Aerodromes - Visual Aids, Markings and Signs — Page 400, Lesson 541

I want to walk you through runway markings now — specifically how a runway gets its number, and what that centre line stripe pattern is all about. This is the visual-aids part of air law, and it's pure identification: you read the runway, you know exactly where you are. Let's start with the runway designation marking. That's the big number you see painted at the threshold. The rule is that it should consist of a two-digit number, and on parallel runways it shall be supplemented with a letter. So on a single runway you just get the number; the letter only appears when there's more than one runway pointing the same way. How do we get that number? It's the whole number nearest to the magnetic bearing of the runway, divided by 10. That magnetic bearing is called the QDM — that's the magnetic heading you'd fly to approach the runway. So if the QDM is 094 degrees, you divide by 10, which gives you 9.4, and you round down to 09. That's why you'll see "09" painted. The rule says "whole number nearest," and the example shows 094 ÷ 10 = 09.4 rounded down to 09. Now, a subtlety: if that calculation gives you a single digit — say the QDM is 045, which divided by 10 is 4.5, rounding to 05 — then it shall be preceded by a zero. So you always paint two digits. But here's a real-world wrinkle: in some states the zero is omitted. The excerpt gives the example of New York JFK, where the south-to-north runways are marked "4R" and "4L" — no leading zero. Now the parallel-runway case. When you have parallel runways, each designation number gets a letter, and the letters are assigned from left to right as you view them from the direction of approach. For two parallel runways, you get "09L" and "09R" — left and right. For three, you get "09L," "09C," "09R" — left, centre, right. For four parallel runways, it gets interesting: you get "09L," "09R," "10L," "10R." Notice the second pair jumps from 09 to 10 — the QDM for one pair is increased to differentiate that pair from the other. So you don't have four runways all labelled 09; you split them into two adjacent pairs, each with its own number. Let me show you what this looks like on the ground — here's a runway at RAF Brize Norton with its markings, and a close-up of the designation markings themselves. Now let's move to the runway centre line marking. This is required on a paved runway — so if the runway is paved, you must have it. It's painted along the centre line of the runway, between the runway designation markings. So it runs down the middle, from one threshold's designation to the other's. What does it look like? It's a line of uniformly spaced stripes and gaps. And here's the precise geometry: the length of a stripe plus a gap shall be not less than 50 metres and not more than 75 metres. So one full cycle — one stripe plus one gap — is somewhere between 50 and 75 metres. And the length of each stripe shall be at least equal to the length of the gap. So the stripe is never shorter than the gap; it's at least as long. That's the core of it: the designation number tells you the runway's magnetic orientation and which parallel it is, and the centre line stripes give you a continuous visual reference down the middle of a paved runway.