B737-800
Aircraft Equipment Problem Less Severe
General Maintenance Action; General Release Refused / Aircraft Not Accepted; General Flight Cancelled / Delayed
Procedure; Aircraft
[I] was briefed by inbound Captain about his write up of Master Warning and Hydraulic (HYD) warning light displaying at rotation. He also briefed me on other hydraulic items that were on placard;  Autopilot/ etc. Captain previously informed Maintenance Control and Dispatch enroute. He asked for Maintenance to meet aircraft; none did. He then briefed me the outbound Captain (excellent job by the inbound Captain communicating the issue). No Maintenance present; I initiated a call. Local Maintenance decided to complete Procedures Manual regarding the A and B System quantity and pressure. No problem found; system full; no fluid added. Aircraft was returned to service; Mechanic asked that I supply additional information if problem were to reoccur. He had no explanation for Master Caution and Hydraulic indication from previous leg; felt confident Hydraulic System was sound. All indications at that time were 'normal'. As aircraft had no open write up and Procedures Manual had been performed; I accepted the aircraft. During departure In addition to normal duties; I focused on the Hydraulic Panel and Pressure lights; the First Officer flew the aircraft. Prior to departure I briefed the First Officer that we would separate the gear up and flaps up movements to help isolate any problems. At rotation; pressure was around 2850 in both systems as a low point utilizing flight controls only. At gear up the pressure dropped to approximately 2750; the Master Warning displayed; the HYD indication displayed in the 6 pack; and ALL 4 hydraulic pumps displayed low pressure lights. After approximately 5 seconds the pressure returned to normal in all 4 pumps (3000 PSI). Gear handle was moved to OFF. Flaps retracted normally with dips in the hydraulic pressure to 2850. No master Warning or HYD was displayed during flap movement. We obtained a safe altitude and assessed our situation using an open CRM template. At this point I had no QRH procedure that applied. The mechanical situation was presently: the aircraft appeared to go into a low pressure mode during periods of high hydraulic demand. After discussing an overweight landing; declaration of emergency; etc. I decided that no eminent danger applied; the aircraft was operating normally at that time. We observed that our destination is a major maintenance base with a long runway; VFR weather and a scheduled daytime landing. First Officer concurred that return was not the best solution and a landing at [our destination] was the best option. First Officer continued to fly and work the radio. We contacted Dispatch [and] relayed the indications detailed above. I also wrote up a detailed description of the event in the aircraft logbook. I noticed that this was an exact repeat of the previous Captain's write up; with additional data observed and detailed. Enroute; I consulted a Hydraulic System schematic to determine what 'could be' common between the A and B System; reactive to load; and able return back to a normal system indication. My gut feeling was either a transfer unit or accumulator head pressure (foaming). Maintenance met aircraft; retrieved the logbook and consulted with Maintenance Control as to how to proceed. Language was a barrier. As a solution; I was told It was determined that the Procedures Manual should be run a second time; then dispatch the aircraft back to base. Concurrently; I had the First Officer do a detailed walk around paying special attention to the accumulator head pressure. My First Officer inspected; and then reported to me the head pressure. It was 1/2 way down; out of the green band. I informed the Mechanic of this Non Normal head pressure indication. He indicated that he understood. Nevertheless the logbook was returned; signed off as 'Procedures Manual completed' (same as before) once again no fluid was added and no problem was discovered. The station began boarding the aircraft. I could not get a complete answer about the head pressure not being in the normal range.It was not until I forced the issue requesting to read the Procedures Manual did I see listed in the procedure that all 'Pressures' must be normal. I asked for clarification if this was system line pressure (3000) and/or accumulator pressure. The Mechanic disappeared again. I was very unhappy with the path of the repairs and continued to research the problem. I consulted my B737 Cockpit companion as an additional source document; in addition to all the company documents. I decided to shut down the APU removing all air input into the A and B System reservoir. The indicated head pressure then dropped to Zero. We had a dead leak to atmospheric pressure in the reservoirs. Station Manager and Operations Agent were pressuring me to leave at this time as the logbook had been cleared. I explained that I had a continuing mechanical issue with the aircraft. After the Mechanic returned he told me it was OK to return the aircraft to the US. I explained that I felt the head pressure issue had not been resolved and explained that I would not be departing until it had been resolved. I explained the same to the flight attendants and the agents. This made me very unpopular. I explained the delay to the passengers in a very general way. I kept them informed. Though I felt comfortable with the fact that I was controlling the pace and decisions of this event; I felt distracted by others trying to make decisions for me that were not within their scope of influence. This distraction was inappropriate. A quiet period that could only be described as tense ensued. I assume the Mechanic once again spoke with Maintenance Control and an honest effort was finally launched to find the nature of the missing head pressure. A flurry of activity; had us working with the Mechanic; supporting the troubleshooting effort. The APU was restarted and placed in High Flow with a single pack on. The Mechanic found a low pressure bleed leak in the line that supplied the head pressure in the A and B reservoirs (he found it feeling for the leak that he could hear) (both APU and engines supply this head pressure bleed). I entered it as a separate numbered write up; for the logbook had been signed off at this point; the Mechanic tightened the B Nut; and cleared the logbook. The Mechanic seemed relieved to have fixed an item before releasing the aircraft; I did not get the feeling that he was content with the previous logbook sign off. I got the strong impression that he did not want to disappoint the entity that was directing the repair process. The Aircraft was now repaired. All systems were returned to normal. Subsequent departure and enroute leg were conducted as a normal operation. 'No items' was entered in the logbook by me upon return to the US. Here are the three questions I would like addressed; if possible? 1) How was re-running the Procedures Manual going to fix the problem that I detailed in my very specific write-up? That solution did not work the first time; why would it work now? 2) Why was so much pressure applied to return this aircraft to service in [our destination] and return it to base? Why did the head pressure issue appear to be ignored? Why was it missed the first time? Dispatch appeared to take the forefront and repair/diagnosis appeared to be pushed aside. 3) What would have happened if I would have experienced an engine failure during second segment climb? If four hydraulic pumps were cavitating under high demand; what would have happened if I was down to one engine and two electric? Would the gear and flaps have come up?
B737-800 experiences Master Warning and Hydraulic (HYD) warning lights when the gear is retracted after takeoff with the hydraulic pressure dropping to 2750 PSI then recovering. The discrepancy had been previously written up and signed off by Maintenance; but the crew elects to continue to destination and not depart until the problem is corrected. Eventually a loose connection to the bleed air system; providing head pressure to the hydraulic reservoirs is found and corrected.