B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 9599; Flight Crew Type 4691
Situational Awareness; Communication Breakdown
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown
Flight Crew Became Reoriented
Aircraft; Human Factors
Approximately 12 minutes prior to push; the APU was started. After APU start; the blue 'APU Maintenance' light illuminated. Per CRM; I advocated to the Captain that; per the FM (Flight Manual); we could legally operate and write it up at our destination. The FM allows continued operation with this light on. The Captain; however; desired to have Maintenance address the problem. Maintenance was then called and the Mechanic arrived at the cockpit in a very timely manner and investigated via the FMC. The FMC Maintenance page read 'APU oil low.' The Captain and the Mechanic then agreed to close the main cabin door while the Mechanic added the oil; and then communicate through the cockpit window. While the Mechanic was adding oil to the APU; a ramp person yelled up to the captain through the cockpit window; 'It's going to be okay.' The Captain interpreted this to mean that it was now okay to start the APU. Unknown to me; the Captain then started the APU. After realizing the error; the Captain immediately shut down the APU. A few minutes later; the mechanic was finished servicing the APU and walked up the cockpit window. The Captain was highly apologetic to the mechanic. Fortunately; neither the mechanic nor anyone else was hurt. The flight then proceeded normally. This was a miscommunication problem. While it's true that the ramp agent was merely trying to be helpful; nonetheless; a communication error occurred somewhere between the Mechanic; the Ramp Agent; and the Captain. I believe the primary factors involved were expectation bias and time pressure. The Captain had an expectation bias and heard what he wanted to hear since we were at/near pushback time. Better monitor and cross check between us could have prevented this mishap.
About 15 minutes prior to scheduled pushback; saw the APU required maintenance. Maintenance determined oil was needed. At push time; I asked mx on the radio if it was OK to close the door and start the APU without anyone coming back to the flight deck. They said it was OK to use the APU (I saw the oil had been serviced) and it did not require a new Maintenance Release; but I didn't ensure the aircraft was clear. I attempted to start the APU and then immediately shut it down when I realized I didn't know if there was still a technician outside the rear of the aircraft by the APU.
B737 flight crew reported unconsciously starting the APU while the unit was still being serviced by a Mechanic.