15.18–15.20 APUs, power plant installation and engine fire protection

Scope

All A level 1, B1 level 2.

• 15.18 APUs: purpose, operation, protective systems.
• 15.19 Power plant installation: firewalls, cowlings, acoustic panels, engine mounts, anti-vibration mounts, hoses, pipes, feeders, connectors, wiring looms, control cables and rods, lifting points and drains.
• 15.20 Fire protection: operation of detection and extinguishing systems.

Auxiliary power units (15.18)

Purpose

• Electrical power (APU generator) and bleed air (air conditioning, engine starting) on the ground, independent of ground equipment.
• In flight (within an altitude/speed envelope): back-up electrical power and, at lower altitudes, bleed air; required for some ETOPS operations.

Construction and operation

• Small constant-speed gas turbine, usually in the tail cone behind a firewall, with intake door (opens before start) and exhaust silencer.
• Load compressor (separate centrifugal compressor with inlet guide vanes controlling bleed flow) or bleed from the power section compressor.
• Accessory gearbox drives the generator (often 115/200 V 400 Hz directly, because the APU runs at constant speed), fuel control, oil pump and cooling fan.
• Started by an electric starter from the battery/APU battery (or AC starter-generator); APU fuel from the aircraft tanks via a DC pump.
• Controlled by an electronic control box (ECB) / APU FADEC: automatic start sequence, speed governing at ~100 %, EGT limiting, bleed/load management (electrical load has priority over bleed when temperature limited).

Protective shutdowns (typical)

Automatic shutdown on: overspeed, underspeed, high EGT, low oil pressure, high oil temperature, fire (on the ground — automatic extinguishing), loss of EGT signal, ECB failure, inlet door not open, generator oil over-temperature, etc. Shutdown is normally preceded by a cool-down period at no-load unless an emergency shutdown. Fire detection with automatic shutdown and external warning horn in the nose wheel well and remote shutdown/fire controls on the ground.

Power plant installation (15.19)

Item	Purpose and maintenance points
Firewalls	Isolate fire zones (engine, APU) from the aircraft; stainless steel or titanium; all penetrations sealed with fireproof grommets/bulkhead fittings
Cowlings	Aerodynamic fairing, access, cooling-airflow guidance; fan cowls and thrust reverser cowls latch with hook latches and hold-open rods; un-latched cowls can separate in flight — check all latches and indications
Acoustic panels	Honeycomb with perforated skins in inlet, fan duct and reverser; inspect for delamination, damage, blocked holes
Engine mounts	Transfer thrust, weight, gyroscopic and torque loads; forward mount (thrust) and aft mount; fail-safe links; mount bolts are critical (torque, inspection)
Anti-vibration mounts	Elastomeric isolators reduce vibration transfer (common on turboprops and APUs)
Hoses, pipes, feeders, connectors	Fire-resistant/fireproof lines in fire zones (fuel, oil, hydraulic) with fire sleeves; correct routing, clamping, clearances and torques; lockwire
Wiring looms	Harnesses with fire-resistant insulation and connectors; secured against chafing and heat; separation from fluid lines
Control cables and rods	Throttle/fuel control linkage on older types (rigging, cushion and spring-back checks); most FADEC engines use electrical thrust-lever signals
Lifting points	Hoist points on the engine for removal/installation with approved slings and bootstrap kits
Drains	Drain lines from fuel, oil and hydraulic seals, combustor and nacelle low points collected in a drain mast/drain collector; check drain rates against AMM limits to detect seal leaks

Quick engine change (QEC) unit: engine pre-dressed with accessories, harnesses and pipes for rapid replacement.