The pressure altimeter is simply an aneroid barometer that measures the pressure of the atmosphere at the level where the altimeter is located, and presents an altitude indication in feet. The altimeter uses static pressure as its source of operation. Air is more dense at the surface of the Earth than aloft, therefore as altitude increases, atmospheric pressure decreases. This difference in pressure at various levels causes the altimeter to indicate changes in altitude.

The dial of a typical altimeter is graduated with numerals arranged clockwise from 0 to 9 inclusive. The shortest hand indicates altitude in tens of thousands of feet; the intermediate hand in thousands of feet; and the longest hand in hundreds of feet, subdivided into 20-foot increments.

Altitude

Altitude is vertical distance above some point or level used as a reference. There may be as many kinds of altitude as there are reference levels from which altitude is measured and each may be used for specific reasons. Pilots are usually concerned, however, with five types of altitudes:

Absolute Altitude

The vertical distance of an aircraft above the terrain.

Indicated Altitude

That altitude read directly from the altimeter (uncorrected) after it is set to the current altimeter setting.

Pressure Altitude

The altitude indicated when the altimeter setting window (barometric scale) is adjusted to 29.92. This is the standard datum plane, a theoretical plane where air pressure (corrected to 15° C) is equal to 29.92 in. Hg. Pressure altitude is used for computer solutions to determine density altitude, true altitude, true airspeed, etc.

True Altitude

The true vertical distance of the aircraft above sea level - the actual altitude. (Often expressed in this manner; 10,900 feet MSL.) Airport, terrain, and obstacle elevations found on aeronautical charts are true altitudes.

Density Altitude

This altitude is pressure altitude corrected for nonstandard temperature variations. When conditions are standard, pressure altitude and density altitude are the same. Consequently, if the temperature is above standard, the density altitude will be higher than pressure altitude. If the temperature is below standard, the density altitude will be lower than pressure altitude. This is an important altitude because it is directly related to the aircraft takeoff and climb performance.

The airspeed indicator is a sensitive, differential pressure gauge which measures and shows promptly the difference between (1) pitot, or impact pressure, and (2) static pressure, the undisturbed atmospheric pressure at level flight. These two pressures will be 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 difference in pressure is registered by the airspeed pointer on the face of the instrument, which is calibrated in miles per hour (MPH), knots, or both.

Airspeeds

Indicated Airspeed

Indicated airspeed (IAS) is the direct instrument reading obtained from the airspeed indicator, uncorrected for variations in atmospheric density, installation error, or instrument error.

Calibrated Airspeed

Calibrated airspeed (CAS) is indicated airspeed corrected for installation error and instrument error.

True Airspeed

The true airspeed indicator (TAS) is calibrated to indicate true airspeed under standard sea level conditions - that is, 29.92 in. Hg. and 15° C.

Airspeed Indicator Markings

The following is a description of the standard color-code markings on airspeed indicators used on single-engine light airplanes:

• White Arc
  FLAP OPERATING RANGE
• Lower limit of the White Arc
  POWER-OFF STALLING SPEED WITH THE WING FLAPS AND LANDING GEAR IN THE LANDING POSITION
• Upper limit of the White Arc
  MAXIMUM FLAPS EXTENDED SPEED
• Green Arc
  NORMAL OPERATING RANGE
• The lower limit of the Green Arc
  POWER-OFF STALLING SPEED WITH THE WING FLAPS AND LANDING GEAR RETRACTED
• Upper limit of the Green Arc
  MAXIMUM STRUCTURAL CRUISING SPEED
• Yellow Arc
  CAUTION RANGE
• Red Line
  NEVER-EXCEED SPEED (the red line)

Other Airspeed Limitations

There are other important airspeed limitations not marked on the face of the airspeed indicator. These speeds are generally found on placards in view of the pilot and in the Airplane Flight Manual or Pilots Operating Handbook.

The following are abbreviations for performance speeds:

• VA
  Design maneuvering speed.
• VFE
  Maximum flap extended speed.
• VNE
  Never-exceed speed.
• VS
  The stalling speed or the minimum steady flight speed at which the airplane is controllable.
• VS0
  The stalling speed or the minimum steady flight speed in the landing configuration.
• VS1
  The stalling speed or the minimum steady flight speed obtained in a specified configuration.
• VX
  Speed for best angle of climb.
• VY
  Speed for best rate of climb.

The Attitude Indicator shows rotation about both the longitudinal axis to indicate the degree of bank, and about the lateral axis to indicate pitch (nose up, level or nose down). It utilizes the rigidity characteristic of the gyro. It is gimballed to permit rotation about the lateral axis indicating pitch attitude, and about the longitudinal axis to indicate roll attitude. Once powered up, the indicator is maintain in a fixed position no matter what the aircraft attitude may be.

