B737-300
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 279
Aircraft Equipment Problem Critical
Flight Crew Returned To Departure Airport; General Maintenance Action
Aircraft
Immediately after raising the gear on departure; we experienced a Master Caution light; with a HYD annunciator; A-system Hydraulic Engine Pump Low Pressure light; and A-system Electric Pump Low Pressure light. We never had any flight control low pressure lights. Shortly after the gear and flaps were raised and the handle was placed in the off position; the Electric Pump Low Pressure light extinguished. A-system fluid level showed slightly under one-half. We notified ATC and asked for a vector back towards the airport. They turned us SE at 8;500 FT allowing us time to diagnose the problem. A-system pressure remained normal and the fluid level stabilized at just under one-half. We followed the QRH for engine driven hydraulic pump failure. I contacted Dispatch; notified them of our problem; and told them we were returning to [the departure airport]. I notified the flight attendant crew of the issue and then informed the passengers of our situation. The First Officer completed all of the before landing details. ATC vectored us for a visual approach.  We completed the Descent and Before Landing Checklists and landed uneventfully and taxied to the gate. At all times; one person was responsible for flying the airplane and communicating with ATC; while the other pilot was either working the aircraft issue or coordinating with outside agencies. We did not declare an emergency as we had normal A-system pressure and still one operable A-system pump.  We were within limits for a normal landing.  After landing; upon inspection; we found that the outer layer of tread on the inner portion of the #2 main tire had separated.  When it did; it damaged a hydraulic line that caused our hydraulic fluid to drain to one-half during gear retraction.  Once the landing gear lever was placed to off; the leak stopped.  No other damage was noted to the aircraft. I made the appropriate entry in the logbook. I discussed it with Maintenance.  I contacted Dispatch and Maintenance Control and explained all of the details. I contacted the Chief Pilot on Duty and informed him of the situation. I also contacted the Tower to inform them we had a partial tread separation and they may want to do a FOD sweep on the runway. Finally; I talked to the entire crew to see if everyone was okay to continue or if anyone wanted to get pulled. Everyone chose to continue the trip. The passengers were deplaned and we received a new airplane within one hour. After boarding the new airplane; I explained to the passengers what had happened; assured them that safety was our number one priority; and explained that we were all trained to handle situations like this. They were all appreciative of the information and we departed.
The pilots of a B737 received a hydraulic system malfunction warning and successfully returned to the departure airport. After landing; they observed that a main tire tread had separated and caused damage to a hydraulic line; causing hydraulic fluid to leak.