Pumps in the "back" of the system create vacuum, while pumps in the "front" the system create pressure.
Directional gyros are almost all air-driven by evacuating the case and allowing filtered air to flow into the case and out through a nozzle, blowing against buckets cut in the periphery of the wheel
Typically, ports are flush mounted on the side of the aircraft where air is undisturbed. [Figure 2]
Some aircraft may utilize heated static ports to mitigate ice.
Dual ports remove errors due to slips and skids.
Responsible for Airspeed Indicator, Altimeter, and Vertical Speed Indicators.
The POH/AFM contains any corrections to the airspeed for the various flaps and landing gear configurations.
Alternate Static Source:
An alternate static air source valve is available for emergencies on some aircraft.
If the alternate source ports inside the airplane, where static pressure is usually lower than outside, selection may result in the following erroneous instrument indications:
The altimeter reads higher than normal.
Indicated airspeed (IAS) reads greater than normal.
VSI momentarily shows a climb.
Many POHs provide a correction table and aircraft-specific instructions.
The alternate static source is not corrected for non-standard pressure (as with an altimeter's Kollsman window).
Using alternate static sources may impact other instruments that rely on static pressure (i.e., autopilots, TCAS, transponder, etc.).
Using alternate static sources can also decrease the accuracy beyond the 75 feet recommendation outlined in the Aeronautical Information Manual.
Instrument Flying Handbook, A Typical Electrically Heated Pitot-Static Head
Static Port
Vacuum and Pressure Systems:
Typical Vacuum System
Suction Gauge
Gyroscopes power several flight instruments.
In some aircraft, all the gyros are vacuum, pressure, or electrically operated.
In other aircraft, vacuum or pressure systems provide the power for the heading and attitude indicators, while the electrical system provides the power for the turn coordinator.
Most aircraft have at least two sources of power to ensure at least one source of bank information is available if one power source fails.
The vacuum or pressure system spins the gyro by drawing a stream of air against the rotor vanes to spin the rotor at high speed, much like a waterwheel or turbine operation.
The vacuum or pressure required for instrument operation varies but is usually between 4.5 "Hg and 5.5 "Hg.
One source of vacuum for the gyros is a vane-type engine-driven pump mounted on the engine's accessory case.
Pump capacity varies in different aircraft, depending on the number of gyros.
A typical vacuum system consists of an engine-driven pump, relief valve, air filter, gauge, and tubing necessary to complete the connections. The gauge is mounted in the aircraft's instrument panel and indicates the amount of pressure in the system (measured in inches of mercury less than ambient pressure).
Air is drawn into the vacuum system by the engine-driven vacuum pump [Figure 3]
It first goes through a filter, which prevents foreign matter from entering the vacuum or pressure system.
The air then moves through the attitude and heading indicators, which causes the gyros to spin.
A relief valve prevents the vacuum pressure, or suction, from exceeding prescribed limits.
After that, the air is expelled overboard or used in other systems, such as for inflating pneumatic deicing boots
Monitoring vacuum pressure during flight is important because the attitude and heading indicators may not provide reliable information when suction pressure is low.
The vacuum, or suction, gauge is generally marked to indicate the normal range. [Figure 4]
Some aircraft have a warning light that illuminates when the vacuum pressure drops below the acceptable level.
The gyroscopic instruments may become unstable and inaccurate when the vacuum pressure drops below the normal operating range.
Routinely cross-checking the instruments is a good habit to develop.
