FLC OF AN ACR LGT LANDED AT DUSK WHEN THE RWY EDGE LIGHTS WERE NOT ILLUMINATED.

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FLYING VISUAL APCH USING ILS BACKUP; TO RWY 4R AT O'HARE. CHKED ON WITH TWR ON THE RADIO AT REKKS FAF AND WERE CLRED TO LAND. WE COULD SEE ACFT AHEAD OF US ON APCH AND THE RWY; ALTHOUGH EDGE LIGHTS WERE VERY LOW; OR APPEARED TO BE. AS WE DSNDED THROUGH 500 FT AGL THE ACFT AHEAD OF US TOLD TWR THE RWY EDGE LIGHTS WERE OFF. TWR SAID THEIR INDICATIONS WERE THEY WERE ON. WE WERE ADVISED OF THE RPT AND WE RESPONDED THAT WE COULD SEE THE RWY FINE AND THEN LANDED. THE TIME WAS 32 MINS AFTER OFFICIAL SUNSET WHICH WE LOOKED UP LATER. WE NEVER DOUBTED THE SAFETY OF THE APCH AND LNDG; AND ONLY HAD A FEW SECONDS FROM 500 FT TO LNDG TO DECIDE THE LEGALITY OF NO LIGHTS; SO WE LANDED UNEVENTFULLY. WE TALKED TO SEVERAL INSTRUCTORS AT OUR COMPANY TRAINING DEPT AND NO ONE COULD FIND OUT THE LEGAL ANSWER; AND MY OWN REVIEW OF FAR AND AIM FINDS NO SPECIFIC MENTION OF LNDG WITHOUT RWY LIGHTS. THE CAPT'S COMMENT TO ME WAS INTERESTING. HE SAID HE'D NEVER SEEN THAT HAPPEN IN 30 YRS OF FLYING; AND THAT I'D PROBABLY NEVER SEE IT AGAIN. EVEN SO; I'D LIKE TO KNOW.