Citation Excel (C560XL)
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Training / Qualification
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Maintenance
General Maintenance Action
Aircraft; Company Policy; Procedure
On taxi-in to hangar; we started the APU to provide heat for our elderly passenger once the cabin door was open.  After the passengers were gone; we shut the APU back down; and the Captain commenced his exterior post-flight.  He opened the aft equipment bay door; and called me to come see.  Jet fuel was dripping from the underside of the titanium APU containment box; and was all over the inside of the compartment door.  The Captain wrote it up; and maintenance worked on it overnight.  Next morning; the mechanic tech told us there was a loose 'B' nut on a fuel line coupling and that fuel was spraying all over the exciter box and burner can.  He couldn't believe we hadn't had a fire.If one crew has come across this twice in 3 days; then it would seem advisable for the company to inspect every aircraft of this type in the fleet for this loose nut on an expedited basis.  Say; 7 days.
I opened the aft equipment bay door; and could smell fuel. Since it was snowing there was water dripping onto the aft inspection door however the fuel was very noticeable. I asked my First Officer to come see what I found and determined this must have been the source of the smell we had in the cabin prior. He also witnessed this issue when they ran the APU prior to finding the source of the leak and thought they would have a fire.
A CE-560XL Captain found an APU fuel leak inside the APU compartment. While investigating Maintenance found a loose 'B' nut spraying fuel inside the compartment and on the APU containment box.