Engine Instrumentation — Page 534, Lesson 630

I want to walk you through engine instrumentation, starting with a very practical problem. Imagine a Boeing 747 with four engines mounted far out on the wings. It would be completely impractical to run an oil feed pipe from that outer engine all the way to a flight deck pressure indicator of the Bourdon tube type. A Bourdon tube is a direct-reading gauge where the pressure physically travels through a pipe to move a needle. That just won't work over that distance. So to overcome this, we use remote-indicating systems. These consist of two main components: a transmitter unit located at the pressure source, and an indicator mounted on the appropriate panel in the cockpit. The transmitter measures the pressure at the engine and sends an electrical signal to the indicator. These systems 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. The transmitters feed varying current to an indicator, and they can be AC or DC in operation. We'll cover those systems in detail later. 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. A differential switch or gauge is subjected to pressure on both sides of its sensor — it compares two pressures rather than measuring one against atmosphere. And on the instrument faces, we use coloured arcs of green, amber, or red to indicate the range and limits of the system. Green is the normal operating range, amber is caution, and red is the limit you must not exceed. Now let's move to engine vibration. 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 we're not just measuring vibration for its own sake — we're watching for a change from the normal baseline. 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 the facility whereby the vibration level of each rotating assembly is monitored separately, so that the source of the vibration can be pinpointed. That means if the fan, the compressor, or the turbine is vibrating, you can tell which one. 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. A piezoelectric crystal generates an electrical charge when it's mechanically stressed — so when the engine vibrates, the crystal produces a signal. In the other design, a magnet moves loosely within a coil, and that movement induces a current. 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. This is crucial — the engine vibrates at many frequencies during normal operation, and we only want to react to the harmful ones. Let me explain the full system. These systems utilize the principle that the magnet and piezo crystal, which are suspended within a fixed coil carrying 115 volts at 400 hertz, will move in sympathy with any vibration suffered by the engine. So the coil is energised with 115 volts AC at 400 hertz — that's the aircraft's standard AC power. When the engine vibrates, the suspended magnet or crystal moves within that coil, and this affects the current flowing through the coil into the amplifier and filter. The filter will erase any output which is normal to the engine, and allow through to the amplifier any frequency that is considered to be harmful to the engine. So the filter is the gatekeeper — it removes the normal background vibration and passes only the damaging frequencies. The result of this amplification is sent to the instrument via the rectifier and warning circuit. The rectifier converts the AC signal to DC so the instrument can use it. 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. Finally, one important detail: vibration is measured and displayed in 'Relative Amplitude', abbreviated Rel Ampl. It's not an absolute physical unit like millimetres — it's a relative scale that indicates how much vibration is occurring compared to a reference. That's what the needle deflection represents. So to summarise the whole picture: we have remote-indicating pressure systems with transmitters and indicators, pressure-operated switches for warnings, and a vibration monitoring system that uses a piezoelectric crystal or a magnet-in-coil to sense vibration, filters out normal frequencies, amplifies the harmful ones, and displays the result as Relative Amplitude with a warning light if it exceeds the limit.