FADEC advances allow better engine performance
Next generation of full authority digital engine controls lets pilots select more precise power and raise fuel efficiency.
By Nihad Daidzic
Cessna Citation XLS operated by Panther Aviation. Power for the Citation XLS is provided by 2 PW615Fs incorporating dual-channel FADEC with health monitoring and diagnostics.
Turbine engines are the backbone of the modern air transportation system. Turbine jet engines are reliable, powerful, efficient and mature air-breathing propulsive devices. In other words, jet engines have come a long way from the original designs of Hans von Ohain and Frank Whittle in the 1930s.
But no matter how good a jet engine design may be, ultimately it is the engine control system that enables it to work efficiently and safely over a wide range of operating conditions. Turbine aero engines are working under extremely variable speeds, atmospheric and environmental conditions.
Control strategies and physical systems in jet engines are very complicated, and we are addressing here only some basics of existing systems and looking into future trends. In the US, engine certification is regulated by FAR Part 33. Engine control systems are covered specifically in FAR 33.28.
A modern jet engine's fuel control unit, together with other control subsystems, plays a crucial role in high operational reliability, flexibility, safety and economy of operation. The obsolete hydromechanical fuel control units have been replaced by redundant multichannel digital electronic control systems incorporating sophisticated control strategies.
At first analog and later digital, electronic engine control (EEC) had a supervisory function, paving the way for today's standard thrust-by-wire full-authority EEC or full-authority digital engine control (FADEC).
Modern FADEC—dual-channel, centralized-architecture, engine-mounted, vibration-isolated, mostly air-cooled, powered by respective accessory-gearbox-mounted dual 28-VDC permanent magnet alternators (PMAs) and backed up by airframe power—delivers required redundancy and relieves pilots from continuous engine monitoring and adjustments.
Modern FADEC serves as a superb digital microcontroller and smart interface between pilot, engine and airframe by using sophisticated sensors and actuators. (See photo on p 79.) FADEC has full control of the engine from start to shutdown. Besides channel cross-talk capability, FADEC incorporates self-testing, adaptive autotuning and failsafe design. It also avoids common-mode failures. Additional features include engine health/condition monitoring and management, as well as fault isolation.
Fuel calculations and delivery through a fuel metering valve (FMV) are among the many functions of modern FADEC. Antisurge (variable) bleed valves (ASBVs), variable inlet guide vanes (VIGVs), variable stator vanes (VSVs), fuel and oil temperatures and flow management, active blade tip clearance control (ACC), IDG operation, etc, are also regulated.
And the main afterburner/reheat fuel valve(s) and exit nozzle opening area of a supersonic propulsion engine are also controlled. Thus, FADEC controls the entire air/gas and fuel/oil flows within the engine. For example, FADEC often provides servo fuel (fueldraulics) for VSV/VIGV and bleed valve actuation.
Many of the control actions are often only scheduled depending on the existing operating condition(s), but sometimes secondary-feedback control loops are incorporated for relevant subsystems.
However, the control complexity of centralized architecture and the sheer number of harnesses for sensors and actuators increases the weight of the engine control system. On average, 15–25% of the engine weight is in engine control and accessories systems.
Engine control systems
FADEC installed on a modern civilian high-bypass turbofan. Cooling and finding a benign location for the central FADEC unit is becoming a limiting factor in modern turbofans.
The basic engine negative feedback (closed-loop) control system consists of the following main components—sensors, actuators and controllers. An illustration of the conceptual single-input single-output (SISO) control system is shown in the diagram on this page.
A sensor measures a controlled variable which is then subtracted from a commanded value or set-point (SP) and the error is delivered to the controller for further action. The controller-regulator is the "brain" of the control system which implements control laws to make corrective decisions to maintain SP values.
Three types of engine controller are typically employed in modern turbine engines—SP, transient and limit-protection. Automatic SP regulators have the main task of maintaining commanded thrust (eg, EPR = 1.53 or N1 = 98.3%).
Modern SP controllers, which are typically of proportional-plus-integral (PI) design, provide accurate steady-state regulation of commanded thrust, eg, idle (ground/flight), takeoff, climb, cruise. Limit-protection controllers (topping governors) have the main task of preventing engine hard-limits being exceeded (say, N2 ≤ 104.5% and/or interstage turbine temperature [ITT] ≤ 855°C).
The derivative part of the proportional-plus-integral-plus-derivative (PID) limit-protection linear control law provides anticipatory regulation preventing engine overboost, overheat and overspeeds. Engine acceleration (surge protection) and deceleration (minimum fuel and lean-blowout) transients are scheduled by engine transient controllers.
Typically, the transient controller will work with a fuel gain-scheduling strategy where maximum or minimum fuel delivery will be a function of operating conditions. FADEC-controlled engine autotrim (autotuning) automatically delivers consistency in engine parameters and thrust despite uneven engine wear and tear and/or individual sensor/actuator/component replacements.
A digital controller output is then often converted into an analog signal, amplified and delivered to an actuator which executes control action (eg, modulating fuel flow). In reality, every control system component works with limited accuracy and in real time, so individual control delays and time constants (lag time) must be included in overall system stability and dynamic response characteristics.
However, negative feedback control has one major drawback—it is reactive control after the fact! Imagine driving a car at 120 mph on a narrow curving road while having forward visibility of only 30 ft! Often, this is what we are asking a feedback control system to do—and it can lead to instabilities, inadequate performance and failures. But now imagine having a GPS map of the road ahead.
That information could be combined with the traditional feedback control, resulting in much more accurate and relaxed control. That is what "feedforward" compensation is for. It pre-empts disturbances before they give feedback control "a hard time." The only problem with feedforward control is that one must have good engine models and know the kind and number of disturbances that may occur.