Prior to the first flight of the day, the Boeing 737 aircraft
cabin was found to contain smoke and fumes. While the crew returned
to the crew room, maintenance personnel inspected the aircraft and
found that the auxiliary power unit (APU) had malfunctioned. The
cabin was cleared of fumes and the aircraft despatched with an
unserviceable APU. For a short time after takeoff, some smoke and
fumes were observed in the cabin but cleared.
At around 6,000 ft on approach to Sydney, fumes were again
detected; most noticeably in the rear of the cabin. A fast approach
and normal landing ensued. Cabin staff reported that the smell
dissipated when the airconditioning packs were selected to
HIGH.
Company maintenance investigation found that the APU malfunction
was the result of a cooling fan shaft failure. The failure allowed
APU turbine oil to leak from around the shaft seal from where it
was sucked into the APU inlet prior to the APU control unit
initiating an auto-shutdown. The oil then entered the
airconditioning system ducting and later exited into the cabin as
fumes and oily smoke during that system's normal operation.