Performance - Introduction — Page 136, Lesson 161

Let’s start with the very first definition on this page, because it sets the tone for the whole performance chapter: Specific Fuel Consumption. That is the fuel flow per unit of thrust. In plain terms, it tells you how much fuel the engine burns to produce each unit of thrust. The lower the value, the more efficient the engine. So when you compare engines, a lower specific fuel consumption means you get more thrust for every kilogram of fuel you burn. Next, the Stopway. This is an area beyond the take-off runway. It must be no less wide than the runway, and it is centred upon the extended centre line of the runway. Its job is to support the aeroplane during an aborted take-off, without causing structural damage to the aeroplane. And importantly, it is designated by the airport authorities for use in decelerating the aeroplane during an aborted take-off. So a stopway is not just any flat ground past the runway — it is a certified, load-bearing surface meant to stop you safely if you reject the take-off. Now, Take-off Distance Available, often abbreviated TODA. It is equal to the Take-off Run Available, TORA, plus any clearway. And there is a hard limit: it cannot be more than one and one half times the TORA, whichever is the less. So you take TORA, add the clearway, but you cap the result at 1.5 times TORA. Next, Take-off Mass. This is the mass of the aeroplane, including everything and everyone contained within it, at the start of the take-off run. So it is the total weight of the aircraft, fuel, passengers, cargo — everything — the moment you begin rolling. Then we have Take-off Power. This is the output shaft power identified in the performance data for use during take-off, discontinued approach, and baulked landing. There are three time limits attached to it. For piston engines, it is 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 is 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 limit is a contingency rating — it only applies if you have lost an engine. Next, 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 is the usable runway length — from where you start rolling to where the ground can no longer support your 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 it is the speed you must reach after becoming airborne to guarantee you can climb out with one engine failed. Next, Take-off Thrust. This is the output shaft thrust identified in the performance data for use during take-off, discontinued approach, and baulked landing. The time limits mirror take-off power exactly: 5 minutes for piston engines, 5 minutes for turbine engines in aeroplanes and helicopters, and 10 minutes for turbine engines in aeroplanes only, in the event of a power unit failure or shutdown. Then we have the Taxiway. This is 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. So it is the route you use to move between the runway and the apron or other areas. Next, Thrust. This is that force acting on an aeroplane, produced by the engine or engines, in a forward direction. It is the force that propels you down the runway and through the air. Then True Airspeed, TAS. This is the airspeed of an aircraft relative to undisturbed air. So it is your actual speed through the air mass, corrected for altitude and temperature effects. Finally, Turbojet. This is an aircraft having a jet engine in which the energy of the jet operates a turbine, which in turn operates the air compressor. So the exhaust jet spins the turbine, and that turbine drives the compressor at the front of the engine. Now, let me tie these together. The stopway and the clearway are the two surfaces beyond the runway that extend your take-off distance available. The clearway is shown in Figure 2.1 — it extends from the end of the runway with an upward slope not exceeding 1.25%. The stopway, in Figure 2.2, is the surface that supports you during a rejected take-off, marked by large yellow markings. And Figure 2.3 illustrates both stopways and clearways together at an aerodrome. So the key distinction: the clearway is for continuing the take-off climb, while the stopway is for stopping during an aborted take-off. And your take-off distance available is TORA plus clearway, capped at 1.5 times TORA. That is the foundation of take-off performance.