Engine Instrumentation — Page 512, Lesson 605

Right, let's get into the engine instrumentation chapter. We're starting with the two fundamental ways you'll see this information presented in the cockpit. There are two types of cockpit display. The first is the analogue display, which we sometimes call the "clockwork cockpit." This is the traditional setup with a multitude of individual mechanical gauges, each one showing you a specific piece of information. The second is the electronic display, the "glass cockpit." Here, the information is shown on cathode ray tubes, or CRTs, or on liquid crystal display panels, which we call LCDs. For the digital readouts within those, we use light emitting diodes, or LEDs. Now, a key point for you as a professional pilot: even in a modern glass cockpit, a small number of conventional gauges are retained. These are kept as a backup in case of failure of the electronic displays. You always have a fallback. Both types of display convey essentially the same information to the pilot. But the flexibility of the glass cockpit system means it is now taking over as the preferred means of showing both flight and engine instrumentation. Now let's move on to the instruments themselves, specifically for measuring thrust and power. Thrust measuring instruments come in two basic types. The first type measures the jet pipe pressure directly. This is the P7 gauge. The "P7" is the designation for the pressure at the jet pipe, and this gauge simply senses and displays that pressure. The second type measures the ratio of two parameters: the jet pipe pressure and the engine air intake pressure. This is the EPR gauge, which stands for Engine Pressure Ratio. So instead of a single absolute pressure, it's showing you the relationship between those two pressures. Now, there's an important distinction here. For propeller-driven aircraft, we don't measure thrust directly. Instead, we measure and indicate Torque. This is our indication of engine power. The reason is that the propeller converts power into thrust, so torque is the meaningful parameter for that type of engine. Let me add a bit more detail on the EPR. On some large turbo-fan engines, the integrated turbine discharge pressure and fan outlet pressure is compared to the compressor inlet pressure. This produces what is called 'integrated' EPR. So it's a more sophisticated comparison of pressures to give you a more accurate picture of the engine's performance. Now, how do we sense these pressures? We use Pitot tubes, suitably positioned, to sense the pressures required to work the system. These 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 you have two possible plumbing arrangements: direct mechanical connection, or electrical transmission via a transmitter. Let's look at the markings on the P7 gauge. It can be marked in inches of mercury, which we abbreviate as inHg, or in pounds per square inch, which is psi, or it can be marked as a percentage of the engine's maximum thrust. So you might see any of those three scales depending on the installation. Finally, let's talk about how the EPR is actually computed and displayed. Although EPR can be indicated by either mechanical or electronic means, it is more normal to find the electronic system in use. This system uses two transducers. These transducers sense the relevant air pressures and vibrate at frequencies proportional to those pressures. So the frequency of vibration is your analog signal. A computer then works out the electrical signal appropriate to those pressures, and that signal is sent to the EPR gauge in the cockpit and also to the engine management system. So the same signal feeds both your display and the automatic engine control. That's the core of how we present thrust and power information. We've got the P7 for jet pipe pressure, the EPR for the pressure ratio, and torque for propeller aircraft.