When the wings are aligned with the horizon bar, the aircraft is in level flight. If the wings are above the horizon bar, the aircraft is in a climb. Wings below the horizon bar indicates a decent. The upper blue part of the ball represents the sky. The miniature airplane wings (fixed to the case) represent the wings of the aircraft. In the past, the instrument has been referred to as "an artificial horizon". When in a left turn, the blue portion of the ball will have rolled to the right, as though you were looking at the horizon over the nose of the aircraft. In a right turn, the blue portion will have rolled to the left.

When the attitude indicator is in operation, gyroscopic rigidity maintains the horizon bar parallel to the natural horizon. When the pitch or bank attitude of the aircraft changes, the miniature aircraft, being fixed to the case, moves with it. These movements of the instrument case with respect to the gyro are shown on the face of the instrument as pitch and bank attitude changes of the miniature aircraft with respect to the horizon bar.

Air is sucked through the filter, then through passages in the rear pivot and inner gimbal ring, then into the housing, where it is directed against the rotor vanes through two openings on opposite sides of the rotor. The air then passes through four equally spaced ports in the lower part of the rotor housing and is sucked out into the vacuum pump or venturi tube.

The chamber containing the ports is the erecting device that returns the spin axis to its vertical alignment whenever a processing force, such as friction, displaces the rotor from its horizontal plane. The four exhaust ports are each half-covered by a pendulous vane, which allows discharge of equal volumes of air through each port when the rotor is properly erected. Any tilting of the rotor disturbs the total balance of the pendulous vanes, tending to close one vane of an opposite pair while the opposite vane opens a corresponding amount. The increase in air volume through the opening port exerts a processing force on the rotor housing to erect the gyro, and the pendulous vanes return to a balanced condition.

The principal parts of interest to the pilot are:

• The miniature wings attached to the case remain parallel to the wings of the aircraft.
• The horizon bar which separates the top (light) and bottom (dark) halves of the ball
• The degree marks on the upper periphery of the dial. The first 3 on both sides of center are 10 degrees apart, then 60 degree bank marks, and 90 degree bank arcs.

Fifteen degrees of bank is called a standard rate turn.

Limits

The limits of the instrument refer to the maximum rotation of the gimbals beyond which the gyro will tumble. The older type vacuum-driven attitude indicators have bank limits of approximately 100° to 110°, and pitch limits of 60° to 70°. If, for example, the pitch limits are 60° with the gyro normally erected, the rotor will tumble when the aircraft climb or dive angle exceeds 60°. As the rotor gimbal hits the stops, the rotor processes abruptly, causing excessive friction and wear on the gimbals. The rotor will normally process back to the horizontal plane at a rate of approximately 8° per minute. The limits of more recently developed vacuum-driven attitude indicators exceed those given above.

Caging

Many gyros include a manual caging device, used to erect the rotor to its normal operating position prior to flight or after tumbling, and a flag to indicate that the gyro must be uncaged before use. Turning the caging knob prevents rotation of the gimbals and locks the rotor spin axis in its vertical position. Because the rotor is spinning as long as vacuum power is supplied, normal maneuvering with the gyro caged wears the bearings unnecessarily. Therefore, the instrument should be left uncaged in flight unless the limits are to be exceeded.

In the caged position, the gyro is locked with the miniature aircraft showing level flight, regardless of aircraft attitude. When uncaged in flight, in any attitude other than level flight, the gyro will tend to remain in an unlevel plane of rotation with the erecting mechanism attempting to restore the rotor to a horizontal plane. Therefore, should it be necessary to uncage the gyro in flight, the actual aircraft attitude must be identical to the caged attitude (that is, straight and level), otherwise, the instrument will show false indications when first uncaged.

Errors

Errors in the indications presented on the attitude indicator will result from any factor that prevents the vacuum system from operating within the design suction limits, or from any force that disturbs the free rotation of the gyro at design speed. Some errors are attributable to manufacturing and maintenance. These include poorly balanced components, clogged filters, improperly adjusted valves, and pump malfunction. Such errors can be minimized by proper installation and inspection.

Other errors, inherent in the construction of the instrument, are caused by friction and worn parts. These errors, resulting in erratic precession and failure of the instrument to maintain accurate indications, increase with the life of the instrument.

Another group of errors, associated with the design and operating principles of the attitude indicator, are induced during normal operation of the instrument. A skidding turn moves the pendulous vanes from their vertical position, processing the gyro toward the inside of the turn. After return of the aircraft to straight-and-level, coordinated flight, the miniature aircraft shows a turn in the direction opposite the skid. During a normal turn, movement of the vanes by centrifugal force causes precession of the gyro toward the inside of the turn.

