B737-800
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 201
Aircraft Equipment Problem Less Severe
Flight Crew Diverted; Flight Crew Landed As Precaution
Aircraft
At cruise; the B low pressure light illuminated. The QRH for B Low Pressure light was accomplished. The B pressure and quantity remained normal. We discussed the low weather at our destination and the possibility of losing the B electric pump next with subsequent total loss of B hydraulics. Using the iPad we reviewed the best weather options for a total loss of the B system with a missed approach with the lights remaining on. As the flight continued; the B quantity and pressure did reduce to zero with no B electric light illumination. We decided to go with the divert plan that was previously just a passing thought. The divert was pretty quick and only one ACARS message to Dispatch was made. The Center gave us vectors for descent; the Loss of B Hydraulics QRH was accomplished. The landing was about 700 pounds overweight but uneventful. The Fire Department cleared us into the gate.
Once the Company mechanics got on board they advised us that we should have shut off the B side electric pump immediately when we noticed the quantity gage decreasing. This would've kept fluid in the pumps and prevented them from cavitating and running dry with no fluid to keep them cool and lubricated; makes sense to me. My concern is that our decision to keep the electric pump running with no fluid remaining could've led to a greater emergency like a fire in the number 2 engine. Perhaps a QRH note in the Hydraulic Low Pressure Checklist that states that if hydraulic fluid quantity decreases rapidly; the system should be considered inoperative and the loss of System A or B Checklist should be initiated even though residual system pressure might remain at or near normal.
B737 flight crew reported diverting to an alternate airport after experiencing loss of the B hydraulic system.