Transitioning to the Performance-Based OFS and OES Framework
A New Era for Aerodrome Safeguarding
The International Civil Aviation Organization (ICAO) has finalized a comprehensive overhaul of Chapter 4 in Annex 14, Volume I, transitioning aerodrome safety from prescriptive, fixed-dimension surfaces to a modern, data-driven safeguarding framework. This update addresses the evolution of aircraft technology, advanced navigation capabilities, and the need for more efficient land-use management.
The Shift to the Aeroplane Design Group (ADG)
The most significant structural change is the removal of the "Code Number" (based on runway field length) as a design parameter for safeguarding surfaces. In its place, the new standards introduce the Aeroplane Design Group (ADG), which classifies aircraft by combining their indicated airspeed at threshold (Vat) and their wingspan. This change is critical because approach speed more accurately reflects the airborne behavior of aircraft and aligns aerodrome standards with the criteria used for instrument flight procedure design. By using these performance-based metrics, the dimensions of safeguarding surfaces now more closely match the actual trajectories of modern aircraft.
The Twofold Concept: Obstacle Free Surfaces (OFS) and Obstacle Evaluation Surfaces (OES)
The new framework replaces the single set of Obstacle Limitation Surfaces (OLS) with a twofold system designed to distinguish between safety-critical airspace and evaluative airspace.
Obstacle Free Surfaces (OFS): These surfaces define a volume of airspace in the immediate vicinity of the runway where obstacles must be strictly limited. They are designed to protect aircraft during the visual phase of approach, go-arounds, and balked landings, where operations are standardized and cannot easily adapt to new obstacles.
Obstacle Evaluation Surfaces (OES): These act as "trigger" surfaces. When an object penetrates an OES, it does not automatically mean the object is prohibited; instead, it triggers a mandatory aeronautical study to evaluate the object's impact on the safety and regularity of specific operations.
Key Additions and Specific Surface Refinements
Several surfaces have been redesigned or added to close historical gaps between aerodrome safeguarding and flight procedure design:
Revised Transitional Surface: Unlike the previous version that ended at the inner horizontal surface, the new transitional surface rises to a fixed height of 60 m above the highest threshold elevation.
Horizontal OES: The old "Inner Horizontal Surface" is replaced by a horizontal OES whose radius and height vary based on the ADG and is measured relative to the aerodrome elevation to protect circling procedures.
Surface for Straight-in Instrument Approaches: This new OES protects common PBN and VOR approaches at aerodromes where a full horizontal surface is not established.
Instrument Departure Surface: A new evaluative surface aligned with PANS-OPS criteria has been added to safeguard omnidirectional instrument departure procedures.
Upgraded Precision Protections: The Obstacle Free Zone (OFZ) height has been increased from 45 m to 60 m to better support Category II and III operations, and the requirement for an OFZ on Category I precision runways has been elevated from a recommendation to a standard.
Comprehensive Benefits for Industry and Communities
The transition to this performance-based framework offers substantial benefits across safety, economics, and the environment:
Operational Efficiency: Statistical modeling of actual aircraft trajectories has allowed for surfaces that are often narrower or shorter than current prescriptive rules, which improves runway accessibility and capacity.
Economic Development: Because surfaces can now be tailored to specific local operations, areas of land not required for flight safety can be released for urban development, creating a "win-win" for aviation and non-aviation stakeholders.
Environmental Impact: By ensuring better runway accessibility and reducing the likelihood of missed approaches due to obstacles, the new standards help reduce additional fuel burn and decrease noise nuisance for residents living under flight paths.
Safety Transparency: The harmonization with PANS-OPS (Doc 8168) ensures that aerodrome operators and procedure designers use consistent criteria, ensuring no hazards are left unnoticed.
Global Implementation Timeline
States are expected to incorporate these updated standards into national regulations over a 3 to 5-year timeframe. While industry stakeholders, including aerodrome operators and airlines, will generally require 1 to 2 years to implement the necessary surfaces and acquire resources for the updated aeronautical study processes.