B737-700
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
Air Traffic Control Provided Assistance; Aircraft Automation Overrode Flight Crew; Flight Crew Landed As Precaution; Flight Crew Landed in Emergency Condition; Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Diverted; General Declared Emergency
Aircraft; MEL
I was pilot monitoring/pilot not flying. We had MEL 30-1C which instructs us to configure the bleeds and isolation valve differently until the temperature outside is at or below +10 degrees C. On our flight this meant having the isolation valve closed with the right bleed air switch in the off position until the temperature outside is at or below +10 degrees C; to comply with the MEL 30-1C.When the temperature outside reached +9 degrees C around FL190 I reconfigured the bleeds air system as per MEL 30-1C. I positioned the right engine bleed switch to ON and the isolation valve switch to AUTO. This was then followed by a right BLEED TRIP OFF light. I went to the QRH and followed the procedure for a bleed trip off. On completion of the QRH procedure both the left BLEED TRIP OFF light and the right BLEED TRIP OFF light were illuminated. This meant that we no longer had any pressurization due to no bleed air. We were just leveling the aircraft at FL250. We declared an emergency and descended to 10;000 FT; at which point we were able to reset both bleed air sources. After getting the BLEED TRIP OFF lights to extinguish; we noticed the temperature was above +10 degrees C and reconfigured the bleed system in order to be compliant with MEL. During this event the cabin never got above a pressure altitude of 7;000 FT. After contacting both Dispatch and Maintenance Control we decided to divert.
A Boeing 737-700 crew; conducting operations under the constraints of an MEL requiring modified bleed and isolation valve operation; experienced pressurization problems when both bleed sources tripped off during level-off at FL250. Specific MEL procedures were followed by the crew; but both bleed air sources inexplicably and automatically tripped off; compelling the crew to declare an emergency and divert to a nearby airport.