Errors in both pitch and bank indications occur during normal coordinated turns. These errors are caused by the movement of the pendulous vanes by centrifugal force, resulting in the precession of the gyro toward the inside of the turn. The error is greatest in a 180° steep turn. If, for example, a 180° steep turn is made to the right and the aircraft is rolled out to straight-and-level flight by visual references, the miniature aircraft will show a slight climb and turn to the left. This precession error, normally 3° to 5°, is quickly corrected by the erecting mechanism. At the end of a 360° turn, the precession induced during the first 180° is canceled out by precession in the opposite direction during the second 180° of turn. The slight precession errors induced during the roll-out are corrected immediately by pendulous vane action.

Acceleration and deceleration also induce precession errors, depending upon the amount and extent of the force applied. During acceleration the horizon bar moves down, indicating a climb. Control applied to correct this indication will result in a pitch attitude lower than the instrument shows. The opposite error results from deceleration. Other errors, such as "transport precession" and "apparent precession," relate to rotation of the earth and are of importance to pilots and navigators concerned with high speed and long-range flight.

The application of the foregoing errors as they affect instrument interpretation will be treated later in Chapter V, "Attitude Instrument Flying - Airplanes."

Electric Attitude Indicators.

In the past, suction-driven gyros have been favored over the electric for light aircraft because of the comparative simplicity and lower cost. However, the increasing importance of the attitude indicator has stimulated development of improved electric-driven gyros suited to light plane installation. Improvements relating to basic gyro design factors, easier readability, erection characteristics, reduction of induced errors, and instrument limitations are reflected in several available types. Depending upon the particular design improvements, the details among different instruments will vary as to the instrument display and cockpit controls. All of them present, to a varying degree, the essential pitch and bank information for attitude reference.

Electric gyros may be remotely located, with the gyro assembly mounted at some convenient location other than behind the instrument panel, and with the indicator assembly on the instrument panel driven through a servo motor. Another type is a simpler unit incorporating the gyroscope motor in the instrument case integral with the indicator assembly. The H-6B attitude indicator and J-8 gyro-horizon are representative of this type.

The turn coordinator is an aircraft instrument which displays to a pilot information about the rate of turn, rate of roll, and the 'quality' or 'coordination' of the turn. The turn coordinator was developed to replace the older turn and bank indicator, which displayed rate and quality of turn but not rate of roll.

The turn coordinator is, like the turn and bank instrument it replaced, a gyroscopic instrument. An internal gyroscope, typically electrically driven, spins at approximately 20,000 rpm. As the aircraft turns, the principle of gyroscopic inertia causes the gyro to tilt. This tilting force works against a spring; thus, a slow rate of turn deflects the gyro slightly while a higher rate of turn deflects it more. The gyroscope's movements are linked to an indicator on the front face of the turn coordinator.

The indicator looks like a little airplane seen from behind: when the airplane is level, the rate of turn is zero; when it is tilted, the amount and direction of tilt show the pilot the rate of turn. The wings of the symbolic airplane line up on white tick marks at the level position to indicate zero rate of turn. There is another set of tick marks below the level pair. When the symbolic aircraft is tilted so as to align with one of the tick marks, the aircraft is said to be turning at standard rate of turn, which is 3 degrees of heading change per second. This is often marked on the face plate of the instrument as '2 minutes', since it takes two minutes to complete a 360° heading change when turning at 3°/s.

The quality of turn is indicated by an inclinometer. This is a glass tube mounted on the face of the instrument, below the symbolic airplane. It is actually a completely separate instrument. The inclinometer consists of a glass tube filled with kerosene, and a steel ball. The tube is curved such that its center is the lowest point, and each end is higher. Normally, the ball will then sit in the center position of the tube, which represents a 'coordinated' turn. This position is marked by two vertical wires on the tube. The ball is said to be 'centered' when it sits perfectly evenly between the two wires.

The ball is used typically to tell the pilot the correct amount of rudder input is being applied, usually during rolls and turns. If the rudder input produces a coordinated turn, the ball will remain centered during a roll maneuver. If the ball deflects into the roll, the rudder input was insufficient, indicating a slip; if it deflects opposite the direction of the roll, the rudder input was excessive, indicating a skid. The old adage "step on the ball" refers to the pilot having to apply rudder in the same direction as the ball is deflected in order to return the aircraft to coordinated flight.

The turn coordinator differs from the older turn and bank indicator in that the turn coordinator has the gyro mounted at a 30-degree tilt. This allows the turn coordinator to respond to roll as well as turn. When the aircraft is rolling, the turn coordinator deflection is proportional to the rate of roll and not the rate of turn. Once the roll has stopped, the turn coordinator deflection will settle back to an amount which indicates the rate of turn. Pilots who are unfamiliar with this principle sometimes have difficulty using the turn coordinator properly, as they may see a roll indication and interpret it as a rate of turn.

The turn coordinator should be used as a performance instrument when the attitude indicator has failed. Called 'partial panel' operations, this can be unnecessarily difficult or even impossible if either (1) the pilot does not understand that the instrument is showing roll rates at some times and turn rates at others, and (2) if the internal dash pot is worn out. In the latter case the instrument is said to be under damped; in turbulence it will indicate large full-scale deflections to the left and right, all of which are roll rate responses. In this condition it may not be possible for the pilot to maintain control of the aircraft in partial-panel operations in instrument meteorological conditions.

