Right, let's pick this up with the electronic engine control systems — Page 385, Lesson 479

Right, let's pick this up with the electronic engine control systems. We've just looked at the hydro-mechanical era, and now we're moving into the digital age, which is where the pilot's workload really starts to change. First, I want to introduce the Supervisory EEC. The EEC is the Electronic Engine Control. Now, this is a supervisory system, which means it's a computer that oversees the engine's operation, but it doesn't have total authority over the fuel delivery. It works like this: the computer receives inputs from the pilot's throttle lever angle (TLA) — that's the physical position of the throttle in the cockpit — and/or from the FMS, the Flight Management System. These inputs tell the computer what the required engine target operating parameters are. So the EEC knows what the pilot wants. Now, how does it achieve that? It signals the Fuel Control Unit (FCU). The FCU is the unit that actually adjusts the fuel delivery to the burners. It does this in one of two ways, depending on the pump type. If the engine has a high-pressure hydro-mechanical pump, the FCU varies the swash-plate angle — that's the angle of the plate that drives the pump's pistons, which changes the pump's stroke and therefore its output. If the engine uses a gear-type pump, the FCU adjusts the return, or bypass, of the fuel back to the pump inlet. So instead of varying the pump's stroke, it recirculates excess fuel. Now, the EEC performs all the functions necessary for engine operation and protection. It monitors EPR, which is Engine Pressure Ratio, the throttle lever angle (TLA), Mach number, and the engine inlet pressure and temperature. And here's the key benefit: it will maintain a constant thrust regardless of changes in air pressure, temperature, or the flight environment. So as the aircraft climbs and the air gets thinner, the EEC automatically compensates to hold the thrust you've selected. But — and this is the critical limitation of a supervisory system — any fault within the EEC will cause the system to revert to manual control. That's the provision for manual reversion. If the computer fails, the pilot or the crew can take over and control the engine directly. That's the whole point of it being "supervisory" rather than "full authority." Now, let's contrast that with the Full Authority Digital Engine Control, or FADEC. This is the system that was born in the 1970s. The FADEC system significantly reduces the pilot's tasks and responsibilities with regard to controlling engine efficiency and monitoring engine performance and condition. NASA and Pratt and Whitney were the first to experiment with a digital FADEC system, and the successful outcome led to the Pratt & Whitney PW2000 being the first civil engine retrospectively fitted with FADEC. Now, to understand FADEC, we need to look at what came before it. Originally, the control and metering of the engine fuel system was carried out by a Fuel Control Unit (FCU) mounted on the engine. That FCU incorporated the throttle, the HP cock — that's the high-pressure cock, the fuel shut-off valve — a hydro-mechanical governor, and other devices to regulate and control fuel flow and power output. Control to the FCU from the flight deck was by mechanical means — cables and linkages. As technology advanced, that complex variable-stroke swash-plate fuel pump and the hydro-mechanical means of controlling engine power output were replaced by a single-channel digital electrical computer-controlled system. That's the FADEC. And the key difference from the Supervisory EEC is this: in the event of EEC failure, the Supervisory system has manual reversion. The FADEC does not. It has full authority over the engine, and there's no manual fallback. That's the fundamental contrast between the two systems. Now, before we go further, I should mention that the FADEC system itself has its own drawbacks, including electronic noise interference. And interestingly, the electronic engine control was first introduced as a component of the Rolls Royce Olympus 593 engine fitted to the Concorde. That was the pioneering application. So to summarise the progression: we had the mechanical FCU, then the Supervisory EEC which could be overridden manually, and finally the FADEC which has full authority and no manual reversion. The FADEC is the system that truly offloads the engine management from the pilot.