What does APU stand for and what does it provide?

• A. Auxiliary Power Unit; Provides only electrical power.
• B. Auxiliary Power Unit; It provides bleed air for the AC packs, electrical power, and engine start.
• C. Attenuated Power Unit; Provides bleed air for the AC packs.
• D. Air Processing Unit; Provides bleed air for pressurization.

Answer: (B)

The APU stands for Auxiliary Power Unit. It’s a compact turbine that supplies essential power when the main engines aren’t running or when extra capacity is needed. It provides electrical power to run avionics, lighting, and other systems, and it also delivers bleed air to the environmental control system to operate the air conditioning packs and cabin pressurization. In addition, that bleed air can be used to start the main engines via the air‑start system. That combination—bleed air for the packs, electrical power, and engine starting—is why this option is the best. The other terms are not correct for this system, and saying it provides only electrical power misses two important functions: bleed air for the ECS and the ability to start engines.