ICAO’s New Obstacle Limitation Surfaces – A Herculean Modelling Task? - Pager Power
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ICAO’s New Obstacle Limitation Surfaces – A Herculean Modelling Task?

ICAO’s New Obstacle Limitation Surfaces – A Herculean Modelling Task?
October 27, 2023 Harry Watson

The International Civil Aviation Organization (ICAO) has proposed new obstacle limitation surfaces (OLS) which are planned to be applicable from November 2028; the CAA has launched a consultation with UK aviation stakeholders so that they may relay their concerns to ICAO.

ICAO’s New Obstacle Limitation SurfaceFigure 1: Boeing 747 on final approach to London Heathrow [4].

What are Obstacle Limitation Surfaces?

Obstacle Limitation Surfaces are a set of imaginary surfaces around an aerodrome, which place a theoretical limit on the height of developments in the vicinity of that aerodrome. Officially, the OLS apply only to licensed aerodromes but in practice they are often used as a way of safeguarding unlicensed aerodromes, despite this not being a requirement. Generally speaking, new developments are not permitted to breach the surfaces and existing obstacles breaching the surfaces should be removed where possible [1]. The existing set of obstacle limitation surfaces has been in place since the 1950s and includes specific surfaces to cover take-off (Take Off Climb Surface), landing (Approach Surface), and other operations in the vicinity of the aerodrome (Transitional Surface, Inner Horizontal Surface, Conical Surface, Outer Horizontal Surface). 

Why are the surfaces changing?

The design of aircraft has changed considerably since the inception of the OLS, and furthermore there are a far greater amount of data available about what paths planes take on their take-off and landing. Airports which were once on the outskirts of some of the world’s largest cities are now being consumed in the urban sprawl, with considerably higher demand for construction in close proximity to airports than ever before. As a result of this, it is both possible and desirable to make it easier to facilitate new development in close proximity to airports. 

How does the proposed new OLS vary from the existing OLS?

In the proposed new OLS, the surfaces are going to be categorised into Obstacle Free Surfaces (OFS) and Obstacle Evaluation Surfaces (OES). 

The OFS will include the Approach Surface, Transitional Surface and Take-Off Climb Surface which are similar to those in the current OLS. In addition, they will contain the Inner Approach Surface, Inner Transitional Surface and Balked Landing Surface from the current Obstacle Free Zones (OFZ). These Surfaces will generally not permit any breaches as it is deemed necessary to keep the airspace above them free of obstructions for the safe operations in the vicinity of the aerodrome, based on the analysis of thousands of flights conducted by the ICAO New OLS Task Force. 

The OES will replace the current Inner Horizontal, Conical and Outer Horizontal Surfaces. These surfaces will consist of the Horizontal Surface, the Surface for Straight-In Instrument Approaches, the Surface for Precision Approaches and the Instrument Departure Surface. These surfaces are in place because obstructions which breach them have a potential to pose a risk to safe operations in the vicinity of the aerodrome. 

Furthermore, the way the dimensions of the surfaces are calculated will be different. Previously, the dimensions of the surfaces were based on runway dimensions alone, but now the dimensions are more performance based, considering the nature of the aircraft which use the aerodrome.

Modelling the new surfaces

It can be useful to create models for the OLS, both to help visualise it and to calculate the height of the surfaces in different locations. 

Creating such a model requires the ability to understand the geometry of the surfaces but also how to create and visualise 3D models in space. Pager Power offers OLS assessments and as such Pager Power has a bespoke model for the existing set of surfaces which calculates surface heights and infringements, and plots the surfaces in two and three dimensions. Pager Power has also worked on a model to visualise the new surfaces as they currently are and has the in-house expertise and experience to amend this model as any changes are made to the proposed new OLS.

Pager Power’s Offer

Pager Power is aware that many aerodromes in the UK and Worldwide are trying to adapt to the new surfaces and are currently trying to model them in order to provide a response to the ICAO consultation on the new surfaces. Pager Power has over 20 years of experience in the aviation sector and is eager to provide assistance in the transition over to the new surfaces, both to the aerodromes it has worked with and any other aerodromes that are facing challenges with designing the new ICAO OLS surfaces. Pager Power is therefore offering aerodromes the chance to have their new obstacle limitation surfaces modelled free of charge between now and December 31st. Any aerodrome, whether licensed or unlicensed, can have one set of surfaces modelled by Pager Power free of charge based on information supplied to Pager Power by the aerodrome [3]. Pager Power will supply a 3D model of the surfaces and a 2D chart, and aerodromes should contact Pager Power directly to receive more information on our offer. Pager Power hopes that by doing this, the layout of the new surfaces can be more clearly understood by aerodromes in the UK and worldwide.

About Pager Power

Pager Power undertakes technical assessments for developers of renewable energy projects and tall buildings worldwide. To find out more about the services we offer, click here.



[1] CAP168, 12th Edition 4.50-4.52, Page 191

[2] ICAO State Letter – Proposal for the amendment of Annex 14, Volume I and PANS-Aerodromes (Doc 9981) relating to aerodrome design and operations

[3] Subject to availability.

[4] Adrian Pingstone, Public domain, via Wikimedia Commons. Last accessed on 26th October 2023.


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