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The Legacy of Safety: Understanding the Classic Obstacle Limitation Surfaces (OLS)

AOIWEB LTD · 27 August 2025

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For decades, the Obstacle Limitation Surfaces (OLS) framework has served as the backbone of aerodrome safeguarding, ensuring that the airspace around airports remains clear of hazards to flight operations. While the aviation industry is currently transitioning to a new performance-based system, the standards detailed in the Ninth Edition of ICAO Annex 14, Volume I, represent the classic prescriptive approach that has defined airport design for generations.

The Prescriptive Foundation: Aerodrome Reference Codes

The traditional OLS framework is built upon the Aerodrome Reference Code. This system uses a Code Number (linked to the aeroplane reference field length) and a Code Letter (linked to wingspan) to determine the specific dimensions and slopes of the protected surfaces. Unlike the modern shift toward approach speeds and the Aeroplane Design Group (ADG), this legacy system relies heavily on the physical size and runway requirements of the aircraft.

Defining the Protected Airspace

The OLS consists of several complex surfaces that project into the airspace around a runway, each designed to protect a specific phase of flight:

  • Outer Horizontal Surface: A surface that extends well beyond the immediate environs of the aerodrome to provide broad protection for aircraft maneuvering.

  • Conical Surface: Sloping upwards and outwards from the inner horizontal surface, this surface protects aircraft in the circling circuit.

  • Inner Horizontal Surface: A horizontal plane located at a fixed height (typically 45 meters) above the aerodrome elevation datum.

  • Approach and Inner Approach Surfaces: These inclined planes precede the threshold and protect the final stages of the landing sequence.

  • Transitional and Inner Transitional Surfaces: Complex surfaces along the sides of the runway strip that slope up to the inner horizontal surface. They are unique because their lower edge elevations are tied directly to the nearest point on the runway centre line, meaning the surface curvature mirrors the runway profile.

  • Balked Landing Surface: An inclined plane specifically designed to protect aircraft during an unexpectedly discontinued landing manoeuvre.

  • Take-off Climb Surface: Located beyond the end of the runway or clearway, this surface safeguards the initial climb path of departing aircraft.

ICAO OLS Diagram

Standardized Dimensions for Global Safety

One of the hallmarks of the classic OLS is its rigid dimensionality, which is strictly defined based on the runway classification. For example, a precision approach runway requires a more extensive set of surfaces, including an Obstacle Free Zone (OFZ) comprising the inner approach, inner transitional, and balked landing surfaces. In this legacy framework, fixed objects are strictly prohibited from penetrating these surfaces except for low-mass, frangible aids required for navigation.

Management of Obstacles

The classic OLS provides clear rules for when objects must be restricted. Any object that extends above a defined surface is generally regarded as an obstacle and should ideally be removed. If removal is not possible, the framework provides for the shielding principle, where an object may be permitted if it is shielded by an existing, permanent, and immovable object. Additionally, objects that cannot be removed must be marked and lighted to reduce hazards to aircraft.

The Path Toward Modernization

This legacy system is now being superseded by a more flexible, data-driven framework. While the Ninth Edition of Annex 14 remains a critical reference, the shift toward Obstacle Free Surfaces (OFS) and Obstacle Evaluation Surfaces (OES) marks a new era where safeguarding is dictated by actual aircraft performance rather than the fixed-dimension "envelopes" of the past. For aviation professionals, understanding these classic OLS standards is the first step in mastering the complex evolution of aerodrome safety.