How Good Are Your Brakes?

Cool videos. The V1 speed is the decision time to determine whether you fly or stay on the runway. V1 speed is determined after weight and balance are calculated for each flight. This is not critical on small general aviation aircraft because their wieght and balance do not change as much as cargo and commercial passenger aircraft. Air density is also figured into the calculation because an airplane can take off in considerably less distance at sea level on a cold dry day than on a mountain top on a hot humid day.

The brakes on big aircraft must convert a huge amount of momentum into heat via the brakes.

Mass = kilograms (kg)
Distance = meters (m)
Time = seconds (s)

Momentum (kg m/s) = mass (kg) x velocity (m/s); also, Force = mass x acceleration.

Kinetic energy (joules) = 1/2 x mass (kg) x velocity (m/s) squared.

Joule = kg x m (squared) / s (squared)

1 BTU = 1055 joules
1 BTU = 778 to 782 lb.ft. force
1 BTU = 1.054 to 1.060 kJ (kilo Joules)
1 BTU = .293071 Wh (Watt hours)

So, you can see that a tremendous amout of kinetic energy is developed by an airliner by the time V1 speed is achieved. That energy is changed to heat energy via the brakes and, to a much lesser degree, air friction. The same holds true on landing. Fuel tanks are filled to the minimum necessary for a flight plus required reserve for reaching alternate destinations. The reason for dumping fuel during unscheduled early landings is to bring down aircraft weight (and, therefore, momentum) so the aircraft can land on a specific length of runway.

Physics is phun.