The magnetic compass, which is the only direction-seeking instrument in the airplane, is simple in construction. It contains two steel magnetized needles fastened to a float around which is mounted a compass card. The needles are parallel, with their north-seeking ends pointed in the same direction. The compass card has letters for cardinal headings, and each 30° interval is represented by a number, the last zero of which is omitted. For example, 30° would appear as a 3 and 300° would appear as 30. Between these numbers, the card is graduated for each 5°.

The float assembly is housed in a bowl filled with acid-free white kerosene. The purposes of the liquid are to dampen out excessive oscillations of the compass card and relieve by buoyancy part of the weight of the float from the bearings.
Jewel bearings are used to mount the float assembly on top of a pedestal. A line (called the lubber line) is mounted behind the glass of the instrument that can be used for a reference line when aligning the headings on the compass card.

Variation

Although the magnetic field of the Earth lies roughly north and south, the Earths magnetic poles do not coincide with its geographic poles, which are used in the construction of aeronautical charts. Consequently, at most places on the Earths surface, the direction-sensitive steel needles which seek the Earths magnetic field will not point to True North but to Magnetic North. The angular difference between True North and the direction indicated by the magnetic compass - excluding deviation error - is variation. Variation is different for different points on the Earths surface and is shown on the aeronautical charts as broken lines connecting points of equal variation. These lines are isotonic lines. The line where the magnetic variation is zero is an agonic line.

Deviation

Actually, a compass is very rarely influenced solely by the Earths magnetic lines of force. Magnetic disturbances from magnetic fields produced by metals and electrical accessories in an aircraft disturb the compass needles and produce an additional error. The difference between the direction indicated by a magnetic compass not installed in an airplane, and one installed in an airplane, is deviation.
Although compensating magnets on the compass are adjusted to reduce this deviation on most headings, it is impossible to eliminate this error entirely on all headings. Therefore, a deviation card, installed in the cockpit in view of the pilot, enables the pilot to maintain the desired magnetic headings.

Using the Magnetic Compass

Since the magnetic compass is the only direction-seeking instrument in most airplanes, the pilot must be able to turn the airplane to a magnetic compass heading and maintain this heading. It will help to remember the following characteristics of the magnetic compass which are caused by magnetic dip. These characteristics are only applicable in the Northern Hemisphere. In the Southern Hemisphere the opposite is true.

If on a northerly heading and a turn is made toward east or west, the initial indication of the compass lags or indicates a turn in the opposite direction. This lag diminishes as the turn progresses toward east or west where there is no turn error.

If on a southerly heading and a turn is made toward the east or west, the initial indication of the compass needle will indicate a greater amount of turn than is actually made. This lead also diminishes as the turn progresses toward east or west where there is no turn error.

If a turn is made to a northerly heading from any direction, the compass indication when approaching north lags behind the turn. Therefore, the roll out of the turn is made before the desired heading is reached.

If a turn is made to a southerly heading from any direction, the compass indication when approaching southerly headings leads behind the turn. Therefore, the roll out is made after the desired heading is passed. The amount of lead or lag is maximum on the north-south headings and depends upon the angle of bank used and geographic position of the airplane with regard to latitude.

When on an east or west heading, no error is apparent while entering a turn to north or south; however, an increase in airspeed or acceleration will cause the compass to indicate a turn toward north; a decrease in airspeed or acceleration will cause the compass to indicate a turn toward south.

If on a north or south heading, no error will be apparent because of acceleration or deceleration.

The magnetic compass should be read only when the aircraft is flying straight and level at a constant speed. This will help reduce errors to a minimum.

If the pilot thoroughly understands the errors and characteristics of the magnetic compass, this instrument can become the most reliable means of determining headings.

In this system, the impact air pressure (air striking the airplane because of its forward motion) is taken from a pitot tube, which is mounted either on the leading edge of the wing or on the nose, and aligned to the relative wind. On certain aircraft, the pitot tube is located on the vertical stabilizer. These locations provide minimum disturbance or turbulence caused by the motion of the airplane through the air. The static pressure (pressure of the still air) is usually taken from the static line attached to a vent or vents mounted flush with the side of the fuselage. Airplanes using a flush-type static source, with two vents, have one vent on each side of the fuselage. This compensates for any possible variation in static pressure due to erratic changes in airplane attitude.

The openings of both the pitot tube and the static vent should be checked during the preflight inspection to assure that they are free from obstructions. Clogged or partially clogged openings should be cleaned by a certificated mechanic. Blowing into these openings is not recommended because this could damage any of the three instruments.

