First, pavement strength — Page 377, Lesson 511

Let’s pick up right where the runway surface itself ends and the numbers that govern how we use it begin. We’ve just been talking about the physical shape of the runway and the geoid — that irregular, undulating model of the Earth’s mean sea level caused by local gravitational disturbances. Now I want to move into something you’ll use every single day as a professional pilot: pavement strength and declared distances. First, pavement strength. Here’s the rule that gates everything: where paved areas — and that means runways, taxiways, and aprons — are used by aircraft with a maximum take-off mass greater than 5700 kilograms, the strength of that pavement is reported using the ACN-PCN system. That’s the Aircraft Classification Number and the Pavement Classification Number. The operating rule is beautifully simple: an aircraft can safely use a paved area if the PCN is equal to or greater than the ACN. So the pavement’s number must meet or beat the aircraft’s number. Let me define each side precisely. The PCN, the Pavement Classification Number, indicates the strength of a runway, taxiway, or apron. And here’s a point that surprises students: it is of primary importance for the apron, because that’s where the aircraft mass will be greatest — you’re parked, fully loaded, static, and the pavement has to hold that entire weight without the benefit of motion. Also note the limitation: PCN is only used for paved areas. Unpaved surfaces are a completely different system. Now the ACN, the Aircraft Classification Number. This is a single unique number expressing the relative effect of an aircraft on a paved area, and it relates specifically to pavement type and thickness. It sits on a continuous scale that increases from zero with no upper limit. Every aircraft has an ACN — it’s a property of the aircraft, not the airport. So when you compare, you’re matching the aircraft’s effect number against the pavement’s strength number. But what about the smaller aircraft? For aircraft with a maximum mass equal to and less than 5700 kilograms, the strength of the pavement is not given by ACN-PCN. Instead, it’s calculated from two things: the maximum allowable mass, or the maximum tyre pressure. So the threshold is that 5700-kilogram figure — above it, ACN-PCN; at or below it, mass or tyre pressure. Now let’s shift to the declared distances. For a runway intended for use by international commercial air transport, the following distances must be calculated to the nearest metre or foot. There are four of them, and you must know each by name and by definition. First, TORA — Take-Off Run Available. Second, TODA — Take-Off Distance Available. Third, ASDA — Accelerate-Stop Distance Available. And fourth, LDA — Landing Distance Available. Let me give you the precise definition of TORA, because it sets the pattern. TORA is the distance between the point at which an aeroplane can commence the take-off run, to the nearest point in the direction of take-off at which the surface is incapable of bearing the mass of the aeroplane under normal operating conditions. So it’s not just the physical length of the runway — it’s the usable length, measured from where you can actually start rolling, up to the point where the surface can no longer support your aircraft’s weight under normal conditions. That’s the key idea: the surface’s bearing capacity defines the end of the available distance, not just where the tarmac visually stops. That’s the foundation. We’ve got the 5700-kilogram threshold, the ACN-PCN comparison rule, the PCN for pavement strength with its apron emphasis, the ACN as the aircraft’s unique effect number, the mass-or-tyre-pressure rule for light aircraft, and the four declared distances with TORA defined precisely. Take a moment to let that settle — the next step will be the definitions of TODA, ASDA, and LDA, and how they build on this same idea of usable distance.