Flight Instruments

Air Data System

Pitot/Static Systems

The air data system provides pitot and static air pressure information from three electrically heated pitot/static tubes located on the forward fuselage. The Captain and standby pitot/static systems are supplied by pitot tubes located on the left side of the forward fuselage while the First Officer system is supplied by a tube located on the right side. The Captain system incorporates an air data computer (ADC) to drive the related flight instruments and to supply information to the EFIS and autopilot systems. The First Officer and standby systems utilize conventional pitot/static instruments. The pitot/static tubes for the Captain and First Officer systems each incorporate two integral static ports designated as S1 and S2. The static port pairs (S1–S1, S2–S2) are interconnected through shutoff valves to provide averaged static pressure sensing and eliminate pressure differences between the systems. The S1 system supplies the ADC while the S2 system supplies the First Officer static instruments and pressurization controller. In case of static line leak, the static systems can be isolated from each other by closing the shutoff valves.

Air Data Computer

The ADC senses air temperature, and also converts pitot and static pressures obtained from the Captain pitot tube to electrical signals. The ADC uses this information for calculating altitude, vertical speed, indicated airspeed, true airspeed, VMO, and static air temperature (SAT) which are supplied to the following instruments and systems:

• Captain flight instruments:

a. Altimeter

b. Vertical speed indicator

c. Airspeed indicator

• Both AHRS

• EFIS display processor unit

• Flight director and autopilot



• Flight data recorder

• WAS (for overspeed warning when VMO is exceeded by two knots)

• Rudder limiter

A separate airspeed sensor in the standby pitot/static system also provides airspeed signals to the rudder limiter system and both AHRS for comparison with the ADC airspeed signals. The ADC is supplied from the Left Avionics Start Bus.

Flight Instruments

Servo Instruments

The Captain flight instruments are servo (electromechanical) indicators and are incapable of indicating any information without control of the ADC and power from the Left Avionics Start Bus. Loss of instrument power or the ADC control signal causes failure flags to appear on the face of the affected instruments.


The Captain altimeter presents altitude indications on a drumcounter display in combination with a sweep pointer. This altimeter also supplies altitude information to the altitude preselector/alerter and the ATC transponders. This instrument may be tested by pressing the TEST button located in the lower left corner of the instrument. The First Officer and standby altimeters are conventional pneumatic instruments. The First Officer altimeter is equipped with a vibrator supplied from the Right Essential Bus to prevent instrument seizure due to rapid pressure or temperature changes.

Airspeed Indicators

The Captain airspeed indicator displays indicated airspeed and VMO. The warning annunciator system generates VMO overspeed alerts based on indicated airspeed signals received from the ADC. This instrument may be tested by pressing the TEST button located in the lower left corner of the instrument. The First Officer and standby airspeed indicators are conventional pitot/static instruments. The First Officer airspeed indicator displays indicated airspeed and VMO and also provides backup airspeed data to the WAS for VMO overspeed alerting. The standby airspeed indicator only provides indicated airspeed indications.

RA/Vertical Speed Indicators

The RA/VSI indicator is an integral part of the TCAS system as well as a vertical speed indicator. Power for these is supplied through the Left Avionic Start Bus while the First Officer VSI is a conventional pneumatic instrument. SAT and TAS Indications Static air temperature is supplied to the ADC from the total air temperature probe located on the nose. Static air temperature and true airspeed are displayed on each EHSI. Altitude Preselector/Alerter (APA) The APA located on the center glareshield facilitates setting target altitudes for use by the AP/FD systems and also provides visual and aural alerting when the aircraft approaches or deviates from the selected altitude. The APA is supplied current altitude information from the Captain altimeter, and the desired alert altitude is selected by the adjacent ALT SET knob. The aural warning (C-Chord), produced by the WAS, is momentarily activated simultaneously with illumination of the amber ALT ALERT light on the APA panel when the aircraft initially approaches within 1,000 feet of the selected altitude. The amber ALT ALERT light extinguishes when the aircraft approaches within 200 feet of the selected altitude. Subsequent deviations greater than 200 feet from the selected altitude trigger visual and aural alerting again.

Attitude Heading Reference Systems (AHRS)

Primary attitude and heading information is obtained from two independent attitude and heading reference systems. A standby horizon and standby compass provide backup attitude and heading information. An AHRS system determines aircraft attitude via an inertia sensor which replaces the conventional attitude instruments. This is accomplished by an inertial measurement unit which consists of an attitude-heading computer that monitors accelerometers and three non-gimballed spinning masses installed at perpendicular angles to each other. Magnetic heading information is received from one of the remote compass systems and automatically compensated for errors induced by aircraft accelerations and banking. The inertial measurement unit and the related cooling fan, magnetic compensator, power supply, etc. are combined to form the attitude heading reference unit (AHRU). The AHRU and the related magnetic sensor and control panels comprise a fully independent AHRS. AHRS is commonly termed a “strapdown” system because each AHRU is securely fastened (strapped down) near the aircraft center of gravity and aligned with the aircraft axes. The main power supply to each AHRS is obtained from the respective Avionic Bus and controlled by the associated AVION switch, and the 26v Inverter Bus supplies power for the nav reference signal to both AHRS. When the AVION switch is selected off, power is automatically transferred to the respective Hot Battery Bus for up to 11 minutes to sustain AHRS initialization.

