Narrative:

#1 INS was written up as inoperative in navigation mode. When second officer arrived at aircraft he initiated system warm up and it appeared to operate normally. Shortly thereafter a mechanic came on board and stated that #1 navigation system and flight instruments would have to be operated on the standby gyro as the INS were totally inoperative. After captain and F/east arrived, all crew discussed situation and then contacted system maintenance controller. After discussing situation, he advised this was correct. We (the crew) were not happy with using a standby system as the primary source for gyro stabilization when it appeared the INS gyro platform was operating. We elected to use INS platform for stabilization with the idea if it failed/did not perform correctly, we would then use the standby gyro. During the flight we continued to question the dispatch of aircraft using a standby system for primary operation. On questioning our dispatcher while en route, we were informed the MEL requires both INS gyro platforms to be operational for dispatch. Supplemental information from acn 85996: no circuit breaker's in cockpit were pulled and capped in cockpit to indicate total failure of #1 INS unit. Later determined through more communication with dispatch that aircraft was improperly dispatched as per MEL. A procedure should be available to flight crews for proper set up of cockpit with assorted components of INS unit inoperative, or for total INS system failure.

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Original NASA ASRS Text

Title: CONFUSION ABOUT CORRECT DISPATCH REQUIREMENT WITH 1 INSTRUMENT SYSTEM INOPERATIVE IN NAVIGATION MODE.

Narrative: #1 INS WAS WRITTEN UP AS INOP IN NAV MODE. WHEN S/O ARRIVED AT ACFT HE INITIATED SYS WARM UP AND IT APPEARED TO OPERATE NORMALLY. SHORTLY THEREAFTER A MECH CAME ON BOARD AND STATED THAT #1 NAV SYS AND FLT INSTRUMENTS WOULD HAVE TO BE OPERATED ON THE STANDBY GYRO AS THE INS WERE TOTALLY INOP. AFTER CAPT AND F/E ARRIVED, ALL CREW DISCUSSED SITUATION AND THEN CONTACTED SYS MAINT CTLR. AFTER DISCUSSING SITUATION, HE ADVISED THIS WAS CORRECT. WE (THE CREW) WERE NOT HAPPY WITH USING A STANDBY SYS AS THE PRIMARY SOURCE FOR GYRO STABILIZATION WHEN IT APPEARED THE INS GYRO PLATFORM WAS OPERATING. WE ELECTED TO USE INS PLATFORM FOR STABILIZATION WITH THE IDEA IF IT FAILED/DID NOT PERFORM CORRECTLY, WE WOULD THEN USE THE STANDBY GYRO. DURING THE FLT WE CONTINUED TO QUESTION THE DISPATCH OF ACFT USING A STANDBY SYS FOR PRIMARY OPERATION. ON QUESTIONING OUR DISPATCHER WHILE ENRTE, WE WERE INFORMED THE MEL REQUIRES BOTH INS GYRO PLATFORMS TO BE OPERATIONAL FOR DISPATCH. SUPPLEMENTAL INFO FROM ACN 85996: NO CB'S IN COCKPIT WERE PULLED AND CAPPED IN COCKPIT TO INDICATE TOTAL FAILURE OF #1 INS UNIT. LATER DETERMINED THROUGH MORE COM WITH DISPATCH THAT ACFT WAS IMPROPERLY DISPATCHED AS PER MEL. A PROC SHOULD BE AVAILABLE TO FLT CREWS FOR PROPER SET UP OF COCKPIT WITH ASSORTED COMPONENTS OF INS UNIT INOP, OR FOR TOTAL INS SYS FAILURE.

Data retrieved from NASA's ASRS site as of August 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.