B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 119; Flight Crew Type 2630
Workload; Troubleshooting; Human-Machine Interface; Confusion; Distraction
Aircraft Equipment Problem Less Severe; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Overcame Equipment Problem
Human Factors
After pushback and engine start; the APU switch was inadvertently switched off prior to selection of engine generator electrical power causing power to drop offline momentarily. Power was quickly restored by selecting the engine generators. Before taxiing; we checked the aircraft systems and FMC to make sure the power fluctuation had not caused any faults or deleted any FMC programming. All seemed normal; however we did not notice the MCP Speed window had reset from 126 knots to 100 knots. While flying the departure I noticed that the command pitch bars seemed to be driving me to a pretty low speed. I stopped following them at around 128 knots and continued to maintain what I believe to be around 125-128 knots. The PM and I discussed the slow speed but in those few seconds couldn't put our finger on exactly what was wrong we just knew it wasn't 'normal'. Unfortunately my mind didn't work fast enough to remember V2 + 20 should have been 146 knots or the OPC computed cut-back pitch calculation (this was only my second ZZZ departure from the left seat.). Since the aircraft was flying; I disregarded the command pitch bars; maintained my current speed; and continued the climb to 3000 feet AGL. At that point the power increased and I asked the PM to 'set speed' which is when the PM noticed the MCP Speed window was set to 100 knots. I did not immediately ask for 'flaps 1' because; although I hadn't figured out why; we seemed too slow. I continued to accelerate to just below the MASI 1 speed before I asked for flap retraction. Later in the flight; as we were discussing what happened on the departure; we figured out the power interruption must have caused the MCP Speed window to reset to 100 knots and was therefore driving the command bars to 120 knots after the initial 15 degrees at rotation.
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A Boeing 737 flight crew reported an inadvertent shutdown of the APU prior to the aircraft generators coming on line. This electrical system anomaly resulted in the flight management computer commanding an erroneous airspeed presentation. The crew recognized the problem and adjusted the aircraft speed and configuration accordingly.