Reverse thrust during the landing roll on a transport jet is most effective at

• A. High Speed, High RPM
• B. Low Speed, Low RPM
• C. High Speed, Low RPM
• D. Idle

Answer: (A)

When thrust reversers are used, the braking effect comes from redirecting the engine exhaust forward, opposing the aircraft’s forward motion. The force produced depends on how much exhaust the engine is pushing out (engine thrust) and how much air is flowing through the engine (which increases with both engine RPM and the aircraft’s forward speed). At a higher landing speed, more air is moving through the engine and more exhaust momentum can be redirected forward, so the braking effect is stronger. Likewise, running the engines at high RPM increases the amount of exhaust and thus the potential reverse thrust. If you’re very slow or the engines are at idle, there’s little exhaust to redirect, so reverse thrust adds only a small braking effect. If you had high speed but low RPM, you wouldn’t get as much reverse thrust as you would with high RPM. Therefore the most effective combination during the landing roll is high speed with high engine RPM.