Performance - Introduction — Page 136, Lesson 161

Let's start with the very first definition on this page: Specific Fuel Consumption. That's fuel flow per unit of thrust. The lower the value, the more efficient the engine. So if an engine burns less fuel to produce the same thrust, its specific fuel consumption is lower, and that's a good thing for range and endurance. Next, Stopway. This is an area beyond the take-off runway. It's no less wide than the runway, and it's centred upon the extended centre line of the runway. It has to be able to support the aeroplane during an aborted take-off without causing structural damage, and it's designated by the airport authorities for use in decelerating the aeroplane during that aborted take-off. So think of it as a paved shoulder at the end of the runway, built to take the weight of the aircraft if you have to stop after you've already started rolling. Now Take-off Distance Available — and I want you to hold onto this one because it's a key performance number. It's equal to TORA plus any clearway. TORA is Take-off Run Available, which we'll get to in a moment. And the key limit here: the Take-off Distance Available cannot be more than one and one half times the TORA, whichever is the less. So you take TORA, add any clearway, but the total is capped at 1.5 times TORA. Let me define Take-off Mass next. That's the mass of the aeroplane, including everything and everyone contained within it, at the start of the take-off run. So it's the all-up weight the moment you begin rolling down the runway. Now Take-off Power. This is the output shaft power identified in the performance data for use during take-off, discontinued approach, and baulked landing. And there are three time limits attached to it. For piston engines, it's limited to a continuous period of not more than 5 minutes. For turbine engines installed in aeroplanes and helicopters, also limited to not more than 5 minutes. And for turbine engines installed in aeroplanes only — when specifically requested — it's limited to not more than 10 minutes, but only in the event of a power unit having failed or been shut down. So the 10-minute extension is a contingency rating for engine failure. Take-off Run Available — TORA. This is the distance from the point on the surface of the aerodrome at which the aeroplane can commence its take-off run, to the nearest point in the direction of take-off at which the surface of the aerodrome is incapable of bearing the weight of the aeroplane under normal operating conditions. So it's the runway length that can physically support your aircraft's weight. Then Take-off Safety Speed. This is a referenced airspeed obtained after lift-off at which the required one-engine-inoperative climb performance can be achieved. So after you rotate and get airborne, you need to reach this speed to guarantee you can climb out if one engine fails. Take-off Thrust — and note this mirrors Take-off Power exactly, but it's thrust instead of shaft power. It's the output shaft thrust identified in the performance data for use during take-off, discontinued approach, and baulked landing. Same three limits: piston engines, 5 minutes; turbine engines in aeroplanes and helicopters, 5 minutes; and turbine engines in aeroplanes only, 10 minutes in the event of a power unit failure or shutdown. Taxiway. A defined path on a land aerodrome established for the taxiing of aircraft, intended to provide a link between one part of the aerodrome and another. Simple — it's the road between the runway and the apron. Thrust. That force acting on an aeroplane produced by the engine(s) in a forward direction. That's the fundamental propulsive force. True Airspeed. The airspeed of an aircraft relative to undisturbed air. So it's your actual speed through the air mass, not what the instrument reads at altitude. And finally Turbojet. An aircraft having a jet engine in which the energy of the jet operates a turbine that in turn operates the air compressor. So the exhaust drives the turbine, the turbine drives the compressor, and that's the core of the turbojet cycle. Now, I want to tie the runway definitions together, because they're related. You have TORA — the runway that bears your weight. Then you have a clearway, which extends beyond the runway with an upward slope not exceeding 1.25% — that's in the figure. And a stopway, which is the paved area for aborted take-offs. The Take-off Distance Available is TORA plus clearway, capped at 1.5 times TORA. So the clearway gives you extra distance for the airborne portion of the take-off, but the stopway is for stopping, not for distance calculation. Let me show you the figures so you can see how these fit together physically. So the key contrasts to remember: clearway is for the airborne climb after lift-off, stopway is for the ground deceleration during a rejected take-off. And the Take-off Distance Available uses the clearway, not the stopway. That's a critical distinction for your performance calculations.