Date: 201702
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: APU Fuel System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Ground Personnel
When detected: Aircraft In Service At Gate
Result: General None Reported / Taken
Contributing factors: Aircraft; Procedure
Primary problem: Procedure

Narrative: At the gate in ZZZ shortly before pushback the Ground Crew came to the flight deck to advise of a leak under the APU. I went to inspect. There was a puddle on the ground and some fuel dripping slowly from the APU fuel drain. It was a few drops a minute but the Ground Crew said it was dripping more earlier upon APU shutdown. I called Maintenance Control and was advised that it was normal for some fuel to come from the APU fuel drain upon shutdown. We started the APU; pushed back and started engines. At about 50% rpm of the way through engine start of the second engine the Push Back Driver advised us that fuel was coming from the APU drain again. All indications in the flight deck were normal. We completed the start; which was essentially complete by the time we finished discussing with the Push Back Driver; and shut down APU. The FO and I discussed the situation. There is no QRH procedure; or aircraft manual guidance for this; and Maintenance Control did not seem concerned or relay any qualifying remarks on the limits in our earlier phone call. We decided that we had done all we could do. We were confident the dripping would stop from our ground experience earlier. We taxied out and took off normally. At cruise we discussed the situation more. We decided to advise Maintenance Control via Dispatch and ACARS in case there was an issue they should address. We chose not to start the APU upon landing as the safest option. We called Maintenance Control when we chocked in and decided with Maintenance Control to MEL the APU. Maintenance said then that any fuel above a very minimal level dripping from the APU fuel drain is an indication of a problem. In retrospect; we were wrong to continue at ZZZ without calling Dispatch and Maintenance Control. We should have MEL'd the APU in ZZZ after pushback. I know the correct procedure; I try to follow all the procedures; but didn't this time. I don't have a good explanation for why I failed to do this. Perhaps a combination of Maintenance's comments about it being normal in certain circumstances (expectation bias); the lack of clear guidance on this condition in the QRH and aircraft manual; and the fact that it occurred right at the end of normal APU operation. None of which excuse this. While we did not accomplish the appropriate administrative procedure at ZZZ; we did do what MEL called for and did not use the APU.Don't have much for corrective action; except for me do my job better in the future. I have learned the limits for the APU fuel drain; which will help me avoid in this in the future. There is no guidance what so ever for the APU fuel drain. I hate to suggest more guidance; or to call Maintenance any time any fluid drips from anywhere on the airplane; ever. But perhaps something about what the APU fuel drain is there for and what small amount of fluid is normal would be a possible improvement.




Synopsis: B737 Captain reported that during engine start ground personnel advised that fuel was leaking from the APU fuel drain.