Aircraft Vacuum Instruments - A quick scan of the six-pack provides the pilot with up-to-date information on aircraft speed, altitude, climb/descent, attitude, heading and pitch/turn. Specifically, the six-pack instruments are:
The instruments in the six-pack are powered by various aircraft systems. ASI, altimeter and VSI use the pitot static system which provides ram air pressure from the pitot tube and ambient pressure from the static port. Only the ASI uses the pitot tube; All three instruments use the static port.
Aircraft Vacuum Instruments
KI, HI and Blinker are gyroscopic instruments that contain an internal gyro driven by vacuum, pressure or electrical energy.
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The ASI uses the pressure difference in the pitot static system to measure and display the aircraft's speed. On most aircraft, the ASI displays speed in knots or miles per hour. A needle points to the aircraft's current indicated airspeed (IAS). Color-coded standard markings provide various critical speed information for this model airplane, including stall, flap adjustment, normal operation, caution, and never exceed speed. The table below shows what the colors on an airspeed indicator mean.
Also known as an artificial horizon, the AI uses a rigidly mounted internal gyro to indicate the aircraft's attitude relative to the horizon. The display consists of a miniature airplane in level flight towards the horizon, with a blue sky above and brown or black ground below.
A vertical scale crossing the horizon indicates the degree of tilt up or down. A curved scale across the top indicates the degree of left or right bank. When the plane changes pitch or direction, the plane essentially rotates around the AI's gyro and the instrument display reacts accordingly.
The altimeter uses the barometric pressure obtained from the static port to display the aircraft's approximate altitude, or height above mean sea level (MSL) in feet. Three hands provide altitude information in increments of 100, 1,000 and 10,000 feet.
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Because barometric pressure changes with altitude and atmospheric conditions, most altimeters include an adjustment knob to adjust the local barometric pressure (also known as the altimeter baro setting).
The TC uses a tilted internal gyro to display both initial roll rate and stabilized rate of turn. An aircraft silhouette mimics the aircraft to indicate the direction of rotation and aligns with a marker on the screen when the aircraft rotates at a standard rate of three degrees per second.
Note that the TC is not intended to indicate bank angle, only speed and direction of turn. The TC may contain a fluid-filled inclinometer that provides slip or slide information.
The HI uses a rotating gyro to show the current direction of the compass rose (also known as heading) in which the aircraft is flying. When using a 360 degree compass chart with North as zero or "N", the HI will display the headings in 5 degree increments, counting every 30 degrees. To reduce capture, the last "zero" is omitted from the header - "3" is 30 degrees, "12" is 120, etc.
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The VSI uses the internal pressure difference to give a visual indication of how fast the aircraft is climbing or descending. A chambered diaphragm connected to the static port expands or contracts in response to climb and descent, causing the instrument to display the rate of climb or descent in hundreds of feet per minute (fpm). A hole in the diaphragm releases the pressure change to return to zero rate when no change occurs.
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Serving more than 70 countries, we operate one of the largest maintenance, overhaul and exchange programs in the world and proudly support customers in general, business and commercial aviation, UAVs, defense and special missions. Gyroscopic instruments are essential instruments used on all aircraft. They provide the pilot with important attitude and directional information and are especially important when flying under IFR. Power sources for these instruments may vary. The main requirement is to rotate the tops at high speed. Originally, gyroscopic instruments were exclusively vacuum driven. A vacuum source drew air over the gyrophone in the instruments to make the gyrophone spin. Electricity was later added as an energy source. The rotating armature of an electric motor also acts as a gyro-rotor. In some aircraft, pressure rather than vacuum is used to turn the gyro. Various systems and power supply configurations have been developed to provide reliable operation of the gyroscopic instruments.
Vacuum systems Vacuum systems are widely used to drive gyroscopes. In a vacuum system, airflow directed against the rotor blades spins the rotor at high speed. The action is similar to a water wheel. Atmospheric compressed air is first drawn through one or more filters. This is then channeled into the instrument and directed to vanes on the gyro rotor. A suction line leads from the instrument housing to the vacuum source. From there the air is released overboard. Either a venturi or vacuum pump can be used to provide the vacuum needed to spin the rotors of the gyroscopes.
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The vacuum level required to operate the instrument is typically between 3½ inches and 4½ inches of mercury. This is usually set by a vacuum relief valve located on the supply line. Some dials require a lower vacuum setting. This can be achieved by using an additional control valve in the gym and bench vacuum supply line.
Venturi Tube Systems The velocity of air flowing through a venturi can create sufficient suction to turn instrument gyros. A conduit runs from the gyro instruments to the throat of the venturi, which is attached to the outside of the airframe. The low pressure in the venturi draws air through the instruments, turns the gyros and pushes the air overboard through the venturi. This gyro current source is used in very simple, early aircraft.
A light single engine airplane can be equipped with a 2 inch venturi (2 inch mercury vacuum capacity) to operate the turn and roll indicator. It may also have a larger 8 inch venturi to power the attitude and course indicators. Simplified representations of these venturi vacuum systems are shown in Figure 1. Normally, the air entering the instruments is filtered.
The advantages of using a venturi as a suction source are the relatively low cost and the ease of installation and operation. It also requires no electrical energy. But there are serious limitations. A venturi is designed to produce the desired vacuum at about 100 mph at standard sea level conditions. Large fluctuations in flight speed or air density will cause the suction force developed to fluctuate. Airflow can also be restricted by ice that can form on the venturi tube. Additionally, since the rotor does not reach normal operating speed until after takeoff, pre-flight operational checks of venturi-driven gyroscopes cannot be performed. For these reasons, alternative vacuum energy sources have been developed.
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Motor Driven Vacuum Pump The fan motor driven pump is the most common source of vacuum for gyros installed on general aviation and light aircraft. One type of engine driven pump is directed to the engine and connected to the lubrication system to seal, cool and lubricate the pump. Another commonly used pump is a dry vacuum pump. It operates without external lubrication and installation does not require connection to the engine oil supply. It also does not require an air-oil separator or check valve found in wet pump systems. In many other respects the dry pump system and the oil lubricated system are the same. [Figure 2]
When a vacuum pump creates a vacuum (negative pressure), it also creates positive pressure at the outlet of the pump. This pressure is compressed air. Sometimes it is used to operate pressure giro instruments. The components for pressure systems are essentially the same as those for a vacuum system, as listed below. In other cases, the pressure developed by the vacuum pump is used to inflate deicing boots or inflatable seals, or is thrown overboard.
An advantage of motorized pumps is their consistent performance on the ground and in flight. Even at low engine speeds, they can generate more than enough vacuum, requiring a regulator in the system to continuously deliver the correct suction power to the vacuum instruments. As long as the engine is running, the relatively simple vacuum system appropriately rotates the instrument gyros for accurate readings. However, an engine failure, especially on single-engine aircraft, can leave the pilot without attitude and directional information at a critical time. To counter this shortcoming, the direction indicator often works with an electrically powered gyroscope that can be briefly powered by the battery. In combination with the aircraft's magnetic compass, sufficient position and direction information is therefore still available.
Multi-engine aircraft typically include independent vacuum systems for the pilot's and co-pilot's instruments, powered by separate vacuum pumps on each of the engines. Should an engine fail, the vacuum system, powered by the engine still running, provides a full array of gyroscopes. A connection
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