While parked on the ground, fixed- and rotary-wing aircraft rely on Auxiliary Power Units (APUs) to start and power aircraft systems without taking the aircraft into the sky. APUs are fully capable small engines that could, if needed, assist with crucial functions during flight.
Independent Start Capability
APUs provide electrical power to run systems that control and monitor the engine start process. This includes fuel pumps, ignition systems, and avionics. APUs, are especially useful at airports or remote locations without access to external ground power units (GPUs).
Engine Start Sequence
When the pilot initiates the start sequence, the APU delivers compressed air to spin up the high-pressure core of the main engine (often the compressor). As the engine core reaches the required RPM, fuel is introduced and the igniter is sparked, beginning combustion. Once the engine stabilizes and reaches idle speed the APU turns off, allowing the main engine to manage power generation.
Emergency Power
The APU also serves as a backup power source, in case of in-flight engine failure or electrical system issues, maintaining essential functions for safe flight and landing.
APU Maintenance
Proper maintenance of APUs prevents unexpected failures, which can lead to delays and increased operating costs. Preventative maintenance minimizes the need for costly repairs and extends the life of the APU. Aviation authorities require strict adherence to maintenance schedules for safety and operational standards.
Key APU Maintenance Practices
- Routine Inspections: Regular inspections of fuel, oil, and cooling systems safeguard against leaks or wear.
- Oil and Filter Changes: Changing oil and filters based on the manufacturer’s specifications prevents overheating and supports smooth operation.
- Performance Testing: Verifying output for consistent power supply and operational capability ensures reliability.
- Component Replacements: Replacing worn parts, such as turbines and compressors, is required to avoid failure during operation.
Regular APU maintenance is essential for aircraft operational readiness, safety, and efficiency.
