FADEC and Engine Fuel Control

A gas turbine is controlled almost entirely through its fuel. The pilot asks for thrust; the engine fuel control decides how much fuel reaches the burners, and how fast that amount may change, so that the engine gives what was asked for without overspeeding, overheating, surging or flaming out. The rest of the fuel system, pumps, heaters, filters and valves, is described in gas turbine engine fuel system.

For decades the job was done by a hydromechanical fuel control unit, a precise assembly of valves, springs, flyweights and servo pistons working on the fuel itself. Electronic controls were then added to trim it. Today's airliner engines use FADEC, full authority digital engine control, in which a dual-channel computer has complete authority over the engine and the hydromechanical parts only carry out its orders.

Purpose of engine fuel control

Whatever its technology, a fuel control has to:

• meter the fuel flow the thrust lever or the autothrust demands;
• keep the engine at the selected speed or thrust as the conditions change, for example on a hot day, when a compressor with less dense air to work on would otherwise speed up at a fixed throttle, or on a cold day, when an uncompensated engine's idle speed falls;
• limit the rate of change of fuel flow, so that the engine accelerates without surging and decelerates without flaming out;
• protect the engine's limits of rotational speed, pressure and temperature;
• compensate for the density of the fuel, whose heat content varies with its specific gravity;
• sequence starting and shutdown.

It does so by varying the fuel delivered by the engine-driven high-pressure (HP) fuel pump. On the CFM56-5B of the A320 this is a gear pump driven by the HP spool through the accessory gearbox, which means it turns slowly during a start and fast at take-off.

Hydromechanical units and governors

A hydromechanical fuel control unit (FCU) senses engine speed, compressor pressures and the throttle position mechanically or hydraulically and meters fuel accordingly. Speed limiting is one example. In one arrangement described in the ATPL texts, the HP pump's output is set by a servo piston; a hydro-mechanical governor in the pump produces a hydraulic pressure proportional to engine speed, and when the HP shaft reaches its limit the excess pressure opens valves that bleed off servo pressure, reducing the fuel delivered.

The next step was the electronic fuel control with limited authority. On the Boeing 737-400, for example, a hydromechanical main engine control (MEC) scheduled fuel from the power lever angle (PLA), and a power management control (PMC) used N1, inlet temperature and pressure to trim it electronically to the N1 wanted. The PMC gave a constant-thrust climb without lever adjustment. If it failed, N1 drifted slowly away from the target before a PMC INOP light came on; with the PMC switched off, the MEC alone controlled the engine.

In a FADEC engine the hydromechanical part survives as the hydromechanical unit (HMU). On the CFM56-5B and V2500 of the A320 the HMU meters fuel to the combustion chamber under FADEC command, positions the variable stator vanes and variable bleed valves, and provides overspeed protection. The FADEC drives it through torque motors, electrohydraulic servo valves that turn electrical commands into hydraulic movement. The 737 NG's EEC likewise meters fuel through an HMU.

Acceleration control

When the thrust lever is advanced, the engine cannot accelerate as fast as fuel can be added. Too much fuel too soon raises the pressure in the combustion chamber, reduces the axial velocity of the air through the compressor and can make it stall or surge; too rapid a reduction can take the mixture below its weak limit and put the flame out (see gas turbine intakes and compressors and combustion chambers and turbines).

An acceleration control unit prevents the first problem. It acts as a second throttle valve in series with the pilot's, adjusting a fuel metering plunger according to the compressor inlet pressure (P1) and delivery pressure (P3), so that fuel flow rises only as fast as the airflow does. A FADEC does the same with acceleration and deceleration schedules in software, which is one reason a high-bypass engine takes several seconds to spool up from idle (see thrust levers, idle modes and engine handling).

Fuel metering and bypass valves

The fuel metering valve (FMV) sets the flow to the burners. On the CFM56-5B the FADEC positions it through a torque motor and servo valve, and a resolver reports its position back to the FADEC, closing the control loop.

The HP pump delivers more fuel than the engine needs at almost every condition. A fuel bypass valve returns the surplus to the low-pressure side and, in doing so, holds a constant pressure drop across the metering valve. With a constant pressure drop, the flow through the valve depends only on its opening, so the metered flow is a direct function of the FMV position.

