Incident Description
While a mechanic was performing a "Predispatch Ops Check" on the airplane from the cockpit, ground personnel notified him that smoke was observed coming from the aft stairwell. Fire extinguishing agents were applied to the area and the fire was contained.
Examination and Findings
Examination of the airplane after the incident revealed that the insulation on a wire to the standby hydraulic system electric motor pump had chafed through, allowing the wire conductor to contact a hydraulic system "A" case drain return line. Approximately four feet above the hydraulic line, a 1-inch by 3/16 inch hole was observed in the backside of a reservoir pressurization line, which was fed by a 13th stage bleed-air line.
According to Equipment Quality Analysis Reports submitted by Boeing, the chafing between the standby hydraulic pump electrical power wire and the hydraulic system "A" case drain return line exposed the standby pump conductor wire and resulted in electrical arcing and puncture to the system "A" return line, followed by ignition to the mist of hydraulic oil leaking from the hole in the line.
Cause
The wire bundle and tube chafing condition appeared to have occurred from inadvertent mis-routing of the wire bundle during maintenance activity.