SEA00IA019
SEA00IA019

On November 11, 1999, approximately 1520 Pacific standard time, Alaska Airlines flight 500, a Boeing 737-400 (N772AS), returned to land when the crew noted fuel low pressure lights and popped circuit breakers after departure and climb-out from Portland International Airport, Portland, Oregon. There were no injuries to the five crewmembers or 140 passengers, and the airplane's electrical wiring system sustained minor damage. The flight was operated under 14 CFR 121, and was on an instrument flight plan, en route to San Francisco, California.

The crew stated during climb-out from Portland, the #2 fuel tank low pressure light illuminated. Thirty seconds later, the center tank fuel plow pressure light illuminated. Circuit breakers for the center right pump and the #2 fuel pump popped. The crew elected to pull the aft pump circuit breaker, and returned to Portland for an uneventful landing.

During the trouble-shooting process, the circuits were repowered. A master caution light illuminated and other circuit breakers opened. Inspection revealed heavily sooted and compromised wires (with melted insulation) in wiring bundles near station 410 in the cargo hold.

The captain was interviewed during the course of the investigation. He noted that the first indication of a problem was when climbing through 11000 feet MSL, when he saw the master caution fuel indication and an indication of low fuel pressure on the right tank. He turned around and noted that the boost pump circuit breakers were in. He then noted low pressure light on the center tank. Anticipating fuel contamination, he opened the crossfeed manifold. The crew leveled at 15000 feet near Deschute. Turning again to the circuit breakers, he noted that the center tank right pump circuit breaker was out, the right tank forward circuit breaker was out, the right tank aft pump was still in. The respective switches were still on.

At that time they had elected to return to land at Portland. During descent, the crew went through the QRH, which included the precaution to not reset the circuit breakers. At that time, the left center pump continued functioning. During descent the aft pump circuit breaker was pulled. Voltages and frequencies for both generators were okay. The P-6 electrical distribution panel circuit breakers were all in.

After landing, the crew performed a normal shutdown. During taxi in, they noted an electrical smell, which they originally attributed to use of the galley ovens. When they elected to disembark the passengers, passengers leaving the airplane who had been seated mid-cabin noted that they had smelled electrical smoke.

Alaska's maintenance crew found that the ground refueling valve was open. That valve was closed by mechanics, but it did not change the problem. The problem appeared to be affecting both the left and right busses. The mechanics tried a bus protection panel and swapping the three generator control units. None of these units seemed to change the problem. At that time, the mechanics started to see various indications in the cockpit, including right wing anti-ice and right wing overheat.

Technicians isolated an electrical short to the forward cargo compartment. When they touched the panel around station 410, the saw some sparks. Power was removed from the aircraft once the damage was found. The technicians opened the ceiling panel to gain access to the area. When they touched the bottom of the raceway clamp, it fell apart.

FAA and NTSB investigators documented the damaged wiring bundles at station 420 above the forward cargo compartment ceiling on the right side of the airplane, and had them sectioned for further inspection and testing. The wire bundle channel and clamp were also removed for inspection and analysis.

A total of 113 conductors were reported by Alaska Airlines to have been involved in the electrical arcing.

Analysis by the Boeing Equipment Quality Analysis (EQA) group could not determine the cause for the electrical arcing that burned numerous wires of the subject bundles in two. According to Boeing EQA, the extent of the arc damage observed and documented appeared to be consistent with arc tracking of the polymide (Kapton) insulated wires. Besides the polymide wires, there were nine Teflon insulated wires that had been severely damaged by the electrical arcing. The insulation on one of the Teflon insulated wires, W336-003-10, had evidence that it might have been damaged prior to the electrical arcing. W336-003-10, and two other 10 gauge wires in the bundles, were conductors for the three-phase 115VAC system A electric hydraulic pump.

According to Boeing analysis, electrical arcing had damaged approximately twelve inches of insulation on wire W336-003-10. The exposed conductors at one end of the wire had sustained arc damage at two locations. The arcing burned several wire strands in two, which became unraveled from the rest of the strands. There was approximately one third of the insulation left in this area of the wire. A small area of the edge of the insulation appeared to have been cut prior to the occurrence of the electrical arcing. According to Boeing EQA, this would suggest that the insulation might have been missing before the electrical arcing occurred.

The wire bundle channel (bracket) and clamp were also analyzed. Evidence indicated that an unidentified wire that was involved in the arcing/burning event had been out of place, along the side of the bracket between the cushioning foam and the plastic (nylon) frame before or during the event. The analysis of these two parts could not determine what type of wire had caused the damage to the side of the cushion clamp or the nylon channel.

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