Engine Instrumentation — Page 524, Lesson 616

Let's pick up with the tacho-indicators themselves, because that's where this passage starts. These are the instruments that show you engine speed. Now, the key point here is that they don't generate their own signal — they receive their speed signals directly from speed sensors, or they get them via servo-operated systems. And because they're electronic instruments, they need a power source, and that comes from the aircraft's airborne power supply. Here's the critical safety feature. If that power supply fails, or if the signal itself fails, the indicator doesn't just freeze or show a wrong value. It's returned to an Off Scale position. That means the needle goes right off the dial, so you immediately know something's wrong. And on top of that, a Power Off Flag may be displayed — a little flag that pops into view to tell you the instrument has lost its power. Now, let's talk about how speed is actually presented. As I mentioned, it's now usually shown as a percentage figure. It's only on piston engine aircraft that you see the actual speed of rotation — the real rpm. On gas turbine engines, the dials show percentage speed, and here's the important definition: 100% corresponds to the optimum turbine speed. So the whole scale is built around that reference point. The dial itself has two scales. There's a main scale calibrated from 0 to 100%, in 10% increments. Then there's a second pointer, or a digital counter, that displays speed in 1% increments — so you get the coarse reading from the main scale and the fine reading from the second one. As well as digital readouts, vertical ribbon displays are used — those are the long vertical bars you see on modern glass cockpits. Now, just like every other instrument, coloured arcs and indicator lines are used to show ranges and limits of engine speed. Green represents the normal operating range. Amber denotes caution. Red shows maximum or minimum speed, and also ranges that are restricted because of excessive vibration. And on a piston engine, the reference rpm should also be placarded — that means the recommended rpm is written on a placard near the instrument for the pilot to see. Let me show you the torquemeter system, because it's a good example of how these engine instruments work mechanically. Now here's the next big topic — synchronization. On multi-engine aircraft, to reduce structural vibration and noise, the speed of all engines must be synchronized. Now, it's impractical to have the pilot adjust the throttle of each engine manually to synchronize the speed, and there's another problem: individual indicators may vary in accuracy. So even if two indicators read the same, the engines might not actually be running at the same speed. To facilitate manual adjustment of speed, an additional instrument is used — it's called a Synchroscope. Let me show you what it looks like. Now, this instrument was designed at the outset for operation from the AC generated by the tachometer system. So it runs off the alternating current that the tachometer system produces. And here's what it does: it provides a qualitative indication of the difference in speeds between two or more engines. Qualitative — that means it shows you the direction of the difference, whether one is faster or slower, not the exact numerical amount. Here's how it works. One engine is selected as the master. The others are slaves to it. The instrument shows clearly whether a slave engine is running faster or slower than the master. So the pilot can see at a glance which throttle to adjust and in which direction. The passage mentions that there are examples of dial presentations for synchroscopes for a twin and a four-engine aircraft, and also a combined tacho and synchroscope — that's an instrument that shows both speed and synchronization in one unit. So to sum up the key points: tacho-indicators need power and fail to Off Scale with a Power Off Flag; gas turbines show percentage speed with 100% as optimum; the dial has a main scale in 10% increments and a fine scale in 1% increments; coloured arcs show normal, caution, and red limits; and the synchroscope uses a master-slave setup to show which engine is faster or slower, running off the AC from the tachometer system.