The simulation of the Litton INS system would not be complete without the simulation of the INS alignment. The Litton LTN-72 uses ultra precise gyroscopes with ring-lasers that need to be aligned with a known position prior to aircraft movement. The status of this alignment process is indicated by an "ALIGN" indication on the status page of the LTN-72RL CDU. The image above shows the the Mode Control Unit (MCU) that is typically mounted on the overhead panel of the aircraft. It is put there so it is out of the way because it really only needs to be touched during alignment prior to aircraft movement.
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The INU starts the process once the switch is lifted out of a locked Standby (STBY) position and moved to the Alignment (ALIGN) position. In the image below you can see that the process starts with a status of 90 and counts down to a status of 2. The INU also estimates the time for alignment to be 14.6 minutes. This value also counts down as we work towards status 2. For the simulation it is important that the simulator does not report aircraft movement or the alignment will not complete or will be wrong.
Once the INU reaches Status 2, the last alignment status, the green READY NAV light comes and indicates that the INU can now be put into navigation mode. Navigation mode should not be entered until we are ready to move the aircraft.
Once the INU is in navigation mode (see image below) the timer starts counting up to give us an indication for how long the INU has been running. For a precision INU, such as the LTN-72, i think the average error is 2NM over a 4 hour period.
Below is the display sequence as published by Litton in the LTN-72 Pilots Operating handbook.
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