Narrative:

Climbing through FL260 to assigned altitude of FL300; the intermittent warning horn sounded and the cabin altitude warning light illuminated. As I put my oxygen mask on the captain (PF) called ATC; requested and initiated an immediate descent. He then gave the aircraft to me as he put his mask on. In the descent toward 10;000 feet; after completing immediate action items; I; as the pm; began running the abnormal pressurization checklist.I don't recall the cabin altitude when I started the checklist since we were in a rapid descent and well below the initial altitude at the onset of the event. ATC asked if we were declaring an emergency and the captain stated 'yes.' I completed the [abnormal pressurization] checklist and continued with the emergency descent checklist since we were already in the descent. The captain called the cabin crew to ascertain the status in the cabin; determine if the masks were deployed (they were not) and to see if there were abnormal sounds or indications in the cabin. The flight attendants advised all was normal with the exception of ear pain complaints. We shortly leveled at 10;000 feet. The only cockpit lights illuminated during the event were the cabin altitude and off schedule descent. We were cleared direct to our destination; and completed a visual approach and landing. I think the descent was initiated too quickly; before we could analyze the problem. Once our oxygen masks were on; running the checklist may have led us to restoring the pressurization. However; since the descent was initiated immediately after the warning horn sounded; by the time the checklist was run by me; it was difficult to determine if pressurization was being maintained. Taking time to analyze the problem would have been helpful and allowed the checklist to be run as intended.

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Original NASA ASRS Text

Title: Upon receiving a Cabin Altitude Warning light and aural warning the flight crew of a B737-500 declared an emergency and initiated an emergency descent.

Narrative: Climbing through FL260 to assigned altitude of FL300; the intermittent warning horn sounded and the Cabin Altitude Warning light illuminated. As I put my oxygen mask on the Captain (PF) called ATC; requested and initiated an immediate descent. He then gave the aircraft to me as he put his mask on. In the descent toward 10;000 feet; after completing immediate action items; I; as the PM; began running the Abnormal Pressurization Checklist.I don't recall the cabin altitude when I started the checklist since we were in a rapid descent and well below the initial altitude at the onset of the event. ATC asked if we were declaring an emergency and the Captain stated 'yes.' I completed the [abnormal pressurization] checklist and continued with the Emergency Descent Checklist since we were already in the descent. The Captain called the cabin crew to ascertain the status in the cabin; determine if the masks were deployed (they were not) and to see if there were abnormal sounds or indications in the cabin. The flight attendants advised all was normal with the exception of ear pain complaints. We shortly leveled at 10;000 feet. The only cockpit lights illuminated during the event were the cabin altitude and off schedule descent. We were cleared direct to our destination; and completed a visual approach and landing. I think the descent was initiated too quickly; before we could analyze the problem. Once our oxygen masks were on; running the checklist may have led us to restoring the pressurization. However; since the descent was initiated immediately after the warning horn sounded; by the time the checklist was run by me; it was difficult to determine if pressurization was being maintained. Taking time to analyze the problem would have been helpful and allowed the checklist to be run as intended.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.