What instrument has 3 needles and indicates rotor speed and the power turbine sections' speed?

The instrument that features three needles and indicates rotor speed as well as the power turbine sections’ speed is the triple tachometer. This instrument is essential in rotorcraft operation, as it provides crucial information on the rotational speeds of different components. The primary needle displays the rotor speed, which is vital for maintaining proper flight dynamics and ensuring safe operation. The additional needles indicate the speeds of the power turbine, which help the pilot to monitor engine performance and changes in power output.

The design of the triple tachometer makes it a comprehensive tool for quickly assessing multiple speeds simultaneously, which is critical during various phases of flight, including takeoff, landing, and maneuvers. By interpreting the readings from all three needles, pilots can make informed decisions to optimize performance and safety.

The other instruments mentioned serve different functions; for example, the altimeter measures altitude, the vertical speed indicator displays the rate of climb or descent, and the torque indicator measures the twisting force on the rotor system. They do not provide the multiple readings of rotor and power turbine speeds that the triple tachometer does.