APUs, Piston, Electric/Hybrid Engines & Engine Control
Auxiliary power units, piston and electric/hybrid propulsion at familiarisation level, then engine control in depth: hydromechanical FCU, supervisory EEC and full-authority digital engine control (FADEC).
Engine types & engine control, summary notes
Free- Sub-section 14.1 covers engines as a family, not just turbines. A B2 engineer needs a systems-level awareness of each type and, at greater depth, of how the engine is controlled electronically.
- Auxiliary Power Units (APUs) are small gas turbines, usually in the tail cone, driving a generator and supplying bleed air for air conditioning and main-engine start on the ground, and providing backup electrical power in flight. The APU has its own start, ignition, fuel, lubrication, bleed, fire protection and automatic shutdown protections, and is controlled from the flight deck through an electronic control unit.
- Piston engines convert reciprocating motion to rotation and are used on light aircraft. Their electrical interfaces of interest to a B2 engineer are the magneto ignition system, the starter, the charging system and the engine instruments (RPM, manifold pressure, cylinder head and exhaust gas temperature, oil pressure and temperature, fuel flow).
- Electric and hybrid propulsion uses electric motors supplied by batteries or fuel cells, or a combination of a thermal engine and an electric motor. Key items are the high-voltage distribution and isolation, the battery management system, thermal management, motor controllers and inverters, and the safety precautions for high-voltage systems.
- Engine control (14.1e) is examined at the higher level. Older engines used hydromechanical fuel control units. Modern engines use Electronic Engine Control: a supervisory EEC trims a hydromechanical unit, whereas a full-authority digital engine control (FADEC) has complete authority over engine operation with no mechanical backup.
- A FADEC has dual redundant channels (an active channel and a standby channel that swap on fault), takes inputs from the thrust lever, air data, and engine sensors (N1, N2, EGT, pressures and temperatures), and drives the fuel metering unit, variable geometry (variable stator vanes, bleed valves), and the start and ignition sequence. It enforces limit protection against overspeed, overtemperature and surge, provides automatic start and relight, and reports faults through built-in test equipment to the maintenance system.
- The FADEC is usually powered by a dedicated permanent-magnet alternator on the engine gearbox once running, so it is independent of the aircraft electrical system, with aircraft power used for starting.
- APU role
- Ground: electrical power + bleed for air-con and engine start · In flight: backup power
- Control evolution
- Hydromechanical FCU > supervisory EEC > full-authority FADEC
- FADEC architecture
- Dual channel (active/standby) · sensor inputs > fuel metering + variable geometry
- FADEC power
- Dedicated permanent-magnet alternator when running; aircraft power for start
What is the practical difference between a supervisory EEC and a FADEC?
A supervisory EEC trims an underlying hydromechanical fuel control unit, which can still run the engine if the electronics fail. A FADEC has full authority with no mechanical backup, so it is built with dual redundant channels and extensive built-in test to achieve the required reliability.
Why is the FADEC supplied by its own permanent-magnet alternator?
So that engine control is independent of the aircraft electrical system once the engine is running. A failure or interruption of aircraft power cannot then cause loss of engine control; aircraft power is used only for starting before the alternator produces sufficient output.
State the main services an APU provides.
On the ground it drives a generator for electrical power and supplies bleed air for air conditioning and for starting the main engines; in flight it can provide backup electrical power and, on some types, bleed air, within its certified operating envelope.
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