Class A Aircraft - Take-off — Page 395, Lesson 489

I want to walk you through the brake energy limit and brake cooling for a Class A aircraft take-off. This is a performance limit that protects the brakes from overheating during a rejected take-off. Let's start with the physics. For an aircraft of mass M, travelling at a true speed of V, the kinetic energy is ½ MV². That's half times the mass times the square of the true speed. Now, if the aircraft is braked to a stop from this speed, a large proportion of that energy will go into the brakes as heat. The energy capacity of the brakes is limited, so for a given mass there will be a limiting speed from which a stop can be made. That limiting speed is the brake energy limit speed, and it's given the symbol VMBE. Now, here's a key point: this speed is a True Ground Speed. Because it's a true ground speed, the corresponding Indicated Air Speed, or IAS, will vary with altitude, temperature and wind. Runway slope will also affect the speed, because a change in height involves a change in potential energy. So the same true ground speed limit will show up as different IAS values depending on those conditions. Now, the critical operating rule: the brake energy limit speed VMBE must not be less than the V1 speed. V1 is the decision speed — the speed up to which you can reject the take-off and stop safely on the runway. If VMBE is less than V1, then the mass must be reduced until V1 and VMBE are the same. In other words, you lighten the aircraft so that the speed from which you can stop on brake energy alone comes up to match your decision speed. The flight manual will give the amount of weight to be deducted for each knot that V1 exceeds VMBE. So it's a precise, quantifiable correction — so many kilograms or pounds per knot of excess. Now, in practice, for most aircraft VMBE will only be limiting in extremely adverse conditions of altitude, temperature, wind and runway slope. If you look at Figure 14.14, you'll notice a grey area in the graph on the top left hand side. If the mass and pressure altitude falls within this grey area, then VMBE will not be limiting — unless you're operating with a tailwind or with improved climb performance. So the grey area tells you when you can ignore the brake energy limit. The same graph can be seen in CAP 698 on page 15 of section 4. Now let's move to brake cooling. The value of VMBE obtained from the data assumes that the brakes are at ambient temperature before the start of take-off. That's a crucial assumption. If a take-off is rejected following a recent landing, or after prolonged taxiing, the brakes will already be at a fairly high temperature, and their ability to absorb further energy will be reduced. So the brake energy limit you calculated is no longer valid — the brakes are already hot and can't take as much additional heat. Data is given in the manual to show the time to be allowed for the brakes to cool. An example of a brake cooling graph is shown in Figure 14.15, and it can also be found in CAP 698 on page 50 of section 4. That graph tells you how long you must wait before the brakes have cooled enough to allow another take-off. So to summarise the whole picture: VMBE is the true ground speed from which you can stop on brake energy alone for a given mass. It must never be below V1, and if it is, you reduce mass until they match. And you must always account for the starting temperature of the brakes — if they're hot from a recent landing or long taxi, you need to allow cooling time before the VMBE data is valid again.