Flight Instruments Vocabulary
Introduction
To safely fly an aircraft, pilots need to understand and operate flight instruments, recognize errors, and be aware of malfunctions.
This chapter covers the pitot-static system, vacuum system, gyroscopic instruments, and the magnetic compass.
Understanding instrument functionality and recognizing malfunctions are key for safe and effective instrument use.
Pitot-Static Flight Instruments
The pitot-static system uses static air pressure and dynamic pressure from the aircraft's motion.
This system powers the airspeed indicator (ASI), altimeter, and vertical speed indicator (VSI).
Pressure Chamber and Lines
The pitot tube measures the combined total pressure (static and dynamic) when the aircraft moves through the air.
Static pressure (ambient pressure) is always present, whether the aircraft is moving or at rest; it's the local barometric pressure.
Dynamic pressure is present only when the aircraft is in motion or facing wind and is related to the speed.
The same dynamic pressure is generated whether the aircraft is moving through still air at knots or facing a knot wind.
The pitot tube captures total pressure through a small opening at the front which enters the pressure chamber.
There is a drain hole in the back of the chamber to remove moisture.
Both openings should be checked before flight to ensure they are not blocked.
The ASI is the primary instrument that uses the pitot tube.
Total pressure from the pitot tube is transmitted to the ASI.
Static pressure is also delivered to the ASI to cancel out the static pressures, leaving only dynamic pressure to be indicated.
Changes in dynamic pressure cause the ASI to register corresponding increases or decreases in airspeed.
The altimeter and VSI use only static pressure, derived from the static port.
Static Pressure Chamber and Lines
The static chamber is vented through small holes to the undisturbed air on the sides of the aircraft.
As atmospheric pressure changes, pressure moves freely in and out of the instruments through lines connected to the static system.
Some aircraft have an alternate static source in case the primary source is blocked.
The air pressure inside the flight deck is lower than the exterior pressure due to the venturi effect of air flowing around the fuselage.
When the alternate static source is used:
The altimeter indicates a slightly higher altitude than actual.
The ASI indicates a greater airspeed than actual.
The VSI shows a momentary climb before stabilizing if altitude is held constant.
Pilots should consult the AFM/POH to determine the exact error introduced when using the alternate static source.
If no alternate static source is available, breaking the glass face of the VSI can introduce static pressure into the system in the event of a blockage, although it renders the VSI inoperative. The VSI is chosen for this because it is the least important instrument dependent on the static source.
Altimeter
The altimeter measures the height of an aircraft above a given pressure level.
It is a vital instrument for indicating altitude.
Pilots need to understand altimeter operation and associated errors to use it effectively.
The altimeter consists of a stack of sealed aneroid wafers, evacuated to an internal pressure of inches of mercury (''Hg).
These wafers expand and contract with changes in static pressure.
A higher static pressure compresses the wafers; a lower static pressure allows them to expand.
This movement is translated into altitude changes via mechanical linkages to the needles on the indicator face.
Static pressure is introduced into the sealed altimeter case, surrounding the aneroid wafers.
If static pressure is higher than the pressure inside the wafers, they compress until pressures equalize.
Conversely, if static pressure is lower, the wafers expand.
Principle of Operation
The pressure altimeter is an aneroid barometer measuring atmospheric pressure and indicating altitude in feet.
It uses static pressure as its source of operation.
Air density decreases with altitude, and atmospheric pressure decreases accordingly.
This pressure difference at various levels causes the altimeter to indicate changes in altitude.
Typical altimeters have a dial graduated from zero to nine, with a long, thin needle indicating tens of thousands of feet, a short, wide needle indicating thousands of feet, and a long needle indicating hundreds of feet.
The indicated altitude is correct only when:
Sea level barometric pressure is standard ( "Hg).
Sea level free air temperature is standard ( degrees Celsius (°C) or degrees Fahrenheit (°F)).
Pressure and temperature decrease at a standard rate with increasing altitude.
Adjustments for nonstandard pressures are made via a barometric scale on the altimeter face (Kollsman window).
The indicated altitude is the altitude read after the barometric pressure setting is dialed into the Kollsman window.
Effect of Nonstandard Pressure and Temperature
Pressure and temperature changes between takeoff and landing must be considered.
