How is the APU started according to the system requirements?

• A. Using an electric starter
• B. Using hydraulic pressure from an aircraft-mounted accumulator
• C. Using manual crank
• D. Using battery power

Answer: (B)

The auxiliary power unit (APU) is started using hydraulic pressure from an aircraft-mounted accumulator as part of its system requirements. This method allows the APU to initiate operation efficiently without relying solely on electrical systems, which can be particularly advantageous in certain operational scenarios where battery levels may be low or other electric systems are inactive.

Hydraulic pressure is often stored in an accumulator and can provide the necessary force to turn the APU's starter motor quickly and reliably. This ensures a quick start-up process, which is crucial when the APU is needed to provide electrical power and pneumatic pressure for engine start and other auxiliary functions while the aircraft is on the ground.

While other options such as an electric starter might be utilized in some contexts, and battery power can support various aircraft systems, the unique design of the APU allows hydraulic systems to play a pivotal role in its starting procedure in many aircraft. Manual cranking, on the other hand, is generally not applicable for modern APUs due to the use of sophisticated start mechanisms designed for efficiency and reliability.