Class A - Additional Take-off Procedures — Page 421, Lesson 520

Right, let’s pick this up where the take-off performance logic left off. We’re now inside the Class A additional take-off procedures, and the specific case I want to walk you through is what happens when the anti-skid system is inoperative. First, let’s make sure we’re clear on what anti-skid actually does. Anti-skid is the system that modulates braking pressure to prevent the wheels from locking up during a rejected take-off or on landing. When it’s working, it lets you apply maximum braking force without skidding. Now, the critical point here is the side effect of losing it. If the anti-skid system is inoperative, the braking capability of the aeroplane is degraded. That means the accelerate-stop distance — the distance required to accelerate to V1, then reject the take-off and bring the aeroplane to a full stop — that distance increases. It gets longer because you simply cannot brake as effectively. But here’s the trade-off I want you to hold onto. The same change that lengthens the accelerate-stop distance also has a side effect on the take-off distance. The take-off distance is the distance required to accelerate to V1, suffer an engine failure, and continue the take-off to a height of 35 feet. And that distance increases as well. So you have both distances growing, and that’s a problem because both must remain within the available field lengths — the physical runway and stopway you have to work with. So how do we resolve this? The answer is to reduce the mass of the aeroplane. If you reduce the take-off mass, both the accelerate-stop distance required and the take-off distance required will decrease. That brings them back within the available field lengths. And there’s a second adjustment that goes hand in hand with the mass reduction: V1 must also be decreased. V1 is the decision speed — the speed up to which you can reject the take-off and stop safely, and beyond which you must continue. So, in summary, with the anti-skid system inoperative, both V1 and the aeroplane mass must be decreased. Now, where do you actually find the numbers for this? The anti-skid inoperative procedure is detailed on page 34 of section 4 in CAP 698. That’s the Civil Aviation Publication that contains the performance schedules. At the top of that page there’s a very clear introduction to the procedure, and below that is the method for calculating the mass reduction and the V1 reduction. So the procedure gives you a systematic way to compute exactly how much you need to bring the mass down and how much to bring V1 down, so that your rejected take-off and your continued take-off both fit within the field. Let me just tie the logic together once more, because this is the core of the whole procedure. Anti-skid inoperative means degraded braking. Degraded braking means longer accelerate-stop distance. But it also lengthens the take-off distance. To fix both, you reduce mass, which shortens both distances, and you reduce V1. So the two levers you pull are mass and V1, and both go down. That’s the entire rationale behind the anti-skid inoperative procedure.