Without adjustments for nonstandard pressure, flying from a high-pressure to a low-pressure area would result in the aircraft's actual altitude being lower than indicated (“GOING FROM A HIGH TO A LOW, LOOK OUT BELOW”).
Conversely, flying from a low-pressure to a high-pressure area would result in the aircraft's actual altitude being higher than indicated.
It is important to frequently obtain current altimeter settings en route to ensure terrain and obstruction clearance.
Many altimeters cannot be adjusted for barometric pressures exceeding "Hg; in such cases, the aircraft's actual altitude is higher than indicated.
Flight operations are not recommended when barometric pressure is below if the altimeter cannot be set to the actual setting.
Adjustments for pressure do not compensate for nonstandard temperature.
Cold air is denser than warm air; when operating in colder-than-standard temperatures, the altitude is lower than indicated (“FROM HOT TO COLD, LOOK OUT BELOW”).
When flying into a cooler air mass while maintaining a constant indicated altitude, true altitude is lower.
In mountainous terrain, a higher indicated altitude may be required to provide adequate terrain clearance in colder-than-standard temperatures.
Altitude corrections for temperature can be computed on the navigation computer.
Setting the Altimeter
Altimeters have a barometric pressure setting window (Kollsman window) that can be adjusted using a knob.
The pressure scale, calibrated in inches of mercury (''Hg) and/or millibars (mb), is adjusted to match the given altimeter setting.
Altimeter setting is station pressure reduced to sea level, but it is accurate only near the reporting station.
The altimeter must be adjusted as the flight progresses from one station to the next.
Air traffic control (ATC) provides updated altimeter settings, local automated weather observing system/automated surface observation system (AWOS/ASOS), or automatic terminal information service (ATIS) broadcasts.
The altimeter setting broadcast by ground stations is station pressure corrected to mean sea level and does not account for irregularities at higher levels, particularly the effect of nonstandard temperature.
If each pilot in a given area uses the same altimeter setting, each altimeter should be equally affected by temperature and pressure variation errors, making it possible to maintain the desired vertical separation between aircraft.
Pilots must maintain a regimented visual scan for intruding air traffic.
When flying over mountainous terrain, atmospheric conditions can cause the altimeter to indicate an altitude of feet or more higher than the actual altitude.
A generous margin of altitude should be allowed for possible altimeter error and downdrafts.
Before takeoff, the pilot receives an altimeter setting from the control tower or ATIS and sets this value in the altimeter setting window.
The altimeter indication should then be compared with the known airport elevation.
If the pilot desires to fly the traffic pattern at approximately feet above the terrain, and the field elevation. An indicated altitude needs to be maintained.
It is important to set the altimeter to the current setting.
When the actual pressure is lower than what is set in the altimeter window, the actual altitude of the aircraft is lower than what is indicated on the altimeter.
When the actual pressure is higher than what is set in the altimeter window, the actual altitude of the aircraft is higher than what is indicated on the altimeter.
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Altimeter Operation
Altimeter pointers move through changes in air pressure and adjustments to the barometric scale.
As the aircraft climbs or descends, changing pressure expands or contracts the aneroid barometer.
A decrease in pressure causes the altimeter to indicate an increase in altitude; an increase in pressure causes a decrease in altitude.
If the aircraft is sitting on the ground with a pressure level of "Hg and the pressure level changes to "Hg, the altimeter shows an increase of approximately feet in altitude.
Pilots must ensure the aircraft is flying high enough to clear terrain or obstructions.
Accurate altitude information is vital when visibility is restricted.
Pilots must be aware of the altitude of the aircraft and the elevation of surrounding terrain.
It is essential to maintain altitude in accordance with air traffic rules to reduce the possibility of a midair collision.
Types of Altitude
Altitude is vertical distance above some point or level used as a reference.
Pilots are mainly concerned with five types of altitudes:
Indicated altitude: Read directly from the altimeter (uncorrected) when it is set to the current altimeter setting.
True altitude: The vertical distance of the aircraft above sea level; expressed as feet above mean sea level (MSL). Airport, terrain, and obstacle elevations on aeronautical charts are true altitudes.
Absolute altitude: The vertical distance of an aircraft above the terrain, or above ground level (AGL).
