B737 flight crew experiences cabin altitude warning horn climbing through FL300. Cabin altitude stabilizes nicely descending through FL240 for FL200 although a diversion is deemed necessary for fuel considerations. The First Officer experienced difficulty using the interphone and the Captain had difficulties with glasses and the headset.

Date: 2009-12 · Aircraft: B737-300 · Phase: cruise

Anomalies: aircraft-equipment-problem-critical

Synopsis

Narrative

Before takeoff; pressurization was set at FL360. We only climbed to FL220; turbulence related. I reset mode control to FL220; then descended to FL200 because of turbulence. After a smooth ride; higher was reported smooth. I reset mode control to FL360 and climbed. We were cleared to FL340. Going through FL300 we got the altitude warning horn. We proceeded with oxygen masks on and 100% and Crew communications. I was able to talk to ATC but not with the Captain. My own error; but we still descended down to FL200 and the cabin pressure stabilized nicely descending through FL240. We came to the conclusion the cabin couldn't keep up with all the flight level changes and decided cautiously to try to climb again. Unfortunately; the cabin started to climb at a non-standard rate so we stopped our climb without event and diverted. When the cabin altitude horn sounded; the cabin altitude was 10;500'. Although the Captain and I had our masks on; the Passenger cabin masks never dropped and no one was alarmed at any time in back. We both agreed later the new briefing about the warning horn sounding in the air was a positive factor; and thus we recognized and reacted in a more timely matter. I think there should be a note under Crew communications. Use flight intercom to establish communications. From talking to other Pilots; I'm not the first First Officer to miss that in the heat of battle. The Captains are used to talking on the flight intercom all the time. The First Officer rarely uses the intercom. Afterwards; you say of course you would; but it's not instinct.

Second reporter narrative

Cruising at FL220 to stay below a turbulence area that had a floor of FL230. Pressure controller set to FL360. Descended to FL200 because of turbulence after setting pressure controller to FL220 to avoid an 'off schedule descent.' Began climb to FL340 after passing turbulence area. At FL300 the Altitude Warning horn came on. No other indication of pressurization problems. Cabin altitude got to 10;500 ft. Had difficulty getting headset in the proper position after donning the O2 mask. First Officer had her volume control all the way down on the flight interphone; so she couldn't hear me. She seemed to be unfamiliar with how to use the flight interphone system including use of the bottom part of the yolk mounted (lower) switch. Advised ATC that we were descending. AT FL240 the Altitude Warning horn stopped. We leveled off at FL200. Cabin pressure was stabilized. Suspected that the pressure controller had become confused with so many altitude selections. It seemed to have reset and was operating normally. Did not have enough fuel onboard to continue at FL200; so we began climbing to FL340 while monitoring the cabin pressure indications. Cabin altitude indication exceeded what it should have been for selected altitude; so we elected to divert. Needed to accomplish an overweight landing and did so using approved procedures and appropriate writeup.The pressurization failure was mechanical and there is nothing that we could have done about that. I believe that our training regarding O2 mask usage is inadequate. It is a much more complicated task to effectively don the mask while dealing with the variables of a headset and glasses than is represented in our training.

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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.

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