B737 Next Generation Undifferentiated
Confusion; Troubleshooting; Communication Breakdown
Aircraft Equipment Problem Critical
General Maintenance Action
Aircraft
Twice in two weeks I've found myself attempting Battery APU starts in B737NG. Neither one of them worked. Turns out there is an Aircraft and DC [Battery] APU Start circuit in the newer model aircraft. Either one is deferrable; but none were written up or deferred at the time. Once we put [Ground] AC power on the aircraft; the APU started and ran fine. I guess my question is that most of our APU starts are with AC power; and we don't have many opportunities to 'try' the DC APU start. The DC part of the APU start could be vital [if dual engine flameouts occur.] Does Maintenance check this with any regularity? Is this DC portion failure a 'trend' we need to check? (Maybe I just got lucky with 2 recent aircraft.) Explore this as a failure trend that needs to be corrected. APU Battery start in Next Generation aircraft.
Reporter stated that 99% of the time flight crews start APUs using Ground AC Power. Unlike the Classic B737s that only have a single point APU Start circuit; the B737NGs have two separate Power circuits for starting APUs. The APU Battery Start capability becomes critical when in the air; especially if dual engine flameouts occur and electrical power for fuel pumps and air for emergency air starts are needed.Reporter stated that one of the guarantees by Boeing was that; there would be enough Battery power to start APUs.  He's not sure if the Ship's Battery is the issue; since the Battery Voltage indicated adequate Battery power was available.  But when Maintenance performed BITE Tests on the APUs; the results showed 'Battery' as a fault. The two times he recently attempted to start the APU; he had come on-board to a 'cold' aircraft. No Ground Power was hooked-up and the APU Battery DC starts failed. When Ground Power was eventually hooked-up to the aircraft; the APUs started right up. He isn't sure if his Air Carrier understands the potential safety concerns that flight crews could be exposed to; especially in flight; if APUs are consistently not starting in the DC Battery mode.
Pilot reports that twice in two weeks; attempted DC Battery starts of APUs had failed on B737NG aircraft. APU Start capability becomes critical in the air if a dual engine failure occurs. Maintenance BITE Tests of APUs after Battery start failures indicate 'Battery' fault. APUs start OK using Ground Power.