Each AHRS receives the following data:

• Heading information from the respective flux detector unit (magnetic sensor) located in the corresponding wing tip

• Airspeed from the standby IAS

• True airspeed and vertical speed information from the ADC

• Flight/ground information from the weight-on-wheels system

Each AHRS supplies attitude and heading data to:

• The on-side display processor unit (DPU)

• Cross-side RMI

• Flight data recorder

• Autopilot/flight director

• Multiprocessor unit (MPU)

The Captain AHRS also provides antenna stabilization signals to the weather radar.

AHRS Initialization

The attitude heading computer requires approximately 70 seconds from initial power application to initialize. During initialization, the aircraft must not be moved and the hydraulic pump should not be operated (to prevent voltage transients). During initialization, the EHSI compass cards rotate clockwise from North. Initialization is complete when all of the following occurs:

• The blue sky and brown earth appear

• The red ATT and HDG flags disappear from the EADI and EHSI, respectively

• The EHSI displays the current aircraft heading

• The EADI displays aircraft attitude HDG SLAVE Pushbutton

Each AHRS incorporates a HDG SLAVE pushbutton to rapidly slave the related compass cards to the current heading detected by the related flux detector. The pushbutton is inhibited when the autopilot is engaged and should not be used during flight in turbulence.

AHRS Failures

AHRS failure may occur due to malfunctions in attitude or heading sensing, or both. Attitude failures are indicated on the affected EADI by:

• Removal of the blue sky and brown earth

• Flashing red ATT for 10 seconds followed by steady illumination

• Flashing red FD appears for 10 seconds on both EADIs if the left AHRS fails

Heading failures are indicated on the affected EHSI by:

• Flashing red HDG for 10 seconds followed by steady illumination

• Display of a red OFF flag on the cross-side RMI compass card

AHRS failures do not trigger master warning alerting. AHRS failures are remedied by selecting X SIDE DATA on the respective EFIS panel. This action selects the opposite AHRS for attitude and heading data. The affected EFIS displays yellow XATT and XHDG on the EADI and EHSI, respectively.

Standby Attitude Indicator

A conventional attitude gyro, supplied from the Emergency Avionic Bus via control of the ESS AVION switch, provides standby attitude information after failure of both AHRS. The gyro may be fast-erected (caged) by pulling the adjacent knob. There is no adjustment for parallax. Instrument failure or power loss is indicated by display of a red GYRO flag.

Standby Compass

A conventional magnetic compass provides emergency heading information in the event of failure of both AHRS. The standby compass is susceptible to errors induced by turns and accelerations and appropriate procedures must be used. A compass deviation card is located on the center windshield post and is stowed when not in use by folding it forward.

Electronic Flight Instrument System (EFIS)

The Collins Proline II EFIS system processes data supplied from the AHRS, airspeed indicators, navigation receivers, and autopilot/flight director systems for presentation on the electronic flight displays located on each pilot instrument panel. The EFIS system is comprised of independent Captain and First Officer subsystems. Each subsystem is comprised of the following major components:

• A display processor unit (DPU) which develops the images displayed on the cathode ray tubes (CRT)

• Two electronic flight displays (CRT), installed one above the other on each pilot instrument panel, which function as:

a. Electronic attitude director indicator (EADI) (on top)

b. Electronic horizontal situation indicator (EHSI) (on bottom)

• A display control panel (DCP) for control of dimming, display format, and DH selection Components common to each subsystem include:

• A CRS - HDG panel for setting course and heading

• A multifunction display (MFD) driven by its own multiprocessor unit (MPU)

Each EFIS subsystem has limited reversionary operating capability after failures. All reversionary switching for each subsystem is controlled by the respective overhead EFIS panel.

Display Processor Unit (DPU)

The heart of each EFIS subsystem is the DPU which forms all symbols, alphanumeric text and colors displayed on the electronic flight displays. Each DPU normally supplies the on-side electronic flight displays and the autopilot/flight director system when selected. Inputs to each DPU includes information from the:

• Respective display control panel

• Radio altimeter (radio height, DH setting)

• Air data computer (airspeed, altitude, TAS, and SAT)

• Respective VOR/ILS/MB receiver (course deviation and marker beacon indications)

• Weather radar (radar echo display)

• Respective DME (distance, ground speed, TTG)

• ADF (NDB bearing)

• Both AHRS (pitch and roll attitudes, magnetic heading)

• Autopilot/flight director (mode annunciation, FD command bar)

DPU Failure

Failure of a DPU is indicated by blanking of both associated electronic flight displays and red flashing XDTA on the remaining EADI. In such cases, the MPU can be substituted to drive the electronic flight display displays by selecting DRIVE XFR on the respective EFIS panel which also illuminates the amber DRIVE XFR light on the affected instrument panel. The electronic flight displays are recovered and a duplicate image of the EHSI is shown on the MFD.

