95-20 Primary Instruments
95-21 Vertical Speed Indicator
The vertical speed indicator indicates rate of ascent or descent. Check vertical speed
indicator for a zero-rate needle indication when rotorcraft is in a stationary hover.
95-22 Airspeed Indicator
The airspeed indicator displays airspeed in knots and miles per hour. The airspeed indicator
operates in forward flight only; backward or lateral flight will not indicate correct airspeed.
95-23 Engine (N2) and Rotor Dual Tachometer
CAUTION
Installation of electrical devices can affect accuracy and
reliability of electronic tachometers.
An electronic engine (N2) and rotor dual tachometer is standard. Engine tachometer
signal is provided by a transducer on the engine gearbox. Rotor tachometer signal is
provided by magnets on the main gearbox tail rotor driveline yoke. Each tachometer is
on a separate circuit with its own circuit breaker.
The tachometer bus receives power from the avionics bus or through the battery
relay bypass circuit as long as the main rotor gearbox has oil pressure. Therefore, the
tachometers will receive power through the bypass circuit whenever the rotors are turning
even if the avionics bus is not powered. The bypass circuit is disconnected from the
tachometer bus when the annunciator test button is depressed. Pressing the test button
while the helicopter is running will confirm that the tachometers are receiving power from
the avionics bus.
NOTE
Do not use magnetized tools.
CAUTION
Protect instrumentation using foam padding or equivalent.
Handle instruments like eggs.
A. Removal
1. Turn battery switch off and pull out TACHS N2 (2 amp) and R (2 amp) circuit
breakers at panel.
2. Remove perimeter screws securing instrument face to console and pull face aft.
3. Remove screws securing console harness connector to F792-1 dual tachometer
and unplug connector.
4. Supporting instrument, remove screws securing tachometer to face and remove
tachometer.
MAY 2015 Chapter 95 Instrument System Page 95.5
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