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 B.  The second and more complete method is the trouble shooting tree. This trouble shooting procedure is a complete procedure, based upon the performance of a simplified operational test of the system. It is presented in a tree-type format to aid in the rapid isolation of any failure in the system. The test steps (heavy-line boxes) are connected by shaded arrows to show the OK condition path. When all steps in the OK condition path check out, the system is operable.
 C.  When a test step does not check out, follow the NOT OK line to the box containing the trouble symptom. Turn to the procedure indicated and continue to follow a single line by analyzing the results of each test step until the required corrective action is determined. Perform the specified corrective action; then repeat the step at which the failure was encountered.
 D.  When all INS are operating properly, slight display variations between systems will normally be observed. This is caused by such factors as gyro drift rate variations, temperature changes and mechanical friction.
 E.  The operation of the WARN lamp indicates that a critical malfunction exists. If the WARN lamp lights during the operational test, the malfunction code analysis procedure (paragraph 3) must be performed. Performing this procedure will allow the particular malfunction to be identified and corrective action taken. The existance of noncritical malfunctions is also noted in the procedure. The malfunction code analysis procedure lists the malfunction code, pilots' action code, and recommended maintenance action. After performing the malfunction code analysis, the step at which the malfunction was encountered should be repeated before completing the simplified operational test procedure.
 F.  On airplanes with PMS, when the INS is in the PMS waypoint control mode, control of the INS is through the PMS CDU, and the INS CDU operates only as a display unit. To operate the INS totally independent of the PMS, open the PMS circuit breakers (Ref 34-62).
 NOTE: In the PMS waypoint control mode the INS CDU FROM/TO display is
____ blank (with the INS CDU AUTO/MAN/RMT switch in MAN).
 2.  Trouble Shooting Tips
_____________________
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 A.  Trouble shooting of the INS is facilitated by the inherent self-testing features of the system which provide one or more warning indications when a failure occurs. The WARN lamp on the control/display unit (CDU) provides a master warning for most malfunctions occurring in the inertial navigation unit (INU). Malfunctions occurring in the mode selector unit (MSU) or CDU are not accompanied by malfunction codes but will normally be obvious because of abnormal indications on displays and lamps. In general, an abnormal display accompanied by a WARM light indicates an INU malfunction; an abnormal display not accompanied by a WARN light indicates a CDU malfunction.
 B.  A battery unit malfunction may disable the INS preventing system turn-on. Should this happen, there will be no warning or other indications on either the MSU or CDU (no keys or indicators will illuminate). If the system does turn on, and then reverts to battery power (due to loss of aircraft power), the CDU amber BATT indicator will illuminate and, 30 seconds later, the ground crew call horn in the nose wheel well will sound. When the INS battery becomes depleted, and the battery voltage drops below 20 volts, the INS will shut down. At this time, both the MSU red BATT and CDU red WARN indicators will illuminate.
 C.  In addition to visual indications on the CDU and MSU, the INS produces separate output voltages to other airplane systems to indicate that true heading, navigation, primary pitch and roll attitude, auxiliary pitch and roll attitude, and platform heading output signals from each INS are valid.
 D.  The INS cannot be turned on unless appropriate battery unit is in place, battery unit circuit breaker is closed and STANDBY IGNITION switch on overhead panel P5 is in the NORM position.
 E.  An overtemperature in the INU will cause the CDU displays to blank and the CDU WARN indicator to light. The operation of the equipment cooling and INS fans should be checked as well as the integrity of the gaskets (air flow seals) on the INU mounting trays. The INU should be replaced if this overtemperature failure persists.
 F.  If a failure occurs that causes the WARN lamp on the CDU to light steady, the fault system indicator on the front panel of the INU should be checked. If this indicator is yellow (normal is black), a hard failure has occured within the INU and the INU should be replaced.
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 (1)  If failure condition is not again detected, however, the INU can be cycled thru alignment to NAV mode.
　
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本文鏈接地址：747飛機維護手冊AMM CHAPTER 34 - NAVIGATION 第34章導航3(9)