Narrative:

I was performing aerial mapping of highway 24 from peterson AFB to calhan. The altitude I was trying to maintain for this project was 17900 ft MSL indicated. My plan for the flight was to remain clear of class a airspace and avoid filing an IFR flight plan. I had received cos local ATIS information and set my altimeter to 29.68. While flying the project flight line down highway 24, I noticed my indicated altitude was reaching FL180 and promptly corrected for 17900 ft MSL to avoid class a airspace. Throughout the whole project, I was not in communication with any controling agency. I was monitoring cos approach. After completing the highway 24 project I left the area and descended to the north for more work in the centennial area. I contacted apa tower for clearance into their airspace. Centennial switched me to 123.7. I contacted the tower and gave them the information for my survey flight and was told to 'proceed as request.' apa tower called for an identify and gave me a phone number to contact ZDV. Upon landing, I contacted the watch supervisor at ZDV. He informed me that I was operating at FL185 in the cos and came within 100 ft of an airliner. I told him that I was attempting to remain clear of class a airspace and suspected my mode C was in error. The supervisor told me my mode C was fine because the identify at apa was correct, and the airliner could visually tell that I was at FL185. Throughout the flight, my intent was to remain clear of class a airspace. In the future, I plan to file an IFR flight plan for any project over 17000 ft MSL to avoid any possible airspace violation.

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Original NASA ASRS Text

Title: A C401 PHOTO PLT ON A NO FLT PLAN FLT HAS AN INCURSION INTO CLASS A AIRSPACE AND HAS AN NMAC WITH AN ACR ACFT NE OF COS, CO.

Narrative: I WAS PERFORMING AERIAL MAPPING OF HWY 24 FROM PETERSON AFB TO CALHAN. THE ALT I WAS TRYING TO MAINTAIN FOR THIS PROJECT WAS 17900 FT MSL INDICATED. MY PLAN FOR THE FLT WAS TO REMAIN CLR OF CLASS A AIRSPACE AND AVOID FILING AN IFR FLT PLAN. I HAD RECEIVED COS LCL ATIS INFO AND SET MY ALTIMETER TO 29.68. WHILE FLYING THE PROJECT FLT LINE DOWN HWY 24, I NOTICED MY INDICATED ALT WAS REACHING FL180 AND PROMPTLY CORRECTED FOR 17900 FT MSL TO AVOID CLASS A AIRSPACE. THROUGHOUT THE WHOLE PROJECT, I WAS NOT IN COM WITH ANY CTLING AGENCY. I WAS MONITORING COS APCH. AFTER COMPLETING THE HWY 24 PROJECT I LEFT THE AREA AND DSNDED TO THE N FOR MORE WORK IN THE CENTENNIAL AREA. I CONTACTED APA TWR FOR CLRNC INTO THEIR AIRSPACE. CENTENNIAL SWITCHED ME TO 123.7. I CONTACTED THE TWR AND GAVE THEM THE INFO FOR MY SURVEY FLT AND WAS TOLD TO 'PROCEED AS REQUEST.' APA TWR CALLED FOR AN IDENT AND GAVE ME A PHONE NUMBER TO CONTACT ZDV. UPON LNDG, I CONTACTED THE WATCH SUPVR AT ZDV. HE INFORMED ME THAT I WAS OPERATING AT FL185 IN THE COS AND CAME WITHIN 100 FT OF AN AIRLINER. I TOLD HIM THAT I WAS ATTEMPTING TO REMAIN CLR OF CLASS A AIRSPACE AND SUSPECTED MY MODE C WAS IN ERROR. THE SUPVR TOLD ME MY MODE C WAS FINE BECAUSE THE IDENT AT APA WAS CORRECT, AND THE AIRLINER COULD VISUALLY TELL THAT I WAS AT FL185. THROUGHOUT THE FLT, MY INTENT WAS TO REMAIN CLR OF CLASS A AIRSPACE. IN THE FUTURE, I PLAN TO FILE AN IFR FLT PLAN FOR ANY PROJECT OVER 17000 FT MSL TO AVOID ANY POSSIBLE AIRSPACE VIOLATION.

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.