Narrative:

Upon initial contact with potomac; I informed them I was 5 miles northeast of the westminister VOR; level at 2000 ft and was requesting my ADIZ and bravo airspace clearance back into bwi. After receiving a transponder code of XXXX; I was told to proceed direct to bwi. The time was xa:00 pm local as my ADIZ flight plan back to my originating airport (bwi) was previously filed for xa:00 pm. At approximately 5 miles south of the VOR; I was given a vector of 120 degrees and told to contact potomac now on frequency 119.00. Upon initial call-up; ATC queried my altitude and after I responded 'level at 2000 ft' he informed me that my mode C readout was intermittent. I informed ATC that I would recycle my transponder and after I did such; ATC responded that the mode C capability had been lost. I was then given a heading of 360 degrees and directed to exit the ADIZ; which; I complied with immediately. After exiting the ADIZ; the controller told me to 'squawk VFR; sorry about this and have a good day.' now outside the ADIZ; I reestablished contact with potomac on 119.70 and asked for a transponder check. The controller replied that he indeed had my 1200 code on radar; 3 miles east of the westminster VOR; but no mode C. Upon taking the aircraft to an avionics shop located on the field; the technicians informed me of two points of interest. 1) the transponder mode C bench test was only operating intermittently at best and that they needed to keep the aircraft over the weekend for further tests. 2) that I was perhaps the 40TH customer within several months stopping at the repair station for the same complaint. Apparently; the technicians voiced their suspicions for a 'dead area' just south of the westminster VOR where even they had experienced first-hand; potomac's inability to properly pick up mode C transponder capabilities at and below 2000 ft MSL. Although my transponder was reported as 'intermittent operation of mode C at best;' I thought this unusual occurrence should be noted.

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Original NASA ASRS Text

Title: A C172 PLT EXPRESSED CONCERN REGARDING MODE C RECEPTION BY PCT S OF EMI VOR.

Narrative: UPON INITIAL CONTACT WITH POTOMAC; I INFORMED THEM I WAS 5 MILES NE OF THE WESTMINISTER VOR; LEVEL AT 2000 FT AND WAS REQUESTING MY ADIZ AND BRAVO AIRSPACE CLRNC BACK INTO BWI. AFTER RECEIVING A XPONDER CODE OF XXXX; I WAS TOLD TO PROCEED DIRECT TO BWI. THE TIME WAS XA:00 PM LOCAL AS MY ADIZ FLT PLAN BACK TO MY ORIGINATING ARPT (BWI) WAS PREVIOUSLY FILED FOR XA:00 PM. AT APPROXIMATELY 5 MILES S OF THE VOR; I WAS GIVEN A VECTOR OF 120 DEGS AND TOLD TO CONTACT POTOMAC NOW ON FREQ 119.00. UPON INITIAL CALL-UP; ATC QUERIED MY ALT AND AFTER I RESPONDED 'LEVEL AT 2000 FT' HE INFORMED ME THAT MY MODE C READOUT WAS INTERMITTENT. I INFORMED ATC THAT I WOULD RECYCLE MY XPONDER AND AFTER I DID SUCH; ATC RESPONDED THAT THE MODE C CAPABILITY HAD BEEN LOST. I WAS THEN GIVEN A HDG OF 360 DEGS AND DIRECTED TO EXIT THE ADIZ; WHICH; I COMPLIED WITH IMMEDIATELY. AFTER EXITING THE ADIZ; THE CTLR TOLD ME TO 'SQUAWK VFR; SORRY ABOUT THIS AND HAVE A GOOD DAY.' NOW OUTSIDE THE ADIZ; I REESTABLISHED CONTACT WITH POTOMAC ON 119.70 AND ASKED FOR A XPONDER CHECK. THE CTLR REPLIED THAT HE INDEED HAD MY 1200 CODE ON RADAR; 3 MILES E OF THE WESTMINSTER VOR; BUT NO MODE C. UPON TAKING THE ACFT TO AN AVIONICS SHOP LOCATED ON THE FIELD; THE TECHNICIANS INFORMED ME OF TWO POINTS OF INTEREST. 1) THE XPONDER MODE C BENCH TEST WAS ONLY OPERATING INTERMITTENTLY AT BEST AND THAT THEY NEEDED TO KEEP THE ACFT OVER THE WEEKEND FOR FURTHER TESTS. 2) THAT I WAS PERHAPS THE 40TH CUSTOMER WITHIN SEVERAL MONTHS STOPPING AT THE REPAIR STATION FOR THE SAME COMPLAINT. APPARENTLY; THE TECHNICIANS VOICED THEIR SUSPICIONS FOR A 'DEAD AREA' JUST S OF THE WESTMINSTER VOR WHERE EVEN THEY HAD EXPERIENCED FIRST-HAND; POTOMAC'S INABILITY TO PROPERLY PICK UP MODE C XPONDER CAPABILITIES AT AND BELOW 2000 FT MSL. ALTHOUGH MY XPONDER WAS RPTED AS 'INTERMITTENT OPERATION OF MODE C AT BEST;' I THOUGHT THIS UNUSUAL OCCURRENCE SHOULD BE NOTED.

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.