Pressure altitude: The altitude indicated when the altimeter setting window (barometric scale) is adjusted to "Hg. This is the altitude above the standard datum plane. Pressure altitude is used to compute density altitude, true altitude, true airspeed (TAS), and other performance data.
Density altitude: Pressure altitude corrected for variations from standard temperature. When conditions are standard, pressure altitude and density altitude are the same. If the temperature is above standard, the density altitude is higher than pressure altitude. If the temperature is below standard, the density altitude is lower than pressure altitude. This is an important altitude because it is directly related to the aircraft’s performance.
A pilot must understand how the performance of the aircraft is directly related to the density of the air.
The density of the air affects how much power a naturally aspirated engine produces, as well as how efficient the airfoils are.
Instrument Check
Prior to each flight, a pilot should examine the altimeter for proper indications in order to verify its validity.
To determine the condition of an altimeter, set the barometric scale to the current reported altimeter setting transmitted by the local airport traffic control tower, flight service station (FSS), or any other reliable source, such as ATIS, AWOS, or ASOS.
The altimeter pointers should indicate the surveyed field elevation of the airport.
If the indication is off more than feet from the surveyed field elevation, the instrument should be referred to a certificated instrument repair station for recalibration.
Vertical Speed Indicator (VSI)
The VSI, sometimes called a vertical velocity indicator (VVI), indicates whether the aircraft is climbing, descending, or in level flight.
The rate of climb or descent is indicated in feet per minute (fpm).
If properly calibrated, the VSI indicates zero in level flight.
Principle of Operation
The VSI operates solely from static pressure and is a differential pressure instrument.
It contains a diaphragm connected to the static line of the pitot-static system inside an airtight case.
The area outside the diaphragm is also connected to the static line but through a restricted orifice (calibrated leak).
When the aircraft is on the ground or in level flight, the pressures inside the diaphragm and the instrument case are equal, and the pointer is at the zero indication.
When the aircraft climbs or descends, the pressure inside the diaphragm changes immediately, but the case pressure changes more slowly due to the metered leak, causing the diaphragm to contract or expand.
The instrument needle indicates this pressure differential as a climb or descent.
When the pressure differential stabilizes, the needle indicates the rate of altitude change.
The VSI displays two different types of information:
Trend information: Shows an immediate indication of an increase or decrease in the aircraft’s rate of climb or descent.
Rate information: Shows a stabilized rate of change in altitude.
The trend information is the direction of movement of the VSI needle.
From the initial change in the rate of climb, the time period until the VSI displays an accurate indication of the new rate, is called the lag.
Rough control technique and turbulence can extend the lag period and cause erratic and unstable rate indications.
Some aircraft are equipped with an instantaneous vertical speed indicator (IVSI), which incorporates accelerometers to compensate for the lag in the typical VSI.
Instrument Check
As part of a preflight check, proper operation of the VSI must be established.
Make sure the VSI indicates a near zero reading prior to leaving the ramp area and again just before takeoff.
If the VSI indicates anything other than zero, that indication can be referenced as the zero mark.
After takeoff, the VSI should trend upward to indicate a positive rate of climb and then, once a stabilized climb is established, a rate of climb can be referenced.
Airspeed Indicator (ASI)
The ASI is a sensitive, differential pressure gauge that measures and indicates the difference between pitot (impact/dynamic pressure) and static pressure.
These two pressures are equal when the aircraft is parked on the ground in calm air.
When the aircraft moves through the air, the pressure on the pitot line becomes greater than the pressure in the static lines.
This pressure difference is registered by the airspeed pointer on the face of the instrument, which is calibrated in miles per hour, knots (nautical miles per hour), or both.
The ASI is the one instrument that utilizes both the pitot, as well as the static system.
The ASI introduces the static pressure into the airspeed case while the pitot pressure (dynamic) is introduced into the diaphragm.
The dynamic pressure expands or contracts one side of the diaphragm, which is attached to an indicating system. The system drives the mechanical linkage and the airspeed needle.
Airspeeds
Pilots need to be very familiar with each type of airspeed.
Indicated airspeed (IAS): The direct instrument reading obtained from the ASI, uncorrected for variations in atmospheric density, installation error, or instrument error. Manufacturers use this airspeed as the basis for determining aircraft performance. Takeoff, landing, and stall speeds listed in the AFM/ POH are IAS and do not normally vary with altitude or temperature.
