Electronic Horizontal Situation Indicator

STUDENT GUIDE FOR T-1A AIRCRAFT SYSTEMS

CHAPTER TEN

less. If radio altitude data becomes invalid, the radio altitude display is blanked and the red

flashing radio altitude invalid label RA is displayed.

Decision Height (DH) Display - The DH (selected on the DSP) is numerically displayed

with a DH annunciation in the lower right corner of the EADI. The DH display flashes when the

aircraft is between DH +50 feet and DH. Below six feet radio altitude, the DH flashing is

inhibited and remains inhibited until DH +100 feet altitude is reached.

Lateral Deviation Display - Lateral deviation from the active navigational course is

shown by the position of a pointer relative to the deviation scale. The scale consists of two dots

left and right of center. The navigation (nav) pointer is a symbol indicating which sensor is

providing active course data. If sensor data becomes invalid, a red lateral deviation warning flag

labeled LOC, FMS, or TCN is displayed above the lateral deviation scale.

Electronic Horizontal Situation Indicator

The EHSI format presents a 360 compass card similar to existing electromechanical HSIs.

Compass Display - The compass display is a full 360 rose with the labels North (N), East

(E), South (S), and West (W) designating the cardinal points and numeric at the 30 points.

Aircraft heading is read against the lubber line. Markings are provided every 45 around the

periphery of the card to aid in procedure turns.

Primary Selected Course - Primary selected course is shown by the relationship of the

solid bar arrow (active course pointer) with respect to the compass card and is repeated digitally

in the upper right corner with the label CRS above the numbers.

Heading Cursor - The selected heading is indicated by two magenta adjacent rectangles

that rotate about the compass card. The heading cursor is rotated to the actual aircraft heading

(under the lubber line) when the DSP heading sync button is pressed.

Bearing Pointer Display - A magenta, shape-coded bearing pointer is displayed along

with the heading sensor annunciation when selected on the DSP. If the heading sensor is VHF

Omni-directional Range (VOR), the pointer is a hexagon. The Flight Management System

(FMS) pointer is star-shaped, the Automatic Direction Finding (ADF) pointer is a cross, and the

Tactical Air Navigation (TACAN) pointer is a TACAN symbol. If the bearing sensor fails, the

bearing pointer is removed from view and the bearing pointer sensor annunciation is boxed and

flashes for 10 seconds.

Nav Data Display - The nav display indicates Time-To-Go (labeled TTG), Groundspeed

(labeled GSP) or Relative Velocity (labeled RVEL with up/down arrows) and relative wind

speed/direction (labeled with a number and direction arrow).

Distance Display - The distance display indicates distance to the selected VOR, TACAN,

or waypoint primary course. Distance digits are displayed below the waypoint/Navigational Aid

(NAVAID) identifier in the same color as the selected course. If invalid distance data is

AVIONICS

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