Gas Turbines - Fuel Systems — Page 376, Lesson 475

Let's pick this up right where the fuel control story gets interesting. We've got the engine running, and now we're looking at the automatic protections that keep it from destroying itself. First, the top temperature control. The whole idea here is that the pilot should be able to demand full power at any moment without worrying about melting the turbine. So the system automatically watches the exhaust gas temperature. Thermocouple probes sit right in the gas stream, either inside the turbine or just after it. These probes produce an electrical output that tells us the temperature at the rear of the engine. That signal goes two places: to the cockpit instruments so you can see it, and to a 'temperature control signal amplifier'. The amplifier's electrical output then powers a solenoid, and that solenoid indirectly controls the fuel flow. So if the temperature climbs too high, the system trims the fuel back automatically. That's your meltdown protection. Now, the power limiter. Think about the compressor. Its casing can only withstand so much internal pressure before the material fails. Exceed the design maximum, and the casing breaks — catastrophic. So the FCU, the fuel control unit, has a power limiter device. It's signalled by two pressures: intake pressure, which we call P1, and compressor delivery pressure, P3. The combination of those two signals works through capsules and levers to control the fuel flow, so the maximum pressure ratio is never exceeded. That's the pressure protection. Finally, the RPM limiter. The compressor spools spin incredibly fast, and if you let them overspeed, the centrifugal forces will tear them apart. There are two basic methods to limit rotational speed. The first one uses an electrical signal proportional to shaft speed. A tacho-generator, or electronic speed sensor, is driven by the appropriate shaft. It sends signals to an amplifier — and that's where the excerpt cuts off, but you get the principle: speed is sensed electrically, and that signal will be used to govern the fuel. So to summarise the three guardians: top temperature control watches the exhaust gas temperature via thermocouples, the power limiter watches the pressure ratio via P1 and P3, and the RPM limiter watches shaft speed via a tacho-generator. Each one can pull fuel back to keep the engine inside its safe envelope.