Calibrated airspeed (CAS): IAS corrected for installation error and instrument error. Although manufacturers attempt to keep airspeed errors to a minimum, it is not possible to eliminate all errors throughout the airspeed operating range. At certain airspeeds and with certain flap settings, the installation and instrument errors may total several knots. This error is generally greatest at low airspeeds. In the cruising and higher airspeed ranges, IAS and CAS are approximately the same. Refer to the airspeed calibration chart to correct for possible airspeed errors.
True airspeed (TAS): CAS corrected for altitude and nonstandard temperature. Because air density decreases with an increase in altitude, an aircraft has to be flown faster at higher altitudes to cause the same pressure difference between pitot impact pressure and static pressure. Therefore, for a given CAS, TAS increases as altitude increases; or for a given TAS, CAS decreases as altitude increases. A pilot can find TAS by two methods. The most accurate method is to use a flight computer. With this method, the CAS is corrected for temperature and pressure variation by using the airspeed correction scale on the computer. Extremely accurate electronic flight computers are also available. Just enter the CAS, pressure altitude, and temperature, and the computer calculates the TAS. A second method, which is a rule of thumb, provides the approximate TAS. Simply add percent to the CAS for each feet of altitude. The TAS is the speed that is used for flight planning and is used when filing a flight plan.
Groundspeed (GS): The actual speed of the airplane over the ground. It is TAS adjusted for wind. GS decreases with a headwind and increases with a tailwind.
Airspeed Indicator Markings
Aircraft weighing pounds or less, manufactured after , and certificated by the FAA are required to have ASIs marked in accordance with a standard color-coded marking system.
This system of color-coded markings enables a pilot to determine at a glance certain airspeed limitations that are important to the safe operation of the aircraft.
For example, if during the execution of a maneuver, it is noted that the airspeed needle is in the yellow arc and rapidly approaching the red line, the immediate reaction should be to reduce airspeed.
ASIs on single-engine small aircraft include the following standard color-coded markings:
White arc: commonly referred to as the flap operating range since its lower limit represents the full flap stall speed and its upper limit provides the maximum flap speed. Approaches and landings are usually flown at speeds within the white arc.
Lower limit of white arc (VS0): the stalling speed or the minimum steady flight speed in the landing configuration. In small aircraft, this is the power-off stall speed at the maximum landing weight in the landing configuration (gear and flaps down).
Upper limit of the white arc (VFE): the maximum speed with the flaps extended.
Green arc: the normal operating range of the aircraft. Most flying occurs within this range.
Lower limit of green arc (VS1): the stalling speed or the minimum steady flight speed obtained in a specified configuration. For most aircraft, this is the power-off stall speed at the maximum takeoff weight in the clean configuration (gear up, if retractable, and flaps up).
Upper limit of green arc (VN0): the maximum structural cruising speed. Do not exceed this speed except in smooth air.
Yellow arc: caution range. Fly within this range only in smooth air and then only with caution.
Red line (VNE): never exceed speed. Operating above this speed is prohibited since it may result in damage or structural failure.
Other Airspeed Limitations
Some important airspeed limitations are not marked on the face of the ASI, but are found on placards and in the AFM/ POH. These airspeeds include:
Design maneuvering speed (VA): the maximum speed at which the structural design's limit load can be imposed (either by gusts or full deflection of the control surfaces) without causing structural damage. Weight is important when referencing this speed. For example, VA may be knots when an airplane is heavily loaded, but only knots when the load is light.
Landing gear operating speed (VLO): the maximum speed for extending or retracting the landing gear if flying an aircraft with retractable landing gear.
Landing gear extended speed (VLE): the maximum speed at which an aircraft can be safely flown with the landing gear extended.
Best angle-of-climb speed (VX): the airspeed at which an aircraft gains the greatest amount of altitude in a given distance. It is used during a short-field takeoff to clear an obstacle.
Best rate-of-climb speed (VY): the airspeed that provides the most altitude gain in a given period of time.
Single-engine best rate-of-climb (VYSE): the best rate-of-climb or minimum rate-of-sink in a light twin-engine aircraft with one engine inoperative. It is marked on the ASI with a blue line. VYSE is commonly referred to as “Blue Line.”
