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 B.  The INS will automatically transfer to battery power if ground power is removed from the airplane with the INS turned on. In order to protect the INS batteries from inadvertent discharge, a ground crew call horn, located in the nose wheel well, activates 30 |10 seconds after the INS transfers to battery power with the airplane sitting on the ground.
 7.  Horizontal Situation Indicator (Fig. 10)
______________________________
 A.  Two horizontal situation indicators (HSI) are installed in the airplane, one on each pilots' instrument panel. Each indicator is a composite instrument used primarily to display airplane heading with respect to true north, or magnetic north as determined by its mode of operation. The HSI also displays information from conventional sources such as the VOR/ILS and magnetic heading reference systems as well as from the inertial navigation system.
 B.  The captain's HSI receives INS navigation information from INS No. 1 or No. 3 and the first officer's HSI receives INS navigation information from INS No. 2 or No. 3 as selected from the INS transfer switch on the captain's flight instrument panel. An illuminated annunciator adjacent to the switch shows which two of the three INS systems are supplying information to the HSI's (Fig. 1).
 C.  The horizontal situation indicators, operating in a true heading-oriented INS mode, display airplane true heading, desired track, actual or ground track, drift angle, distance to the next waypoint, track angle error, and ground speed (Fig. 10). BCD signals representing distance and ground speed are decoded in separate digital displays.
 (1)  
True heading is displayed by the azimuth card relative to the lubber line.

 (2)  
Track angle error is displayed by the angle between the course arrow and the drift angle bug.

 EFFECTIVITY AIRPLANES WITH LTN-72 INS  
  CONFIG  01 A Page 24  Apr 25/86

34-41-00
HEADING 
WARNING  TRUE/MAG HEADING 
FLAG  ANNUNCIATOR 

TYPICAL HEADING - ORIENTED HSI DISPLAY
Horizontal Situation Indicator
Figure 10

EFFECTIVITY
AIRPLANES WITH LTN-72 INS
CONFIG 2 799 Page 25
34-41-00
Apr 25/86
 (3)  
Ground track information is displayed by the drift angle bug read against the azimuth card relative to true north (000.).

 (4)  
Drift angle is displayed by the angle between the drift angle bug and the lubber line.

 (5)  
Crosstrack deviation is displayed by the course deviation bar relative to the course deviation scale.

 (6)  
Desired track information is displayed by the course arrow read against the azimuth card.

 D.  Two separate digital readouts are provided in each HSI, one in each of the upper corners of the instrument bezel. The left-hand readout normally displays distance to the next waypoint indicated in the TO display of the CDU. The right-hand digital readout displays ground speed information from INS No. 1. The digital displays are controlled through the CDU.
 E.  The HSI provides annunciators to indicate operating status.
 (1)  
A meter-operated VOR-ILS flag appears in the upper left area of indicator when a radio or magnetic system is being used as the source of navigation information. In the INS mode of operation the VOR-ILS flag retracts to reveal an INS legend.

 (2)  
A data source annunciator is located below the VOR-ILS/INS annunciator. The data source annunciator displays the numbers 1, 2, or 3 which correspond to the system in use.

 (3)  
A heading information annunciator is located behind the lubber line at the top of the indicator. A meter-operated MAG flag appears when magnetic heading is displayed. In the INS mode the MAG flag retracts to reveal a TRUE legend which signifies a true heading-oriented display.

 (4)  
An ALERT annunciator is located in the lower left portion of the instrument face. When the INS signifies that the airplane is within 2 minutes of its selected waypoint or destination, the ALERT annunciator illuminates.

 F.  The HSI furnishes warning flags to annunciate heading and navigation failures.
 (1)  
A HEADING flag drops into view behind the lubber line and obscures part of the azimuth card in event of loss of the heading valid signal from the input data source, loss of heading servo power, or excessive heading servo error signal.

 (2)  
A meter-driven navigation failure warning flag is located adjacent to the course deviation bar. The flag appears when navigation data is invalid.

 G.  The HSI biases the markers (bugs) out of the way when the information is invalid.
　
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