Engine Instrumentation — Page 524, Lesson 616

Right, let's pick this up with the tacho-indicators and how they actually behave in the aircraft. These speed indicators receive their signal either directly from the speed sensors, or through a servo-operated system. Either way, they need a power source from the aircraft's airborne power supply. Now, here's a critical failure behaviour you need to know: if that power supply fails, or if the signal itself fails, the indicator is driven to an Off Scale position. That's deliberate — it makes the failure obvious. And on top of that, a Power Off Flag may be displayed, so you get a clear visual warning that the instrument is no longer trustworthy. Now, about how speed is presented. As I mentioned, on modern aircraft you'll almost always see speed shown as a percentage figure. It's only on piston engine aircraft that you'll see the actual speed of rotation displayed, in rpm. Gas turbine engines use dial displays showing percentage speed, where 100% corresponds to the optimum turbine speed. So the dial is telling you how fast the turbine is spinning relative to its design optimum, not an absolute rpm figure. Let me walk you through the dial itself, because there are two scales. There's a main scale calibrated from 0 to 100%, in 10% increments — that's the coarse reading. Then there's a second pointer, or a digital counter, that displays speed in 1% increments — that's the fine reading, so you can read the speed precisely. And alongside the digital readout, you'll also see vertical ribbon displays used on some instruments. Now, like every other instrument in the cockpit, these use coloured arcs or indicator lines to show ranges and limits. Green marks the normal operating range. Amber denotes caution — you're getting into a region you need to watch. Red shows the maximum or minimum speed, and also ranges that are restricted because of excessive vibration. So red isn't just about overspeed — it can mark a band you must avoid because the engine vibrates badly there. And on a piston engine, the reference rpm should also be placarded, meaning the recommended or limiting rpm is physically marked on the instrument for the pilot to see. Now let's move to a multi-engine consideration. On an aircraft with more than one engine, you want all engines running at the same speed to reduce structural vibration and noise. But it's impractical to have the pilot adjust each throttle manually to synchronise the speeds — and on top of that, individual indicators may vary in accuracy, so you can't trust them to match perfectly by eye. To make manual adjustment easier, there's an additional instrument called a Synchroscope. Here's how it works. It was designed from the outset to operate from the AC generated by the tachometer system — so it's powered by the alternating current the tachometer itself produces. The instrument gives a qualitative indication of the difference in speeds between two or more engines. Qualitative, meaning it shows you the direction and magnitude of the difference, not an exact number. One engine is selected as the master, and the others are slaves to it. The instrument shows clearly whether a slave engine is running faster or slower than the master, so you know which throttle to adjust and in which direction. The dial presentations vary — there are versions for twin-engine aircraft, for four-engine aircraft, and combined tacho and synchroscope units that integrate both functions into one instrument. You can see those presentations in Figure 38.10. So to tie it together: the tacho gives you the percentage speed with its failure flags, and the synchroscope gives you the relative speed between engines so you can match them manually. Both are essential for smooth, quiet, structurally sound multi-engine operation.