Obstacle Limitation Surfaces Series: An Overview of Guidance

Overview

The following article presents an overview of the guidance surrounding Obstacle Limitation Surfaces (OLS) for aerodromes in the UK and worldwide. The most common surface types will be listed, followed by a description of relevant guidance documents. Key similarities and differences from each guidance document are highlighted.

What are Obstacle Limitation Surfaces?

Obstacle Limitation Surfaces (or Protected Surfaces) are established around aerodromes all over the world for safeguarding purposes.

In order to protect aircraft against potential collision risk, it is necessary to implement rules regarding tall structures surrounding aerodromes. This is achieved via the implementation and safeguarding of Obstacle Limitation Surfaces, which are imaginary planes defined in 3D around the aerodrome.

Protected Surfaces must not be infringed. This is illustrated in the image above, which depicts the relevance of OLS to structures such as high-rise buildings and wind turbines.

Types of Protected Surfaces

In most parts of the world, the OLS comprises the following protected surfaces:

• The Approach Surface
• The Take-Off Climb Surface
• The Transitional Surface
• The Inner Horizontal Surface
• The Conical Surface
• The Outer Horizontal Surface

For a full overview of each protected surface and what it entails, please see this article.

Other Surfaces
It is worth noting that there are other, more complex surfaces used in specific jurisdictions. The Federal Aviation Administration (FAA) in the US and London City Airport in the UK both have sets of surfaces which deviate from convention in some way. These are not discussed at length here.

UK and Worldwide Safeguarding Guidance

Different countries, and the organisations located within them, use variations of Obstacle Limitation Surfaces guidance.

There are four sets of guidance that are currently in use in the UK:

1. UK Civil Aviation Authority (CAA): Civil Aviation Publication (CAP) 168.
This guidance document covers the licensing of aerodromes in the UK, and it applies to licensed aerodromes, including Duxford and Retford Gamston. 

2. UK Civil Aviation Authority (CAA): Aerodromes 139.
This guidance document is applicable to certificated aerodromes in the UK, and the guidance contained within it is equivalent to the EASA guidance which applies in Europe. Certificated aerodromes are those which have an aerodrome certificate; this list includes some of the largest aerodromes in the UK, such as London Heathrow and Manchester Airport.

3. Ministry of Defence (MOD) and Military Aviation Authority (MAA): Regulatory Article 3512 (RA 3512).
This guidance document has been applicable to all military aerodromes in the UK since 2018. It has replaced Issue 8 of the Manual of Aerodrome Design & Safeguarding (MADS).

4. UK Civil Aviation Authority (CAA): Safeguarded and Obstacle Limitation Surfaces – London City Airport.
This guidance document is applicable to London City Airport, and it was written in 2004. It sets out unique surface shapes and dimensions which recognise the unique context in which London City Airport operates and is situated. The surface shapes have not changed since then, but the dimensions will have, as a result of changes to declared distances and threshold locations which have taken place in the intervening years.

In addition to these documents, we introduce three guidance documents that are currently in force outside of the UK:

1. International Civil Aviation Authority (ICAO): Annex 14 to the Convention on International Civil Aviation, Volume I
This guidance sets the international standard for Obstacle Limitation Surfaces, and is the guidance upon which that produced by local or national authorities is based. 

2. European Aviation Safety Agency (EASA): Certification Specifications and Guidance Material for Aerodrome Design. (CS-ADR-DSN)
This guidance is applicable to EASA member states, which consist of countries in the European Union (EU) along with Switzerland, Iceland, Norway and Liechtenstein. Aerodromes 139 is very similar to this guidance, which in turn is almost identical (for OLS purposes at least) to ICAO Annex 14.

3. Federal Aviation Administration (FAA): Federal Aviation Regulation Part 77
This guidance is applicable to Airports in the US. The dimensions of surfaces presented within vary significantly from those of other aerodromes worldwide.

