Narrative:

At 37;000 ft; we experienced a dual controller failure of the left pack. With approximately 2 hours remaining to our destination; we were faced with several options. We could continue as planned at altitude on an over water route and land with 5;000 pounds of fuel or go inland and arrive with approximately 4;000 pounds of fuel. Knowing that aircraft dispatched for single pack operations are limited to 25;000 ft; I felt both of these options unwise. While the QRH seems to allow us to continue in a single pack operation as high as 41;000 ft; I felt it safer to descend to the dispatchable limit for several reasons: we had just experienced complete in-flight failure of one of two critical components; cause unknown. Useful consciousness at that altitude is about 15 seconds. Potential pressurization related injuries to passengers would be much more severe at the higher altitudes. Why expose ourselves by putting all our faith in a single pack at high altitude and still arrive at our filed destination with minimal fuel. We need more clarification on the disparity between the QRH and the MEL. Common sense tells me the MEL limitation is there for a good reason. Regardless; we were unable to make it to our destination at the lower altitude with the fuel on board. With both a system failure and a fuel divert; I declared an emergency. We landed uneventfully; refueled; had the pack placarded; and departed 40 minutes later at 24;000 ft. Since dispatch with a single pack operation is common; most pilots are familiar with the limitations. A note in the QRH clarifying the capabilities of a single pack and/or the direction to continue at altitude would be helpful.

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Original NASA ASRS Text

Title: A B737-800 at FL370; diverted to an enroute airport for fuel after a left pack failed because with a distant destination; over water flight or a revised overland routing at FL250 would have resulted in a minimum fuel situation.

Narrative: At 37;000 FT; we experienced a dual controller failure of the left pack. With approximately 2 hours remaining to our destination; we were faced with several options. We could continue as planned at altitude on an over water route and land with 5;000 LBS of fuel or go inland and arrive with approximately 4;000 LBS of fuel. Knowing that aircraft dispatched for single pack operations are limited to 25;000 FT; I felt both of these options unwise. While the QRH seems to allow us to continue in a single pack operation as high as 41;000 FT; I felt it safer to descend to the dispatchable limit for several reasons: We had just experienced complete in-flight failure of one of two critical components; cause unknown. Useful consciousness at that altitude is about 15 seconds. Potential pressurization related injuries to passengers would be much more severe at the higher altitudes. Why expose ourselves by putting all our faith in a single pack at high altitude and still arrive at our filed destination with minimal fuel. We need more clarification on the disparity between the QRH and the MEL. Common sense tells me the MEL limitation is there for a good reason. Regardless; we were unable to make it to our destination at the lower altitude with the fuel on board. With both a system failure and a fuel divert; I declared an emergency. We landed uneventfully; refueled; had the pack placarded; and departed 40 minutes later at 24;000 FT. Since dispatch with a single pack operation is common; most pilots are familiar with the limitations. A note in the QRH clarifying the capabilities of a single pack and/or the direction to continue at altitude would be helpful.

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.