Date: 201212
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: B737-300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Descent
Aircraft component: Pressurization System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer
Human factors: Communication Breakdown
Second reporter function: Captain
Anomaly: Aircraft Equipment Problem Critical
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; General Declared Emergency
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: While descending through 22;000 feet; we got the Cabin Altitude Warning horn. Captain called to don our masks. We complied with the emergency action Items. Looking at the cabin altitude; it showed below 10;000 FT (9;500 FT). I had looked at it just as we started the descent out of 24;000 FT and it was at 5;400 FT; which seemed where it should be for the landing altitude. I asked for an immediate descent and re-clarified an emergency descent. After about 30-60 seconds; the horn was out and the cabin was below 8;000 FT. The whole time the cabin was descending. We told the Controller we had a pressurization issue but it was under control. We were now at 13;000 FT MSL.    Communication was difficult at first as I had not gone to the mask position [on the comm panel] and the Captain wasn't hitting the interphone button to talk. We thought it odd that we felt no pressure spike in our ears. I think after a long day and 12 years at another carrier with the full masks that go to mask [mic] automatically; I forgot to go to mask at first (5 seconds).


Second narrative: Number two engine was slow to start but within limits. We sent an ACARS message to Maintenance enroute to have them meet the aircraft on arrival. Maintenance arrived and started to investigate. We assisted in engaging the starter on the #2 engine. Initially they thought 'it might be the turbofan. A while later they came back to the jetway and said it was the right pack and they were going to MEL it under 21-1-01. We talked with Dispatch and Maintenance Control and got a new Release. We departed and were in cruise at FL240 and the pressurization was normal throughout the takeoff; climb; and cruise phase. We were given a descent clearance for the arrival to 16;000 FT. We were on the VNAV path. At approximately FL220; we got the Cabin Altitude Warning horn. We immediately donned our masks and told ATC that we needed an immediate descent clearance to below 16;000 feet. As we descended below FL190; the cabin was back below 10;000 feet at approximately 8;000 FT. We talked to the F/A's and they were unaware of any pressurization concerns or ear pain. The Passengers did not have any concerns either. Neither the F/O nor I noticed any ear pain either. We landed uneventfully. One concern on the emergency descent was some difficulty communicating between Pilots. The F/O has a new Bose A20 headset and it took longer than we would expect to establish communications between us.I believe that perhaps there was another maintenance issue related to the right pack that caused the left pack not to be able to hold pressurization once the thrust levers were retarded for the descent. I have dispatched and flown a good number of flight over my career with a single pack operation and have never had a problem with holding the cabin.


Synopsis: B737-300 Flight Crew reports a cabin altitude warning horn descending through FL220. Masks are donned and communications established; with difficulty. The cabin is quickly below 10;000 feet and the flight continues to destination. The aircraft had been dispatched with a single pack.