Narrative:

Launched from ZZZ for aerial advertising purposes in the commercial airship. Cruised west to a position about 8 NM by onboard GPS with detroit wayne metropolitan airport selected. About 1 hour alter when overflying pontiac, ptk, tower passed a telephone for TRACON quality control. Spoke with mrs X. She advised approach indicated the ship was inside class B airspace at 6 NM from dtw. The next morning after discussing the situation with mrs X, I began another 5 hour sortie at about XA00 am. This time, I was in contact with approach. Upon returning to the exact point of 045 degree radial, 7.2 NM. The approach controller gave me a range of 7.5 NM. I asked him if I was outside or inside class B. He said with his class B overlay, that I was inside. There seems to be some discrepancy between the detroit sectional, the detroit terminal and the apparent class be overlay used by ATC. I checked 4 other ranges in different locations in the nne quadrant with up to 1.5 NM discrepancies between our GPS distance and the range read by ATC. The detroit sectional shows 7 NM 8000 ft to sfc ring to be centered on the airport and is drawn that way on the sectional chart. The detroit terminal shows the 7 NM 8000 ft to sfc ring which appears to be centered on the 'detroit metropolitan DME antenna' location indicated by (.) at the approach end of runway 22L. Using a measuring tool, the DME antenna is about 1.5 NM from the airport reference point which seems to match the database information in our GPS which is our only distance reading instrument. In any case, map reading shows line features which puts the ford proving ground outside the 7 NM ring shown on the charts and ATC said I was 7.5 NM, and only when he looked at the class B overlay did it appear I was inside class B. Every effort was made on my part in this very large but very slow moving blimp to remain beyond 7 NM. Could it be that the 7 NM 8000 ft-sfc curtain should be centered on the airport reference point rather than the VOR/DME 'dxo' position on the 'dtw' DME antenna position? To me, there appears to be 2 different points of reference for where the actual 8000 ft-sfc curtain is located. Callback conversation with reporter revealed the following information: reporter stated was flying company executives near a plant east of dtw airport. After being advised by ATC his aircraft was in class B airspace, he disregarded his GPS and continued the flight visually using landmarks to navigation clear of class B airspace. He agreed he should not have used the airport center to measure distance. It was explained the class B segments are marked off from the ILS/DME antenna (idtw) which is clearly marked on the terminal chart.

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Original NASA ASRS Text

Title: BLIMP PLT BELIEVES THERE IS A DISTANCE DISPARITY BTWN THE DTW CLASS B TERMINAL CHART AND THE CLASS B ON THE DTW SECTIONAL CHART.

Narrative: LAUNCHED FROM ZZZ FOR AERIAL ADVERTISING PURPOSES IN THE COMMERCIAL AIRSHIP. CRUISED W TO A POS ABOUT 8 NM BY ONBOARD GPS WITH DETROIT WAYNE METRO ARPT SELECTED. ABOUT 1 HR ALTER WHEN OVERFLYING PONTIAC, PTK, TWR PASSED A TELEPHONE FOR TRACON QUALITY CTL. SPOKE WITH MRS X. SHE ADVISED APCH INDICATED THE SHIP WAS INSIDE CLASS B AIRSPACE AT 6 NM FROM DTW. THE NEXT MORNING AFTER DISCUSSING THE SIT WITH MRS X, I BEGAN ANOTHER 5 HR SORTIE AT ABOUT XA00 AM. THIS TIME, I WAS IN CONTACT WITH APCH. UPON RETURNING TO THE EXACT POINT OF 045 DEG RADIAL, 7.2 NM. THE APCH CTLR GAVE ME A RANGE OF 7.5 NM. I ASKED HIM IF I WAS OUTSIDE OR INSIDE CLASS B. HE SAID WITH HIS CLASS B OVERLAY, THAT I WAS INSIDE. THERE SEEMS TO BE SOME DISCREPANCY BTWN THE DETROIT SECTIONAL, THE DETROIT TERMINAL AND THE APPARENT CLASS BE OVERLAY USED BY ATC. I CHKED 4 OTHER RANGES IN DIFFERENT LOCATIONS IN THE NNE QUADRANT WITH UP TO 1.5 NM DISCREPANCIES BTWN OUR GPS DISTANCE AND THE RANGE READ BY ATC. THE DETROIT SECTIONAL SHOWS 7 NM 8000 FT TO SFC RING TO BE CTRED ON THE ARPT AND IS DRAWN THAT WAY ON THE SECTIONAL CHART. THE DETROIT TERMINAL SHOWS THE 7 NM 8000 FT TO SFC RING WHICH APPEARS TO BE CTRED ON THE 'DETROIT METRO DME ANTENNA' LOCATION INDICATED BY (.) AT THE APCH END OF RWY 22L. USING A MEASURING TOOL, THE DME ANTENNA IS ABOUT 1.5 NM FROM THE ARPT REF POINT WHICH SEEMS TO MATCH THE DATABASE INFO IN OUR GPS WHICH IS OUR ONLY DISTANCE READING INST. IN ANY CASE, MAP READING SHOWS LINE FEATURES WHICH PUTS THE FORD PROVING GND OUTSIDE THE 7 NM RING SHOWN ON THE CHARTS AND ATC SAID I WAS 7.5 NM, AND ONLY WHEN HE LOOKED AT THE CLASS B OVERLAY DID IT APPEAR I WAS INSIDE CLASS B. EVERY EFFORT WAS MADE ON MY PART IN THIS VERY LARGE BUT VERY SLOW MOVING BLIMP TO REMAIN BEYOND 7 NM. COULD IT BE THAT THE 7 NM 8000 FT-SFC CURTAIN SHOULD BE CTRED ON THE ARPT REF POINT RATHER THAN THE VOR/DME 'DXO' POS ON THE 'DTW' DME ANTENNA POS? TO ME, THERE APPEARS TO BE 2 DIFFERENT POINTS OF REF FOR WHERE THE ACTUAL 8000 FT-SFC CURTAIN IS LOCATED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED WAS FLYING COMPANY EXECUTIVES NEAR A PLANT E OF DTW ARPT. AFTER BEING ADVISED BY ATC HIS ACFT WAS IN CLASS B AIRSPACE, HE DISREGARDED HIS GPS AND CONTINUED THE FLT VISUALLY USING LANDMARKS TO NAV CLR OF CLASS B AIRSPACE. HE AGREED HE SHOULD NOT HAVE USED THE ARPT CTR TO MEASURE DISTANCE. IT WAS EXPLAINED THE CLASS B SEGMENTS ARE MARKED OFF FROM THE ILS/DME ANTENNA (IDTW) WHICH IS CLRLY MARKED ON THE TERMINAL CHART.

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.