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46   Air Regulations, 1938

or by visual signal, it being understood that when permission has been asked by visual signal the reply shall always be by visual signal. The visual signal shall consist of a repetition of the same three-letter sign made with signalling lights of the aerodrome.
These signalling lights shall be constituted either by a group of lights arranged on a horizontal plane at the apexes of an equilateral triangle, each side of which measures from 3 to 10 feet, or by a luminous beam directed at the aircraft.
The colour green shall be used to give permission to land and the colour red to prohibit landing.

17. At every aerodrome, the firing of a red pyrotechnical light or the display of a red flare from the ground, whether by day or by night and notwithstanding any previous permission, shall be taken as an instruction to aircraft in flight that they are not to land for the moment and to aircraft manoeuvring on the landing area that they are to stop moving.
At aerodromes provided with the triangular device provided for in paragraph 16 (c) above, the emission by such device of intermittent red lights shall, whether by day or by night and notwithstanding any previous permission, instruct aircraft in flight that they are not to land for the moment. 

18. To require an aircraft to land, the following signals shall be used:—
(a) By day: a series of projectiles discharged at intervals of ten seconds, each showing, on bursting, black smoke.
(b) By night: a series of projectiles discharged at intervals of ten seconds, showing, on bursting, white lights or stars.
In addition, if it is necessary to distinguish, amongst several, the aircraft which is to land, an intermittent white luminous beam shall be directed at that aircraft.
Provided that, when the authority who desires to give the order to land is able to establish a radio-electric communication with the aircraft, this order may be given by using the means of communication established.