(2)
The navigation unit of an INS should be replaced only if the system can be shown to consistently either not accept or not transmit data. The solution for a system that intermittently rejects or alters data is to enter it several times.
3. Significance of Action/Malfunction Codes
________________________________________
A. In order to ensure dispatchability and prevent unnecessary component removals, a knowledge is required of the causes of some malfunction codes or what these codes will do to the operation of the system.
B. Loss of Attitude Valid will cause the ADI to shut down with the warning flag in view and the sphere tumbling. Any of malfunctions 33 through 39 will cause a loss of the Attitude Valid signal from the INS. If any of these malfunctions do not appear in Temporary or Semipermanent Malfunction Record then the problem should not be the INS nav unit.
C. Malfunctions 42, 45, 57 or 60, when appearing in either this entire group or several appearing together would indicate that the system was moved or taxied in ALIGN. Under extreme conditions, malfunctions 15 or 16 may appear, but again this would be along with several of the others. A realignment will normally correct these codes. However, if NAV was selected with these malfunctions present, it may take several alignments (and going to NAV each time) to cure them.
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D. Malfunctions 32, 59 and 63, are computer self-check malfunctions. In order to verify the existence of any other problem, it is necessary to align the system to at least PI=8. The computer is at idle during PI=9 (or STBY) and will detect only computer malfunctions. Until PI=8 the only other things that can be seen are the stored malfunctions from the last time the system was operating. These can be extinguished with the Push to Test and if they were hard failures, will not reappear until the computer goes into full operation (PI=8).
E. Malfunctions 41, 43 and 45 are checks of the present position that is loaded into the system. Codes 41 and 45 are absolute checks of loaded versus calculated latitude and 43 is a relative check of loaded latitude and longitude between the systems. Malfunction 41 will occur upon
___ entering PI=6 if the difference between loaded and calculated latitude is greater than 3.5 degrees. Malfunction 45 will occur 5 minutes after entering PI=6 if the difference between loaded and calculated latitude is greater than 42 minutes. If either of these malfunctions appear, the first thing to check would be loaded latitude. If it is correct, then it is possible that the INS navigation unit is not calculating latitude correctly. However, if this happens on all of the systems then it is more probably an incorrect latitude that has been inserted. Malfunction 43 occurs when the loaded latitude or longitude in any of the systems
__
differs by more than 0.7 minute.
4. Action Code/Malfunction Code Display Procedure______________________________________________(See figure 101.)
A. Current Malfunctions
(1) The digital computer unit (DCU) performs data reasonableness checks and self-tests while monitoring warning signals from the IRU and IRUE. The DCU also produces pilots' action codes and malfunction codes and illuminates the WARN lamp on the control/display unit (CDU) when malfunctions and out-of-tolerance conditions are detected. The digital computer unit records the codes, even if the malfunction is momentary, for display on the CDU. The following steps describe the procedure for interrogating the DCU to determine the cause of the warning. It may be possible to correct one class of malfunctions (those associated with action codes 04 and 05) by following the recommended maintenance action. Figure 101 describes all possible action and malfunction codes.
(a)
Place data selector switch in DSRTK/STS position. Observe mode status/actual performance index number, desired performance index number, and pilots' action code in right-hand data display. (See figure 101.)
(b)
Press and release TEST switch. Observe that action code display (01 through 06) is replaced by malfunction code display (12 through 63).
NOTE: If WARN lamp goes out, failure was intermittent and has
____ been cleared. If cleared, disregard the action code.
(c) Repeat step (b) recording each malfunction code displayed until display goes blank or until the action code reappears. Perform action for most severe malfunction recorded.
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B. INS under PMS Waypoint Control (On airplanes with PMS).
(1)
Malfunction code 52 is set when the INS is under PMS waypoint control, and a PMS transmitted waypoint is not received from the PMS.
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