Obstacle Limitation Around Airports; why so Restrictive?

21 Oct Obstacle limitation around airports; why so restrictive?


Real estate and housing developers, planners and wind energy companies face stringent height restrictions around airports. At first sight, these height restrictions seem disproportionate to the altitude of the aircraft that pass overhead. Also, the height of buildings away from the take-off and landing paths is significantly restricted. This raises the question whether the height restrictions are too restrictive and could be alleviated. Before we can answer this question we need to take a closer look on how the height restrictions are established. The determining factors are:

  • Obstacle clearance, ensure an obstacle free flight path.
  • Reliability, integrity and availability of communication, navigation and surveillance (CNS) systems.

Obstacle clearance

Height restrictions to guarantee obstacle clearance are determined by a series of obstacle limitation surfaces* for approach, departure and circling procedures. In principal, the lowest surface determines the maximum building height at a given location. One of the obstacle limitation surfaces is the take-off climb surface. This is an inclined surface, starting near the runway end, protecting aircraft during climb out. The (recommended) slope of this surface is 1.6% (i.e., the minimum climb gradient). Comparing the slope of this surface to the ‘real life’ climb gradient might suggest that the slope is too conservative and the height restrictions are too restrictive. A real life climb gradient in excess of 10% is more rule than exception. The 1.6% slope originates from the 2.4% one engine inoperative climb gradient requirement for twin engine aircraft reduced by a 0.8% safety margin. This exceptional case that one engine is inoperative is determining the height restrictions instead of the nominal case. This principal is applied for all obstacle limitation surfaces to warrant an obstacle free flight in almost every situation.

CNS: Communication, Navigation, and Surveillance Systems

The possible interference with CNS systems is responsible for even more building restrictions. CNS refers to a combination of ground based stations/antennas and systems on board the aircraft exchanging radio signals for communication, navigation, or surveillance. Take for instance the Instrument Landing System (ILS) that provides aircraft with both vertical and horizontal guidance during approach. Any obstacle between the ground based antenna and the aircraft can interfere, reflect or otherwise disrupt these signals. This may have a serious impact on the reliability or the coverage of the CNS systems and therefore reduce the flight safety and the capacity of the airport. CNS surfaces* are defined to identify which planned objects may have such negative impacts.

Exceeding the obstacle limitation surfaces

In reality, buildings or other objects often exceed the obstacle limitation surfaces. This may be acceptable as long as the safety and capacity impact has been analysed and minimised. Existing buildings that penetrate these surfaces will rarely be removed. Instead operational measures should have been taken. Such measures guarantee an acceptable level of safety, but often have a negative impact on capacity. For example the usable runway length may be reduced or the required climb gradients may be raised. The consequence can be that certain aircraft types cannot use the airport, or cannot be fully loaded.

New developments above obstacle limitation surfaces are usually discouraged by authorities as the safety and capacity generally decreases with every new penetration. Moreover, an aeronautical study to analyse these negative consequences is expensive; establishing new operational measures to reduce them even more.

Nevertheless, when your new development penetrates an obstacle limitation surface you can always apply for an exception to be made. The authorities may initiate an aeronautical study to analyse the impact of the planned object on flight safety and capacity. Also keep in mind: your building might just meet the requirements, but the crane that is required to build it might not. In that case a permit to temporarily penetrate the obstacle limitation surface will be required. In the end it comes down to the difficult question which sacrifices on flight safety and capacity are acceptable when weighed against the interests served by the new development.

*Definitions and information on obstacle limitation surfaces are published in internationally adopted standard documents like ICAO Doc Annex 14, ICAO Doc 8168 (PANS-OPS) and EUR Doc 015 (CNS). In most countries additional local regulations and exceptions apply.

About To70. To70 is one of the world’s leading aviation consultancies, founded in the Netherlands with offices in Europe, Australia, Asia, and Latin America. To70 believes that society’s growing demand for transport and mobility can be met in a safe, efficient, environmentally friendly and economically viable manner. For more information please refer to www.to70.com

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Kjeld Vinkx
Kjeld Vinkx
I have joined To70 in 2003 and now jointly manage our Dutch office and aim to contribute to become the best global aviation consultants. I have a strong focus on data driven analysis, strategic advice and providing solutions that work. By delivering outstanding research and consultancy services, To70 enables aviation and society to cope with the challenges for airport and airspace operations.
  • Fouad
    Posted at 18:07h, 20 April Reply

    Great article, explaining the airspace issues in layman terms. We provide the obstacle analysis and aeronautical studies in the UAE and refer our customers to this article if they need some explanation. Thanks.

    • Kjeld Vinkx
      Posted at 11:24h, 28 April Reply

      Thanks Fouad, good to hear that you appreciate this blog!

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