When is it essential to run the APU during a mission?

Running the Auxiliary Power Unit (APU) during a mission is essential in specific scenarios, primarily when ground support is required or during engine start. The APU provides electrical power and pneumatic air, which are crucial for several operations before engine start, such as running systems that require power while the engines are not yet running.

Additionally, using the APU during ground support operations, such as when powering onboard systems (heating, air conditioning, and avionics), ensures that the main engines do not need to be running, thus saving fuel and reducing wear on the engines. This capability enhances operational safety and facilitates ground operations without reliance on external power sources.

In contrast, the other choices suggest limitations that do not reflect the full operational needs of the aircraft. For instance, relying on the APU solely when the aircraft is grounded or only during takeoff and landing does not account for the critical role it plays during various stages of the mission. Proper use of the APU ensures that all systems function smoothly and efficiently throughout the mission profile.