보조 동력 장치 (APU)

Overview

The Auxiliary Power Unit is a small gas turbine engine, typically installed in the aircraft tail cone, that operates independently of the main propulsion engines to provide electrical power and pneumatic bleed air for ground operations and as an airborne backup. On the ground, the APU enables the flight crew to start main engines, power cabin air conditioning and lighting, operate avionics and electrical systems, and conduct pre-departure checks without reliance on ground support equipment (GPU) or an external air cart. This self-sufficiency capability is particularly valuable at remote airports with limited ground support infrastructure.

The APU is certified as part of the aircraft type certificate but operates on its own FADEC and fire protection system. It has different operating envelope limitations from the main engines — most APUs are limited in operation above certain altitudes (typically FL250 for start, FL410 for operation), beyond which the main engines must supply all electrical and pneumatic needs.

How It Works

An APU is structurally similar to a turboshaft engine: a centrifugal or axial compressor driven by a turbine, with a combustion chamber between them. Unlike the main propulsion engines, the APU's turbine output is not used to produce thrust. Instead, the compressor drives a gearbox that powers a generator (or combined starter-generator on modern aircraft) and provides load compressor output for the aircraft's pneumatic system.

On a conventional APU, the load compressor is a separate section providing bleed air at regulated pressure and temperature for engine starting and air conditioning. A load control valve modulates the bleed extraction to protect the APU from overload. The generator section produces 115V 400Hz AC power (or 28V DC on smaller aircraft) via a constant-speed drive or, on newer designs, a variable-frequency generator with power electronics. APU start is typically electric, using a dedicated starter-generator or a dedicated starter motor, and the sequence is managed automatically by the APU FADEC with no required crew action other than activation and monitoring.

Key Components

• APU Engine Core: Centrifugal compressor, annular combustor, and single-stage turbine in a compact package. Modern APUs from suppliers such as Honeywell (131-9), Pratt & Whitney Canada (APS 3200), and Safran achieve thermal efficiencies comparable to purpose-built turbogenerators.
• Load Compressor Section: Separate compressor stage (or dedicated stage) providing regulated pneumatic output for engine start and air conditioning. Load Control Valve (LCV) modulates extraction.
• APU Generator: 90–120 kVA AC generator on most commercial aircraft (larger on widebodies such as the A380). Connected to the AC bus through an APU Bus Tie Breaker.
• APU FADEC / Electronic Control Box (ECB): Manages start sequencing, governs speed and load, enforces EGT and speed limits, and controls shutdown. Communicates with aircraft maintenance systems for fault reporting.
• APU Fire Detection and Suppression: Separate continuous element detector loop around the APU bay, connected to a dedicated extinguisher bottle. Automatic shutdown on fire detection on ground; crew-commanded shutdown in flight.
• Inlet and Exhaust: Flush-mounted ram air inlet (typically a scoop or door-covered inlet in the tail cone) and a separate exhaust duct. Inlet door opens automatically on APU start and closes on shutdown.

Aircraft Applications

• Boeing 737-800 — Honeywell 131-9B APU; single generator (90 kVA) and pneumatic output for start and aircon
• Airbus A320-200 — Honeywell 131-9A APU; automatic start monitoring via ECAM; widely used for self-contained airport operations
• Boeing 777-300ER — Honeywell 331-500 APU; 120 kVA generator; pneumatic output for large-diameter main engine starts
• Boeing 787-9 — Honeywell 131-9B APU modified for bleed-free operation; provides electrical power only (225 kVA); electric engine start via main starter-generators
• Airbus A380-800 — two APUs (Pratt & Whitney Canada APS 3200) for the massive electrical and pneumatic demands of the double-deck aircraft

Advantages and Limitations

The APU provides genuine operational flexibility, enabling aircraft to operate independently at destinations without ground power infrastructure and reducing turnaround time by allowing systems to be powered while refuelling and loading. It significantly reduces dependence on ground diesel generators and air carts, with associated noise and emissions benefits at the gate — though the APU itself produces noise and exhaust that are subject to increasing airport regulation.

APU fuel consumption is a meaningful operational cost: a typical narrowbody APU burns approximately 100–130 kg/hour at idle, and airline APU ground time runs from less than 30 minutes (best case, with full gate power available) to several hours on turns at airports without ground power. Airlines increasingly use Fixed Electrical Ground Power (FEGP) and Pre-Conditioned Air (PCA) to minimise APU run time for environmental and cost reasons. APU reliability is critical: an aircraft with an inoperative APU may face Minimum Equipment List restrictions and require ground air carts for engine starting at all departures, with operational and scheduling implications.