MD80 AND B727 HAD LESS THAN LEGAL SEPARATION.

Narrative

I WAS THE COPLT ON A COMMERCIAL AIR TRANSPORT FLT ON DEC/WED/98. APPROX 1 HR 45 MINS INTO THE FLT I EXCUSED MYSELF FROM THE COCKPIT TO GO TO THE LAVATORY. I WAS GONE FROM THE COCKPIT 3-4 MINS. WHILE IN THE LAVATORY I NOTICED THAT THE ACFT WAS BEGINNING A DSCNT. AS I RETURNED TO THE COCKPIT AND SAT DOWN I SAW THAT THE ACFT WAS IN A DSNDING TURN TO THE R. I SAW THAT THE ALT WAS SET AND ARMED TO LEVEL OFF AT FL300 AND THAT THE HEADING WAS SET TO SOMEWHERE AROUND 090 DEGS. THE CAPT BEGAN TO BRIEF ME ON OUR STATUS; WHEN ATC ASKED FOR OUR ALT. I BELIEVE THEY SAID WE SHOULD BE AT FL310. THE CAPT IMMEDIATELY DISCONNECTED THE AUTOPLT AT FL306 AND RETURNED TO FL310. I SAW ON TCASII THAT A TARGET WITH A HOLLOW BLUE DIAMOND WAS 3-4 MI BEHIND US AND 1600 FT BELOW US. WE WERE ON APPROX A 090-100 DEG HDG. THE TARGET WAS MOVING FROM OUR 5 O'CLOCK POS TO OUR 7 O'CLOCK POS. FROM THE TIME THAT I ENTERED THE COCKPIT TO WHEN WE LEVELED OFF AT FL310 WAS APPROX 30 SECONDS. I HAD NOT EVEN HAD TIME TO FASTEN MY SEAT BELT OR PUT ON MY HEADSET. I WAS ABLE TO HEAR ATC BECAUSE MY OVERHEAD SPEAKER WAS ON. SUPPLEMENTAL INFO FROM ACN 423601: MD80 ON A 100 DEG VECTOR FOR SPACING; WAS TOLD TO DSND TO FL310 FROM FL330. THE MD80 READ BACK THE CLRNC CORRECTLY. THE MD80 THEN DSNDED THROUGH FL310; AND HALTED HIS DSCNT AT FL306 WHEN ASKED IF HE WAS LEVEL AT FL310 AND REMINDED THAT FL310 WAS HIS DSCNT CLRNC. THE MD80 THEN ADVISED THAT HE WAS DSNDING TO FL300. THE MD80 PROMPTLY CLBED BACK TO FL310. WHILE AT FL306; THE MD80 LOST APPROVED SEPARATION WITH A B727 NBOUND AT FL290.