Typical Vacuum System
Suction Gauge
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
Vacuum Pressure:
Gyro pressure gauge, vacuum gauge, or suction gauge are all terms for the same gauge used to monitor the vacuum developed in the system that actuates the air driven gyroscopic flight instruments
Air is pulled through the instruments, causing gyroscopes to spin
The speed at which the gyros spin needs to be within a certain range for correct operation
This speed is directly related to the suction pressure that is developed in the system
The suction gauge is extremely important in aircraft relying solely on vacuum operated gyroscopic flight instruments
Vacuum is a differential pressure indication, meaning the pressure to be measured is compared to atmospheric pressure through the use of a sealed diaphragm or capsule
The gauge is calibrated in inches of mercury
It shows how much less pressure exists in the system than in the atmosphere [Figure 1]
A physical failure may manifest with the indicator becoming stuck
A vacuum failure may manifest with the instrument acting erratically
Vacuum System Failures:
Vacuum systems are recognized by a low indication on the vacuum gauge or unusual instrument indications
Reduces or eliminates effectiveness of the turn coordinator, attitude indicator, and heading indicator
When the primary air inlet is blocked, the backup inlet automatically opens due to pressure
Occurs when the vacuum pump fails or when both intakes are blocked
Pressure Systems:
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
Gyroscope Systems:
Instrument Flying Handbook, A venturi tube system that provides necessary vacuum to operate key instruments
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
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
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.
Common Training Aircraft Pitot-Static System Characteristics:
Piper Arrow:
Composed of a heated Pitot tube on the lower left wing
Two static ports are located on each side of the fuselage
Alternate static air (below the pilot control yoke) provides static pressure from inside the cabin
Cessna 172:
Composed of a heated Pitot tube on the lower surface of the left wing
An external static port is located on the lower left side of the forward fuselage
Pitot Tube consists of a heating element, a 5-amp switch/breaker, and associated wiring
Alternate static (below throttle) provides pressure from inside the cabin
Common Training Aircraft Vacuum System Characteristics:
Cessna-172:
Provides "power" to the attitude indicator and directional indicator
The desired suction range is 4.5 to 5.5 inches Hg
Transducers measure vacuum output at each pump
If output of the left or right pump drops below 3.0 in. HG then L VAC R will flash for 10 seconds before becoming steady
There are 2 engine driven vacuum pumps
Piper Arrow:
Protected by a vacuum regulator (protects gyros)
Normal operation reads 4.8 to 5.1 inches of mercury
"Powers" air driven gyro instruments
Vacuum pump is a dry type pump
Zero pressure may indicate a sheered pump drive
Reduction in pressures may indicate dirty filters and screens
Private Pilot (Airplane) Operation of Aircraft Systems Airman Certification Standards:
Objective: To determine the applicant exhibits satisfactory knowledge, risk management, and skills associated with safe operation of systems on the airplane provided for the flight test.
Private Pilot (Airplane) Operation of Aircraft Systems Skills:
The applicant exhibits the skill to:
PA.I.G.S1:
Operate at least three of the systems listed in K1a through K1l appropriately.
PA.I.G.S2:
Complete the appropriate checklist(s).
Private Pilot (Airplane) Systems and Equipment Malfunctions Airman Certification Standards:
Objective: To determine the applicant exhibits satisfactory knowledge, risk management, and skills associated with system and equipment malfunctions appropriate to the airplane provided for the practical test
Causes and remedies for smoke or fire onboard the aircraft.
PA.IX.C.K4:
Any other system specific to the airplane (e.g., supplemental oxygen, deicing).
PA.IX.C.K5:
Inadvertent door or window opening.
Private Pilot (Airplane) Systems and Equipment Malfunctions Risk Management:
The applicant is able to identify, assess, and mitigate risk associated with:
PA.IX.C.R1:
Checklist usage for a system or equipment malfunction.
PA.IX.C.R2:
Distractions, task prioritization, loss of situational awareness, or disorientation.
PA.IX.C.R3:
Undesired aircraft state.
PA.IX.C.R4:
Startle response.
Private Pilot (Airplane) Systems and Equipment Malfunctions Skills:
The applicant exhibits the skill to:
PA.IX.C.S1:
Describe appropriate action for simulated emergencies specified by the evaluator, from at least three of the elements or sub-elements listed in K1 through K5 above.
PA.IX.C.S2:
Complete the appropriate checklist(s).
Vacuum System Case Studies:
Conclusion:
The vacuum system is an important component for many aircraft as it runs instrumentation and potentially other systems
Of note however, is that these systems can be expensive to maintain and there is the option to run many if not all of the systems requiring vacuum on electric
