Harrisburg, PA, USA
N277FE
Boeing 727-233
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 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/puncture to the system "A" return line, followed by ignition to the mist of hydraulic oil leaking from the hole in the line. The wire bundle/tube chafing condition appeared to have occurred from inadvertent mis-routing of the wire bundle during maintenance activity.
On May 25, 2001, about 0624 eastern daylight time, a Boeing 727-233, N277FE, operated by Federal Express Corp., as flight 1503, sustained minor damage after a fire ensued near the aft stairwell door while the airplane was standing on the ramp at the Harrisburg International Airport, Harrisburg, Pennsylvania. The certificated airline transport flight crew was not injured. Visual meteorological conditions prevailed and an instrument rules flight plan was filed for the scheduled cargo flight conducted under 14 CFR Part 121. According to a representative of the operator, the airplane landed, was taxied to the ramp area, parked, and the engines were shut down. A mechanic installed the tail jack, the cargo was off loaded, and the flightcrew disembarked. Neither the mechanic nor the flightcrew recalled observing any fire or smoke. The mechanic then proceeded to the cockpit to perform a "Predispatch Ops Check" on the airplane for a pending flight. During the check, other ground personnel notified the mechanic that smoke was observed coming from the aft stairwell. Fire extinguishing agents were applied to the area from where the smoke was emitting, and the fire was contained. One of the items to be completed during the "Predispatch Ops Check" performed by the mechanic, was to place the system "A" rudder switches in the "OFF" position, resulting in the operation of the hydraulic standby pump. 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 4 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. Inspection of the airplane also revealed that fire damage was concentrated outboard and above the hydraulic standby reservoir. Heat damage was observed in the following locations: lower surface of the upper torque box, frame webs (BS 1203 and 1223), stringer S8L-S9L, aluminum skin panels, and associated wiring located in the area. Burned paint was also observed on a titanium panel inside the number one engine pylon. Additionally, the external surface of the number one engine high stage bleed regulator was coated with "coked" hydraulic fluid. Following the incident, the operator initiated a fleet wide inspection program after a second Boeing 727 airplane in their fleet was found with similar chafing to the hydraulic system "A" case drain return line. The damaged portion of the hydraulic system "A" case drain return line, the wire harness, and the damaged portion of the reservoir pressurization line, were forwarded to the Boeing Company, Seattle, Washington, and examined under the presence of Safety Board personnel on August 20, 2001. According to the Equipment Quality Analysis Reports submitted by Boeing, there was evidence of chafing and arcing from the wire harness rubbing against the hydraulic system "A" case drain return line, P/N 65-17844-146, as indicated by localized melting of the line and copper deposits around the periphery of the hole in the line. No evidence of preexisting external damage to the reservoir pressurization line was observed. The wire harness, which chafed against the hydraulic system "A" case drain return line, was identified as W344. The harness contained six wires, which were held together by plastic tie-wraps. The specific wire that arced was identified as W344-002-12, a 12-guage wire, was connected to Pin B of connector D308 at the standby hydraulic system electric motor pump. The wire exhibited arcing damage in two locations, about 3/4 inches apart. Two tie-wraps were observed approximately 2-3 inches from the arcing damage. The maximum continuous current carried by the wire did not exceed 26 Amps; however, the "inrush current," reached a maximum current of 167 Amps during the initial 2-seconds of startup of the standby hydraulic system electric motor pump. Chemical analysis confirmed that the wire met specifications even though its physical appearance was different then the other wires in the harness. The reason for the difference was not determined during the examination; however, "possible explanations for this can be that different vendor wire was used during the manufacture of this harness or the wire may have been replaced in service at some time." Two other wires in the wire harness displayed damaged insulation. The damage was such that the copper conductor was exposed; however, no evidence of arcing was noted. Boeing concluded that 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/puncture to the system "A" return line, followed by ignition to the mist of hydraulic oil leaking from the hole in the line. On September 28, 2001, Boeing sent a message to all 727 Field Service Bases advising them of the fire that occurred to N277FE, and to recommend corrective action. The recommended corrective action included the following statements: "As noted, the wire bundle/tube chafing condition is believed to have occurred from inadvertent mis-routing of the wire bundle during maintenance activity. Boeing therefore encourages operators to check their airplanes at the next convenient maintenance opportunity for a similar chafing/tube damage condition, take the action necessary to correct any damage found and record results." "During investigation of this subject, it is believed that one or more of the support clamps were likely missing, resulting in additional 'slack' in the wire bundle and allowing it to chafe against the hydraulic tube." "Verify that a minimum clearance of 0.25 inch is present between wire bundle W344 and adjacent tubing/structure."
The inadvertent misrouting of a wire harness by maintenance personnel, which resulted in chafing to a hydraulic line and electrical wire, and subsequent fire.
Source: NTSB Aviation Accident Database
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