Auxiliary power unit oil System:

A sump at the bottom of the gearbox collects the returning oil, in some APU's the rear face of the sump is finned and let into the intake plenum to act as the oil cooler. The oil is drawn up by the oil pump and pressurised, it then passes through the oil filter before being distributed to the bearings. The oil returns to the sump by gravity. The oil system is monitored by a low oil pressure switch and a high oil temperature switch, either of which can shut the engine down.