General Principles - Take-off — Page 148, Lesson 172

We're starting the take-off section of the performance syllabus, and the very first thing I want to establish is the fundamental definition of the take-off itself, because everything else hangs off it. The take-off part of the flight is defined as the distance from the brake release point, which we abbreviate as BRP, to the point at which the aircraft reaches a defined height. That defined height is a specific technical term called the "screen height." Now, the screen height is not a universal number — it varies by aircraft class. For Class A aeroplanes, the screen height is 35 feet. For Class B aeroplanes, it's 50 feet. So when we talk about the take-off distance, we're always measuring from the moment the brakes are released to the moment the aircraft climbs to that screen height. For any particular take-off, there's a hard safety requirement: it must be shown that the distance required for take-off in the prevailing conditions does not exceed the distance available at the take-off aerodrome. In other words, what the aircraft needs to perform the take-off must fit within what the aerodrome can offer. But here's the catch — there are various terms used to describe the available distances at an aerodrome, and that's where the complexity comes in. Some aerodromes have extra distances associated with the main runway, and these are used in a variety of ways. Before we look at those distances, we need to define two areas associated with the runway. The first is the clearway. A clearway is an area beyond the runway, not less than 152 metres — that's 500 feet — wide, centrally located about the extended centre line of the runway, and under the control of the airport authorities. Now, the clearway is expressed in terms of a clearway plane. That plane extends from the end of the runway with an upward slope not exceeding 1.25%, and above that plane no object or terrain may protrude. There is one exception: threshold lights may protrude above the plane, but only if their height above the end of the runway is 0.66 metres — that's 26 inches — or less, and only if they're located to each side of the runway. Here's the key conceptual point: clearways are not physical structures. They are simply an area of defined width and length which are free of obstacles. So a clearway is essentially protected airspace beyond the runway, not a paved surface. Now the second area is the stopway. A stopway is an area beyond the take-off runway, no less wide than the runway, and centred upon the extended centre line of the runway. It must be able to support the aeroplane during a Rejected Take-off — abbreviated RTO — without causing structural damage to the aeroplane. And it must be designated by the airport authorities for use in decelerating the aeroplane during a rejected take-off. Now contrast this with the clearway: stopways are physical structures and are usually paved. However — and this is an important limitation — stopways are not as strong as the main length of runway. That's why they are only used to help bring an aeroplane to a stop in the event of a rejected take-off. They are not for normal operations. Stopways are identified by large yellow chevrons on either end of the main runway. So let me pull the contrast together, because it's the heart of this section. The clearway is obstacle-free airspace beyond the runway — it's for the aeroplane to climb through, it's not a physical structure. The stopway is a physical, usually paved, surface beyond the runway — it's for the aeroplane to roll over and decelerate during a rejected take-off, and it's weaker than the main runway. Understanding stopways and clearways is essential when examining the available distances published at aerodromes. There are four principal aerodrome distances, although only three will apply to the take-off, and those are what we'll discuss next. So hold onto these two definitions — clearway and stopway — because the available distances we're about to look at are built directly on top of them.