A B737 FLC DECLARES AN ELECTRICAL PWR SOURCE EMER IN ORDER TO GET A LOWER ALT WHILE DIVERTING TO BHM AFTER LOSS OF THEIR APU.

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WE WERE SCHEDULED TO FLY 6 LEGS. MAF-HOU-SAT-HOU-MSY-IND-MDW. WE WERE THE ORIGINATING CREW FROM DEPARTING MAF AT AM45 CDT. THE ACFT CHK OUT OK AND AFTER PUSHBACK AND ENG START THE CAPT WENT TO PUT THE R ENG GENERATOR ON THE #2 BUS AND HE HAD TO USE THE SWITCH 2 TIMES FOR IT TO TAKE. THE FLT FROM MAF-HOU WAS NORMAL. AFTER PUSHBACK AT HOU THE CAPT TRIED TO PUT THE R ENG GENERATOR ON THE #2 BUS AND IT WOULD NOT STAY ON-LINE. WE WENT BACK TO THE GATE AND MAINT CAME OUT TO THE ACFT AND REPLACED A RELAY. THEN WE PUSHED BACK AGAIN AND STILL HAD THE SAME PROB AND WE WENT BACK TO THE GATE AGAIN. MAINT CAME BACK OUT AGAIN AND DEFERRED THE R ENG GENERATOR. NOW WE HAD TO USE THE APU GENERATOR FOR THE #2 BUS; AS PER THE MEL. WE FLEW FROM HOU-SAT-HOU-MSY WITH NO PROBS. ON THE LEG FROM MSY-IND WE WERE LEVEL AT 33000 FT MSL JUST S OF THE MERIDIAN VOR (MEI) WHEN THE #2 BUS TRIPPED OFF. THE CAPT TURNED THE APU GENERATOR SWITCH BACK ON. AFTER ABOUT 4 MINS IT DID THE SAME THING AGAIN; BUT THIS TIME AFTER RESETTING THE SWITCH IT TRIPPED OFF AFTER ABOUT 30 SECONDS. ALSO AT THIS SAME TIME WE COULD FEEL A SLIGHT RUMBLE OF THE ACFT; LIKE WHEN THE SPD BRAKES WERE DEPLOYED; BUT IN THIS CASE THEY WERE NOT. THE FLT ATTENDANTS CALLED US AND SAID THAT THEY COULD HEAR A NOISE COMING FROM THE AFT SECTION OF THE ACFT THAT KIND OF SOUNDED LIKE A CAR BACKFIRING AND THAT SOME OF THE PAX WERE GETTING WORRIED. AFTER THE CAPT AND I TALKED ABOUT WHAT WAS GOING ON AND WHAT WE HAVE AND DON'T HAVE AND RUNNING THE QRH WE GUESSED THE APU GENERATOR WAS GONE AND WE WERE NOW DOWN TO 1 PWR SOURCE PLUS BATTERY PWR. WE SHUT DOWN THE APU. AT THIS POINT WE TALKED TO DISPATCH AND DECIDED TO DIVERT TO BHM. I TOLD CTR THAT WE WERE DIVERTING TO BHM AND THAT WE NEED A LOWER ALT. CTR TOLD ME TO SWITCH FREQ AND MAKE THE REQUEST WITH THEM. I CHK ON AT FL330 AND TOLD HIM THAT WE WERE DIVERTING TO BHM AND THAT WE NEEDED A LOWER ALT. HE SAID THAT WE KIND OF TOLD HIM LATE ABOUT DIVERTING AND HE HAD A LOT OF TFC AND WOULD WORK US DOWN. I WENT OFF FREQ TO TALK TO THE FLT ATTENDANTS TO TELL THEM THAT WE WERE DIVERTING TO BHM AND IF THEY WOULD TELL THE PAX AND ALSO THAT THE CAPT WOULD TALK TO THE PAX IN A FEW MINS. WHEN I CAME BACK ON FREQ THE CAPT HAD RECEIVED CLRNC FOR A LOWER ALT; BUT THE ONLY WAY HE COULD GET ONE QUICKLY WAS BY DECLARING AN EMER. CTR ASKED WHAT THE NATURE OF THE EMER WAS AND THE CAPT TOLD HIM THAT WE'RE DOWN TO 1 GENERATOR. CTR HAD US TURN ABOUT 15 DEGS TO THE L AND GAVE US A LOWER ALT. AFTER I GOT THE ATIS AND RAN THE PERFORMANCE DATA I CHK ON WITH BHM APCH AND TOLD THEM THAT WE DO NOT NEED ANY EMER EQUIP. THE REST OF THE FLT AND LNDG AT BHM WAS NORMAL AND UNEVENTFUL. THE ONLY REASON ON DECLARING AN EMER WAS BECAUSE OF CTR'S DELAY IN GRANTING US A LOWER ALT WITH US ONLY BEING 130 NM FROM BHM WE NEEDED TO START DOWN. THE CREW DID A GOOD JOB WORKING TOGETHER AND WITH DISPATCH ON GETTING THE ACFT AND ITS PEOPLE DOWN SAFELY. THE ONLY THING THAT COULD HAVE BEEN BETTER WAS WITH CTR WHEN WE TOLD HIM THAT WE WERE DIVERTING TO AN ARPT WAY SHORT OF OUR DEST AND THERE WAS NO WX AROUND HE SHOULD HAVE FIGURED THAT SOMETHING ELSE WAS UP. WHICH CTR IT WAS I DO NOT REMEMBER BECAUSE WE WERE CHANGING SECTORS AND FREQS VERY FAST; BUT IT WAS RIGHT NEAR MERIDIAN VOR.