Narrative:

While on a course to and from northwest of the st. Louis area from and returning to st. Louis downtown parks airport, small aircraft was flown at an altitude between 1500-1800 ft MSL as shown on the altimeter. Both rtes the altimeter was set to the st. Louis downtown parks ATIS. This flight path was under the 2000 ft floor of the st. Louis class B airspace along the mississippi river, west of the chain of rocks bridges, just north of the highway 67 fort bellefontain missouri river bridge, just south of the pwrplant near portage de sioux, over the bluff and northwest. On the return trip, while approaching the 270 degree radial off the troy VOR, an orange large body jet airliner was noticed as traffic high and passing above, left to right. Having had the encoder box of my transponder adjusted by radio shop only 10 days prior, I have reasons to believe that I may have been indicating as much as 1200 ft higher than actual altimeter altitude. During the check, the technician was having trouble with the equipment he was using. The batteries died and he connected to AC power. His reading showed minus 1000 ft when pressure altitude at that time was plus 200 ft. He adjusted the encoder screw to obtain a reading of plus 200 ft while continuing to have trouble with his equipment throughout the check. If the encoder was reading high, as much as 1200 ft or more, that could have possible triggered a proximity warning at the st. Louis TRACON with the airline traffic off the troy VOR or other traffic in the area. Enclosed is a copy of the report from another shop which verifies the incorrect setting which was suspected, and note of corrections made.

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Original NASA ASRS Text

Title: SMA MODE C GIVES FALSE ALT INFO.

Narrative: WHILE ON A COURSE TO AND FROM NW OF THE ST. LOUIS AREA FROM AND RETURNING TO ST. LOUIS DOWNTOWN PARKS ARPT, SMA WAS FLOWN AT AN ALT BTWN 1500-1800 FT MSL AS SHOWN ON THE ALTIMETER. BOTH RTES THE ALTIMETER WAS SET TO THE ST. LOUIS DOWNTOWN PARKS ATIS. THIS FLT PATH WAS UNDER THE 2000 FT FLOOR OF THE ST. LOUIS CLASS B AIRSPACE ALONG THE MISSISSIPPI RIVER, W OF THE CHAIN OF ROCKS BRIDGES, JUST N OF THE HWY 67 FORT BELLEFONTAIN MISSOURI RIVER BRIDGE, JUST S OF THE PWRPLANT NEAR PORTAGE DE SIOUX, OVER THE BLUFF AND NW. ON THE RETURN TRIP, WHILE APCHING THE 270 DEG RADIAL OFF THE TROY VOR, AN ORANGE LARGE BODY JET AIRLINER WAS NOTICED AS TFC HIGH AND PASSING ABOVE, L TO R. HAVING HAD THE ENCODER BOX OF MY XPONDER ADJUSTED BY RADIO SHOP ONLY 10 DAYS PRIOR, I HAVE REASONS TO BELIEVE THAT I MAY HAVE BEEN INDICATING AS MUCH AS 1200 FT HIGHER THAN ACTUAL ALTIMETER ALT. DURING THE CHK, THE TECHNICIAN WAS HAVING TROUBLE WITH THE EQUIP HE WAS USING. THE BATTERIES DIED AND HE CONNECTED TO AC POWER. HIS READING SHOWED MINUS 1000 FT WHEN PRESSURE ALT AT THAT TIME WAS PLUS 200 FT. HE ADJUSTED THE ENCODER SCREW TO OBTAIN A READING OF PLUS 200 FT WHILE CONTINUING TO HAVE TROUBLE WITH HIS EQUIP THROUGHOUT THE CHK. IF THE ENCODER WAS READING HIGH, AS MUCH AS 1200 FT OR MORE, THAT COULD HAVE POSSIBLE TRIGGERED A PROX WARNING AT THE ST. LOUIS TRACON WITH THE AIRLINE TFC OFF THE TROY VOR OR OTHER TFC IN THE AREA. ENCLOSED IS A COPY OF THE RPT FROM ANOTHER SHOP WHICH VERIFIES THE INCORRECT SETTING WHICH WAS SUSPECTED, AND NOTE OF CORRECTIONS MADE.

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.