B737-300
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 153; Flight Crew Type 153
Human-Machine Interface
Aircraft Equipment Problem Critical
Flight Crew Returned To Departure Airport; General Declared Emergency; General Maintenance Action
Aircraft
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.
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.