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

Let's start with the take-off itself, because everything else hangs off that definition. The take-off part of the flight is defined as the distance from the brake release point — that's the BRP, the exact spot where you release the brakes and start rolling — to the point at which the aircraft reaches a defined height. That defined height is called the "screen height." And here's a key number for you: the screen height varies from 35 ft for Class A aeroplanes to 50 ft for Class B aeroplanes. So Class A, which is your jet transport category, uses 35 feet. Class B, the smaller propeller-driven types, uses 50 feet. Now, for any particular take-off, you must show 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 you need must fit inside what you've got. But the catch is, there are various terms used to describe the available distances at an aerodrome, and some aerodromes have extra distances associated with the main runway which 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 m — that's 500 ft — wide, centrally located about the extended centre line of the runway, and under the control of the airport authorities. So it's a defined strip of ground beyond the end of the runway. The clearway is expressed in terms of a clearway plane, extending from the end of the runway with an upward slope not exceeding 1.25%, above which no object or terrain protrudes. So imagine an invisible sloping plane rising gently off the end of the runway at 1.25%, and nothing may stick up through that plane. There is one exception: threshold lights may protrude above the plane if their height above the end of the runway is 0.66 m — that's 26 inches — or less, and if they are located to each side of the runway. So the lights are allowed to poke through, but only within those limits. Now, here's the crucial point: clearways are not physical structures. They are simply an area of defined width and length which are free of obstacles. There's nothing paved, nothing built — it's just clear airspace and clear ground. 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 — that's an RTO — without causing structural damage to the aeroplane, and it's designated by the airport authorities for use in decelerating the aeroplane during a rejected take-off. So unlike the clearway, stopways are physical structures and are usually paved. But — and this is important — stopways are not as strong as the main length of runway. Therefore they are only used to help bring an aeroplane to a stop in the event of a rejected take-off. You would never use a stopway for normal acceleration or rotation; it's purely for stopping. Stopways are identified by large yellow chevrons on either end of the main runway. So let me contrast the two for you. Clearway: not a physical structure, at least 152 m wide, used for the airborne part of the take-off — you climb over it. Stopway: physical, paved, at least as wide as the runway, used only for decelerating during a rejected take-off, marked with yellow chevrons. 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.