Comparator Alerting

Each DPU compares the cross-side data for discrepancies with the on-side data. The monitored parameters and associated comparator annunciations and thresholds are as follows:

• Pitch (PITCH) and heading (ROLL): (greater than four degrees difference in either parameter; three degrees if FD/AP is in APP MODE)

• Localizer (LOC): (greater than 1/3 dot difference)

• Glideslope (GS): (greater than1/2 dot difference)

• Radio altitude (RA): (threshold varies linearly with height from 30 to 170 feet)

• Flight director (FD): (greater than five degrees difference in pitch or roll steering commands)

Comparator discrepancies trigger master caution alerting with flashing amber AVIONICS on the CWP, and the corresponding yellow annunciator flashes on the EADI until master caution alerting is cancelled. If a XSIDE DATA selection is made on either EFIS switch or TAKEOFF INHIBIT MODE is selected on the WAS, then no comparator alerts are given for pitch, roll or heading discrepancies.

Display Control Panel (DCP)

Each EFIS subsystem display is controlled by the related display control panel supplied from the respective Avionic Start Bus. Each DCP contains controls for adjustment and selection of:

• EHSI declutter

• EHSI display format (bearing pointer, weather radar, 2nd course displays, and ROSE/SECTOR/MAP MODES)

• EHSI and EADI dimming

• DH setting

• Radio altimeter testing

• Flight director command bar display

• Elapsed time start, stop, and reset

DCP failures are indicated by flashing yellow DCP on both electronic flight displays with master caution alerting and flashing amber AVIONICS on the CWP. Electronic Flight Instrument Displays Color Coding Navigation and data symbols used on the electronic flight displays and the MFD are color coded for identification: Nav 1 indications and sources are displayed in blue; Nav 2 indications and sources are displayed in green. Miscellaneous data that is not oriented left or right (such as TAS) is displayed in white. Digital information is normally displayed according to the data color code. The related digits are replaced by normal color dashes without other alerting if the signal is lost or system failure occurs.


The EADI displays primary attitude information that may be integrated with flight director commands. The Captain EADI is supplied from the Left Avionic Start Bus while the First Officer EADI is supplied from Right Avionic Start Bus. Attitude indications are reliable throughout 360° roll and 90° pitch. Red chevrons (that point toward the horizon) come into view when pitch attitude exceeds +40° or –30°. If the pitch attitude exceeds +30° or –20° or bank angle exceeds 65° a declutter feature is automatically activated. When activated, all information except the aircraft symbol, blue and brown colors, and the pitch and roll scales are removed to enhance orientation. When the nav source is tuned to a localizer frequency, the EADI displays a localizer deviation scale with a runway symbol to supplement the EHSI nav display. When the MAP MODE is selected, the EADI displays a course deviation indicator (CDI) with an octagonal symbol in addition to the EHSI indications.


The EHSI displays VOR, LOC and ILS navigation information. An NDB bearing pointer may be optionally displayed as desired. Three display formats are available:

• ROSE MODE–normal display with a 360° compass rose

• SECTOR MODE with a 80° span of the compass rose centered on the current heading. Weather radar may be displayed in the SECTOR MODE

• MAP MODE (pictorial nav display) in which VORTAC stations, distance, and courses are shown relative to the aircraft position. The VORTAC is displayed as an octagonal shaped symbol with a course line to the station and a dashed course line from the station

• Weather radar information may be displayed in the SECTOR MODE. MAP MODE requires that VORTAC stations be tuned. Symbols and course lines are not shown when:

a. A VOR station is tuned (no DME)

b. The DME is set DME HOLD

c. There is no reception, or VORTAC failure occurs

When a localizer has been selected the EHSI automatically returns to SECTOR MODE and displays the standard VOR course pointer. The desired format is selected via the SECTOR knob located on the related display control panel (DCP). The Captain EHSI is supplied from the Left Avionic Start Bus while the First Officer EHSI is supplied from the Right Avionic Start Bus.

Electronic Flight Display Failures

Failure of an electronic flight display results in blanking of the affected display. After such failures the attitude and navigation displays can be combined on the remaining electronic flight display in a composite format via selection of ADI REV or HSI REV, as appropriate, on the respective EFIS panel. Nav indications may be integrated with the COMPOSITE MODE by selecting a localizer frequency or by selecting the MAP MODE via the SECTOR knob and pushing the DEV pushbutton.

Standby VOR/ILS Indicator

The Captain instrument panel is equipped with a conventional VOR/ILS indicator for use as a supplemental display to the EHSI. Course selection is made via the adjacent OBS knob and all navigation and status indications are made by mechanical indicators and flags. The standby indicator is supplied from the Essential Avionic Bus. Nav displays on this indicator are not dependant on 26v inverter power.

Multifunction Display (MFD)

The MFD may be used to display weather radar indications and TCAS. MFD displays are developed through an independent video processor known as the multifunction processor unit (MPU). In addition, the MPU can also drive the pilot electronic flight displays via DRIVE XFR selection if one or both DPU fail. After DRIVE XFR selection, the affected electronic flight displays are recovered and a duplicate image of the EHSI is shown on the MFD.