• Master Switch - Most often actually two separate switches, the Battery Master and the Alternator Master. The Battery Master activates a relay (sometimes called the battery contactor) which connects the output of the battery to the aircraft's main electrical bus. The alternator master activates the alternator by applying power to the alternator field circuit. These two switches provide electrical power to all the systems in the aircraft.
• Throttle - Sets the desired engine power level. The throttle controls the volume of fuel/air mixture delivered to the cylinders.
• Pitch Control - Adjusts the Constant Speed Unit, which turn adjusts the propeller pitch & regulates the engine load as necessary to maintain the set R.P.M.
• Mixture Control - Sets the amount of fuel added to the intake airflow. At higher altitudes the air pressure (and therefore the oxygen level) declines so the fuel volume must also be reduced to give the correct air/fuel mixture. This process is known as "leaning".
• Ignition Switch - Activates the magnetos by opening the grounding or 'p-lead' circuit; with the p-lead ungrounded the magneto is free to send its high-voltage output to the spark plugs. In most aircraft the ignition switch also applies power to the starter motor during engine start. In piston aircraft engines, the battery does not generate the spark for combustion. This is accomplished using devices called magnetos. Magnetos are connected to the engine by gearing. When the crankshaft turns, it turns the magnetos which mechanically generate voltage for spark. In the event of an electrical failure, the engine will continue to run. The Ignition Switch has the following positions:
  Off - Both magnetos are connected to electrical ground.
  Right - The right magneto is connected to its spark plugs. The left magneto is grounded.
  Left - The left magneto is connected to its spark plugs. The right magneto is grounded.
  Both - Both magnetos are connected to their spark plugs. This is the normal operating configuration.
  Start - The pinion gear on the starter motor is engaged with the flywheel and the starter motor runs to turn the engine over.
• Tachometer - A gauge to indicate engine speed in revolutions per minute (RPM) or percentage of maximum.
• Manifold Pressure Gauge – Used to measure the pressure in the intake manifold.
• Oil Temperature Gauge - Indicates the engine oil temperature.
• Oil Pressure Gauge - Indicates the supply pressure of the engine lubricant.
• Exhaust Gas Temperature Gauge – Indicates the temperature of the exhaust gas just after combustion. Used to set the fuel/air mixture (leaning) correctly.
• Cylinder Head Temperature Gauge - Indicates the temperature of at least one of the cylinder heads. Used to set the fuel/air mixture.
• Carburetor Heat Control - Controls the application of heat to the carburetor venturi area to remove or prevent the formation of ice in the throat of the carburetor as well as bypassing the air filter in case of impact icing.
• Alternate Air - Bypasses the air filter on a fuel-injected engine.

An avionics instrument used in aircraft navigation to determine an aircraft's lateral position in relation to a track. If, for example, the location of the aircraft is to the left of course, the needle deflects to the right, showing the direction to steer to correct for course deviations. Correction is made until the needle centers, and the aircraft is on course. The deflection of the needle is proportional to the course deviation, but sensitivity and vary depending on the system being used.

When used with a GPS it shows actual distance left or right of the programmed course line. Sensitivity is usually programmable or automatically switched, but 5 nautical miles deviation at full scale is typical for en route operations. Approach and terminal operations have a higher sensitivity up to frequently .3 nautical miles at full scale.

When used with a VOR or VORTAC the course line is selected by turning an "OMNI Bearing Selector" or "OBS" knob usually located in the lower left of the instrument. It then shows the number of degrees deviation from the desired course to the Navigational aid (navaid), and is used to intercept and fly TO or FROM any of the 360 compass "radials" that emanate from the navaid. Sensitivity is 10 degrees at full scale. (See Using a VOR for usage during flight.)

When used for instrument approaches using a LDA or ILS the OBS knob does not function, since the course line is usually the runway heading, and is determined by the ground transmitter. Many CDI's also incorporate a second, horizontal, needle. This is used to provide vertical guidance when used with a precision ILS approach, and the decent course line (usually 3 degrees) is also determined by the transmitter located on the ground.

A CDI is normally not used with an ADF, which receives information from a normal AM radio station or an NDB. An ADF indicator or Radio Magnetic Indicator (RMI) is used instead, both of which provide direction or heading information.

Older CDI's were designed to receive a signal from a VOR, LDA or ILS receiver. These receivers outputted a signal composed of two AC voltages. An internal converter converted the signal to drive the needle left or right. These CDI units are not compatible with GPS units. Many modern VOR/LDA/ILS receivers include the converter, and output a differential voltage to drive a "converterless" CDI. The CDI's without a converter are compatible with aviation GPS units.

A transponder-based radio navigation technology that measures distance by timing the propagation delay of VHF or UHF radio signals.

It was invented by Edward George "Taffy" Bowen whilst employed as Chief of the Division of Radiophysics of the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Australia. Another Australian world-first, engineered version of the system was deployed by Amalgamated Wireless Australasia Limited in the early 1950's operating in the 200 MHz VHF band. This Australian domestic version was referred by the Federal Department of Civil Aviation as DME(D) (or DME Domestic), and the later international version adopted by ICAO as DME(I).

