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

Let's start with the very foundation of take-off performance — the distances an aerodrome gives you to work with. These are the "available" distances, and they're the yardsticks against which we measure the aeroplane's actual performance. First, the Take-off Run Available, or TORA. This is the distance from the point on the aerodrome surface where the aeroplane can commence its take-off run, to the nearest point in the direction of take-off where the surface is incapable of bearing the weight of the aeroplane under normal operating conditions. In plain terms, it's the length of runway you can actually roll along on the ground before the surface can no longer support you. At most aerodromes, the TORA is simply the length of the runway from threshold to threshold. Next, the Take-off Distance Available, or TODA. This is the take-off run available plus any clearway — so TORA plus clearway. If there's no clearway, then the TODA equals the TORA. Now, what's a clearway? It's an area beyond the end of the runway, and it's not meant for rolling on the ground — it's airspace above ground that's clear of obstacles, so the aeroplane can climb over it. The TODA is what we compare against the aeroplane's actual take-off distance. The requirement is that the aeroplane must be able to complete the take-off within the take-off distance available. But here's a critical limit: although clearways can be of any length, there's a cap on how much clearway you can count when calculating TODA. The maximum length of clearway you can use cannot be more than half the length of the TORA. Then we have the Accelerate-stop Distance Available, or ASDA, also called the Emergency Distance Available, or EMDA. This is the take-off run available plus any stopway — TORA plus stopway. If there's no stopway, then the ASDA equals the TORA. A stopway is an area beyond the runway that's strong enough to support the aeroplane during a rejected take-off — that's when you abort the take-off and brake to a stop. The ASDA is compared against the aeroplane's actual accelerate-stop distance, and the requirement is that the aeroplane's accelerate-stop distance must not exceed the accelerate-stop distance available. So to summarise the picture: TORA is the runway you roll on. TODA is TORA plus clearway — the runway plus the obstacle-free airspace you climb through. ASDA is TORA plus stopway — the runway plus the strong ground you can stop on if you abort. And remember the clearway limit: it can't exceed half the TORA. Now, one more thing from the summary figure — there's a minimum value of 152 metres, and it's specified for Class A aircraft. That's the minimum length for something in this take-off distance picture, and it applies specifically to Class A aircraft — the large transport category jets and turboprops you'll be flying. That 152-metre figure is a floor, a minimum that must be respected in the take-off distance available calculation. So when you're planning a take-off, you're always comparing three things: the aeroplane's actual take-off distance against the TODA, the aeroplane's actual accelerate-stop distance against the ASDA, and you're respecting that clearway can't exceed half the TORA, with that 152-metre minimum for Class A aircraft in the mix.