A FLT ATTENDANT'S COCKPIT ENTRY CAUSED THE CAPT TO BECOME DISTRACTED FROM MONITORING; AND FAIL TO DETECT THE FO'S ALTDEV.

Narrative

WE WERE GIVEN A DSCNT CLRNC TO CROSS 40 MI OUT OF ATL AT 12000 FT 250 KTS. AFTER READING BACK CLRNC FLT ATTENDANT CAME TO COCKPIT WITH A PAX BAGGAGE PROB. I WAS BUSY WITH HER AND CALLING THE COMPANY. THE FO WAS FLYING THE AIRPLANE AND HAD SET 11000 FT IN THE ALT WINDOW. HE SAID HE THOUGHT IT WAS A STRANGE ALT BUT THAT HE HEARD 11000 FT OVER THE RADIO. HE SAW THAT I WAS BUSY WITH THE PAX PROB AND DID NOT QUESTION ME. AFTER LEVELING AT 11000 FT ZTL CALLED AND SAID MAINTAIN 11000 FT AND ASKED IF HE HAD GIVEN US 12000 FT OR 11000 FT. I TOLD HIM HE GAVE US 12000 FT. HE SAID IT WOULD BE OKAY TO STAY AT 11000 FT. THERE WAS NO OTHER TFC NEAR US AND CAUSED NO PROBS. MY SUGGESTION TO KEEP THIS FROM HAPPENING AGAIN IS FOR AIRLINE MGMNT TO LESSEN FLC INVOLVEMENT IN PAX PROBS EXCEPT 'EMERS.' ALSO ON DSCNT THE SEATBELT SIGN SHOULD GO ON AT 18000 FT; ONCE ON; NO FLT ATTENDANT IS TO BRING ANY PAX PROBS TO THE CREW (EXCEPT EMERS) UNTIL ON THE GND AT THE GATE. I REALIZE THERE IS NO GOOD EXCUSE FOR AN ALTDEV. BUT AS LONG AS THERE ARE DISTRS THERE WILL BE DEVS. SUPPLEMENTAL INFO FROM ACN 304147: (I HAD PROGRAMMED 12000 FT INTO THE COMPUTER PREVIOUSLY AS THIS WAS A PUBLISHED ALT AND SPD STANDARD FOR THE LA GRANGE SIX STAR). HOWEVER; WHEN WE WERE GIVEN THIS SPD AND ALT RESTR (ASSIGNMENT); I MISUNDERSTOOD THE ALT TO BE 11000 FT (CAPT HAD ANSWERED THE CALL FROM ATL). EVEN THOUGH I THOUGHT THIS WAS AN UNUSUAL ALT FOR THE STAR; I DID NOT QUESTION THE CAPT (BECAUSE HE WAS TALKING TO A FLT ATTENDANT THAT HAD COME TO THE COCKPIT WITH A PAX PROB). I SET 11000 FT IN THE ALT WINDOW WHICH HE DIDN'T NOTICE BECAUSE OF THE FLT ATTENDANT DISTR.