DME is similar to Secondary Radar, except in reverse. The system was a post-war development of the IFF (Identification Friend or Foe) systems of World War II. To maintain compatibility, DME is functionally identical to the distance measuring component of TACAN.

A flight deck instrument display system in which the display technology used is electronic rather than electromechanical. Although cathode ray tube (CRT) displays were used at first, liquid crystal displays (LCD) are now more common.

The complex electromechanical attitude director indicator (ADI) and horizontal situation indicator (HSI) were the first candidates for replacement by EFIS. However, there are now few flight deck instruments for which no electronic display is available.

EFIS installations vary greatly. A light aircraft might be equipped with one display unit, on which are displayed flight and navigation data. A wide-body aircraft is likely to have six or more display units.

Typical EFIS displays and controls can be seen at this B737 technical information web site. The equivalent electromechanical instruments are also shown here.

An EFIS installation will have the following components:

• displays
• controls
• data processors

A basic EFIS might have all these facilities in the one unit.

Display units

On the flight deck, the display units are the most obvious parts of an EFIS system, and are the features which give rise to the name "glass cockpit". The display unit taking the place of the ADI is called the primary flight display (PFD). If a separate display replaces the HSI, it is called the navigation display.

The names Electronic Attitude Director Indicator and Electronic Horizontal Situation Indicator are used by some manufacturers.[1] However, a simulated ADI is only the centerpiece of the PFD. Additional information is both superimposed on and arranged around this graphic.

Multi-function displays can render a separate navigation display unnecessary. Another option is to use one large screen to show both the PFD and navigation display.

The PFD and navigation display (and multi-function display, where fitted) are often physically identical. The information displayed is determined by the system interfaces where the display units are fitted. Thus, spares holding is simplified: the one display unit can be fitted in any position.

LCD display units generate less heat than CRTs; an advantage in a congested instrument panel. They are also lighter, and occupy a lower volume.

Control panels

The pilots are provided with controls, with which they select display range and mode (for example, map or compass rose) and enter data (such as selected heading).

Where inputs by the pilot are used by other equipment, data buses broadcast the pilot's selections so that the pilot only needs to enter the selection once. For example, the pilot selects the desired level-off altitude on a control unit. The EFIS repeats this selected altitude on the PFD and by comparing it with the actual altitude (from the air data computer) generates an altitude error display. This same altitude selection is used by the automatic flight control system to level off, and by the altitude alerting system to provide appropriate warnings.

Data processors

The EFIS visual display is produced by the symbol generator. This receives data inputs from the pilot, signals from sensors, and EFIS format selections made by the pilot. The symbol generator can go by other names, such as display processing computer, display electronics unit, etc.

The symbol generator does more than generate symbols. It has (at the least) monitoring facilities, a graphics generator and a display driver.[2] Inputs from sensors and controls arrive via data buses, and are checked for validity. The required computations are performed, and the graphics generator and display driver produce the inputs to the display units.

An integrated system used in modern aircraft to provide aircraft crew with aircraft engines and other systems instrumentation and crew annunciations.

Components

EICAS typically includes instrumentation of various engine parameters, including for example RPMs, temperature values, fuel flow and quantity, oil pressure etc. Typical other aircraft systems monitored by EICAS are for example hydraulic, pneumatic, electrical, deicing, environmental and control surface systems. As EICAS has high connectivity, it provides data acquisition and routing.

EICAS is a key function of a Glass cockpit system, which replaces all analog gages with software-driven electronic displays. Most of the display area is used for navigation and orientation displays, but one display or a section of a display is set aside specifically for EICAS.

The Crew Alerting System (CAS) is used in place of the annunciator panel on older systems. Rather than signaling a system failure by turning on a light behind a translucent button, failures are shown as a list of messages in a small window near the other EICAS indications. The CAS system is, in essence, an electronic version of the Idiot light.

The Garmin G1000 is an integrated flight instrument system manufactured by Garmin typically composed of two display units, one serving as a primary flight display, and one as a multi-function display. It serves as a replacement for most conventional flight instruments and avionics.

An aircraft with a basic Garmin G1000 installation contains two LCD displays (one acting as the primary flight display and the other as the multi-function display) as well as an integrated communications panel that fits between the two.

Beyond that, additional features are found on newer and larger G1000 installations, such as in business jets. This includes:

• A third display unit, to act as a co-pilot PFD
• An alphanumeric keyboard
• An integrated flight director/autopilot (without it, the G1000 interfaces with an external autopilot)

Depending on the airplane manufacturer and whether or not a GFC 700 autopilot is installed, the G1000 system will be comprised of either 2 GDU 1040 displays (no autopilot), a GDU 1040 PFD/GDU 1043 MFD (GFC 700 autopilot installed), or a GDU 1045 PFD/GDU 1045 MFD (GFC 700 autopilot installed with VNAV).