Downstream, an HP fuel shut-off valve in the HMU starts and stops the engine's fuel. On the A320, setting the ENG MASTER switch OFF sends closing signals directly to the LP and HP fuel valves, bypassing the FADEC, so that the engine can always be shut down even if the FADEC has failed. The shut-off valves are described in gas turbine engine fuel system.

FADEC and the EEC

FADEC is the whole control system; its computer is the electronic engine control (EEC), called the electronic control unit (ECU) on CFM56-powered A320s. "Full authority" means that the computer controls the engine throughout its operating range and that no mechanical path bypasses it. On the A320 there is no linkage between the thrust levers and the engines: the lever positions are sensed electrically and sent to the FADEC as a thrust lever angle (TLA), which the computer treats as the crew's demand. When the A320 was certified this was new enough to need a special condition, listed as SC P-01 in its EASA type-certificate data sheet.

The FADEC is built for redundancy. Each engine has its own, on the A320 mounted on the engine fan case, with two channels: one controls the engine while the other stands by, ready to take over if the active channel fails. The 737 NG's EEC changes channel at each start attempt, and the A320 alternates channels, together with igniters, on successive automatic starts. Each FADEC has its own engine-driven alternator; the A320's becomes self-powered at 15 % N2, and falls back on aircraft 28 V DC if the alternator fails.

The FADEC takes in the thrust lever angle and autothrust demand, air data from the aircraft, bleed air demand and the engine's own sensors. From them it:

For the crew, the result is that a thrust lever position corresponds to a thrust, whatever the temperature and altitude, and that the limits are protected without constant adjustment. The indications the FADEC drives are described in engine indications and condition monitoring, and its partnership with the autothrust in autothrottle.

Degraded modes

The Boeing 737 NG shows how a FADEC degrades. In normal mode the EEC uses sensed flight conditions and bleed demand to calculate N1 ratings, then adjusts fuel flow until the actual N1 matches the commanded N1. If the signals it needs are lost, it changes automatically to soft alternate mode, using the last valid flight conditions so that thrust does not change at once; rating shortfalls or exceedances may then develop as the conditions change. Retarding the thrust lever to idle, or selecting ALTN on the EEC switch, enters hard alternate mode. Its thrust schedule gives equal or more thrust than the normal mode for the same lever position and the EEC no longer limits thrust, so maximum rated thrust can be reached before the forward stop; the thrust levers go to or near full forward only when terrain contact is imminent. Loss of either display electronics unit sends both EECs to alternate mode, so that the two engines are not controlled from a single source of data.

Overspeed protection

Engine overspeed protection keeps the spools below their structural limits. The A320 FADEC protects N1 and N2 at all times, and the 737 NG's EEC gives N1 and N2 redline protection in both normal and alternate modes. On the CFM56-5B the N1 red line is 104 % and the N2 indication turns red above 105 %.

A second, independent layer protects against a failure of the control itself. The CFM56-5B's HMU contains a hydromechanical overspeed governor, independent of the FADEC, which opens the fuel bypass valve if N2 exceeds 107.2 %, diverting fuel away from the burners. On the Embraer E190-E2 the FADEC shuts the engine down if N2 reaches 102 % or more three times within 30 seconds.

Engine limit protection and power limiting

Engine limit protection means that the crew cannot overboost the engine through the thrust levers. On the A320, pushing the levers to TOGA gives the certified take-off rating and no more. On the 737 NG, full rated take-off thrust is reached before the lever's forward stop and the maximum rated thrust at the stop, and the EEC limits the maximum thrust according to the aircraft model. In a windshear escape the crew can therefore push the levers fully forward in normal mode without overboosting the engines.

The protection is not complete. Neither type's control protects the EGT limit in flight. The A320 FADEC monitors EGT and aborts an automatic start on the ground for a hot start, an overtemperature, a stall or a failure to light; it does not abort an in-flight start, and in a manual ground start it intervenes only if the start EGT limit is exceeded before 50 % N2; the 737 NG's abnormal start protection works only during ground starts. EGT limits must be observed by the crew in both of the 737 EEC's modes.

Older fuel controls added a power limiter. If the pressure at the HP compressor outlet exceeded its maximum, a pressure sensor signalled the fuel control, which reduced fuel flow and engine speed before the casing and compressor could be overstressed.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.