Instrument Flying Systems

Electrical Systems
Many general aviation aircraft that use pneumatic attitude indicators use electric rate indicators and vice versa. Some instruments identify their power source on their dial, but it is extremely important that pilots consult the POH/AFM to determine the power source of all instruments to know what action to take in the event of an instrument failure.
 
Direct current (d.c.) electrical instruments are available in 14- or 28-volt models, depending upon the electrical system in the aircraft. Alternating current (a.c.) is used to operate some attitude gyros and autopilots. Aircraft that have only d.c. electrical systems can use a.c. instruments by installing a solid-state d.c. to a.c. inverter, which changes 14 or 28 volts d.c. into three-phase 115-volt, 400-Hz a.c.
 
Pneumatic Systems
Pneumatic gyros are driven by a jet of air impinging on buckets cut into the periphery of the wheel. This stream of air is obtained on many aircraft by evacuating the instrument case and allowing filtered air to flow into the case through a nozzle to spin the wheel.

Venturi Tube Systems
Aircraft that do not have a pneumatic pump to evacuate the instrument cases can use venture tubes mounted on the outside of the aircraft. Air flowing through these tubes speeds up in the narrowest part, and according to Bernoulli's principle, the pressure drops. This location is connected to the instrument case by a piece of tubing. The two attitude instruments operate on approximately 4" Hg suction; the turn-and-slip indicator needs only 2" Hg, so a pressure-reducing needle valve is used to decrease the suction. Filtered airflow's into the instruments through filters built into the instrument cases. In this system, ice can clog the venturi tube and stop the instruments when they are most needed.

Wet-Type Vacuum Pump Systems
Steel-vane air pumps have been used for many years to evacuate the instrument cases. The discharge air is used to inflate rubber deicer boots on the wing and empennage leading edges. The vanes in these pumps are lubricated by a small amount of engine oil metered into the pump and this oil is discharged with the air. To keep the oil from deteriorating the rubber boots, it must be removed with an oil separator.
 
The vacuum pump moves a greater volume of air than is needed to supply the instruments with the suction needed, so a suction-relief valve is installed in the inlet side of the pump. This spring-loaded valve draws in just enough air to maintain the required low pressure inside the instruments, as is shown on the suction gauge in the instrument panel. Filtered air enters the instrument cases from a central air filter. As long as aircraft fly at relatively low altitudes, enough air is drawn into the instrument cases to spin the gyros at a sufficiently high speed.