A question for A320 experts

[Chock 17,769]

So, today I was doing the road marshalling for an A320 which was being pushed out for departure and something unusual occurred. The aeroplane in question required an air start unit to be attached to crank its number 2 engine on stand. Because of this, as is normal in such circumstances, I went out onto the roadway behind and to the side of it with marshalling wands to ensure no traffic passed behind it. So, the anti-collision beacons go on, the air start unit cranks up to high power and the right engine begins cranking. At this time, fuel began pouring from the left wing, so I signalled the headset guy to alert him to this, by giving an emergency stop signal, but he did not see me because he was watching the number two engine, and the air start guy didn't see me either because he was facing the headset guy following his signals. I could not leave the road because the engine was running and so I had to stay there to prevent traffic passing behind the aeroplane, eventually the fuel stopped pouring out of the wing. I was waving the marshalling wands trying to get anyone on the stand's attention, but to no avail, so the air start unit got disconnected and the aircraft pushed out with me having to stay there until it reached the taxiway to ensure nothing got in the way. It was only then that I was able to attract someone's attention and advise them that A, we needed to report a fuel spill on stand and B, tell the crew that they had been losing fuel out of the left wing. This we did and the aircraft stopped on the taxiway after having been disconnected from the tug, but eventually it continued on its way. To make matters more confusing, another A321 was also coming onto the stand at the same time, so we had to hit the emergency stop on the safe dock system because of the fuel spill until we knew it was all okay for that one to come on stand, meaning that the A321 had to be marshalled onto the stand manually.

So, my question is this: What caused the A320's fuel tank to vent like that? I am assuming that there was some overpressure on the system from the air start unit which caused the fuel level to surge to a point where it vented some fuel from the left wing, but this is just a guess. Anyone know for sure what the reason was for that fuel coming out?

[martin-w 3,235]

NACA Duct. Pukes fuel out from vent tank if tank over full?

[Bob Scott 7,721]

If the acft is equipped with center tank jet pumps instead of boost pumps (all 320neo and some retrofitted 320seo acft), and the fuel xfer mode switch is inadvertently left in "manual", the xfer valve auto shutoff logic is bypassed, so when the boost pumps are turned on at engine start, it initiates motive flow to the jet pumps, which then will push fuel through the open xfer valve to the already-full wing tanks, filling the surge/vent tank(s) and then overflowing out through the NACA port(s).

[Chock 17,769]

Sounds about right. Cheers. It was a NEO variant incidentally.

Edited July 23, 2021 by Chock