Narrative:

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.

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Original NASA ASRS Text

Title: B737 flight crew reported unconsciously starting the APU while the unit was still being serviced by a Mechanic.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.