Narrative:

On 7/mon/90, the reporter departed downtown parks airport, cahokia, il, in an IFR flight ot northeast philadelphia airport. Tower instructed the reporter to turn 20 degrees to the right and visly avoid the 'arch' in downtown st louis and to contact departure. Departure instructed the reporter to turn right to a heading of 20 degrees with an altitude assignment. My HSI was not giving me the right headings. Departure then told me to turn 20 degrees to the right and asked my heading which I was reading from compass which was floating since we were in climb. I reset the slaved HSI thinking to declare an instrument malfunction. However, the HSI and compass then stayed in sync and on climb stayed in sync. A few days before, FBO at love field, the compass was swung since the HSI was not holding with the compass on flight from natchez, ms. FBO advised that the compass, a '$39' item needs swinging. The HSI is right, a '$4000' item. On return home I inquired of a former naval officer about the strange behavior of the HSI. It is supposed to be correct according to FBO. I was told that since the cps FBO people at downtown parks had repositioned the plane overnight, the HSI would take about 15 mins to reposition itself even though it is a slaved HSI. I have tried in the past to find material on the qualities of the HSI. Mfr on request sent me pages from the poh for the HSI. No instruction was there. If I had been in IMC I would have declared an instrument malfunction and asked for vectors to the nearest ILS. I am enclosing a copy of the letter I wrote to the pilots association. There should be available to pilots more detailed material about many of the items of equipment used in aircraft to explain how the equipment works, its capabilities and what its negatives are.

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Original NASA ASRS Text

Title: PLT OF SMA DEPARTING CPW ON AN IFR FLT PLAN IS QUESTIONED ABOUT HIS HEADING. PLT HAD DIFFICULTY TELLING WHETHER THE HSI WAS GIVING HIM ACCURATE HEADING DATA, AND WHETHER ITS AUTOMATIC SLAVING FEATURE WAS WORKING. REPORTER SAYS THERE IS VERY LITTLE DOCUMENTATION ON THE SLAVING FEATURE OF HIS HSI.

Narrative: ON 7/MON/90, THE RPTR DEPARTED DOWNTOWN PARKS ARPT, CAHOKIA, IL, IN AN IFR FLT OT NE PHILADELPHIA ARPT. TWR INSTRUCTED THE RPTR TO TURN 20 DEGS TO THE R AND VISLY AVOID THE 'ARCH' IN DOWNTOWN ST LOUIS AND TO CONTACT DEP. DEP INSTRUCTED THE RPTR TO TURN R TO A HDG OF 20 DEGS WITH AN ALT ASSIGNMENT. MY HSI WAS NOT GIVING ME THE R HDGS. DEP THEN TOLD ME TO TURN 20 DEGS TO THE R AND ASKED MY HDG WHICH I WAS READING FROM COMPASS WHICH WAS FLOATING SINCE WE WERE IN CLB. I RESET THE SLAVED HSI THINKING TO DECLARE AN INSTRUMENT MALFUNCTION. HOWEVER, THE HSI AND COMPASS THEN STAYED IN SYNC AND ON CLB STAYED IN SYNC. A FEW DAYS BEFORE, FBO AT LOVE FIELD, THE COMPASS WAS SWUNG SINCE THE HSI WAS NOT HOLDING WITH THE COMPASS ON FLT FROM NATCHEZ, MS. FBO ADVISED THAT THE COMPASS, A '$39' ITEM NEEDS SWINGING. THE HSI IS R, A '$4000' ITEM. ON RETURN HOME I INQUIRED OF A FORMER NAVAL OFFICER ABOUT THE STRANGE BEHAVIOR OF THE HSI. IT IS SUPPOSED TO BE CORRECT ACCORDING TO FBO. I WAS TOLD THAT SINCE THE CPS FBO PEOPLE AT DOWNTOWN PARKS HAD REPOSITIONED THE PLANE OVERNIGHT, THE HSI WOULD TAKE ABOUT 15 MINS TO REPOSITION ITSELF EVEN THOUGH IT IS A SLAVED HSI. I HAVE TRIED IN THE PAST TO FIND MATERIAL ON THE QUALITIES OF THE HSI. MFR ON REQUEST SENT ME PAGES FROM THE POH FOR THE HSI. NO INSTRUCTION WAS THERE. IF I HAD BEEN IN IMC I WOULD HAVE DECLARED AN INSTRUMENT MALFUNCTION AND ASKED FOR VECTORS TO THE NEAREST ILS. I AM ENCLOSING A COPY OF THE LETTER I WROTE TO THE PLTS ASSOCIATION. THERE SHOULD BE AVAILABLE TO PLTS MORE DETAILED MATERIAL ABOUT MANY OF THE ITEMS OF EQUIP USED IN ACFT TO EXPLAIN HOW THE EQUIP WORKS, ITS CAPABILITIES AND WHAT ITS NEGATIVES ARE.

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.