Key Variations and Similarities Between Guidance

UK CAP 168 to UK Aerodromes 139
The comparison of the two main guidance documents, which set out OLS dimensions for civil aerodromes, is explored in more detail in this article.

UK CAP 168 to UK RA3512
Both being in the UK, the documents provide similar guidance. There are, for example, differences in the radii of arcs used to draw the Inner Horizontal Surface. CAP168 uses a radius of 4000m in pretty much all cases except for code 1 and 2 non-instrument runways. RA3512, however, has rules which are more in line with those in ICAO Annex 14, having a broader spectrum of inner horizontal surface radii for smaller runways. It was previously the case that UK military rules had a lower Outer Horizontal Surface (OHS), at 145m above the reference point, compared to 150m for the rules presented in CAP 168. This difference has now been removed, and in both cases the OHS is taken to be at such a height as to meet the top of the conical surface.

ICAO Annex 14 vs EASA CS-ADR-DSN vs UK Aerodromes 139
These three guidance documents share all of the same surfaces, and the overwhelming majority of the dimensions presented within also match. UK Aerodromes 139 was taken from EASA CS-ADR-DSN, which in turn is based on ICAO Annex 14. At the time of writing, the only discrepancy found between ICAO Annex 14 and EASA CS-ADR-DSN is for the inner width of the approach surface, which has arisen from ICAO Annex 14 having a recent amendment which has not yet been adopted into EASA CS-ADR-DSN.

Safeguarded and Obstacle Limitation Surfaces – London City Airport
The surfaces for London City Airport vary slightly from those at other aerodromes. London City Airport has an Outer Transitional Surface established in lieu of the Conical Surface that can be found at most other aerodromes, and an approach surface gradient of 5%, steeper than the 2.5% that would normally be given for a precision instrument runway of the length found at London City Airport. The Inner Horizontal Surface (IHS) also takes a different shape – it is rectangular as opposed to circular. Despite this, it has a height of 45 metres above the lowest threshold, which is the standard among other guidance.

Federal Aviation Administration (FAA): Federal Aviation Regulation Part 77
The surfaces for aerodromes in the US vary significantly from those at other aerodromes worldwide. Runways are allocated surface dimensions based on the purpose of use instead of just on runway length and approach type. Some surfaces, including the IHS (or Horizontal Surface as it is known here), are still present but of different dimensions. The Horizontal Surface, for example, has a radius of 10,000 feet for larger runways, which compares to 4,000m (13,123 feet) for most other aerodromes. It should be noted that imperial equivalents are used when measuring surface dimensions throughout the guidance.

Conclusions

Failing to consider Obstacle Limitation Surfaces can jeopardise the development of a project. It is therefore important to understand the specific OLS for a given aerodrome, how the rules vary from country to country, and between the different organisations within each country.

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If you would like to discuss a particular project or need more information regarding OLS or any other aspect of your wind or building development, follow this link to make an enquiry. Alternatively, we would be more than happy to have a discussion with you over the phone on +44 (0)1787 319001.

References

Civil Aviation Authority. Civil Aviation Publication 168. Issue 13 – December 2025.

Civil Aviation Authority. UK Regulation (EU) 139/2014.

Civil Aviation Authority. Safeguarded and Obstacle Limitation Surfaces – London City Airport. 2004.

Ministry of Defence and Military Aviation Authority. Regulatory Article (RA) 3512: permanent fixed wing aerodrome: obstacle environment. Issue 3, March 2023.

ICAO. Annex 14 to the Convention on International Civil Aviation – Aerodromes Volume I: Aerodrome Design and Operations. Ninth Edition, July 2022.

ICAO. Aerodrome Services Manual Part 6. 1983.

EASA. Certification Specifications and Guidance Material for Aerodrome Design (CS-ADR-DSN). Issue 7, May 2025.

FAA. Federal Aviation Regulations Part 77 – Objects Affecting Navigable Airspace. Amendment 11, 1989.