(7)
If the PMS failure is corrected, the malfunction code(s) can be cleared from the INS CDU, and (if the ground speed is less than 75 knots) PMS waypoint control may be regained. If the ground speed is greater than 75 knots the PMS is locked out until the INS IS down-moded to STBY.
NOTE: If the INS reverts to INS waypoint control due to a command
____ from the PMS, PMS waypoint control may be regained regardless of ground speed and without down-moding the INS.
(8) When the INS reverts to INS waypoint control (if caused by a PMS transmission failure or when commanded by the PMS) the INS operates as an independent system.
E. INS Instrument Switching
(1) The transfer switching for INS navigation data is shown in Fig. 14. The navigation display signals normally are applied from INS 1 to the captain's HSI and from INS 2 to the first officer's HSI. Selection of an alternate INS may be made by operating the INS transfer switch, which positions the associated ledex relay. The VOR-ILS/INS switch determines whether radio navigation data or INS navigation data will be displayed.
EFFECTIVITY AIRPLANES WITH LTN-72 INS
CONFIG 01 A Page 40 Aug 25/87
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(2)
The INS pitch and roll attitude signals drive the attitude display sphere in the ADI. A flag signal is provided to the ADI's in case of loss or failure of attitude reference signals (Fig. 15). The attitude signals are normally routed from INS 1 to the captain's ADI and from INS 2 to the first officer's ADI. INS 3 is selected as an alternate source for either ADI by operating the ATT/COMP STAB transfer switch which positions the appropriate transfer relay.
(3)
Platform heading signals from the INS are used by the magnetic heading reference system (MHRS) for compass stabilization. Refer to 34-21-00. Platform heading is normally applied from INS 1 to MHRS compass coupler 1 and from INS 2 to MHRS compass coupler 2 (Fig. 15). INS 3 is selected as an alternate source for either compass coupler by operating the ATT/COMP STAB transfer switch which positions the appropriate transfer relay.
15. INS Power Interface (Fig. 16 and 17)
___________________
A. Power for the INS system is derived from the standby bus, the fail-op bus and from the essential and No. 2 flight instrument buses. Control is through the electronic circuit breaker panel (P7), the overhead circuit breaker panel (P126) and the main power circuit breaker panel (P6).
B. The system is energized by closing the circuit breakers which supply power to the navigation units, central air data computers (CADC), horizontal situation indicators (HSI), attitude director indicators (ADI), 26-volt ac autotransformers, and flight instrument switching relays. The INS battery units contain individual thermal breakers which must also be closed to energize the system.
C. System power requirements include 115-volt ac primary power, 26-volt ac synchro power, 28-volt dc relay control power and 5 volts ac for panel lighting.
D. By operating the INS BAT PARALLEL switch on the flight engineer's panel (P4), battery unit No. 3 can be switched in parallel with battery unit No. 1. This increases the emergency operating time of INS 1 to approximately 60 minutes.
16. Operation
_________
A. Functional Description, Operating Mode Control
(1) General
EFFECTIVITY AIRPLANES WITH LTN-72 INS
CONFIG 01 A Page 41 Aug 25/87
34-41-00
(a)
The INS automatically initiates a self-alignment sequence when the mode selector switch is set to STBY during INS startup procedures. The self-alignment sequence must be completed before the aircraft is moved. The INS is not significantly affected by gusty wind conditions and movement caused by fueling or cargo and passenger loading. During the self-alignment sequence, the pilot performs predeparture procedures which include display test, present and waypoint position entries, and alignment status check. Display test and present position entries procedures are performed with the mode selector switch set to STBY. Switching to align mode before present position latitude and longitude are inserted will cause the INS alignment to fail and will cause the WARN annunciator to indicate that present position is not inserted or is incorrect. Waypoint entries and alignment status check procedures are performed with the mode selector switch set to ALIGN. Semiautomatic autofill procedures allow waypoint coordinates to be manually inserted into one INS and automatically transferred to the remaining INS.
(b)
The mode selector and control/display units are the primary INS cockpit instruments. The MSU (Fig. 7) is used to start and align the system before takeoff. The MSU contains a remote access switch (labeled OFF, TEST and BIAS) located behind a removable access cover. The switch must be in the OFF position.
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