Final answer:
Flight crews cannot use REV IDLE on landing when there is an engine failure or when specific landing conditions require a normal idle power approach.
Step-by-step explanation:
Flight crews cannot use REV IDLE on landing when the aircraft has an engine failure or when specific landing conditions require a normal idle power approach. REV IDLE, also known as reverse idle, refers to the lowest setting of engine thrust in reverse during landing. It is used to provide additional deceleration to the aircraft.
Engine failures can occur during landing and, in such cases, the flight crew must utilize other available means to safely decelerate the aircraft, such as brakes, spoilers, and thrust asymmetry compensation systems.
Specific landing conditions, such as contaminated runways or strong crosswinds, may also restrict the use of REV IDLE. These conditions affect the effectiveness and safety of using reverse thrust, and alternative landing techniques are employed to ensure a safe landing.