The aircraft's fuel system, covered back in the fuel-system module, ends at the engine's doorstep; this lesson follows the fuel from there to the nozzles, and then covers the intelligence that decides how much of it flows: the fuel control unit, the family of engine control systems, and the turbine world's FADEC, whose piston cousin the piston module introduced.
Three questions from this lesson — one straightforward, one that applies it, one that pushes a little further. Nothing is recorded, and you can draw a new set as often as you like.
The HP (high-pressure) fuel cock is:
The engine fuel filter is fitted with a bypass because:
In cruise, one of the duplicated sensor inputs of a FADEC (full-authority digital engine control) fails. The consequence is:
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LP (low-pressure) pump — the engine-driven backing pump: it guarantees the HP pump a positive supply, protecting against vapour and cavitation whatever the tanks and altitude are doing.
Filters — protect the fine machinery downstream; a bypass opens if a filter clogs (ice being the classic cause), because dirty fuel is better than no fuel.
HP (high-pressure) pump — delivers the pressure the nozzles need to atomise fuel into a burning chamber.
Fuel control (hydromechanical unit) — the metering brain, portioned out in its own section below.
HP fuel cock — the shut-off between control and engine: the cockpit's fuel lever or switch, the definitive way to stop a turbine.
Fuel manifold and nozzles — the manifold rings the engine distributing fuel evenly; the nozzles atomise it into the combustion chamber's primary zone.
The two classic HP pump types:
Gear-type pump — two meshing gears carrying fuel around their casing: simple, robust and of fixed displacement.
Swash-plate type — an axial piston pump whose angled plate strokes its pistons, the plate angle variable so the pump's delivery can itself be varied.
The FCU's tasks: meter the fuel that turns the crew's thrust or power demand into a steady setting; schedule fuel through accelerations and decelerations so the compressor never surges nor the flame ever starves (its acceleration schedule is exactly the acceleration line of the compressor map); hold the engine inside its rpm and temperature limits; and govern idle. Its inputs are what those tasks require: thrust lever angle, engine rpm (each spool), compressor outlet pressure, and engine inlet pressure and temperature, from which the ambient conditions follow.
The engine control system's tasks wrap around the FCU's: turn the lever's demand into thrust while protecting every limit, and manage the engine's accessories, variable vanes, bleeds and clearances, through every phase from start to shutdown. Four generations of the system exist side by side:
Hydromechanical — cams, springs, capsules and levers do all the computing mechanically: robust, and entirely blind to anything it cannot feel through a linkage.
Hydromechanical with electronic supervisor — a limited-authority electronic box trims the mechanical unit's output, sharpening accuracy and limit protection but able to be overridden or failed back to pure mechanics.
Single-channel FADEC with hydromechanical backup — the computer has full authority in normal operation, with a mechanical reversion behind it.
Dual-channel FADEC with no backup — the modern standard: two computing channels and no mechanical fallback at all, the arrangement the rest of this lesson describes.
A FADEC (full-authority digital engine control, as the piston module established) is, in the turbine world, a full-authority dual-channel system: an electronic engine control unit (EEC) with its wiring and sensors, commanding not just fuel but the variable vanes, the active clearance control and the bleed configuration of the earlier lessons, receiving the thrust lever's position as a purely electrical TLA (thrust lever angle) signal, and protecting the engine automatically, including an EGT protection function and overspeed protection.
Redundancy by channels — two identical channels each capable of running the engine alone; one is in control, the other in active standby, and control swaps between them on alternating flights or at the first fault.
Everything cross-checked — all input and output data is checked by both channels, so a disagreement is detected the moment it appears.
A single input failure has no operational effect — the system substitutes the other channel's value, or a value synthesised from related parameters, and the engine runs on undisturbed; the crew typically learns of it from a maintenance message, not a symptom.
Own sensors, some aircraft data — the FADEC trusts its own dedicated sensors first, using aircraft systems' data (air data above all) where it helps.
Own electrical power — a dedicated permanent-magnet alternator (PMA) on the engine's gearbox powers the FADEC whenever the engine turns, with aircraft power only for starting and backup: the aircraft's electrical system can die entirely and the engine's brain keeps working.
The component chain and the system family are recall; the FCU's scheduling logic and the FADEC's redundancy behaviour carry the understanding marks. The overview, following the learning objectives:
Name the engine fuel system's components and their functions
Name the two HP pump types
State the FCU's tasks and list its input parameters
State the engine control system's tasks and list the four system types
Describe the FADEC as a full-authority dual-channel system with its functions
Explain channel redundancy, cross-checking, single-input-failure behaviour, sensor sources and the FADEC's own power