Flight without reference to a visible horizon can be safely accomplished by the use of gyroscopic instrument systems
These systems include attitude, heading, and rate instruments, along with their power sources
These instruments include a gyroscope (or gyro) that is a small wheel with its weight concentrated around its periphery
Gyroscope Characteristics:
Instrument Flying Handbook, A venturi tube system that provides necessary vacuum to operate key instruments
Characteristics of gyroscopes:
Rigidity
Precession
Rigidity:
When this wheel is spun at high speed, it becomes rigid and resists tilting or turning in any direction other than around its spin axis
Attitude and heading instruments operate on the principle of rigidity
For these instruments, the gyro remains rigid in its case and the aircraft rotates about it
Precession:
Rate indicators, such as turn indicators and turn coordinators, operate on the principle of precession
In this case, the gyro processes (or rolls over) proportionate to the rate the aircraft rotates about one or more of its axes
Instrument Flying Handbook, A venturi tube system that provides necessary vacuum to operate key instruments
Power Sources:
Power Sources Aircraft and instrument manufacturers have designed redundancy in the flight instruments so that any single failure will not deprive the pilot of the ability to safely conclude the flight
Gyroscopic instruments are crucial for instrument flight; therefore, they are powered by separate electrical or pneumatic sources
Pneumatic Systems Pneumatic gyros are driven by a jet of air impinging on buckets cut into the periphery of the wheel
On many aircraft this stream of air is obtained by evacuating the instrument case with a vacuum source 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 case can use venturi tubes mounted on the outside of the aircraft, similar to the system shown in Figure 3-27
Air flowing through the venturi tube 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 of suction; the turn-and-slip indicator needs only 2" Hg, so a pressure-reducing needle valve is used to decrease the suction
Air flows 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
Vacuum Pump Systems:
Wet-Type Vacuum Pump:
Steel-vane air pumps have been used for many years to evacuate the instrument cases
The vanes in these pumps are lubricated by a small amount of engine oil metered into the pump and discharged with the air
In some aircraft the discharge air is used to inflate rubber deicer boots on the wing and empennage leading edges
To keep the oil from deteriorating the rubber boots, it must be removed with an oil separator like the one in Figure 3-28
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
Dry Air Vacuum Pump:
As flight altitudes increase, the air is less dense and more air must be forced through the instruments
Air pumps that do not mix oil with the discharge air are used in high flying aircraft
Steel vanes sliding in a steel housing need to be lubricated, but vanes made of a special formulation of carbon sliding inside carbon housing provide their own lubrication in a microscopic amount as they wear
Instrument Flying Handbook, Single-engine instrument vacuum system using a steel-vane wet-type vacuum pump
Instrument Flying Handbook, Twin-Engine Instrument Pressure System Using a Carbon-Vane Dry-Type Air Pump
Pressure Indicating Systems:
Figure 3-29 is a diagram of the instrument pneumatic system of a twin-engine general aviation airplane
Two dry air pumps are used with filters in their inlet to filter out any contaminants that could damage the fragile carbon vanes in the pump
The discharge air from the pump flows through a regulator, where excess air is bled off to maintain the pressure in the system at the desired level
The regulated air then flows through in-line filters to remove any contamination that could have been picked up from the pump, and from there into a manifold check valve
If either engine should become inoperative or either pump should fail, the check valve isolates the inoperative system and the instruments are driven by air from the operating system
After the air passes through the instruments and drives the gyros, it is exhausted from the case
The gyro pressure gauge measures the pressure drop across the instruments
Electrical Systems:
Many general aviation aircraft that use pneumatic attitude indicators use electric rate indicators and/or the reverse
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
A.C. is used to operate some attitude gyros and autopilots
Aircraft with only D.C. electrical systems can use A.C. instruments via installation of 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.
