Narrative:

While climbing out passing approximately FL290; the cabin altitude horn sounded with the cabin altitude warning light. We immediately donned our masks and complied with the warning horn or warning light cabin altitude checklist. The first officer was the pilot flying and we transferred aircraft control; so he could run the checklist and monitor the pressurization panel. We ran the cabin altitude warning/abnormal pressurization checklist and could not control the cabin altitude. We then initiated an emergency descent and declared an emergency with ATC. ATC asked if we wanted to return and we confirmed that was our plan. We received a clearance direct to the airport and descended to their MVA of 16;000 feet.we informed the flight attendants of the situation during the descent and asked them if they could see anything that would cause us to lose pressurization since both packs were operating as advertised and there was no cockpit indication of why we were losing pressurization (door open lights; etc.). They claimed everything was fine in the cabin. Then we informed the passengers with a brief synopsis of the incident and our plan to return. The cabin descended below 10;000 feet a few minutes after we leveled off at 16;000 feet. The first officer contacted operations of our return and reason for return. The rest of the arrival and landing was uneventful. Emergency equipment was standing by and; while taxiing to the gate; we terminated the emergency. Maintenance met us at the gate and we thoroughly briefed them on the incident.during a maintenance walk around of the aircraft after the incident; there was a bungee cord hanging out of the aft cargo door. We had an MEL on the cargo door hold open mechanism/device. I will now ask ramp specifically about door condition prior to pushback if there is an MEL dealing with any aircraft access. Pressurization was checked on climb checklist. No one 'felt' the cabin climbing. I need a better scan to include cabin altimeter during the climb up to altitude.

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Original NASA ASRS Text

Title: A B737-300 flight crew declared an emergency and returned to their departure airport upon receipt of a cabin altitude warning as they climbed through FL290.

Narrative: While climbing out passing approximately FL290; the Cabin Altitude Horn sounded with the Cabin Altitude Warning light. We immediately donned our masks and complied with the Warning Horn or Warning Light Cabin Altitude Checklist. The First Officer was the Pilot Flying and we transferred aircraft control; so he could run the checklist and monitor the pressurization panel. We ran the Cabin Altitude Warning/Abnormal Pressurization Checklist and could not control the cabin altitude. We then initiated an emergency descent and declared an emergency with ATC. ATC asked if we wanted to return and we confirmed that was our plan. We received a clearance direct to the airport and descended to their MVA of 16;000 feet.We informed the flight attendants of the situation during the descent and asked them if they could see anything that would cause us to lose pressurization since both packs were operating as advertised and there was no cockpit indication of why we were losing pressurization (door open lights; etc.). They claimed everything was fine in the cabin. Then we informed the passengers with a brief synopsis of the incident and our plan to return. The cabin descended below 10;000 feet a few minutes after we leveled off at 16;000 feet. The First Officer contacted Operations of our return and reason for return. The rest of the arrival and landing was uneventful. Emergency equipment was standing by and; while taxiing to the gate; we terminated the emergency. Maintenance met us at the gate and we thoroughly briefed them on the incident.During a Maintenance walk around of the aircraft after the incident; there was a bungee cord hanging out of the aft cargo door. We had an MEL on the cargo door hold open mechanism/device. I will now ask Ramp specifically about door condition prior to pushback if there is an MEL dealing with any aircraft access. Pressurization was checked on Climb Checklist. No one 'felt' the cabin climbing. I need a better scan to include cabin altimeter during the climb up to altitude.

Data retrieved from NASA's ASRS site as of July 2013 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.