Engine Instrumentation — Page 524, Lesson 614

I want to walk you through the electrical generator system for measuring engine speed — this is one of the classic ways we read how fast an engine is turning, and it's still found on large aircraft today. At its heart, this system uses a small three-phase generator called a tacho-generator, driven directly by the engine. The output of that generator goes to an indicator, and inside the indicator there's an asynchronous motor turning a drag-cup assembly. That assembly moves a pointer over a scale, just like the mechanical system we looked at earlier. Now, what does the indicator actually show you? Two options. It can display the actual revolutions per minute — though that's not too common — or, much more typically, it shows speed as a percentage of maximum engine speed. That's the percentage tacho-indicator you'd see on the flight deck. Here's where the terminology gets important. On twin or triple spool engines, you can display the speed of the high, intermediate, and low pressure compressors separately. These are called N3, N2, and N1 — N being the SI symbol for rotational speed. And remember this key point: N1 and EPR are the parameters used to measure thrust in turbojets. So N1 isn't just a compressor speed — it's a thrust parameter. There's also a clever feature called an overspeed pointer, sometimes called a trailing pointer or limit pointer. It's fitted concentrically with the main pointer — meaning it sits on the same axis, behind it. Initially it's positioned at the appropriate maximum rpm graduation. If the main pointer exceeds that position, the limit pointer is carried along with it. But when speed reduces, the limit pointer stays at the maximum speed reached — it records the peak. To reset it, you apply a separate 28 V DC supply to a solenoid inside the indicator. Now, there's a practical problem. There's always provision on the HP compressor spool to drive a tacho-generator through the high speed gear box. But facilities may not always be available for driving tacho-generators from the intermediate and low pressure compressor shafts. When that's the case, we use a speed probe — and it works very well. Let me explain how the speed probe works. It's positioned on the compressor casing, in line with either a phonic wheel or the actual fan blades. As the spool rotates, the magnetic flux in the probe — or sensor head — is altered. That changes the current flowing in the coil fitted inside the probe. The frequency with which that current changes is directly related to the spool speed. That frequency is fed to an indicator in the cockpit to show the spool rotational speed. Now, here's a really important operational point. Both the tacho-generator and the speed probe can do more than just show speed. They can provide a signal that illuminates a warning lamp on the engine start control panel. This tells the pilot two things: not only that the engine is turning, but also whether it's turning in the correct direction. That's particularly critical during engine start, because it's used to inform the pilot when to open the HP Fuel Cock. And note — this lamp is only illuminated during the start cycle. Finally, there are two advantages to this system worth remembering. First, there's a reduction in moving parts required in the engine. Second, a number of separate electrical outputs, additional to those needed for speed indication, can be provided — for example, for automatic power control and flight data acquisition systems. So to tie it together: the tacho-generator is the older, mechanical-style approach driven through gearboxes, while the speed probe is the modern alternative using magnetic flux and frequency to read spool speed directly. Both feed the cockpit, both can drive the start warning lamp, and both give you the N1, N2, N3 readings you need — with N1 and EPR being your thrust parameters.