Flight Instruments -
Electronic Horizontal Situation Indicator (EHSI)

The Horizontal Situation Indicator (HSI) presents an electronically generated color display of navigational data. Each HSI is capable of showing the airplane's progress on a dynamic map display.

Display Orientation

During normal operation, heading reference data is supplied to each HSI from the respective IRS. The center IRS provides that data as an alternate source. During pre-flight, heading/track data is unavailable until the associated IRS has completed alignment and entered the navigation mode. Heading flags do not appear in this case.

Heading Reference Switch

The compass rose can be referenced to magnetic north or true north. The heading reference switch on the center Instrument panel is used to manually select magnetic or true reference for both HSIs, both RDMIs, the AFDS and both FMCs.

If the AFDS is in the HDG SEL mode and the Heading Reference Switch position is changed, the AFDS mode changes to HDG HOLD.

When the HSI is referenced to true north and the airplane descends 2000 ft at more than 800 fpm, the box changes from white to amber, flashes for 10 seconds, then remains amber. The box returns to white when the airplane climbs 2000 feet at more than 500 feet per minute. A green box is displayed around the M for 10 seconds when the HSI is returned to magnetic referencing.

Track Reference

Airplane track data is supplied by the onside FMC during normal operation, with the opposite FMC is available as an alternate source. If track information from the FMC is unreliable, it is automatically supplied by the IRS..

Airplane track data is supplied by the CDU when in Alternate Navigation.

MAP Mode

EHSI - Map

The MAP mode is recommended for most phases of flight.

Presented heading up, this mode shows airplane position relative to the route of flight against a moving map background.

Presented track up, this mode shows airplane position relative to the route of flight against a moving map background.

MAP Mode allows automatic VOR and DME tuning.

Displayed information can include:

Map Switches

Map Switches
  • NAV AID
    • Displays all (only) high attitude VHF navigation aids stored in the FMC database when HSI range is 80m or greater.
    • Displays all VHF navigation aids stored in the FMC database when HSI range is 40m or less.
  • AIRPORT Displays all airports stored in the FMC database.
  • ROUTE DATA Displays estimated time of arrival and any attitude constraint for each waypoint in the displayed flight plan.
  • WAYPOINT
    • When HSI range is 40 nm or less: displays all waypoints stored in the FMC database.
    • The switch is inactive when HSI range is 80 nm or greater.

VOR Mode

EHSI - VOR Mode. The VOR mode is presented heading up. The VOR mode displays track, heading, and wind speed and direction with VOR navigation information. A conventional full compass rose VOR mode is also available.

Note: VOR Mode selects manual VOR and DME tuning on the associated VOR/DME Panel (automatic tuning is inhibited).


ILS Mode

EHSI - ILS Mode. The ILS mode is presented heading up. The ILS mode displays track, heading, and wind speed and direction with ILS approach information. A conventional full compass rose ILS mode is also available.

Note: ILS Mode selects manual VOR tuning on the associated VOR/DME Panel (automatic tuning is inhibited). DME is manually tuned on the ILS Panel.

APP Mode

The APP mode is presented heading up. The APP mode displays track, heading, and wind speed and direction with ILS approach information.


PLAN Mode

EHSI - PLAN Mode The PLAN mode is presented true north up. The active route may be viewed using the STEP prompt on the FMC-CDU LEGS pages.

The top part of the HSI maintains a display of track and heading information as in the MAP mode.

PLAN Mode allows automatic VOR and DME tuning.

Center Mode

The CENTER MAP mode provides the same date as the MAP mode with the exception that the Airplane Symbol is located at the center of the display so that data behind the airplane is within view.

Traffic

Traffic information from the TCAS can be displayed on the VSI.

Traffic information from the TCAS can be displayed on the HSI.

Traffic Switch

PUSH:
  • displays or removes TCAS traffic information on the HSI.
  • removes TCAS FAIL message if displayed on the associated HSI.

Weather Radar Display

Display of weather radar returns on the HSI is enabled or disabled by the WXR switch on the respective EFIS control panel.

Terrain Display

The HSI can display look ahead terrain alerting.

Predictive Windshear

The HSI can display predictive windshear warnings.

EFIS Failures Flags and Annunciations

In addition to the normal EFIS displays, various failure annunciations, flags, or indications may be displayed on the ADI or HSI.

The location of the different failure flags and annunciations is depicted in the ADI and HSI Failure Flags and Annunciations figures included in the EFIS Controls and Indicators, section 10 of this chapter.

Not all EFIS displays will be replaced by a failure flag or annunciation if the signal from the sending unit has failed. In theses instances, failure is indicated by removal of the data or the affected portions of the display.

During preflight, heading/track data is unavailable until the associated IRS has completed alignment and entered the navigation mode. Heading flags do not appear in this case.

If an FMC FAIL message is observed on a CDU, a MAP flag will appear on the associated HSI when viewing the MAP mode. Selecting ALTN with the FMC switch will restore the map display.

If an FMC FAIL message is observed on a CDU, a MAP flag will appear on the associated HSI when viewing the MAP mode. Selecting the opposite FMC with the NAV selector will restore the map display. If both FMCs fail, selecting CDU on the NAV selector will allow the CDU to provide limited map data to the HSI.

If an FMC FAIL message is observed on a CDU, a MAP flag will appear on the associated HSI when viewing the MAP mode. Selecting the opposite FMC with the NAV selector will restore the map display. However, the range selector for the opposite side pilot now controls the displayed range for both HSIs. If the failed side pilot selects a range that is not the same as the opposite side pilot, map information will again be lost and a MAP RANGE DISAGREE message will be displayed. If both FMCs fail, selecting CDU on the NAV selector will allow the CDU to provide limited map data to the HSI.

Various fault messages can also be displayed on the HSIs. For example, a WXR/MAP RANGE DISAGREE message is displayed when the ranges for the FMC and weather radar disagree with the range selected on the HSI control panel.

An EXCESS DATA message is displayed if the quantity of information to the display exceeds the HSI’s capability to provide a normal display. If in the MAP mode, deselecting the map switches may correct the condition and remove the message.

Light Sensing and Brightness Control

Ambient light sensors automatically adjust the brightness of the EFIS displays. Once the desired brightness is set using the EFIS brightness controls, little or no adjustment is needed throughout a wide range of ambient light conditions both outside and inside the flight deck.

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