Date: 201210
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Weather elements / visibility: Rain; Icing; Turbulence
Light: Daylight
Aircraft: B737-400
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: Air Conditioning and Pressurization Pack
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Instrument; Flight Crew Flight Engineer; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Second reporter function: Captain; Pilot Not Flying; Check Pilot
Anomaly: Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Other / Unknown; Inflight Event / Encounter Weather / Turbulence
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Declared Emergency
Contributing factors: Aircraft; Weather
Primary problem: Aircraft

Narrative: Right Pack Deferred under MEL 21-1. While at FL240; aircraft experienced left pack trip and almost simultaneously a left bleed trip. Aircraft subsequently lost pressurization sources. We donned oxygen masks and established communications. We contacted ATC and requested weather at the destination. Due to the current weather; destination weather; and location; we elected to return. When we began the descent to 10;000 FT and the right turn; an emergency was declared. We followed up with the checklist items. We made further contact with the flight attendants and passengers. We also attempted to call company; but attempts were unsuccessful. As a result of the weather; we were unable to maintain a high rate of descent due to turbulence (choosing instead to maintain the structural integrity of the aircraft). As we passed approximately 14;000 FT en route to 10;000 FT; the oxygen masks dropped throughout the aircraft cabin. We continued further descent to 8;000 FT and completed the remaining checklists to include the pack light illuminated checklist and the bleed trip light illuminated checklist. It is our position that the existing weather at the time was a contributing factor to the descent rate and decision to turn back immediately. In addition; the accumulation of ice further exacerbated the condition. The decision to cruise at FL240 was within the MEL limitations and had been filed by company dispatchers. The use of the autopilot during the emergency descent is not listed within the QRH. Due to the hazardous meteorological conditions that existed at the time; we did not engage the autopilot for the emergency descent. Of primary concern was to maintain safe control of the aircraft at all times which was successfully accomplished. No passengers or crew were injured during the event. The aircraft landed without further incident.


Second narrative: En route while at FL240; aircraft experienced left pack trip and shortly after a left bleed trip. Aircraft subsequently lost pressurization due to a previous MEL that had already deferred the right pack. We donned oxygen masks and established communications. We contacted ATC and requested weather at the destination. Due to the current weather; destination weather; and location; we elected to return. When we began the descent to 10;000 FT and the right turn; an emergency was declared. We followed up with the checklist items. No passengers or crew were injured during the event. The aircraft landed without further incident.


Synopsis: B737-400 flight crew reports being dispatched with the right pack deferred. En route at FL240 the left pack fails and the crew elects to return to the departure airport after declaring an emergency. Turbulence prevents a rapid descent and the passenger oxygen masks are deployed.