Narrative:

Due to mechanical problems, the first aircraft that we were supposed to fly had to be replaced by a substitute aircraft. The 'new' second aircraft was towed over from a maintenance hangar. We were late. Things were rushed, but all required procedures were followed by the 3 pilot crew members. The aircraft maintenance log had several 'deferred' items. The most serious to us being an open item that the 'wet' compass was unreliable and off by as much as 20 degrees. Well, the widebody transport uses 3 separate INS (inertial reference) systems for compass stabilization for all phases of flight. All 3 were functioning normally. In day to day operations the 'wet' magnetic compass is rarely ever even looked at so we weren't too concerned about the 'bad' wet compass write up. On the widebody transport the INS systems align the pilots compass displays (HSI and RMI) using magnetic flux valves located in the wings to sense magnetic north, and then the INS system uses the stabilized 'true north' information from the INS. Both of these are fed to the air data computers (CADC) and the subsequent magnetic or true heading information is displayed on the pilots respective instruments (HSI). With all of this complicated equipment, the basic final check before takeoff is to verify that HSI and RMI headings agree with the heading of the runway. A very simple xchk that we all forgot in our rush to get going since we were late due to the earlier mechanical problems. (Our basic airmanship habits learned from day one got lost in the high-tech rush of today's system). When we took off we didn't noticed that the HSI's were not yet aligned to the proper heading. Then, (as fate would have it) the compass systems (both!) were showing the wrong headings. There was then a compass system malfunction but no corresponding flags. The captain's HSI repeats heading information to the first officer's RMI and the first officer's HSI repeats to the captain's RMI. We turned to the proper heading as required on the SID. Then the tower asked us what heading we were on. The captain's HSI showed the appropriate heading of the SID but both RMI's and my HSI showed different headings. But no flags. (This isn't supposed to happen). We then attempted to get heading information from the magnetic (wet) compass but were afraid to rely on it. So now, were reading true north information from the INS display and mentally adding variation and wind drift to get an approximately correct heading. After switching from tower, to departure control through 2 sectors we informed the controllers of our situation. We continued climb out and once we got up to high altitude the compass systems 'settled down' (except the still broken wet compass). The entire flight continued uneventfully. We continued because our route was going to be VFR at destination (dtw), and we determined that all 3 INS systems were operating normally. It was just the HSI displays that were suspect.

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Original NASA ASRS Text

Title: HDG TRACK DEV BY WDB ON DEP PROC DUE TO ACFT EQUIP MALFUNCTION PROBLEM.

Narrative: DUE TO MECHANICAL PROBLEMS, THE FIRST ACFT THAT WE WERE SUPPOSED TO FLY HAD TO BE REPLACED BY A SUBSTITUTE ACFT. THE 'NEW' SECOND ACFT WAS TOWED OVER FROM A MAINT HANGAR. WE WERE LATE. THINGS WERE RUSHED, BUT ALL REQUIRED PROCS WERE FOLLOWED BY THE 3 PLT CREW MEMBERS. THE ACFT MAINT LOG HAD SEVERAL 'DEFERRED' ITEMS. THE MOST SERIOUS TO US BEING AN OPEN ITEM THAT THE 'WET' COMPASS WAS UNRELIABLE AND OFF BY AS MUCH AS 20 DEGS. WELL, THE WDB USES 3 SEPARATE INS (INERTIAL REF) SYSTEMS FOR COMPASS STABILIZATION FOR ALL PHASES OF FLT. ALL 3 WERE FUNCTIONING NORMALLY. IN DAY TO DAY OPS THE 'WET' MAGNETIC COMPASS IS RARELY EVER EVEN LOOKED AT SO WE WEREN'T TOO CONCERNED ABOUT THE 'BAD' WET COMPASS WRITE UP. ON THE WDB THE INS SYSTEMS ALIGN THE PLTS COMPASS DISPLAYS (HSI AND RMI) USING MAGNETIC FLUX VALVES LOCATED IN THE WINGS TO SENSE MAG N, AND THEN THE INS SYS USES THE STABILIZED 'TRUE N' INFO FROM THE INS. BOTH OF THESE ARE FED TO THE AIR DATA COMPUTERS (CADC) AND THE SUBSEQUENT MAGNETIC OR TRUE HDG INFO IS DISPLAYED ON THE PLTS RESPECTIVE INSTS (HSI). WITH ALL OF THIS COMPLICATED EQUIP, THE BASIC FINAL CHK BEFORE TKOF IS TO VERIFY THAT HSI AND RMI HDGS AGREE WITH THE HDG OF THE RWY. A VERY SIMPLE XCHK THAT WE ALL FORGOT IN OUR RUSH TO GET GOING SINCE WE WERE LATE DUE TO THE EARLIER MECHANICAL PROBLEMS. (OUR BASIC AIRMANSHIP HABITS LEARNED FROM DAY ONE GOT LOST IN THE HIGH-TECH RUSH OF TODAY'S SYS). WHEN WE TOOK OFF WE DIDN'T NOTICED THAT THE HSI'S WERE NOT YET ALIGNED TO THE PROPER HDG. THEN, (AS FATE WOULD HAVE IT) THE COMPASS SYSTEMS (BOTH!) WERE SHOWING THE WRONG HDGS. THERE WAS THEN A COMPASS SYS MALFUNCTION BUT NO CORRESPONDING FLAGS. THE CAPT'S HSI REPEATS HDG INFO TO THE FO'S RMI AND THE FO'S HSI REPEATS TO THE CAPT'S RMI. WE TURNED TO THE PROPER HDG AS REQUIRED ON THE SID. THEN THE TWR ASKED US WHAT HDG WE WERE ON. THE CAPT'S HSI SHOWED THE APPROPRIATE HDG OF THE SID BUT BOTH RMI'S AND MY HSI SHOWED DIFFERENT HDGS. BUT NO FLAGS. (THIS ISN'T SUPPOSED TO HAPPEN). WE THEN ATTEMPTED TO GET HDG INFO FROM THE MAG (WET) COMPASS BUT WERE AFRAID TO RELY ON IT. SO NOW, WERE READING TRUE N INFO FROM THE INS DISPLAY AND MENTALLY ADDING VARIATION AND WIND DRIFT TO GET AN APPROX CORRECT HDG. AFTER SWITCHING FROM TWR, TO DEP CTL THROUGH 2 SECTORS WE INFORMED THE CTLRS OF OUR SITUATION. WE CONTINUED CLB OUT AND ONCE WE GOT UP TO HIGH ALT THE COMPASS SYSTEMS 'SETTLED DOWN' (EXCEPT THE STILL BROKEN WET COMPASS). THE ENTIRE FLT CONTINUED UNEVENTFULLY. WE CONTINUED BECAUSE OUR RTE WAS GOING TO BE VFR AT DEST (DTW), AND WE DETERMINED THAT ALL 3 INS SYSTEMS WERE OPERATING NORMALLY. IT WAS JUST THE HSI DISPLAYS THAT WERE SUSPECT.

Data retrieved from NASA's ASRS site as of July 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.