within 22 NM of VORs.

Application Example

On a segment with MEA 6,000 feet and MOCA *3,800 feet, pilots within 22 NM of the VOR may descend to 3,800 feet; otherwise, they must climb back to at least 6,000 feet.

Off-Route Obstruction Clearance Altitude (OROCA)

What is OROCA?

Off-Route Obstruction Clearance Altitude (OROCA) is a grid-based minimum on IFR enroute charts. It guarantees:

• At least 1,000 feet of clearance in non-mountainous regions
• 2,000 feet in mountainous areas

OROCA is not tied to any airway or navigation fix and does not guarantee navigation aid or ATC coverage.

Where is OROCA Used?

OROCA is used for off-route, direct, or emergency IFR navigation, and is depicted as bold blue numbers in each grid on IFR enroute charts.

How is OROCA Determined?

Each one-degree quadrangle is surveyed for the highest obstacle or terrain; the required buffer is added, and the value is rounded up to the next 100 feet.

Application Example

A direct route across a grid with an OROCA of 7,900 feet requires flight at or above this altitude in that area.

Obstacle Clearance Altitude (OCA) / Obstacle Clearance Height (OCH)

What is OCA/OCH?

Obstacle Clearance Altitude (OCA) and Obstacle Clearance Height (OCH) are approach-specific minima:

• OCA: The minimum altitude above mean sea level (MSL) on an approach segment ensuring adequate obstacle clearance.
• OCH: The corresponding height above the runway threshold or aerodrome elevation.

They ensure safety during approach and missed approach procedures.

Where is OCA/OCH Used?

OCA/OCH are published on instrument approach charts and are critical during descent under instrument meteorological conditions.

How is OCA/OCH Determined?

Authorities analyze the approach path and protected areas, adding a buffer (often 250 feet for non-precision approaches) above the highest obstacle to set the OCA/OCH.

Application Example

If the OCA for an RNAV approach is 1,240 feet MSL and the OCH is 265 feet above threshold, the aircraft must not descend below these unless visual reference is established.

Maximum Elevation Figure (MEF)

What is MEF?

Maximum Elevation Figure (MEF) is a value shown on VFR sectional charts, representing the highest known elevation (including obstacles) within each chart quadrant.

Where is MEF Used?

MEF is used in VFR navigation and planning, particularly for night or cross-country operations.

How is MEF Determined?

The highest terrain or obstacle in each quadrant is identified, a buffer of 100 feet is added, and the figure is rounded up to the next 100 feet (e.g., “57” = 5,700 feet).

Application Example

A VFR pilot notes an MEF of 7,300 feet and selects a cruising altitude of 8,500 feet for additional safety margin.

Regulatory Background

United States

• 14 CFR § 91.119: Sets basic minimum altitudes for all operations.
• 14 CFR § 91.177: Specifies IFR altitude minimums—1,000 feet above highest obstacle (2,000 feet in mountainous areas) or published MEA/MOCA/OROCA.
• ICAO Annexes 2, 11, Doc 8168: Establish international standards for obstacle clearance and minima.

How Clearance Altitudes Are Used

Flight Planning

Pilots review charts to identify MEA, MOCA, OROCA, or MEF for each segment. For IFR, cruise altitude must be at or above MEA/MOCA; for VFR, MEF helps guide safe altitude selection.

Enroute Navigation

Pilots must remain at or above published minima unless cleared by ATC or in an emergency. Off-route, OROCA sets the minimum safe altitude.

Approach and Departure

During approach, OCA/OCH limits must be respected. Departures may require minimum climb gradients or compliance with obstacle departure procedures.

In-Flight Adjustments

• Weather/Icing: Temporary use of lower MOCA may be allowed if within 22 NM of navigation aid.
• Emergencies: OROCA offers a quick-reference safe altitude grid for immediate terrain clearance.
• Night/Marginal Visibility: Flying above OROCA or MEF is strongly advised for added safety.

Comparison Table: Key Minimum Obstacle Clearance Altitudes

Summary

Clearance altitude is a fundamental element of aviation safety. It ensures aircraft safely clear all obstacles and terrain throughout their flight, from takeoff to landing, whether flying under IFR or VFR. By understanding and adhering to MEA, MOCA, OROCA, OCA/OCH, and MEF, pilots maintain regulatory compliance and minimize risk, even in dynamic or emergency situations.

For more information on how clearance altitudes protect your operations, contact us or schedule a demo of advanced flight planning tools designed to automate and enhance clearance management.