Minimum control speed (VMC): the minimum flight speed at which a light, twin-engine aircraft can be satisfactorily controlled when an engine suddenly becomes inoperative and the remaining engine is at takeoff power.
Instrument Check
Prior to takeoff, the ASI should read zero.
However, if there is a strong wind blowing directly into the pitot tube, the ASI may read higher than zero.
When beginning the takeoff, make sure the airspeed is increasing at an appropriate rate.
Blockage of the Pitot-Static System
Errors almost always indicate blockage of the pitot tube, the static port(s), or both.
Blockage may be caused by moisture (including ice), dirt, or even insects.
During preflight, make sure the pitot tube cover is removed.
Then, check the pitot and static port openings.
A blocked pitot tube affects the accuracy of the ASI, but a blockage of the static port not only affects the ASI, but also causes errors in the altimeter and VSI.
Blocked Pitot System
The pitot system can become blocked completely or only partially if the pitot tube drain hole remains open.
If the pitot tube becomes blocked and its associated drain hole remains clear, ram air is no longer able to enter the pitot system. Air already in the system vents through the drain hole, and the remaining pressure drops to ambient (outside) air pressure.
Under these circumstances, the ASI reading decreases to zero because the ASI senses no difference between ram and static air pressure. The ASI no longer operates since dynamic pressure cannot enter the pitot tube opening. Static pressure is able to equalize on both sides since the pitot drain hole is still open. The apparent loss of airspeed is not usually instantaneous but happens very quickly.
If both the pitot tube opening and the drain hole should become clogged simultaneously, then the pressure in the pitot tube is trapped. No change is noted on the airspeed indication should the airspeed increase or decrease. If the static port is unblocked and the aircraft should change altitude, then a change is noted on the ASI. The change is not related to a change in airspeed but a change in static pressure. The total pressure in the pitot tube does not change due to the blockage; however, the static pressure will change.
Airspeed indications rely upon both static and dynamic pressure together, the blockage of either of these systems affects the ASI reading.
The ASI has a diaphragm in which dynamic air pressure is entered. Behind this diaphragm is a reference pressure called static pressure that comes from the static ports. The diaphragm pressurizes against this static pressure and as a result changes the airspeed indication via levers and indicators.
When both dynamic and static air pressure are equal at zero speed with increased speed, dynamic pressure must include two components: static pressure and dynamic pressure.
The airspeed indication must be based upon a relationship between these two pressures.
An ASI uses the static pressure as a reference pressure and as a result, the ASI's case is kept at this pressure behind the diaphragm. On the other hand, the dynamic pressure through the pitot tube is connected to a highly sensitive diaphragm within the ASI case.
An aircraft in zero motion (regardless of altitude) results in a zero airspeed, the pitot tube always provides static pressure in addition to dynamic pressure. Therefore, the airspeed indication is the result of two pressures: the pitot tube static and dynamic pressure within the diaphragm as measured against the static pressure in the ASI’s case.
If the aircraft were to descend while the pitot tube is obstructed, the pressure in the pitot system, including the diaphragm, would remain constant. But as the descent is made, the static pressure would increase against the diaphragm causing it to compress, thereby resulting in an indication of decreased airspeed. Conversely, if the aircraft were to climb, the static pressure would decrease allowing the diaphragm to expand, thereby showing an indication of greater airspeed.
The pitot tube may become blocked during flight due to visible moisture. Some aircraft may be equipped with pitot heat for flight in visible moisture. Consult the AFM/POH for specific procedures regarding the use of pitot heat.
Blocked Static System
If the static system becomes blocked but the pitot tube remains clear, the ASI continues to operate; however, it is inaccurate.
The airspeed indicates lower than the actual airspeed when the aircraft is operated above the altitude where the static ports became blocked because the trapped static pressure is higher than normal for that altitude.
When operating at a lower altitude, a faster than actual airspeed is displayed due to the relatively low static pressure trapped in the system.
Revisiting the ratios that were used to explain a blocked pitot tube, the same principle applies for a blocked static port.
If the aircraft descends, the static pressure increases on the pitot side showing an increase on the ASI. This assumes that the aircraft does not actually increase its speed. The increase in static pressure on the pitot side is equivalent to an increase in dynamic pressure since the pressure cannot change on the static side.
