Engine Instrumentation — Page 512, Lesson 608

Let's start with the EPR gauge, because that's where the real trap lies for an engine operator. EPR stands for Engine Pressure Ratio. It's the ratio of two pressures: the engine intake pressure and the jet pipe pressure. So the gauge isn't showing you a single pressure — it's showing you the relationship between those two. Now, here's the subtlety. The engine intake pressure will vary with changing ambient pressure, and it will also vary with changing airspeed. So during take-off, as your forward airspeed increases, you'd expect the intake pressure to rise. But here's the counter-intuitive part: an increase in forward airspeed during take-off causes a drop in the reading on the EPR gauge. Why? Because it's only an apparent drop. The actual ratio of the two pressures is what's changing. Let me walk you through the mechanism. As forward airspeed increases during take-off, the engine intake pressure rises relative to the jet pipe pressure. The jet pipe pressure, at low airspeeds, is essentially unaffected. So when the numerator of your ratio goes up and the denominator stays the same, the ratio itself falls. That's the apparent drop you see on the gauge. Now, here's the danger. An inexperienced engine operator, seeing that EPR drop, might open the throttles further to try to restore the loss. They're fixated on that one parameter. But while they're doing that, the other parameters — N1, N2, N3, and EGT — are in danger of exceeding their limits without the operator even knowing. N1, N2, and N3 are the rotational speeds of the three spools of the engine, and EGT is Exhaust Gas Temperature. All of those can be pushed past their limits while the operator is chasing the EPR reading. To prevent this, most operators require that EPR is set before the aircraft reaches approximately 60 knots. After that speed, no increase in engine power is allowed unless it's an emergency. So the procedure is: set your EPR early, before 60 knots, and then leave it alone. Now, what happens after take-off? As airspeed increases beyond V2 — that's the take-off safety speed — the increase in engine intake pressure is passed through the engine to the jet pipe. So now the jet pipe pressure rises too, and the ratio changes back to what was set on take-off. The apparent drop corrects itself. Now let's move to the second major topic: Engine Torque. Turboprops and turboshaft engines produce torque rather than thrust. That's a fundamental difference. The systems that produce thrust indications for turbojet and turbofan engines are vastly different from those that produce torque indications for turboprop and turboshaft engines. The torquemeter measures, and its indicator displays, the power being produced by the engine. So instead of showing you thrust, it shows you the actual power output. And here's the definition of torque, which I want you to hold onto: torque, by definition, is a force applied at a distance to a turning point. That's the core concept — a force, applied at some distance from the axis of rotation, creating a turning effect. That's what the torquemeter is measuring. So to summarize the key contrasts: EPR is a ratio of two pressures, and it can give you an apparent drop during take-off that isn't a real loss of power. Torque, on the other hand, is a direct measure of power for turboprop and turboshaft engines, defined as force applied at a distance to a turning point.