PLT WAS RETURNING FROM A NIGHT PROFICIENCY FLT AND DISCOVERED THAT THE XPONDER WAS INOP AT THE SAME TIME HE WAS ENTERING THE 30 NM CLASS B MODE C VEIL. THE FLT WAS CONTINUED TO THE DEST AND THE LATER INSPECTION REVEALED THAT THE XPONDER WAS INOP. ACFT EQUIP PROB LESS SEVERE. FAR MODE C VEIL XPONDER OP.

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I DEPARTED ARB ON A LCL PROFICIENCY FLT APPROX 30 MINS AFTER SUNSET. I PRACTICED STEEP TURNS APPROX 15 NM W OF ARB FOR 30 MINS. I THEN LANDED AT JXN WHERE I COMPLETED 4 FULL STOP TKOFS AND LNDGS FOR NIGHT CURRENCY BEFORE RETURNING TO ARB. IT WAS A BEAUTIFUL CLR NIGHT. AS I APCHED ARB THE LORAN FLASHED A WARNING ABOUT THE DTW CLASS B AIRSPACE AND THE MODE C VEIL. AT THAT TIME I NOTICED THAT THE XPONDER WAS NOT FLASHING. I CHKED THE DIM SWITCH; THE XPONDER TEST BUTTON; CIRCUIT BREAKER AND SECURITY OF THE UNIT IN THE PANEL; BUT STILL NO LIGHT. I WAS NOT SURE IF A BULB WAS BURNED OUT OR IF THE XPONDER WAS INOP. THE TWR HAD NOT MENTIONED A PROB ON DEP OR ON AN EARLIER FLT THAT DAY. THE UNIT WAS CHKED IN NOVEMBER FOR COMPLIANCE WITH FARS. THAT MEANT LESS THAN 10 HRS OF OP. THE TWR WAS NOW CLOSED; I WAS WORKING CTAF ON THE TWR FREQ AND SETTING UP FOR LNDG. I COULD HAVE CALLED DTW APCH; BUT BY THEN I WAS NEARLY IN THE PATTERN AND THAT WOULD ONLY HAVE DISTRACTED ME FROM LNDG. I DID GET THE UNIT CHKED RIGHT AWAY AND FOUND THAT IT WAS INOP. I DO NOT KNOW WHEN THE UNIT FAILED. IT WAS NOT DARK WHEN I LEFT AND THE LIGHT IS NOT VERY BRIGHT. USUALLY I FLY IFR AND ATC OR APCH LETS ME KNOW RIGHT AWAY IF THERE IS A PROB.