Instruments & Avionic Systems

Pitot-static and gyro/AHRS instruments, the dual tachometer and needle split, torque and low-rotor-RPM warning.

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Main ideas

Helicopters carry the usual pitot-static instruments (airspeed, altitude, VSI) and gyroscopic/AHRS attitude and heading references feeding EFIS displays, plus the standard communication, navigation and surveillance avionics.
The key rotary-wing addition is a DUAL TACHOMETER showing both ROTOR RPM (Nr) and engine/power-turbine RPM on concentric needles: in normal drive the needles are 'joined', and a needle SPLIT shows the freewheel has disengaged (engine failure / autorotation).
Power is set and monitored by TORQUE (and by engine gas temperature/N1), not by EPR; a low-rotor-RPM warning (horn/light) is a critical caution.
Vibration and gearbox data are shown via HUMS; radio altimeter is important for low-level and hovering operations.
Instrument pitot-static errors and blockages behave exactly as on fixed-wing (static feeds ALT/ASI/VSI; pitot feeds the ASI).
⚠ Exam trap: the DUAL tachometer shows rotor RPM (Nr) and engine RPM together — a needle SPLIT indicates the freewheel has disengaged (autorotation); helicopter power is read as TORQUE, and low rotor RPM triggers a critical warning.

Solved examples

During flight the rotor-RPM and engine-RPM needles of the dual tachometer suddenly 'split', with engine RPM dropping and rotor RPM steady. What has happened?
The needles splitting means the freewheel unit has disengaged the engine from the rotor — typically an engine failure or power loss. The rotor is now free-wheeling (driven by the airflow) while the engine winds down. The pilot must immediately enter autorotation (lower the collective) to preserve rotor RPM. A joined-needle indication is normal powered flight.

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