The GDU 1040 is the standard base bezel with no autopilot/flight director mode selection keys below the heading bug. The GDU 1043 has autopilot/flight director keys for all GFC 700 modes except VNAV. The GDU 1045 is essentially identical to the GDU 1043 except for the addition of an autopilot/flight director mode for VNAV. Depending on how the units are installed, an MFD failure may, or may not, have an impact on autopilot or flight director use. If a GDU 1040 is used as a PFD in an airplane equipped with a GFC 700 autopilot, a failure of the MFD (which houses the autopilot mode selection keys) will leave the autopilot engaged, but the modes cannot be changed because no autopilot keys are present on the PFD. But, if an MFD failure occurs in an airplane with the GFC 700 autopilot and either a GDU 1043 or a GDU 1045 bezel installed as a PFD, the pilot will have full use of the autopilot through the keys on the PFD.

Both the PFD and MFD each have two slots for SD memory cards. The top slot is used to update the Jeppesen aviation database every 28 days. The aviation database must be current to use GPS for navigation during IFR instrument approaches. The bottom slot houses the World terrain and Jeppesen obstacle databases. While terrain information rarely changes or needs to be updated, obstacle databases can be updated every 56 days through a subscription service. The top card can be removed from the G1000 system following an update, but the bottom card must stay in both the PFD and MFD to ensure accurate terrain awareness and TAWS-B information.

Primary flight display (PFD) Screenshot of the PFD on the G1000The primary flight display shows the basic flight instruments, such as the airspeed indicator, the altimeter, the heading indicator, and course deflection indicator. A small map called the "inset map" can be enabled in the corner. The buttons on the PFD are used to set the squawk code on the transponder. The PFD can also be used for entering and activating flight plans.

Multi-function display (MFD) The MFD usually shows engine instrumentation and a moving map.The multi-function display typically shows a moving map on the right side, and engine instrumentation on the left. Most of the other screens in the G1000 system are accessed by turning the knob on the lower right corner of the unit. Screens available from the MFD other than the map include the setup menus, information about nearest airports and NAVAIDs, Mode S traffic reports, terrain awareness, XM radio, flight plan programming, and GPS RAIM prediction.

The G1000 system consists of several integrated components which sample and exchange data or display information to the pilot.

GDU 1040 Display

The GDU 1040 Display acts as the primary source of flight information for the pilot. Each display can interchangeably serve as a primary flight display (PFD) or multi-function display (MFD). The wiring harness within the aircraft specifies which role each display is in by default. All of the displays within an aircraft are interconnected using a high-speed Ethernet data bus.

In normal operation, the display in front of the pilot is the PFD and will provide aircraft attitude, airspeed, altitude, vertical speed, heading, rate-of-turn, slip-and-skid, navigation, transponder, inset map view (containing map, traffic, and terrain information), and systems annunciation data. The second display, typically positioned to the right of the PFD, operates in MFD mode and provides engine instrumentation and a moving map display. The moving map can be replaced or overlaid by various other types of data, such as satellite weather, checklists, system information, waypoint information, weather sensor data, and traffic awareness information.

Both displays provide redundant information regarding communications and navigation radio frequency settings even though each display is only paired with one GIA 63 Integrated Avionics Unit. In the event of a single display failure, the remaining display will adopt a combined "reversionary mode" and automatically become a PFD combined with engine instrumentation data. A red button labeled "reversionary mode" or "display backup," located on the instrument panel, is also available to the pilot to select this mode manually if desired.

The GDU 1043, a variant of the GDU 1040, has extra keys on the face for control of Garmin's GFC 700 autopilot.

GMA 1347 Audio Panel

The GMA 1347 panel provides buttons for selecting what audio sources are heard by each member of the cockpit. It also includes a button for forcing the integrated cockpit into a fail-safe mode.

GIA 63 Integrated Avionics Unit

The GIA 63 unit is a combined communications and navigation radio. It provides a two-way VHF communications transceiver, a VHF navigation receiver with glideslope, a GPS receiver, and a variety of supporting processors. Each unit is paired with a GDU 1040 display, which acts as a controlling unit.

GDC 74A Air Data Computer

The GDC 74A computer replaces the internal components of the pitot-static system in traditional aircraft instrumentation. It measures airspeed, altitude, vertical speed, and outside air temperature. This data is then provided to all the displays and integrated avionics units.

GRS 77 Attitude and Heading Reference System (AHRS)

The GRS 77 system measures aircraft attitude, rate of turn, and slip and skid indications. This data is then provided to all the integrated avionics units. Unlike many competing systems, the AHRS can be rebooted and recalibrated in flight during turns of up to 20 degrees.

GMU 44 Magnetometer

The GMU 44 Magnetometer measures aircraft heading and is a digital version of a traditional compass.

GTX 32/33 Transponder

Either the GTX 32 or GTX 33 transponder can be used in the G1000 system although the GTX 33 is far more common. The GTX 32 provides standard mode-C replies to ATC interrogations while the GTX 33 provides mode-S bidirectional communications with ATC and therefore can indicate traffic in the area as well as announce itself spontaneously via "squitting" without prior interrogation.

GEA 71 Engine/Airframe Unit

The GEA 71 unit measures engine RPM, manifold pressure, oil temperature, cylinder head temperature, exhaust gas temperature, and fuel level in each tank. This data is then provided to the integrated avionics units.

Backup systems

As a condition of certification, all aircraft utilizing the G1000 integrated cockpit must have a redundant airspeed indicator, altimeter, artificial horizon, and magnetic compass. In the event of a failure of the G1000 instrumentation, these backup instruments become primary.

In addition, a secondary power source is required to power the G1000 instrumentation for a limited time in the event of a failure of the aircraft's alternator and primary battery.

A Machmeter is an aircraft pitot-static system flight instrument that shows the ratio of the true airspeed to the speed of sound, a dimensionless quantity called Mach number. This is shown on a Machmeter as a decimal fraction. An aircraft flying at the speed of sound is flying at a Mach number of one, expressed as "Mach 1.0".

As an aircraft in transonic flight approaches the speed of sound, it first reaches its critical mach number, where air flowing over low-pressure areas of its surface locally reaches the speed of sound, forming shock waves. The indicated airspeed for this condition changes with ambient temperature, which in turn changes with altitude. Therefore, airspeed is not entirely adequate to warn the pilot of the impending problems. Mach number is more useful, and most high-speed aircraft are limited to a maximum Mach number, also known as as a "Mach limit".

For example, if the Mach limit is Mach 0.83, at 30,000 feet where the speed of sound under standard conditions is 590 knots, the corresponding true airspeed is 489 knots. The speed of sound varies with air temperature, so at Mach 0.83 at 10,000 feet, where the air is much warmer, the corresponding true airspeed would be 530 knots.

A radar altimeter, radio altimeter, low range radio altimeter (LRRA) or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft. This type of altimeter provides the distance between the plane and the ground directly below it, as opposed to a barometric altimeter which provides the distance above a pre-determined datum, usually sea level.

Radar altimeters are frequently used by commercial aircraft for approach and landing, especially in low-visibility conditions (see instrument flight rules, Autoland) and also automatic landings, allowing the autopilot to know when to begin the flare manuever.

In civil aviation applications, radio altimeters generally only give readings up 2,500' above ground level (AGL).

(TCAS is not to be confused with Radar Altimeter.)
(TCAS uses a transponder with an encoding altimeter, or ADC for IFF up to the aircraft's maximum altitude.)
Today, almost all airliners are equipped with at least one and usually several radio altimeters, as they are essential to autoland capabilities (determining height through other methods such as GPS is not permissible under current legislation). Even older airliners from the 1960's, such as Concorde and the BAC 1-11 were so equipped and today even smaller airliners in the sub-50 seat class are supplied with them (such as the ATR 42 and BAe Jetstream series).

Radio altimeters are an essential part in ground proximity warning systems (GPWS), warning the pilot if the aircraft is flying too low or descending too quickly. However, radar altimeters cannot see terrain directly ahead of the aircraft, only that directly below it; such functionality requires either knowledge of position and the terrain at that position or a forward looking terrain radar which uses technology similar to a radio altimeter.

It is interesting to note that whilst they are called altimeters, the information they provide is not called altitude in aviation; altitude is specifically height above sea level which is usually obtained from a pressure altimeter. The term height when used in aviation refers to the height above the terrain directly below the aircraft, that from a radio altimeter, in order to avoid confusion. "Radar-altitude" is used on some instrumentation but for communication purposes, the term height is always used.

The Traffic alert and Collision Avoidance System (or TCAS) is a computerized avionics device which is designed to reduce the danger of mid-air collisions between aircraft. It monitors the airspace around an aircraft for other aircraft equipped with a corresponding active transponder, independent of air traffic control, and warns pilots of the presence of other transponder-equipped aircraft which may present a threat of mid-air collision (MAC). It is an implementation of the Airborne Collision Avoidance System mandated by International Civil Aviation Organization to be fitted to all aircraft over 5700 kg or authorized to carry more than 19 passengers.

Official definition from PANS-ATM (Nov 2007): ACAS / TCAS is an aircraft system based on secondary surveillance radar (SSR) transponder signals which operates independently of ground-based equipment to provide advice to the pilot on potential conflicting aircraft that are equipped with SSR transponders.

In modern glass cockpit aircraft, the TCAS display may be integrated in the Navigation Display; in older glass cockpit aircraft and those with mechanical instrumentation, a TCAS display replaces the mechanical Instantaneous Vertical Speed Indicator (which indicates the rate with which the aircraft is descending or climbing).