Signal square

A signal square is an aerodrome equipment internationally defined at the annex 14 of the Convention on International Civil Aviation by the International Civil Aviation Organization.

It contains ground symbols to indicate visually the conditions on the aerodrome to over-flying aircraft. There are four basic signals.

In the UK, the square internal sides should measure 12 metres, "bordered by a white strip 30 centimetres wide" and "so located on the aerodrome that it is visible from all directions of approach". The Schedule 1, Section 9 of the Rules of the Air Regulations 2007 further illustrates the dimensions of each particular signal, e.g. both arms of the landing T should be 4 metres long and 40cm thick, and the dummbell sign should consist of three sections, each 1.5m long. The sides of the red panel with the yellow cross should be 3m, with the crossing lines being 50cm wide. All dimensions shall be "subject to a tolerance of 10 per cent, plus or minus". The same documents include the H sign indicating a landing area for helicopters, commonly used on a helipad (but not a circle around the letter H), to be displayed in the Signals Area. According to the UK Civil Aviation Authority, these visual aids can be used at aerodromes with a significant amount of general aviation movements.