Engine Instrumentation — Page 512, Lesson 608

I want to walk you through engine instrumentation, starting with a really important concept: engine pressure ratio, or EPR. Let me set the scene. The engine intake pressure — that's the pressure of the air being sucked into the front of the engine — will vary with changing ambient pressure, which is the pressure of the surrounding atmosphere, and also with changing airspeed. So as the aircraft moves faster, that intake pressure changes. Here's the subtle trap. During take-off, as forward airspeed increases, you'll see a drop in the reading on the EPR gauge. But I want you to understand this is only an "apparent drop." It's not that the engine is losing power. It's only the ratio of two pressures that is changing — the ratio between engine intake pressure and jet pipe pressure. Let me unpack that. EPR is literally a ratio: engine intake pressure divided by jet pipe pressure. During take-off, as forward airspeed increases, the engine intake pressure rises relative to the jet pipe pressure. The jet pipe pressure, being unaffected at low airspeeds, stays roughly the same. So when the numerator goes up and the denominator stays put, the ratio between the two pressures falls. That's why the EPR gauge shows a reduction. Now here's the danger, and this is a classic trap for the inexperienced engine operator. Seeing that EPR drop, an inexperienced pilot might open the throttles further to try to restore the loss — having eyes for only that one parameter. Meanwhile, the other parameters — N1, N2, N3, and EGT — are in danger of exceeding their limits without his knowledge. N1, N2, and N3 are the rotational speeds of the engine's compressor and turbine spools, and EGT is exhaust gas temperature. If you're fixated on EPR, you can overboost the engine and blow past those limits without ever noticing. To prevent this, most operators require that EPR is set before the aircraft has reached approximately 60 knots. After that speed, no increase in engine power is allowed unless in an emergency. So the procedure is: set your EPR early, before 60 knots, and then leave it alone. Now, after take-off, as airspeed increases beyond V2 — that's the take-off safety speed — something changes. The increase in engine intake pressure is passed through the engine to the jet pipe. So the jet pipe pressure catches up, and the ratio changes back to that set on take-off. The EPR reading returns to what you originally set. Now let's shift to a completely different kind of engine: engine torque. Turboprops and turboshaft engines produce torque rather than thrust. This is a fundamental contrast. The systems that produce indications of thrust for turbojet engines and turbofan engines are vastly different from those which produce indications of torque for turboprop and turboshaft engines. The torquemeter measures, and its indicator displays, the power being produced by the engine. So on a turboprop, you're not reading thrust — you're reading torque, which is a measure of power. Let me define torque precisely, because this is the definition you need. Torque, by definition, is a force applied at a distance to a turning point. So think of a lever arm — a force pushing at some distance from a pivot point. That's torque. If applied... and the excerpt cuts off there, but that's the foundation: force times distance from the turning point. So to tie it together: on a turbojet or turbofan, you monitor EPR and spool speeds. On a turboprop or turboshaft, you monitor torque — the twisting force that represents the power the engine is producing. Different engines, different parameters, and each has its own traps.