Incident Overview
On 16 February 2000, at approximately 23:35 MESZ, a Swearingen SA 227 AC operated by a German airline was on a cargo flight from Luton, England to Paderborn-Lippstadt, Germany. The crew consisted of a captain, a first officer, and a loadmaster. During the final approach, when the landing gear was extended, the loadmaster observed smoke or fog in the cargo compartment and notified the flight crew. The pilots donned their oxygen masks and completed the landing without further difficulty. After engine shutdown and door opening, the cabin air cleared. The standby fire brigade was not required to intervene.
Aircraft Examination
Following a temporary repair at Paderborn and subsequent ferry to the company's maintenance facility, the German Federal Bureau of Aircraft Accident Investigation (BFU) inspected the aircraft. Examination revealed that an electrical cable (TLTLAV2-16) for the toilet pump motor was in contact with a hydraulic line (angle tube PN 27-81006-489) for the nose gear down circuit underneath the forward right seat row floorboard (ATA 25-20). At the probable contact point, the cable insulation had a defect and the tube wall showed a crater with a hole approximately 0.2 mm in diameter. The circuit breaker (CB542) for the affected circuit had not tripped. The hydraulic reservoir for the affected circuit was empty. Hydraulic fluid had deposited and accumulated on the floor structure and around the damage. Additionally, the braided polyamide sleeving of wire bundles appeared scorched in several places.
Laboratory Analysis
The damaged cable and tube were examined at the Lower Saxony State Criminal Police Office (LKA Niedersachsen) laboratory. Copper particles (from the cable conductor) were found melted onto the hydraulic tube at the damage site. The cable insulation (PTFE) had a defect matching the cylindrical shape of the tube; the color of the probable contact surface corresponded to white plastic melt. The cable sleeving (polyamide) had melted away at various points, but cable ties (polyethylene) were only slightly browned and cable insulations hardly discolored, and only on their upward-facing side. The cable insulation was PTFE (melting point 327 °C), sleeving polyamide (melting 185-265 °C), and cable ties polyethylene (melting 105-138 °C).
Test Results
In a gas flame test, polyamide fabric melted and ignited, while PTFE insulation did not appear to change until prolonged flame exposure caused charring; the plastic stopped burning when the flame was removed. Self-sustained combustion did not occur with horizontal cable bundles. Polyamide fabric showed no visible changes after three days in contact with hydraulic fluid (Brayco 882, MIL H 83282) at temperatures up to 150 °C. The hydraulic fluid was identified as a complex mixture of high-boiling hydrocarbons.
Assessment and Cause
The BFU determined that three distinct events occurred: (1) The cable bundle sleeving melted and burned from above at various locations, but temperature and heat flow were too localized to affect cable insulation or adjacent ties. (2) The cable insulation melted due to temperatures above 300 °C or was abraded by vibration against the hydraulic tube, exposing the copper conductor. Repeated contact of the bare conductor with the metal tube created sparks eroding a crater in the tube wall via 28 V DC spark erosion. (3) Hydraulic oil leaked out and atomized into the underfloor area and eventually the cabin after the spark erosion created a hole and the landing gear system pressure (120-150 bar) was applied. No in-flight fire occurred, as the ignition source power was too low to ignite the aerosol. The BFU concluded that the incident was due to improper routing and handling of electrical cables in the aircraft.