Date: 201202
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization Control System
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting; Situational Awareness; Training / Qualification
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural MEL / CDL; Inflight Event / Encounter Fuel Issue
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; General Maintenance Action; General Declared Emergency
Contributing factors: Aircraft; MEL
Primary problem: Ambiguous

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.


Second narrative: Closer review of the procedure shows we could legally have continued the flight; but again; it just didn't make sense. There should be a note clarifying the crew's ability to continue flight at cruise altitude even if it is above (in our case well above) the dispatchable FL250. I think the Captain chose the most prudent and safe route by declaring an emergency and diverting. Clarify the QRH Pack procedure. In step 3 of the procedure; at the end of the first option (Both Pack lights extinguish); 'Continue normal operation' is specified. That statement is NOT found at the end of the next option where a single Pack light remains illuminated. If flight at high cruise altitude is a recommended outcome at the end of step 3; then the procedure should state so.


Synopsis: 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.