Gas Turbines - Fuel Systems — Page 389, Lesson 486

I want to walk you through the FADEC system — that's Full Authority Digital Engine Control. We're picking this up right at the point where FADEC can take control of the engine. Let me set the scene. FADEC has the authority to initiate an Engine Shutdown — abbreviated ESD. That's the final closing down of the engine. But notice the qualifier: the final closing down is a pilot action. So even though FADEC is "full authority," the actual last step of shutting the engine down is done by the pilot. This happens if the engine exceeds certain limits. Those limits are: N1, N2, Acceleration, and EGT. Let me define those for you, because they're the core engine parameters. N1 is the low-pressure compressor/turbine speed — the fan speed, essentially. N2 is the high-pressure compressor/turbine speed. Acceleration is the rate at which the engine spools up. And EGT is Exhaust Gas Temperature — the temperature of the gases leaving the turbine. If any of these exceed their limits, FADEC can initiate the shutdown sequence, but the pilot performs the final closing down. Now let's look at how this works in a typical application — a normal flight. Prior to flight, the flight crew enters the data appropriate for the day's flight into the Flight Management System, the FMS. The FMS takes environmental data — things like temperature, wind, runway length, runway condition, cruise altitude — and it calculates power settings for the different phases of flight. To initiate take-off, the flight crew advance the throttles to a take-off detent, or they select an auto-throttle take-off if it's available. The FADECs — plural, because there's one per engine — compute the required take-off thrust setting and apply it to the engines. Here's the key point I want you to grasp: there is no direct linkage between the throttle and the engine fuel control to open fuel flow. When the flight crew moves the throttle, they are merely sending an electronic signal to the EEC/ECU — that's the Electronic Engine Control or Engine Control Unit, the computer that is the heart of FADEC. That EEC/ECU subsequently controls and monitors the fuel flow. So the throttle is just a signal generator, not a mechanical fuel valve. The FADECs compute the appropriate thrust settings and apply them for climb, cruise, and all other phases of flight. During flight, small changes in operation are constantly being made to maintain efficiency. Maximum thrust is available for emergency situations if the throttle is advanced to full, but the FADEC system will control the engine acceleration to ensure that operating limitations are not exceeded. So even in an emergency, FADEC won't let you overspeed or overtemp the engine. Now, an important operational reality: the flight crew has no means of manually overriding the FADECs. They must accept whatever the FADECs provide. However, they do retain the facility to manually shut the engine down if and when it is required. So the pilot can always kill the engine, but cannot manually fly the fuel schedule. FADECs today are employed by almost all current generation jet engines, and increasingly in newer piston engines, fixed-wing aircraft, and helicopters. So this isn't just a jet engine technology anymore. Let me walk you through the advantages of the FADEC system, because this is a list you need to know cold. First, improved engine efficiency due to the precise management and control of the fuel system. Second, automatic engine protection against out-of-tolerance operations — that's the limit protection we just talked about. Third, fault-tolerant systems that function even if they are degraded — so if a sensor fails, the system keeps working, just at a reduced capability. Fourth, improved safety because the FADEC computer is dual-channel and receives multiple inputs that provide redundancy in case of failure. So there are two independent channels, and multiple sensors feeding each. Fifth, semi-automatic engine starting and restarting — and the ability to abort or recycle an engine start. Sixth, better system integration with engine and airframe systems. Seventh, long-term health monitoring and fault diagnostics — the system watches the engine over its life and diagnoses faults. Eighth, a reduction in the number of parameters to be monitored by the flight crew — because the computer does the monitoring. And ninth, automatic engine emergency responses, such as an automatic thrust increase to avert a stall. Now the disadvantages — and this is the flip side of "full authority." True full authority digital engine controls have no form of manual override. If a total FADEC failure occurs, the engine fails. The pilot has no way of manually controlling the engines other than to shut them down. And as with any single point of failure, the risk can be mitigated by providing in-built redundancy — which is exactly why the system is dual-channel with multiple inputs, as we saw in the advantages. So the whole design philosophy is: give the computer full authority for precision and protection, but build in redundancy so that a single failure doesn't kill you, and always leave the pilot the ultimate power to shut the engine down. There's a figure in your materials — Figure 26.6, a typical FADEC structure — that shows you how all these components connect. Take a look at it when you get a chance.