Which instruments continue to operate if both the alternator and battery switch are turned off in the Schweizer 300c?

When both the alternator and battery switches are turned off in the Schweizer 300C helicopter, certain instruments still continue to function independently of electrical power. This is due to their mechanical or inertial nature rather than reliance on electrical systems.

The correct choice includes the tachometers, airspeed indicator, compass, altimeter, and manifold pressure gauge because these instruments operate using mechanical means or static air pressure rather than electrical power.

For instance, the airspeed indicator operates based on the differential pressure between static air and the dynamic air flow created by the helicopter’s movement. The altimeter measures atmospheric pressure changes and is therefore unaffected by electrical systems as long as it is mechanically calibrated. The compass relies on magnetic fields to indicate direction and does not require electricity.

In contrast, other instruments such as navigation instruments do rely on electrical power to function. Thus, when the electrical systems are turned off, these instruments would not be operational. The design of the Schweizer 300C ensures that critical flight data remains available even when electrical failures occur, ensuring the pilot can still monitor essential performance metrics.