Engine Instrumentation — Page 534, Lesson 630

I want to walk you through engine instrumentation, starting with a problem that every multi-engine aircraft designer faces. Take a Boeing 747 — its outer engine is a long way from the flight deck. It would be completely impractical to run an oil feed pipe all the way from that engine to a pressure indicator of the Bourdon tube type on the flight deck. That's the classic direct-reading gauge, where the pressure physically pushes on a tube. So instead, we use remote-indicating systems. These systems have two main components. First, a transmitter unit, which is located at the pressure source — right at the engine. Second, an indicator, which is mounted on the appropriate panel in the cockpit. The transmitter feeds a varying current to the indicator, and these systems can be AC or DC in operation. They have distinct advantages over direct-reading gauges. For example, the pressure of hazardous fluids can be measured at their source and not brought into the cockpit — that's a big safety win. Also, weight can be saved by reducing the length of pipelines. We'll cover these systems in more detail later in the chapter. Now, beyond just indicating actual pressures, we can display warnings to the pilot using pressure-operated switches. These switches can operate for low pressure, high pressure, or incorrect differential pressures. Let me explain that last one. A differential switch or gauge is subjected to pressure on both sides of its sensor — so it's comparing two pressures, not just reading one. And to help the pilot read the situation at a glance, we use coloured arcs — green, amber, or red — to indicate the range and limits of the system. Green is the normal operating range, amber is caution, red is the limit. Now let's move to engine vibration, which is a whole different kind of monitoring. Vibration monitoring equipment — abbreviated VME — is fitted to almost all commercial jet-engined aircraft. Here's the key principle: although gas turbine engines have an extremely low vibration level, any change in that level is usually indicative of damage which may lead to failure. So a small change in vibration is a big deal. Warnings will be given in the cockpit if the vibration levels are exceeded, and some systems have a continuous read-out of vibration levels. The latest engines have a facility whereby the vibration level of each rotating assembly is monitored separately, so that the source of the vibration can be pinpointed. That's important — you don't just want to know something is vibrating, you want to know which rotating assembly is causing it. Now, how does VME actually work? The principle requires either an input from a Piezoelectric crystal mounted strategically on the engine, or an input from a coil which will be affected by the movement of a Magnet mounted loosely within it. Let me unpack that. A piezoelectric crystal generates an electrical signal when it's mechanically stressed — so when the engine vibrates, the crystal produces a signal proportional to that vibration. Alternatively, a magnet suspended loosely within a coil will move in sympathy with the vibration, and that movement induces a current in the coil. In either case, the frequency of the incoming vibrations will be filtered so that only those frequencies that are indicative of damage occurring will affect the output. Here's the detailed operating principle. The magnet and piezo crystal are suspended within a fixed coil carrying 115 volts at 400 hertz. When the engine vibrates, these elements move in sympathy with that vibration. This movement affects the current flowing through the coil into the amplifier and filter. The filter's job is to erase any output which is normal to the engine — the benign, expected vibration — and allow through to the amplifier any frequency that is considered to be harmful to the engine. So the filter is a gatekeeper: it only passes the dangerous frequencies. The result of this amplification is sent to the instrument via the rectifier and warning circuit. The needle will show the appropriate deflection for the amount of vibration being suffered by the engine at that time. If the level of vibration exceeds a predetermined amount, a warning light on the instrument illuminates. And here's the unit we use: vibration is measured and displayed in 'Relative Amplitude', abbreviated Rel Ampl. That's the scale you'll see on the instrument face. Let me pull that together for you. We have two families of instrumentation here. Pressure systems use remote transmitters feeding indicators, with pressure-operated switches for warnings and coloured arcs for range. Vibration systems use either a piezoelectric crystal or a magnet-in-coil to sense vibration, filter out the normal frequencies, amplify the harmful ones, and display the result as Relative Amplitude with a warning light if it exceeds the limit. Both are about giving you, the pilot, early warning of trouble before it becomes a failure.