General Principles - Take-off — Page 148, Lesson 174

Let's start with the very foundation of take-off performance: the distances the aerodrome gives you to work with. These are the "available" distances, and they are fixed properties of the runway and its surroundings. You don't calculate them; you look them up for the airport you're using. First, the Take-off Run Available, or TORA. This is the distance from the point on the aerodrome surface where the aeroplane can begin its take-off run, to the nearest point in the direction of take-off where the surface can no longer bear the weight of the aeroplane under normal operating conditions. In plain terms, it's the length of runway you can physically roll on before the ground can't support you. At most aerodromes, the TORA is simply the runway length from threshold to threshold. Next, the Take-off Distance Available, or TODA. This is the TORA plus any clearway. So, TODA = TORA + clearway. If there's no clearway, then the TODA equals the TORA. Now, what is a clearway? It's an area beyond the end of the runway, over which the aeroplane can continue its initial climb. It's not a surface you roll on; it's airspace you fly through. The key point here is that the TODA must be compared to the aeroplane's actual take-off distance. The regulations require that the aeroplane must be able to complete its take-off within the TODA. But here's a critical limit: even though clearways can be any length, there's a cap on how much clearway you can count when calculating TODA. The maximum clearway length you can use cannot be more than half the length of the TORA. So, the clearway you credit is limited to 50% of your take-off run available. Now, the third distance: the Accelerate-stop Distance Available, or ASDA, sometimes called the Emergency Distance Available, or EMDA. This is the TORA plus any stopway. So, ASDA = TORA + stopway. If there's no stopway, then the ASDA equals the TORA. A stopway is a surface beyond the end of the runway, designed to support the aeroplane during a rejected take-off. It's marked with large yellow chevrons. The ASDA must be compared to the aeroplane's actual accelerate-stop distance. The requirement is that the aeroplane's accelerate-stop distance must not exceed the ASDA. In other words, if you decide to abort the take-off, you must be able to stop within the ASDA. Let me tie these together. You have three distinct available distances: TORA for the ground roll, TODA for the full take-off including the climb over a clearway, and ASDA for the rejected take-off stopping distance. Each one is compared against a different performance requirement of the aeroplane. The take-off distance must fit within the TODA, and the accelerate-stop distance must fit within the ASDA. And remember the clearway limit: no more than half the TORA can be credited as clearway. There's also a figure that summarises all of this, showing the take-off run available, the clearway, the accelerate-stop distance available, and the first significant obstacle, with a minimum of 152 metres for Class A aircraft. That's the summary of the available distances at an aerodrome.