APU

Understanding the Auxiliary Power Unit (APU)

The Auxiliary Power Unit (APU) is a compact gas turbine that supplies electrical and pneumatic power independently of the main engines. In transport-category aircraft, the APU typically provides 115/200 V AC, 400 Hz electrical power and bleed air for air conditioning and engine starting. An electronic control unit (ECU/ECB) manages fuel scheduling, start logic, and protective functions, while valves such as the APU inlet door, load control valve (LCV), and bleed/isolations valves regulate air intake and pneumatic output. ATPL-level knowledge focuses on how the APU integrates with aircraft systems, normal indications (N%, EGT, oil), and the sequence from start command to “APU AVAIL” or “ON SPEED” status, indicating stabilized governed speed and electrical/bleed availability.

Standard procedures emphasize starting with a verified power source (battery or external), monitoring for a correct EGT rise and acceleration, and waiting for stabilization before loading the APU with packs or engine start air. Many procedures require packs off or reduced pack demand during engine start to ensure adequate duct pressure. Before shutdown, crews typically remove pneumatic and high electrical loads and allow a short cooldown period to prevent thermal stress, unless the system provides automatic cooldown. Operating limitations—published in the AFM/FCOM and governed by aviation regulations—cover start altitude/airspeed envelopes, maximum continuous EGT, generator load limits, and bleed availability in flight (often more limited than on the ground). Dispatch considerations under the MEL may permit flight with an inoperative APU if suitable ground electrical and pneumatic support is available.

Protective logic is central to APU systems questions. The ECU monitors for overspeed, EGT overtemperature, low oil pressure, high oil temperature, and door or bleed faults, commanding automatic shutdown and load shedding as required. Fire protection typically includes dual fire detection loops and an extinguishing bottle; on many aircraft the APU will automatically shut down and discharge the bottle on the ground, while in flight the crew must discharge manually—details vary by type and must be learned per aircraft. Fuel is normally drawn from a main tank via an electric pump (often DC-powered for battery starts). Environmental and safety factors include tailwind or crosswind start limits, FOD risk at the intake, high-temperature exhaust zones, and ramp procedures to keep personnel and equipment clear of the hazard area. Understanding these systems and procedures enables correct decision-making when selecting the combination of true statements often seen in ATPL-style exams.

What the APU question bank covers

• APU functions and components: ECU/ECB, starter-generator, inlet door, LCV, bleed and isolation valves, surge/pressure control.
• Normal procedures: start sequence, stabilization, applying/removing electrical and bleed loads, cooldown and shutdown.
• Operating limitations: EGT, N% indications, generator load, start cycle limits, altitude/airspeed envelopes, and environmental constraints.
• Pneumatic/electrical integration: supplying packs and engine starts, crossbleed considerations, effects on duct pressure and EGT.
• Protection and fire safety: automatic shutdown logic, fault indications, ground vs in-flight fire response, and crew actions.
• Regulatory and operational aspects: AFM/FCOM procedures, MEL dispatch with APU inoperative, and best practices per aviation regulations.