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 November article, in which we presented a high-level exploration into how and why the surfaces might be changing, over the next few months we will go into the minutia of each surface to uncover what will stay the same and any specific changes. We will also look at what has been removed from the model, and any new criterion that have been added. In this edition we will be focusing our attention on the Take Off Climb Surface (TOCS).
Figure 1: Three aircraft take off at Beijing Capital International Airport from parallel runways — Suparna Airlines Boeing 737-300QC, Emirates Boeing 777-300ER, China Eastern Airlines Airbus A330 [1]
What is the Purpose of the Existing TOCS?
The TOCS already exists in the current set of obstacle limitation surfaces with the purpose of safeguarding departures. It is a sloped surface that originates a specified distance beyond the end of the Take Off Distance Available (TODA) of the runway before sloping upwards and outwards at a specified gradient and splay, along the extended runway centreline. It is one of the most restrictive surfaces contained in the OLS for a typical aerodrome, with the surface normally having a shallower gradient than either the approach surface or the transitional surface [2]. This is reflective of the heightened collision risk that aircraft face during the take-off phase of flight. Not only is the TOCS one of the most restrictive surfaces in terms of its dimensions, it is also the most restrictive in terms of its enforcement. In the UK for instance, guidance published in CAA Aerodromes 139 states that “New objects or extensions of existing objects should not be permitted above a take-off climb surface except when the new object or extension would be shielded by an existing immovable object.” [3] This effectively disallows obstacles being constructed which would breach the TOCS.
The New TOCS – a Departure?
The TOCS is going to be incorporated into the new OLS as one of the obstacle free surfaces (OFS). Like the existing TOCS, it safeguards departures and for larger aircraft at least, it is shallower than the approach surface. As an OFS, the enforcement of the TOCS will be similar to that of the existing surface. Indeed, the ICAO state letter [4] of 30 May 2023 sets out that: “The obstacle free surfaces (OFS) will provide a volume of airspace necessary for safe and accessible operations near the runway and in the vicinity of the aerodrome. As such, the volume of airspace is kept free from obstacles, except for existing obstacles and/or terrain which would have been assessed earlier.”
Figure 2: Plan view of the new TOCS, taken from page B-40 of [4].
For many runways, it seems the new and existing TOCS will be very similar, carrying out similar functions, having similar dimensions and similar rules regarding their enforcement. The differences arise out of their edge cases. The existing TOCS is very standardised, and except for a change of bearing up to 15 degrees to allow for any noise abatement procedures, there isn’t any other circumstance under which the shape of the TOCS changes.
With the new surface, the set of occasions on which this may happen is made much broader. Among them include changes to the slope of the surface. Section 4.2.6.6 and 4.2.6.7 b) of [4] state that “a higher slope should be adopted for the take-off climb surface where such slope would be consistent with the operational characteristics of the critical aeroplane operating out of the runway and the local conditions.” This is consistent with the aims of the introduction of the new surfaces, one of which is to facilitate development near airports at a time when many airports, which were once a long way out of their respective cities, now have increasing amounts of development occurring within their vicinity.
The track the TOCS follows can also be changed to follow departure tracks more closely than is possible with the existing TOCS. This is in keeping with another aim of the new surfaces, which is to interface more closely with existing instrument flight procedures. As is set out in 4.2.6.3 of [4]: “The [TOCS] shall vary when take-off flight paths involving turns are utilized; two sides originating at the end of the inner edge and diverging uniformly at a specified rate from the extended centre line of the take-off ground track to a specified final width, and extending thereafter parallel to the take-off ground track for the remainder of the length of the take-off climb surface.” In other words, the TOCS could be curved if the departure procedure in question required it.
It seems then that one of the more straightforward surfaces of the existing OLS now has the possibility to be rather complicated, depending on the aerodrome in question. Pager Power has taken the step to develop a model for the new surfaces and is actively looking at ways to reconcile the comparative difficulties faced in modelling the new surfaces. With this capability Pager Power strives to continue in its quest to forge positive relationships between developers and airports.
More on the new ICAO OLS surfaces can be found here.
More on Pager Power’s new ICAO OLS Modelling can be found here.
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References
[1] Danny Yu, Three aircraft take off at Beijing Capital International Airport from parallel runways — Yangtze River Express (now Suparna Airlines) Boeing 737-300QC, Emirates Boeing 777-300ER, China Eastern Airlines Airbus A330, http://www.airliners.net/photo/Yangtze-River-Express/Boeing-737-330(QC)/2669724/L/
[2] ICAO, Annex 14 to the Convention on International Civil Aviation, Ninth Edition, July 2022 Volume I
[3] CAA, Aerodromes, UK Regulation (EU) 139/2014
[4] ICAO State Letter (Reference AN 4/1.1.58-23/33)
