Engine Instrumentation — Page 512, Lesson 605

Let’s start with the big picture. In the cockpit, we have two families of display. The first is the analogue display — you’ll often hear it called the “clockwork cockpit.” That’s the traditional layout where you have a multitude of separate mechanical gauges, each one showing its own piece of information. The second is the electronic display — the “glass cockpit.” Here, the information is shown on cathode ray tubes, or CRTs, and on liquid crystal display panels, or LCDs, with light emitting diodes, LEDs, used for the digital readouts. Now, an important design point: even in a glass cockpit, we keep a small number of conventional gauges. Why? As a fallback, in case the electronic displays fail. So the clockwork gauges are retained as a safety net. Both types of display convey essentially the same information to the pilot. But the glass cockpit has flexibility that the analogue system doesn’t, and that flexibility is why it’s now taking over as the preferred way to show both flight and engine instrumentation. Now let’s move to the heart of this chapter — thrust and power measuring instruments. There are two basic types of thrust measuring instrument, and then a third concept for propeller aircraft. The first type measures jet pipe pressure directly. That’s the P7 gauge. The “P7” is the pressure in the jet pipe, and the gauge simply measures that pressure. The second type measures the ratio of two parameters: the jet pipe pressure and the engine air intake pressure. That’s the EPR gauge — Engine Pressure Ratio. So instead of one absolute pressure, we’re comparing two pressures as a ratio. Then, for propeller-driven aircraft, we measure and indicate Torque. Torque is an indication of engine power. The key relationship here is that the propeller converts power into thrust. So on a prop aircraft, we don’t measure thrust directly — we measure the torque, which tells us the power, and the propeller turns that power into thrust. Now, there’s a refinement on some large turbo-fan engines. Here, the integrated turbine discharge pressure and fan outlet pressure is compared to the compressor inlet pressure. That produces what’s called “integrated” EPR. So instead of just jet pipe versus intake, we’re combining the turbine discharge and fan outlet pressures, and comparing that combined value to the compressor inlet pressure. How do we sense these pressures? With Pitot tubes, suitably positioned. The Pitot tubes sense the pressures required to work the system. And here’s the key design choice: the tubes can either be connected directly to the indicator in the cockpit, or they can be connected to a pressure transmitter, which is then electrically connected to the indicator. So we have two possible plumbing paths — direct pneumatic, or via a transmitter with an electrical link. Let’s talk about how the P7 gauge is marked. The P7 system gauge can be marked in inches of mercury — that’s inHg — or in pounds per square inch — psi — or as a percentage of the engine’s maximum thrust. So three possible calibrations for the same gauge. Now, for the EPR system. EPR can be indicated by either mechanical or electronic means, but in practice it’s more normal to find the electronic system in use. Here’s how that electronic system works. It uses two transducers. Each transducer senses one of the relevant air pressures, and each vibrates at a frequency proportional to that pressure. So the vibration frequency is the analog of the pressure. Then a computer works out the electrical signal appropriate to those pressures. That signal is sent to two places: the EPR gauge in the cockpit, and the engine management system. So the same signal drives the pilot’s display and feeds the engine’s control computer. Let me tie that together. The P7 gauge gives you one absolute pressure — jet pipe pressure. The EPR gauge gives you a ratio — jet pipe pressure over intake pressure. On a prop aircraft, you read torque, which is power, and the prop converts that power to thrust. On big turbo-fans, you may see integrated EPR, which compares combined turbine discharge and fan outlet pressure to compressor inlet pressure. And the sensing is done by Pitot tubes, feeding either directly to the cockpit indicator or through a pressure transmitter with an electrical connection. That figure shows the EPR indicating system — the layout of the transducers, the computer, and the gauge. Take a moment to trace the signal path: pressures in, transducers vibrating, computer computing, signal out to the cockpit gauge and the engine management system. That’s the complete picture for this section.