Narrative:

About 15 yrs ago I installed an HSI. It was not compatible with the RNAV and companion DME I installed about 5 yrs ago, so I replaced it with a X RNAV. I was very pleased with the equipment until about 2 yrs ago when it became apparent from increasing calls from ATC telling me that I was left of course. At first this seemed to occur on southbound legs most notably between sav and crg. I at first thought the distance from the station was the problem but I tried setting the obs 10 degrees to either side of on course indication. I found that the indication was sometimes full scale and sometimes 1/3 or less movement of the indicator on one side and full scale on the other. The RNAV was suspected and was found to be weak. FBO worked on it at the factory and reprogrammed the computer. I thought it was ok until I missed V44 when flying the 100 degree right of rbv en route to block island, ri. Then in 12/90 flying again to florida from the philadelphia, PA, area I noticed the needle was almost dead in the 'on course indication'. I carefully wrote the conditions down and took the airplane to daytona beach radio shop. I showed them my notes and they said the problem was partly in the HSI 100. The RNAV was weak but not the primary problem. They took the HSI apart and found that the meter and several other parts of the resolution system are connected to the gyro assembly by slip rings. These were dirty and corroded and one was bent. The technician replaced the bent ring and cleaned the rest. What was happening (the opinion of the radio tech) was that the on course indication is a zero voltage condition. When the higher resistance parts of the slip ring were delivering power to give off course indication the meter received insufficient current and gave incorrect (read lesser) indication. The 'navigation disabled' flag did not show because the signal was ok as processed. I don't believe there is any way to verify that the CDI is indicating normally in this type of instrument. I am now unsure that I can use this or any HSI or F/D with confidence in a single pilot navigation setup. I became disoriented on a night proficiency flight in the daytona beach area in the second week of 12/90 as a result of my reliance on the RNAV indication when returning to spruce creek from the daytona arsa. (Spruce creek is under the arsa) and overflew the spruce creek airport and almost went back into the daytona arsa when reversing course from a point north of where my RNAV indicated. Are all HSI's subject to this kind of error. I worry now and have replaced my unit with an overhauled unit. I have not heard of this problem existing, and the radio shop was unaware of the consequences of slip ring corrosion or damage. Thank you for reading this. I hope it makes sense. I hope it helps.

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Original NASA ASRS Text

Title: HSI INDICATION ON CENTERLINE WHEN DISPACED SEVERAL MILES.

Narrative: ABOUT 15 YRS AGO I INSTALLED AN HSI. IT WAS NOT COMPATIBLE WITH THE RNAV AND COMPANION DME I INSTALLED ABOUT 5 YRS AGO, SO I REPLACED IT WITH A X RNAV. I WAS VERY PLEASED WITH THE EQUIP UNTIL ABOUT 2 YRS AGO WHEN IT BECAME APPARENT FROM INCREASING CALLS FROM ATC TELLING ME THAT I WAS L OF COURSE. AT FIRST THIS SEEMED TO OCCUR ON SBND LEGS MOST NOTABLY BTWN SAV AND CRG. I AT FIRST THOUGHT THE DISTANCE FROM THE STATION WAS THE PROB BUT I TRIED SETTING THE OBS 10 DEGS TO EITHER SIDE OF ON COURSE INDICATION. I FOUND THAT THE INDICATION WAS SOMETIMES FULL SCALE AND SOMETIMES 1/3 OR LESS MOVEMENT OF THE INDICATOR ON ONE SIDE AND FULL SCALE ON THE OTHER. THE RNAV WAS SUSPECTED AND WAS FOUND TO BE WEAK. FBO WORKED ON IT AT THE FACTORY AND REPROGRAMMED THE COMPUTER. I THOUGHT IT WAS OK UNTIL I MISSED V44 WHEN FLYING THE 100 DEG R OF RBV ENRTE TO BLOCK ISLAND, RI. THEN IN 12/90 FLYING AGAIN TO FLORIDA FROM THE PHILADELPHIA, PA, AREA I NOTICED THE NEEDLE WAS ALMOST DEAD IN THE 'ON COURSE INDICATION'. I CAREFULLY WROTE THE CONDITIONS DOWN AND TOOK THE AIRPLANE TO DAYTONA BEACH RADIO SHOP. I SHOWED THEM MY NOTES AND THEY SAID THE PROB WAS PARTLY IN THE HSI 100. THE RNAV WAS WEAK BUT NOT THE PRIMARY PROB. THEY TOOK THE HSI APART AND FOUND THAT THE METER AND SEVERAL OTHER PARTS OF THE RESOLUTION SYS ARE CONNECTED TO THE GYRO ASSEMBLY BY SLIP RINGS. THESE WERE DIRTY AND CORRODED AND ONE WAS BENT. THE TECHNICIAN REPLACED THE BENT RING AND CLEANED THE REST. WHAT WAS HAPPENING (THE OPINION OF THE RADIO TECH) WAS THAT THE ON COURSE INDICATION IS A ZERO VOLTAGE CONDITION. WHEN THE HIGHER RESISTANCE PARTS OF THE SLIP RING WERE DELIVERING PWR TO GIVE OFF COURSE INDICATION THE METER RECEIVED INSUFFICIENT CURRENT AND GAVE INCORRECT (READ LESSER) INDICATION. THE 'NAV DISABLED' FLAG DID NOT SHOW BECAUSE THE SIGNAL WAS OK AS PROCESSED. I DON'T BELIEVE THERE IS ANY WAY TO VERIFY THAT THE CDI IS INDICATING NORMALLY IN THIS TYPE OF INSTRUMENT. I AM NOW UNSURE THAT I CAN USE THIS OR ANY HSI OR F/D WITH CONFIDENCE IN A SINGLE PLT NAV SETUP. I BECAME DISORIENTED ON A NIGHT PROFICIENCY FLT IN THE DAYTONA BEACH AREA IN THE SEC WK OF 12/90 AS A RESULT OF MY RELIANCE ON THE RNAV INDICATION WHEN RETURNING TO SPRUCE CREEK FROM THE DAYTONA ARSA. (SPRUCE CREEK IS UNDER THE ARSA) AND OVERFLEW THE SPRUCE CREEK ARPT AND ALMOST WENT BACK INTO THE DAYTONA ARSA WHEN REVERSING COURSE FROM A POINT N OF WHERE MY RNAV INDICATED. ARE ALL HSI'S SUBJECT TO THIS KIND OF ERROR. I WORRY NOW AND HAVE REPLACED MY UNIT WITH AN OVERHAULED UNIT. I HAVE NOT HEARD OF THIS PROB EXISTING, AND THE RADIO SHOP WAS UNAWARE OF THE CONSEQUENCES OF SLIP RING CORROSION OR DAMAGE. THANK YOU FOR READING THIS. I HOPE IT MAKES SENSE. I HOPE IT HELPS.

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.