What are approach-light structures?
The posts, masts and towers that hold approach lights up at the right height — built to carry the light but to give way if an aircraft ever touches them.
Approach lights are placed as nearly as practicable in a horizontal plane through the threshold, and nothing except an ILS or MLS azimuth antenna is allowed to rise through that plane within 60 m of the centreline. Where the ground falls away, a structure carries the light up to that plane. Every elevated light and its support must be frangible: it is designed to break, distort or yield on impact so that an aircraft that lands short meets as little resistance as possible.
Beyond 300 m from the threshold the rule is relaxed for tall supports. Where a structure is higher than 12 m, only the top 12 m must be frangible. Where it is surrounded by non-frangible objects, only the part that rises above them must be. Low mass is part of what makes a structure frangible, and a light or structure that is not conspicuous enough is suitably marked.
Approach Lighting Support Structure
An approach lighting support structure is the post, mast or tower that holds each light up to a horizontal plane through the threshold. Under ICAO Annex 14 every elevated light and its support must be frangible, so it breaks or yields on impact. Beyond 300 m from the threshold the rule is relaxed for tall supports: where a structure is higher than 12 m, only the top 12 m must be frangible, and where it is surrounded by non-frangible objects only the part above them must be. Frangible means low mass, designed to break, distort or yield so as to present the minimum hazard to aircraft, and a light or structure that is not conspicuous enough is suitably marked. Check the current ICAO Annex 14 text and your State’s rules before relying on any figure.
Frequently Asked Questions
Q1What are approach-light structures?
Approach-light structures are the posts, masts and towers that hold approach lights at the correct height along the extended runway centreline. They must keep the lights visible to the pilot while posing as little hazard as possible to an aircraft.
Q2Why must approach-light structures be frangible?
The approach lights sit in the area an aircraft may touch if it lands short. A frangible structure breaks or yields on impact, so it absorbs as little energy as possible and reduces the danger to the aircraft.
Q3Does the whole tower have to be frangible?
Not always. Beyond 300 m from the threshold, where a supporting structure is taller than 12 m, the frangibility requirement applies only to the top 12 m. Where the structure is surrounded by non-frangible objects, only the part that rises above them must be frangible.
Q4Where should the approach lights be mounted in height?
As nearly as practicable in a horizontal plane through the threshold. Structures are used to bring lights up to that plane where the ground falls away. No object other than an ILS or MLS azimuth antenna is allowed to rise through that plane within 60 m of the centreline, and an antenna that does is treated as an obstacle, marked and lit.
Q5Do approach-light structures need to be marked?
Yes, where an elevated light or its supporting structure is not conspicuous enough on its own, it must be suitably marked so that pilots can see it.
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