Confusion
Aircraft Equipment Problem Critical; Inflight Event / Encounter Other / Unknown
Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification; General Maintenance Action
Aircraft; MEL
B737-700 aircraft; in cruise at FL400; Center directed a descent to FL360. Shortly after initiating the descent; I noticed the cabin altitude climbing at 1;500 FPM. The Captain and I began scanning for reasons for this loss of pressurization; however all indications seemed normal (Bleed pointers sensed; outflow valve closed; pressurization controller in alternate per MEL). I donned my mask in anticipation of receiving the Cabin Altitude (Alt) Horn and got the QRH ready. Before the cabin exceeded 10;000 FT; the aircraft leveled off at our assigned altitude of FL360. The resulting increase in thrust immediately restored control of our cabin. We asked ATC for a constant descent at a reduced rate so that we could keep thrust on the engines; and had an uneventful approach and landing. The ALT Warning horn never went off. Maintenance later identified the cause of the failure to be a loose clamp in the AFT pressure bulkhead. System malfunction.
Reporter stated the APU was 'Off'; not operating. Only engine bleeds were being used for the air conditioning packs. He felt a pressure change in his ears shortly after they started their descent from FL400. That's when he looked up at the overhead P-5 panel expecting to see some indication for what he felt in his ears. But all indications were normal; except for the 1;500 FPM cabin rate of climb. Both engines were at flight idle when they were descending. When the B737-700 leveled off at FL360 and the thrust increased; cabin control was restored.Reporter stated the auto mode for the cabin pressure controller was previously deferred (MEL'd); so they were using the Alternate (ALT) Mode per their MEL. On the ground; Maintenance pressurized the aircraft during a two-hour layover and said the APU body fuselage duct was leaking at a clamp where an APU duct shroud is located at the aft pressure bulkhead.
Pilot reports that shortly after initiating descent from FL400; he noticed the cabin altitude climbing at 1;500 FPM on their Next Gen B737-700 aircraft. Before the cabin altitude exceeded 10;000 FT; the aircraft leveled off at FL360 with an increase in thrust that immediately restored control of the cabin. Maintenance found a loose clamp in aft pressure bulkhead.