Date: 201808
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: Mixed
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Air Conditioning and Pressurization Pack
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Other / Unknown; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Landed As Precaution
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: The First Officer and I were flying the last day of four together. We were climbing out when sometime between FL300 and FL320 we received a Master Caution and a right bleed trip off light. It was the First Officer's leg so he continued to fly; and I looked up Right Bleed Trip Off in the QRH. We followed the QRH procedure and initially the right bleed trip off light extinguished. Shortly thereafter we received the Right Bleed Trip Off light again. We followed the QRH procedure again and continued the flight operating on the left pack. Sometime just prior to level off; we received another Master Caution and a left pack trip off light. Obviously the cabin started depressurizing rapidly; so the First Officer (FO) reached for his oxygen mask and asked me for a lower altitude. I told Center we had a pressurization problem and needed a lower altitude immediately. They initially cleared us to FL360. I donned my oxygen mask; and the First Officer and I established communication. ATC then issued a vector and gave us a lower altitude (I believe FL240). I referenced the QRC. We had accomplished the first two steps of the Rapid Depressurization Checklist; and the outflow valve was closed; so I continued with the Emergency Descent Checklist. At that point we got the Cabin Altitude warning horn; which we were expecting; so I silenced it. We were also getting vectors from ATC; and deviating around weather; so we were busy. ATC wanted to know how low we wanted to go; and we told them 10;000 feet. I assumed that the oxygen masks had dropped in the cabin; and we started planning on diverting to [a nearby alternate]. Eventually we were able to regain control of the cabin pressurization by descending and pushing the trip reset switch. The left pack started to pressurize the cabin; but by that point we were fairly low and still descending to our cleared altitude of 10;000 feet. [Our intended alternate] was right off our nose; and we elected to divert. This was the first time I had to call the Flight Attendants; and I was pleasantly surprised to learn that the oxygen masks in the cabin had not dropped. In fact; no one in the cabin even knew anything was wrong until I made an announcement; and that we would have to divert.




Synopsis: B737-700 Captain reported diverting to an alternate airport as a result of a loss of cabin pressure.