If an aircraft begins to climb after a static port becomes blocked, the airspeed begins to show a decrease as the aircraft continues to climb. This is due to the decrease in static pressure on the pitot side, while the pressure on the static side is held constant.
A blockage of the static system also affects the altimeter and VSI.
Trapped static pressure causes the altimeter to freeze at the altitude where the blockage occurred.
In the case of the VSI, a blocked static system produces a continuous zero indication.
Some aircraft are equipped with an alternate static source in the flight deck.
In the case of a blocked static source, opening the alternate static source introduces static pressure from the flight deck into the system.
Flight deck static pressure is lower than outside static pressure. Check the aircraft AOM/POH for airspeed corrections when utilizing alternate static pressure.
Electronic Flight Display (EFD)
Advances in technology have brought about changes in the instrumentation found in all types of aircraft; for example, Electronic Flight Displays (EFDs) commonly referred to as “glass cockpits.”
EFDs include flight displays such as primary flight displays (PFD) and multi-function displays (MFD).
This has changed not only what information is available to a pilot, but also how the information is displayed.
In addition to the improvement in system reliability, which increases overall safety, EFDs have decreased the overall cost of equipping aircraft with state-of-the-art instrumentation.
Primary electronic instrumentation packages are less prone to failure than their analogue counterparts.
Multi-panel digital flight displays combine all flight instruments onto a single screen that is called a primary flight display (PFD).
The traditional “six pack” of instruments is now displayed on one liquid crystal display (LCD) screen.
Airspeed Tape
The ASI is located on the left side of the screen and is displayed as a vertical speed tape.
As the aircraft increases in speed, the larger numbers descend from the top of the tape.
The TAS is displayed at the bottom of the tape through the input to the air data computer (ADC) from the outside air temperature probe.
Airspeed markings for VX, VY, and rotation speed (VR) are displayed for pilot reference.
An additional pilot-controlled airspeed bug is available to set at any desired reference speed.
The electronic airspeed tape displays the color-coded ranges for the flap operating range, normal range, and caution range.
The number value changes color to red when the airspeed exceeds VNE to warn the pilot of exceeding the maximum speed limitation.
Attitude Indicator
The larger attitude indicator on EFD has an artificial horizon that spans the entire width of the PFD.
This expanded instrumentation offers better reference through all phases of flight and all flight maneuvers.
The attitude indicator receives its information from the Attitude Heading and Reference System (AHRS).
Altimeter
The altimeter is located on the right side of the PFD.
As the altitude increases, the larger numbers descend from the top of the display tape, with the current altitude being displayed in the black box in the center of the display tape.
The altitude is displayed in increments of .
Vertical Speed Indicator (VSI)
The VSI is displayed to the right of the altimeter tape and can take the form of an arced indicator or a vertical speed tape.
Equipped with a vertical speed bug.
Heading Indicator
The heading indicator is located below the artificial horizon and is normally modeled after a Horizontal Situation Indicator (HSI).
The heading indicator receives its information from the magnetometer, which feeds information to the AHRS unit and then out to the PFD.
Turn Indicator
The turn indicator takes a slightly different form than the traditional instrumentation.
A sliding bar moves left and right below the triangle to indicate deflection from coordinated flight.
Reference for coordinated flight comes from accelerometers contained in the AHRS unit.
Tachometer
The sixth instrument normally associated with the “six pack” package is the tachometer.
This is the only instrument that is not located on the PFD.
The tachometer is normally located on the multi-function display (MFD).
In the event of a display screen failure, it is displayed on the remaining screen with the PFD flight instrumentation.
Slip/Skid Indicator
The slip/skid indicator is the horizontal line below the roll pointer.
Like a ball in a turn-and-slip indicator, a bar width off center is equal to one ball width displacement.
Turn Rate Indicator
The turn rate indicator, illustrated in Figure 8-12, is typically found directly above the rotating compass card.
Tick marks to the left and right of the lubber line denote the turn (standard- rate versus half standard-rate).
Typically denoted by a trend line, if the trend vector is extended to the second tick mark the aircraft is in a standard-rate turn.
