(12)
 Present position (POS) is the actual latitude and longitude position of the airplane.

 (13)
 Cross track distance (XTK) is the shortest distance between the airplane's present position and the desired track.

 (14)
 Track angle error (TKE) is the angle between the airplane's present track and the desired track.

 (15)
 Distance (DISTANCE) is the distance to go measured between the present position of the airplane and the next waypoint or destination.

 F.  VORTAC (VOR/DME) position updating of the inertial navigation system allows the system to be operated in terminal and approach areas as a radio navigation (R-NAV) system. The VORTAC station may be manually tuned at the VHF navigation control panel or automatically tuned by the inertial navigation unit.
 G.  For an area navigation (R-NAV) approach, the INS approach mode can be enabled so that the inertial navigation unit may provide a magnetic heading output to the horizontal situation indicator. In this mode the approved VORTAC reference station for a particular approach must be manually tuned.
 2.  Basic Operational Description (Fig. 4)
_____________________________
 A.  The inertial navigation system (INS) uses 115 volts ac power from the airplane electrical system. A battery unit is provided, which will operate the INS in the event of an interruption in airplane electrical power. A battery charger is located inside the inertial navigation unit (INU). A mode selector unit (MSU) controls the operating mode of the INS.
 B.  On airplanes with LTN-72RL INS, the INU includes an inertial reference unit (IRU) that comprises a gimbal assembly supporting an X, Y and Z platform. The platform contains a X-Y gyro, a Z gyro and X, Y and Z accelerometers. The gyros maintain the platform horizontal regardless of airplane attitude. The IRU provides pitch and roll signals and platform heading signals to other airplane systems.
 EFFECTIVITY

 AIRPLANES WITH LTN-72RL OR  CONFIG LTN-92 INS  01 Page 5  Apr 25/91

34-41-00
 On airplanes with LTN-92 INS, the INU includes an inertial reference unit (IRU) that comprises a laser gyro assembly and gyro interface and control circuit mounted on a platform. The laser gyros maintain the platform horizontal regardless of airplane attitude. The IRU provides pitch and roll signals and platform heading signals to other airplane systems.
 C.  For the INS to navigate the airplane, the starting point and a destination (each in terms of latitude and longitude) must be entered into the INS. When the starting point latitude and longitude and the destination latitude and longitude are entered, the digital computer unit (DCU) determines the desired track to be flown. The final destination with intermediate destinations (known as waypoints) may be entered before the flight begins.
 D.  The accelerometers supply acceleration signals to the computer. If acceleration is integrated the result is velocity that can be displayed as ground speed. If velocity is integrated the result is distance. If the distance flown is added to the starting point, the airplane's present position is known. If the present position is known and the desired track is known, the computer can develop navigation data (steering commands) to keep the airplane on the desired track so that the airplane can reach any desired destination.
 E.  The digital computer also provides navigation data for display on the CDU. The CDU displays data and also transfers data and instructions to the computer.
 3.  System Functional Description_____________________________
 A.  Present position latitude and longitude must be entered into the system before alignment of the inertial platform will take place. Present position latitude and longitude is entered via the CDU data entry pushbuttons and then inserted into the INU by pressing the CDU ENT pushbutton (when the MSU is set to STBY). Entry of present position also verifies that the INS circuits are functioning properly because the CDU and INU will not accept the entered data if malfunctions exist. Present position must be entered accurately; incorrect insertion of present position may prevent proper alignment of the inertial platform.
 B.  Switching to the align mode is performed after present position latitude and longitude is inserted. Either of two methods of switching to align mode are possible.
 (1)  On airplanes with LTN-72RL INS, one method is to set the MSU mode switch to ALIGN after inserting present position. When ALIGN is selected, the MSU READY NAV annunciator comes on when the alignment sequence is complete, the INS is then ready to be switched to the navigate mode. When the MSU mode switch is set to NAV the INS goes into the navigate mode.
 EFFECTIVITY
　
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