Narrative:

While performing a check of my century III autoplt during the ILS runway 18 approach at olv; I was cleared for the approach to maintain 2500 ft until established. My century III autoplt captured the localizer. Localizer needle was steady with good identify. GS needle was cycling full scale above and below at approximately 12 DME from runway threshold placing the aircraft about 5 mi from FAF at 2500 ft. I engaged the localizer/GS mode of the ciii and it immediately captured both needles which at that time appeared to be centered. I got a green GS capture light on the century III. Distraction and with expectation bias; I allowed the autoplt to descend the aircraft at approximately 500 FPM just like it had properly captured the GS. It had not! Passing 1700 ft MSL; I suspected something was wrong and noticed a GS flag had appeared in the CDI and the absence of a vertical descent indicator on my GNS 480. I immediately disconnect the autoplt and retuned to 2500 ft and a proper GS. Approach notified me of a low altitude alert at 1700 ft and I indicated that I was correcting to the required altitude and GS. Upon talking to garmin technical support I learned that the green GS light and glide path coupler are not inhibited by the presence of a GS flag in the CDI! Furthermore; we discussed the possible causes of an unreliable GS after localizer capture when within the service volume of the ILS transmitter. I plan to investigate the antenna placement and wiring of the newly installed equipment. Callback conversation with reporter revealed the following information: the reporter has been in contact with the manufacturer of the autoplt and his own maintenance shop. They are troubleshooting the equipment to determine the source of the anomaly. They feel confident it is related to the individual installation on the reporter's aircraft. Reporter faulted his own complacency in not cross checking his position relative to the GS intercept point on the approach and allowing the GS to engage well outside the FAF.

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Original NASA ASRS Text

Title: BE35 PLT PERFORMING TEST OF AUTOPLT APCH COUPLER DISCOVERS GS ENGAGED AND DESCENT INITIATED DESPITE RED GS FLAG INDICATING INADEQUATE SIGNAL STRENGTH.

Narrative: WHILE PERFORMING A CHK OF MY CENTURY III AUTOPLT DURING THE ILS RWY 18 APCH AT OLV; I WAS CLRED FOR THE APCH TO MAINTAIN 2500 FT UNTIL ESTABLISHED. MY CENTURY III AUTOPLT CAPTURED THE LOC. LOC NEEDLE WAS STEADY WITH GOOD IDENT. GS NEEDLE WAS CYCLING FULL SCALE ABOVE AND BELOW AT APPROX 12 DME FROM RWY THRESHOLD PLACING THE ACFT ABOUT 5 MI FROM FAF AT 2500 FT. I ENGAGED THE LOC/GS MODE OF THE CIII AND IT IMMEDIATELY CAPTURED BOTH NEEDLES WHICH AT THAT TIME APPEARED TO BE CTRED. I GOT A GREEN GS CAPTURE LIGHT ON THE CENTURY III. DISTR AND WITH EXPECTATION BIAS; I ALLOWED THE AUTOPLT TO DSND THE ACFT AT APPROX 500 FPM JUST LIKE IT HAD PROPERLY CAPTURED THE GS. IT HAD NOT! PASSING 1700 FT MSL; I SUSPECTED SOMETHING WAS WRONG AND NOTICED A GS FLAG HAD APPEARED IN THE CDI AND THE ABSENCE OF A VERT DSCNT INDICATOR ON MY GNS 480. I IMMEDIATELY DISCONNECT THE AUTOPLT AND RETUNED TO 2500 FT AND A PROPER GS. APCH NOTIFIED ME OF A LOW ALT ALERT AT 1700 FT AND I INDICATED THAT I WAS CORRECTING TO THE REQUIRED ALT AND GS. UPON TALKING TO GARMIN TECHNICAL SUPPORT I LEARNED THAT THE GREEN GS LIGHT AND GLIDE PATH COUPLER ARE NOT INHIBITED BY THE PRESENCE OF A GS FLAG IN THE CDI! FURTHERMORE; WE DISCUSSED THE POSSIBLE CAUSES OF AN UNRELIABLE GS AFTER LOC CAPTURE WHEN WITHIN THE SVC VOLUME OF THE ILS XMITTER. I PLAN TO INVESTIGATE THE ANTENNA PLACEMENT AND WIRING OF THE NEWLY INSTALLED EQUIP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER HAS BEEN IN CONTACT WITH THE MANUFACTURER OF THE AUTOPLT AND HIS OWN MAINTENANCE SHOP. THEY ARE TROUBLESHOOTING THE EQUIPMENT TO DETERMINE THE SOURCE OF THE ANOMALY. THEY FEEL CONFIDENT IT IS RELATED TO THE INDIVIDUAL INSTALLATION ON THE REPORTER'S ACFT. REPORTER FAULTED HIS OWN COMPLACENCY IN NOT CROSS CHECKING HIS POSITION RELATIVE TO THE GS INTERCEPT POINT ON THE APCH AND ALLOWING THE GS TO ENGAGE WELL OUTSIDE THE FAF.

Data retrieved from NASA's ASRS site as of January 2009 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.