Air Data Computer (ADC)
EFDs utilize the same type of instrument inputs as traditional analogue gauges; however, the processing system is different.
The pitot static inputs are received by an ADC.
The ADC computes the difference between the total pressure and the static pressure and generates the information necessary to display the airspeed on the PFD.
Outside air temperatures are also monitored and introduced into various components within the system, as well as being displayed on the PFD screen.
The ADC is a separate solid state device that, in addition to providing data to the PFD, is capable of providing data to the autopilot control system.
In the event of system malfunction, the ADC can quickly be removed and replaced in order to decrease downtime and decrease maintenance turn-around times.
Altitude information is derived from the static pressure port just as an analogue system does; however, the static pressure does not enter a diaphragm.
The ADC computes the received barometric pressure and sends a digital signal to the PFD to display the proper altitude readout.
EFDs also show trend vectors, which show the pilot how the altitude and airspeed are progressing.
Trend Vectors
Trend vectors are magenta lines that move up and down both the ASI and the altimeter.
The ADC computes the rate of change and displays the -second projection of where the aircraft will be.
Pilots can utilize the trend vectors to better control the aircraft’s attitude.
By including the trend vectors in the instrument scan, pilots are able to precisely control airspeed and altitude.
Gyroscopic Flight Instruments
Several flight instruments utilize the properties of a gyroscope for their operation.
The most common instruments containing gyroscopes are the turn coordinator, heading indicator, and the attitude indicator.
To understand how these instruments operate requires knowledge of the instrument power systems, gyroscopic principles, and the operating principles of each instrument.
Gyroscopic Principles
Any spinning object exhibits gyroscopic properties.
A wheel or rotor designed and mounted to utilize these properties is called a gyroscope.
Two important design characteristics of an instrument gyro are great weight for its size, or high density, and rotation at high speed with low friction bearings.
There are two general types of mountings; the type used depends upon which property of the gyro is utilized.
A freely or universally mounted gyroscope is free to rotate in any direction about its center of gravity. Such a wheel is said to have three planes of freedom. The wheel or rotor is free to rotate in any plane in relation to the base and is balanced so that, with the gyro wheel at rest, it remains in the position in which it is placed.
Restricted or semi-rigidly mounted gyroscopes are those mounted so that one of the planes of freedom is held fixed in relation to the base.
There are two fundamental properties of gyroscopic action: rigidity in space and precession.
Rigidity in Space
Rigidity in space refers to the principle that a gyroscope remains in a fixed position in the plane in which it is spinning.
An example of rigidity in space is that of a bicycle wheel.
Gimbal rings allow the gyro to rotate freely in any direction where if the gimbal rings are tilted, twisted, or otherwise moved, the gyro remains in the plane in which it was originally spinning.
Precession
Precession is the tilting or turning of a gyro in response to a deflective force.
The reaction to this force does not occur at the point at which it was applied; rather, it occurs at a point that is ° later in the direction of rotation.
This principle allows the gyro to determine a rate of turn by sensing the amount of pressure created by a change in direction.
The rate at which the gyro precesses is inversely proportional to the speed of the rotor and proportional to the deflective force.
Precession can also create some minor errors in some instruments.
Precession can cause a freely spinning gyro to become displaced from its intended plane of rotation through bearing friction, etc. Certain instruments may require corrective realignment during flight, such as the heading indicator.
Sources of Power
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 the operation of a waterwheel or turbine.
The amount of vacuum or pressure required for instrument operation varies, but is usually between "Hg and "Hg.
One source of vacuum for the gyros is a vane-type engine-driven pump that is mounted on the accessory case of the engine.
Pump capacity varies in different aircraft, depending on the number of gyros.
A typical vacuum system consists of an engine-driven vacuum 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 (vacuum is measured in inches of mercury less than ambient pressure).
Air is drawn into the vacuum system by the engine-driven vacuum pump.
Relief valve prevents the vacuum pressure, or suction, from exceeding prescribed limits.
It is important to monitor vacuum pressure during flight, 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.
Some aircraft are equipped with a warning light that illuminates when the vacuum pressure drops below the acceptable level.
When the vacuum pressure drops below the normal operating range, the gyroscopic instruments may become unstable and inaccurate.
Turn Indicators
Aircraft use two types of turn indicators: