FCU

A fuel control unit is a hydromechanical or electronic regulator mounted on or integral to a turbine engine that manages the flow of fuel to the engine's fuel nozzles. It receives signals from engine sensors, throttle inputs, and environmental conditions, then adjusts fuel metering to maintain stable combustion across a wide envelope of flight regimes, altitudes, and temperatures. On most transport aircraft, the FCU is the single most critical link between the flight deck thrust lever and the engine itself.

Hydromechanical FCUs, still common on older turboprops and some business jets, use a centrifugal governor that senses engine speed via a shaft connection and adjusts fuel supply through mechanical linkages and servo valves. Modern FCUs employ full-authority digital engine control (FADEC) architecture, in which a dedicated engine control unit receives inputs from thermocouples, pressure transducers, and speed sensors, then commands electronically controlled fuel metering valves with microsecond precision. Some engines use hybrid systems that blend hydromechanical backup with electronic primary control.

Common failure modes and maintenance

FCU faults typically manifest as hard starts, surges during acceleration, flameout at altitude, or overfueling on the ground. Maintenance technicians troubleshoot using engine parameters logged by the flight data recorder, borescope inspection of nozzles for carbon deposits, and bench testing of fuel control components. Scheduled service intervals involve replacing fuel nozzle seals, inspecting filter screens for particulate, and verifying metering accuracy within manufacturer tolerances, often expressed in pounds of fuel per hour at specific engine speeds. Complete FCU overhaul or replacement is typically performed by the engine manufacturer or approved repair facility and requires test cell validation before return to service.

The FCU must operate reliably across an extreme working range: fuel inlet temperatures from minus 40 to plus 60 degrees Celsius, inlet pressures from 20 to 60 pounds per square inch, and output metering from idle (around 50 pounds per hour on a large turbofan) to full authority (10,000 pounds per hour or more). Failure to regulate fuel correctly can result in compressor stall, combustor flameout, or uncontrolled overspeed, making the FCU one of the most scrutinized components in aircraft certification and maintenance protocols.