Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 176; Flight Crew Type 10800
Training / Qualification; Distraction; Workload; Confusion
Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Aircraft Equipment Problem Dissipated; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action; General Maintenance Action
Aircraft
Descending to and leveling at FL340; the Cabin Altitude Warning horn sounded. After a few seconds of identifying the nature of the horn; we donned the oxygen masks and established communication with each other. We discovered the cabin altitude had reached 10;000. In the confusion of the emergency; the aircraft had descended through the altitude FL340.  Recognizing that we had in fact a cabin altitude warning quickly; such as a warning light that is not installed on this aircraft. We could have assessed the situation in a calm; coordinated manner. Until you experience a true pressurization emergency; it is hard to describe the confusion associated with something so simple. When you do the cabin altitude warning brief at the beginning of the day; really think about the sound and what that means you are going to do. Try donning your mask someday with the headsets and glasses and 'establish communication.' It is more distracting than you think when you are just briefing it. Because of this event; the lack of a Cabin Altitude Warning light in some aircraft seems to be an oversight on safety. When you get a horn or any warning for that matter; there needs to be a visual cue associated. I strongly advise the installation of the light on all aircraft.
When we received the aircraft from the inbound Crew; they complained the cabin was hot and had difficulty cooling it down.  We were flying to XXX and then XXX so we didn't consider the hot cabin to be a problem. The plane did feel warm; especially for a -700.  Shortly after takeoff; I selected the packs to high and around the mid 20s we received a right pack trip off. We ran the QRH and reset the tripped pack and placed the packs back to auto. Fast forward an hour and we were given a descent from FL 400 to FL 340. I was commenting to the Captain what an unusually clear day it was when the autopilot disengaged and the Cabin Altitude Warning horn went off. At this time; we were probably at FL344 still descending with a clearance to FL340. The autopilot disengaging confused us and the Captain was trying to silence the horn believing it was the Autopilot Disconnect horn. It suddenly dawned on me that it was the Cabin Altitude horn and I said out loud; 'don the masks.'   We put on masks and established Crew communication. At this time; we had flown through our altitude and were around FL335. I looked at the TCAS and there was no conflict of traffic; the closest aircraft within 40 NM was 3;500 feet below us. ATC asked us to say altitude and I told him we needed lower. We ran the [checklist] and noted the cabin was indicating at 10;000 feet. In the mid 20s; the Cabin Altitude Warning horn stopped.  We removed the masks and were able to communicate much more easily. The cabin was stable and decreasing so we stopped the descent at 16;000 feet and continued to XXX.  There were no lights associated with the horn and nothing on the pressurization panel. The masks did not drop.  After an uneventful landing in XXX; Maintenance grounded the plane.
A B737-700 pressurization warning alerted at the same time the autopilot disconnected while descending through FL345 to FL340. The Cabin Altitude Warning horn confused the crew who thought for a while it was the autopilot disconnect warning. Once the pressurization anomaly was detected the descent continued to 16;000 feet and the QRH completed.