MD269 HELI PLT FAILED TO ASSURE THAT THE XPONDER WAS TURNED ON DURING FLT UNDER CLASS B OUTER RING. HE NOTICED THE LACK OF THE GREEN LIGHT BLINKING AND RECYCLED THE ON AND OFF SWITCH TO THE ON POS.

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AT APPROX XA00 ON NOV/XA/01; I DEPARTED HWD DESTINED FOR RHV. THE FLT WAS CONDUCTED IN A HELI AT 500-700 FT MSL. APPROX ABEAM SONOL PASS; I NOTICED THAT THE 'REPLY' LIGHT ON THE RADAR XPONDER WAS NOT FLASHING. THE XPONDER ON/OFF/MODE SWITCH MAY HAVE BEEN SET BTWN THE 'ON' AND THE 'ALT' POS (1/2 WAY BTWN THE SWITCH DETENTS). I CYCLED THE SWITCH TO THE 'OFF' AND BACK TO THE 'ALT' POS; AND THE 'REPLY' LIGHT BEGAN FLASHING. I MAY HAVE BEEN OPERATING INSIDE THE SFO MODE C VEIL FOR APPROX 10 MINS WITHOUT AN ACTIVE XPONDER OR WITHOUT REPLYING WITH MODE C ALT DATA. MY RTE OF FLT WAS UNDER THE CLASS B AIRSPACE SEGMENTS; AND AT NO TIME DID I ENTER CLASS B AIRSPACE. SUGGESTIONS FOR PREVENTION: THE ONLY POSITIVE INDICATION THAT A XPONDER IS OPERATING IS THE FLASHING 'REPLY' LIGHT. SINCE HELIS OFTEN OPERATE AT LOW ALT; IT IS CONSEQUENTLY COMMON THAT THE LIGHT ISN'T FLASHING EVEN THOUGH THE XPONDER IS OPERATING NORMALLY. USE OF THE 'TEST' FUNCTION ISN'T PRACTICAL DUE TO THE NEED TO CONTINUALLY 'HAND FLY' MOST HELIS. THERE SHOULD BE SOME OTHER INDICATION THAT A XPONDER IS OPERATING THE 'REPLY' LIGHT AND THE MODE SWITCH POS.