So you’ve got a minimum size: 3.6 m long, 0.9 m across the mouth — Page 390, Lesson 536

We’re moving into a new part of the aerodrome equipment rules now — the physical markers and signalling devices. Let’s start with the wind direction indicator, because that’s the first thing we’re given in detail. The wind direction indicator — the windsock — must be in the form of a truncated cone made of fabric. A truncated cone just means a cone with the pointed end cut off, so it’s a tapering tube, open at both ends. Its length must be not less than 3.6 metres, and the diameter at the larger end must be not less than 0.9 metres. So you’ve got a minimum size: 3.6 m long, 0.9 m across the mouth. It’s constructed so that it gives a clear indication of the direction of the surface wind, and a general indication of the wind speed. The direction comes from which way the sock points — it streams away from the wind. The speed is general, not precise — the more the sock is blown out horizontal, the stronger the wind. Now the colour. The colour or colours must be selected so that the wind direction indicator is clearly visible and understandable from a height of at least 300 metres, having regard to the background. So you’re choosing it against whatever’s behind it — buildings, grass, sky. Where practicable, a single colour should be used, preferably white or orange. The location of at least one wind direction indicator — and this one is called the ‘master’ wind sock — should be marked by a circular white band. That band is 15 metres in diameter and 1.2 metres wide. So it’s a big white ring painted on the ground around the mast, so you can spot where the master sock is. And provisions should be made for illuminating at least one wind indicator at an aerodrome intended for use at night. So if the aerodrome operates at night, you need lighting on at least one windsock. Now, next item — the Landing Direction Indicator. This is provided when the aerodrome chooses to have one, and when it is provided, it shall be located in a conspicuous place on the aerodrome. If a signal square is provided, a landing “T” will always be included in the signs in the square. The landing direction indicator should be in the form of a “T” — that’s the shape. The colour of the “T” should be either white or orange, and the choice depends on which colour contrasts best with the background against which the indicator will be viewed. Where required for use at night, the landing “T” is to be either illuminated or outlined by white lights. Then we have the Signalling Lamp. This must be provided at a controlled aerodrome, in the aerodrome control tower, for the purpose of showing light signals to aircraft either in the air or on the ground, as defined in Chapter 6. The lamp — usually an Aldis lamp, specially designed for signalling — must be capable of three things. First, producing red, green and white light, and being aimed manually at any target as required. Second, giving a signal in any one colour followed by a signal in either of the other two colours. And third, being operated to transmit a message in any one of the three colours by Morse code, up to a speed of at least four words per minute. So to tie it together: the windsock gives you wind direction and a general sense of speed, marked with that 15-metre white band for the master one, lit at night. The landing “T” tells pilots the landing direction, white or orange depending on background, lit or outlined at night. And the Aldis signalling lamp in the tower gives the controller a way to send red, green, or white light signals to aircraft in the air or on the ground, including Morse code messages up to four words per minute.