Aerodromes - Physical Characteristics — Page 377, Lesson 504

Let’s start with the big picture. The aerodrome reference code is the system we use to match an aerodrome’s physical design to the aeroplanes that will use it. It’s not a single number — it’s made of two elements, and each element is tied to a specific performance characteristic or physical dimension of the aeroplane. Element 1 is a number, and it’s based on the aeroplane reference field length. Element 2 is a letter, and it’s based on two things: the aeroplane’s wing span, and the outer main gear wheel span — that’s the distance between the outside wheels of the undercarriage. So when we design an aerodrome, we first identify which aeroplanes the aerodrome is intended to serve, then we determine the two elements of the code for those critical aeroplanes. The code letter or number selected for design purposes is related to the critical aeroplane characteristics for which the facility is provided. A particular specification is then related to the more appropriate of the two elements, or to an appropriate combination of both. Now let’s walk through the table, because this is where the actual values live. For Element 1, the code number 1 applies when the aeroplane reference field length is less than 800 metres. Code number 2 applies when it’s 800 metres or more, but less than 1200 metres. Code number 3 applies when it’s 1200 metres or more, but less than 1800 metres. And code number 4 applies when it’s 1800 metres or more. For Element 2, the code letter A applies when the wing span is up to but not including 15 metres, and the outer main gear wheel span is up to but not including 4.5 metres. Letter B applies when the wing span is 15 metres or more but less than 24 metres, and the outer main gear wheel span is 4.5 metres up to but not including 6 metres. Letter C applies when the wing span is 24 metres or more but less than 36 metres, and the outer main gear wheel span is 6 metres up to but not including 9 metres. Letter D applies when the wing span is 36 metres or more but less than 52 metres, and the outer main gear wheel span is 9 metres up to but not including 14 metres. Letter E applies when the wing span is 52 metres or more but less than 65 metres, and the outer main gear wheel span is 9 metres up to but not including 14 metres. And letter F applies when the wing span is 65 metres up to but not including 80 metres, and the outer main gear wheel span is 14 metres up to but not including 16 metres. Notice the pattern in the boundaries — "up to but not including" versus "or more, but less than." That’s deliberate. It tells you exactly which category a value falls into when it sits right on the edge. A wing span of exactly 15 metres goes into B, not A, because A is "up to but not including 15 metres." A field length of exactly 800 metres goes into code 2, not 1, because code 1 is "less than 800 metres." Now, the aeroplane reference field length itself — that’s the foundation for Element 1. It’s defined as the minimum field length, which is the take-off distance, required for take-off at maximum certificated take-off mass, at sea level, with standard atmospheric conditions, still air, and runway slope. And that value comes from the appropriate Aeroplane Flight Manual — the AFM — as prescribed by the certifying authority, or from equivalent data provided by the aeroplane manufacturer. So the key idea here is that the reference field length is a standardised, worst-case take-off distance. It’s not the actual runway length at any particular airport. It’s the distance the aeroplane needs under a fixed set of conditions — sea level, standard atmosphere, still air, and a runway slope — at its maximum certificated take-off mass. That standardisation is what lets us compare aeroplanes and assign them to code numbers consistently. Let me put it together for you. The aerodrome reference code is a two-part label. The number tells you about the take-off performance — the reference field length. The letter tells you about the physical size — the wing span and the outer main gear wheel span. When you design an aerodrome, you identify the critical aeroplanes, you look up their reference field length and their dimensions, and you assign the code. Then every facility on the aerodrome — the runway, the taxiways, the aprons — is designed to match that code. That’s how the whole system stays consistent between the aeroplane and the ground infrastructure.