Event Summary
On July 17, 2010, Delta Air Lines flight 1188, an MD-88 (registration N990DL), departed Hartsfield-Jackson Atlanta International Airport (ATL) at 2210 eastern daylight time, bound for Bradley International Airport. During the climb to cruise at approximately 11,000 feet, the crew detected an electrical odor. They donned oxygen masks and declared an emergency, returning to ATL, where an uneventful landing was made. After landing, the crew observed small flames emanating from the Captain's side of the glareshield and extinguished them with a fire extinguisher. None of the 106 passengers and 5 crewmembers on board were injured.
Investigation Findings
The National Transportation Safety Board (NTSB) did not travel to the scene. Delta Airlines inspected the aircraft and found that the right wing landing light switch displayed evidence of severe heat damage. No other source for the fire was identified. The switch was removed by Delta maintenance personnel and sent to the NTSB for investigation; no anomalies with respect to switch installation were reported.
Both the “Right Wing Landing Light” (B1-15) and “Right Wing Landing Light Control” (B1-16) circuit breakers were found open following the event. The right wing landing light circuit provides 115 Volts AC electrical power for the light, and the control circuit provides power to extend and retract the light fixture. Both circuits are controlled by the right wing landing light switch.
The switch (Item No. S1-6) is located on the Captain's side of the glareshield control panel and has three positions: retract light and power off, extend light and power off, and extend light and power on. It is a two-pole three-position switch, part number 8906K1268, manufactured by Eaton in week 52 of 1986. Designed to meet MS25201 specifications, the switch can use AC or DC power; in this application it was subjected to 115 volts AC. Per specification, the switch must demonstrate an electrical endurance test life of 10,000 cycles and a mechanical endurance test life of 20,000 cycles.
Detailed Examination
A Systems Group examined the switch at the Eaton facility in Milwaukee, WI on January 25, 2011. The switch was severely fire-damaged: a portion of the phenolic casing covering the A1 and B1 contactors on the terminal face was missing, with remaining edges charred. Terminal lugs A1 and B1 were separated from the switch, and the respective contacts remained attached to the terminal lugs. The B1 contact was deformed and pitted. The A1 and B1 contactor arms were charred in that area, and the B1 contactor pad on the arm had no thickness remaining. A detailed examination report is contained in the Public Docket.
Maintenance History Search
Eaton performed a search for quality issues related to the part number between March 2004 and November 2010 and for issues related to lot number 8652; no issues related to this failure mode were discovered. Delta Airlines searched log book entries related to ATA codes 331 (Lights-Flight Compartment Lighting) and 334 (Lights-Exterior Lighting) dating back to January 1, 2008, for the event aircraft. The search revealed that a broken wire for the right wing landing light was repaired on September 2, 2008. No other right wing landing light discrepancies for this aircraft were identified.