Take Off and Landing Performance Questions and Answers

Answer: D

The forces along the direction of motion are thrust, drag and a part of both weight and lift.

On the ground roll the aircraft is pushed forward by thrust and held back by aerodynamic drag, but also by rolling friction at the wheels. The friction force is \(\mu(W - L)\): it depends on the part of the weight not yet carried by the wing, so both weight and lift enter the force balance along the runway.

\[ m\frac{dV}{dt} = T - D - \mu(W - L) \]

Thrust alone or drag alone leaves out the opposing forces, and 'thrust and drag' leaves out wheel friction, which is what brings weight and lift into the horizontal balance.

Answer: A

The reason is that the thrust is directly proportional to free-stream density.

Hot air is less dense. An engine's thrust falls with air density (in the simple model it is taken as directly proportional to it), so on a hot day the aircraft accelerates more slowly. The thinner air also raises the speed needed to make lift equal weight, so the aircraft must reach a higher speed with less thrust, and the ground roll lengthens.

Thrust does not depend on the aircraft's weight. The lift-off distance is inversely related to density (it grows as density falls), not directly proportional to it. Runway friction is not what changes on a hot day; the density of the air is.