Narrative:

This was the second leg of the day in this aircraft. During the first leg the aircraft press system worked fine. While climbing out from ZZZ at approximately FL350 the cabin altitude warning horn sounded. We immediately donned our oxygen masks and performed the loss of cabin pressurization checklist. While the captain was performing the checklist I leveled off and asked ATC for lower. I noticed the cabin continuing to climb rapidly; regardless of our actions. ATC gave us several 4000 ft leveloffs while clearing traffic below. We did declare an emergency during this process and started a turn towards ZZZ1 while continuing the emergency descent checklist. The flight attendants confirmed that the masks had dropped and everyone was fine. I was focusing on getting the aircraft to a lower altitude and setting up for an approach to ZZZ1 while the captain was working the checklist; getting landing data; coordinating with dispatch; etc. We landed uneventfully at ZZZ1. Maintenance met us at the gate and told us that a seal had broken. While I'm not sure what caused the seal to fail or if the cargo bin was just not shut properly; possibly we could have some sort of pictures of an improperly sealed cargo door for the pilots; rampers; etc; to reference.callback conversation with reporter revealed the following information: the reporter stated that he immediately identified the cabin altitude warning horn and did not confuse it with the takeoff configuration warning. A mechanic at the gate said a cargo door seal failed but the reporter almost immediately left for a backup aircraft and did not have time to investigate for himself. The most frustrating part of this event was the inability of ATC to clear the aircraft to a lower altitude more quickly because of traffic. They were given lower altitude in 4000 ft increments. The reporter does not believe the cabin got above about 20000 ft but he was busy flying and communicating while the captain was running the checklist and attempting to regain the pressurization.

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Original NASA ASRS Text

Title: A B737-300 RAPIDLY LOST CABIN PRESSURIZATION AT FL350 AFTER A CARGO DOOR SEAL FAILED.

Narrative: THIS WAS THE SECOND LEG OF THE DAY IN THIS AIRCRAFT. DURING THE FIRST LEG THE ACFT PRESS SYSTEM WORKED FINE. WHILE CLIMBING OUT FROM ZZZ AT APPROXIMATELY FL350 THE CABIN ALTITUDE WARNING HORN SOUNDED. WE IMMEDIATELY DONNED OUR OXYGEN MASKS AND PERFORMED THE LOSS OF CABIN PRESSURIZATION CHECKLIST. WHILE THE CAPTAIN WAS PERFORMING THE CHECKLIST I LEVELED OFF AND ASKED ATC FOR LOWER. I NOTICED THE CABIN CONTINUING TO CLIMB RAPIDLY; REGARDLESS OF OUR ACTIONS. ATC GAVE US SEVERAL 4000 FT LEVELOFFS WHILE CLEARING TRAFFIC BELOW. WE DID DECLARE AN EMERGENCY DURING THIS PROCESS AND STARTED A TURN TOWARDS ZZZ1 WHILE CONTINUING THE EMERGENCY DESCENT CHECKLIST. THE FLT ATTENDANTS CONFIRMED THAT THE MASKS HAD DROPPED AND EVERYONE WAS FINE. I WAS FOCUSING ON GETTING THE AIRCRAFT TO A LOWER ALTITUDE AND SETTING UP FOR AN APPROACH TO ZZZ1 WHILE THE CAPTAIN WAS WORKING THE CHECKLIST; GETTING LANDING DATA; COORDINATING WITH DISPATCH; ETC. WE LANDED UNEVENTFULLY AT ZZZ1. MAINTENANCE MET US AT THE GATE AND TOLD US THAT A SEAL HAD BROKEN. WHILE I'M NOT SURE WHAT CAUSED THE SEAL TO FAIL OR IF THE CARGO BIN WAS JUST NOT SHUT PROPERLY; POSSIBLY WE COULD HAVE SOME SORT OF PICTURES OF AN IMPROPERLY SEALED CARGO DOOR FOR THE PILOTS; RAMPERS; ETC; TO REFERENCE.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT HE IMMEDIATELY IDENTIFIED THE CABIN ALTITUDE WARNING HORN AND DID NOT CONFUSE IT WITH THE TAKEOFF CONFIGURATION WARNING. A MECHANIC AT THE GATE SAID A CARGO DOOR SEAL FAILED BUT THE REPORTER ALMOST IMMEDIATELY LEFT FOR A BACKUP ACFT AND DID NOT HAVE TIME TO INVESTIGATE FOR HIMSELF. THE MOST FRUSTRATING PART OF THIS EVENT WAS THE INABILITY OF ATC TO CLEAR THE ACFT TO A LOWER ALT MORE QUICKLY BECAUSE OF TFC. THEY WERE GIVEN LOWER ALT IN 4000 FT INCREMENTS. THE REPORTER DOES NOT BELIEVE THE CABIN GOT ABOVE ABOUT 20000 FT BUT HE WAS BUSY FLYING AND COMMUNICATING WHILE THE CAPT WAS RUNNING THE CHECKLIST AND ATTEMPTING TO REGAIN THE PRESSURIZATION.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.