The International Civil Aviation Organization (ICAO) has proposed new obstacle limitation surfaces (OLS) which are expected to be brought into force from November 2028. Following our previous articles in which we explored some of the new Obstacle Free Surfaces (OFS) including the Take-Off Climb Surface and Approach surface, we now examine the new Horizontal Surface. This will be an Obstacle Evaluation Surface (OES), which will have a similar purpose to the existing Inner Horizontal Surface (IHS).
What is the difference between OFS and OES?
The new OLS is expected to be split into two types of surfaces. The OFS aim to protect airspace in the immediate vicinity of the aerodrome, and will provide protection for all designated airspace that aircraft will travel through or near when arriving or departing. The rules are expected to be enforced quite rigidly, with 4.4.4 of the ICAO State letter [3] stating that:
“New objects or extensions of existing objects shall not be permitted above the approach surface, take off climb surface and transitional surfaces and the complex surface extending between the lower edges of the transitional surfaces [obstacle free surfaces].”
This places an effective ban on any new obstructions breaching the OFS. The Horizontal Surface is however one of the surfaces with less strict criteria governing its safeguarding, with paragraph 4.4.8 [3] stipulating:
“States shall ensure that obstacles penetrating the obstacle evaluation surfaces are only permitted when, after aeronautical study, it is determined that the obstacles do not adversely affect the safety or significantly affect the regularity of the existing and intended operations of aeroplanes.”
With the OES, further study may reveal that a breach of the surface and aircraft operations can coexist. This is commensurate with the differing purposes of the surfaces, with OES generally inclined towards safeguarding airspace that some aircraft might use, as opposed to that which all aircraft will use.
What is the purpose of the existing Inner Horizontal Surface?
The current IHS is designed to safeguard aircraft undertaking visual manoeuvring in the vicinity of an aerodrome at which they are about to land [2, p492]. It is a flat surface, which is present for all aerodromes, and it is situated 45 metres above the lowest runway threshold of an aerodrome.
The lateral extent of the surface varies depending on the declared distances of the runway at the aerodrome. For runways with declared distances of less than 1860 metres, the surface is a circle centred on the midpoint of the longest runway, with a radius which varies between 2000 and 4000 metres depending on the length of the runway and whether instruments are used on the approach into the aerodrome.
For aerodromes with longer runways, the shape becomes more complicated. It consists of arcs starting at the thresholds of the main runway and sufficiently long subsidiary runways, which are then joined together to form a locus which forms the outer boundary of the surface.
There can be areas in the vicinity of an aerodrome in which the IHS is the most restrictive, particularly in locations away from the runway centreline where the obstruction isn’t under the Take-Off Climb Surface or the Approach Surface. Compared to the other surfaces we have looked at, the enforcement of the current IHS is slightly more lenient, with UK Aerodromes 139 stating that:
“New objects or extensions of existing objects should not be permitted above the conical surface and the inner horizontal surface except when an object would be shielded by an existing immovable object, or if after a safety assessment, it is determined that the object would not adversely affect the safety or significantly affect the regularity of operations of aeroplanes.”
Regular followers of the New OLS article series published previously will recall that these rules are in common with the section on the Approach Surface which is more than 3000 metres from the runway threshold on the existing OLS. It should also be noted that the mention of “safety assessment” in the existing IHS text in [2] is similar to the “aeronautical study” required of OFS in [3].
Figure 1: Pager Power modelling output for potential Horizontal Surfaces at Norwich Airport, overlaid on Google Earth imagery. This consists of three surfaces at heights 45, 60 and 90 metres above the aerodrome reference elevation, corresponding to the different aerodrome groups landing at the aerodrome. This figure is given for illustrative purposes only [1].
The New Inner Horizontal Surface – A more tailored approach
The new OLS will have the Horizontal Surface as an Obstacle Evaluation Surface. It is ostensibly the replacement for the IHS. Both surfaces are flat and both surfaces have the protection of visual manoeuvring areas as their primary purpose.
The new HS is at the same height or higher as the existing IHS, varying between 45 and 90 metres above the aerodrome reference point elevation depending on the aerodrome design group (ADG) of aircraft using the aerodrome. If aircraft from multiple ADGs use the aerodrome and each of these ADGs have different HS dimensions, a separate HS is established for each of them. This means that aerodromes can have multiple HS established at different heights. The establishment of the particular HS height above the aerodrome reference point elevation specifically is new, and disambiguates between the different height data points used for the IHS previously, which could include the aerodrome reference point elevation, the lowest runway threshold elevation, the highest runway threshold elevation, or any other appropriate datum [5, p3].
The HS is also expected to have a larger radius than the current IHS, and in all cases the surface is now constructed by drawing arcs at the runway thresholds and joining them. This radius is in line with the radius of the Visual Manoeuvring (Circling) area (VM(C)) for each ADG, as calculated based on its indicated airspeed at the runway threshold from the methodology presented in PANS-OPS Volume II [7, I-4-7]. This is as opposed to the current situation in which the surface is constructed in this way only for larger aerodromes, with shorter runways having a circular IHS centred on the midpoint of the runway.
The dimensions of the HS are generally expected to be less uniform, with paragraph 4.3.2.2 of [3] stating that:
“The characteristics of the surface can only be a recommendation, since the shape of the horizontal surface may be varied to match the operations”
In this case, the operations are most likely the visual manoeuvring areas promulgated in the aeronautical information publication (AIP) for a particular aerodrome, and this suggests that the heights may be amended to match those given in the AIP, less the required minimum obstacle clearances.
How can Pager Power help?
Pager Power has a model for the new ICAO OLS as it is set out in [3]. Pager Power is able to undertake initial modelling of these surfaces and can offer initial consultation to developers and airports who require clarity on the new rules that are to be brought into force, having over 20 years of experience in the aviation sector.
More on Pager Power’s new ICAO OLS Modelling can be found here.
About Pager Power
Pager Power undertakes technical assessments for developers of renewable energy projects and tall buildings worldwide. For more information about what we do, please get in touch.
References
[1] Pager Power modelling output, visualised in Google Earth.
[2] CAA, Aerodromes, UK Regulation (EU) 139/2014
[3] ICAO State Letter (Reference AN 4/1.1.58-23/33)
[4] ICAO, Annex 14 to the Convention on International Civil Aviation, Ninth Edition, July 2022 Volume I
[5] ICAO Aerodrome Services Manual, Part 6
[6] CAA CAP 168, Licensing of Aerodromes
Thumbnail image accreditation: Unsplash.com (Sept 2022). Last accessed on 12th August 2024. Available at: https://unsplash.com/photos/passenger-airplane-flies-near-the-sun-glare-with-a-color-rainbow-gradient-on-landing-with-the-wings-of-a-steam-jet-vortex-1xokiBaJIac
