Instrumentation

Engine Instrumentation
38
The tacho-indicators mentioned in the text above receive their speed signals directly from
speed sensors or via servo-operated systems. These indicators require a power source from the
aircraft airborne power supply. In the event of power supply or signal failure, the indicator is
returned to an Off Scale position, and a Power Off Flag may be displayed.
As previously stated, presentation of speed is now usually displayed as a percentage figure. It is
only piston engine aircraft where the actual speed of rotation is displayed. Gas turbine engines
have dial displays which show percentage speed, with 100% corresponding to the optimum
turbine speed. Two scales are displayed. A main scale is calibrated 0 to 100% in 10% increments.
A second pointer or digital counter displays speed in 1% increments. As well as digital read out,
vertical ribbon displays are used.
In line with other instruments, coloured arcs or indicator lines are used to show ranges and
limits of engine speed. Green represents normal operating range, with amber denoting caution.
Red shows maximum or minimum speed, and ranges that are restricted because of excessive
vibration. On a piston engine the reference rpm should also be placarded.
On multi-engine aircraft, to reduce structural vibration and noise the speed of all engines must
be synchronized. It is impractical to have the pilot adjust the throttle of each engine manually
to synchronize the speed, and individual indicators may vary in accuracy. In order to facilitate
manual adjustment of speed an additional instrument known as a Synchroscope (Figure 38.10)
is used. The instrument was designed at the outset for operation from the AC generated by the
tachometer system. The instrument provides qualitative indication of the difference in speeds
between two or more engines. One engine is selected as a master, the others are slaves to
it. The instrument shows clearly whether a slave engine is running faster or slower than the
master. An example of the dial presentation for synchroscopes for a twin and four-engine
aircraft, and combined tacho and synchroscope are shown.
Engine Instrumentation 38
Figure 38.10 Synchroscopes
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