FLC OF B757 UNABLE TO PRESSURIZE CABIN WITHOUT USE OF AIR CONDITIONING PACK WHICH IS MEL'ED. CONCURRING WITH DISPATCH THEY DECIDE TO DIVERT AND CHANGE PLANES.

Narrative

I PICKED UP THE DISPATCH RELEASE FOR OUR FLT FROM DTW TO SEA AND NOTICED THAT WE WERE DISPATCHED WITH 1 AIR- CONDITIONING PACK MEL'ED AS BEING INOP. ONBOARD THE ACFT; I CHKED THE MEL BOOK FOR ANY RESTRS THAT THERE MIGHT BE FOR FLT WITH 1 OF THE 2 AIR-CONDITIONING PACKS INOP AND FOUND OUT THAT THE MAX ALT WAS RESTR TO FL350. THE FLT PLAN WAS FILED FOR A CRUISE ALT OF FL390. I CONTACTED DISPATCH AND HAD THE CRUISE ALT AMENDED TO FL350. AIRBORNE; I CHKED THE CABIN ALT ONCE WE WERE LEVELED OFF AT FL350; AND NOTICED THAT THE CABIN ALT WAS AT 7000 FT AND STILL CLBING AT 200 FPM. NORMAL CABIN ALT AT FL350 SHOULD BE 5600 FT. THE ONLY WAY TO STOP THE SLOW CABIN CLB WAS TO TURN ON THE R (MEL'ED INOP) AIR-CONDITIONING PACK. WITH THE R AIR-CONDITIONING PACK ON; THE CABIN ALT DSNDED BACK TO 5600 FT. DISPATCH WAS CONTACTED AND THE DECISION WAS MADE TO DIVERT TO OUR MAIN BASE SINCE THE ACFT WAS UNABLE TO MAINTAIN A NORMAL CRUISE ALT ON 1 AIR- CONDITIONING PACK. ENRTE TSTMS AND MOUNTAINOUS TERRAIN ALONG THE LATTER HALF OF THE RTE WERE MAJOR CONSIDERATIONS. ONCE ON THE GND AT OUR MAIN BASE; THE DECISION TO CHANGE ACFT WAS MADE. THE PAX WERE ADVISED THAT THE ACFT TO BE USED FOR THE CONTINUATION OF THE FLT WAS AT THE ADJACENT GATE. I ENDED UP BEING THE LAST PERSON OFF OF THE INBOUND ACFT. AS I ENTERED THE WAITING AREA; I MADE A MENTAL NOTE THAT THERE DIDN'T SEEM TO BE VERY MANY OF OUR PAX IN THE GATE AREA AND I WONDERED WHERE THEY MIGHT HAVE GONE OFF TO. AS I WAS TALKING TO THE AGENT HANDLING OUR FLT; ONE OF OUR FLT ATTENDANTS WALKED UP TO THE AGENT AND BEGAN TO CHASTISE HER FOR HAVING BOARDED OUR PAX ON THE OUTBOUND ACFT WITHOUT WAITING FOR THE CABIN CREW TO BOARD FIRST. THAT'S WHY THERE WERE SO FEW PAX IN THE GATE AREA! AFTER TALKING WITH THE AGENT I FOUND OUT THAT SHE HAD LITTLE EXPERIENCE WITH BOARDING FLTS -- SHE USUALLY WORKED IN THE STOCKROOM. SHE HAD BEEN CALLED OUT TO HANDLE OUR FLT SINCE WE ARRIVED AFTER THE AGENTS THAT NORMALLY HANDLED FLTS WENT HOME FOR THE NIGHT. LESSONS LEARNED: NEVER ASSUME EVERYTHING IS OK. THE FILED ALT ON THE FLT PLAN WAS WRONG. THE 'GOOD' AIR-CONDITIONING PACK WAS INCAPABLE OF MAINTAINING CABIN ALT. THE REASON WHY SO FEW PAX WERE IN THE GATE AREA WAS THAT THE AGENT HAD LET THEM ON BOARD THE ACFT WITHOUT HAVING THE FLT ATTENDANTS ON BOARD FIRST.