NTSB Factual Report

On July 17, 2010, the crew of a Delta Air Lines MD-88, registration number N990DL, flight number 1188, declared an emergency and returned to the departure airport after they smelled an electrical odor during the climb to cruise. The scheduled domestic passenger flight to Bradley International Airport was operating under the provisions of Title 14 CFR Part 121. The flight departed Hartsfield-Jackson Atlanta International Airport (ATL) at 2210 eastern daylight time. During the climb to cruise, at an altitude of approximately 11,000 feet, the crew noticed the electrical smell. The crew donned oxygen masks and returned 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 the flames with a fire extinguisher. None of the 106 passengers and 5 crewmembers on board were injured.

The National Transportation Safety Board (NTSB) did not travel to the scene of this event. Upon inspection by Delta Airlines it was revealed that the right wing landing light switch displayed evidence of severe heat damage. No other source for the fire was identified.

The right wing landing light switch was removed by Delta Airlines maintenance personnel and sent to the NTSB for the investigation. No anomalies with respect to switch installation were noted by the personnel who removed the switch.

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 right wing landing light and the “right wing landing light control” circuit provides 115 Volts AC electrical power to extend and retract the right wing landing light fixture. Both electrical circuits are controlled by the right wing landing light switch.

The right wing landing light switch (Item No. S1-6) is located on the Captain's side of the glareshield control panel. The switch controls the right wing landing light on the aircraft. The switch has three positions; one to retract the light fixture and power off the light, one to extend the light fixture and power off the light, and one to extend the light fixture and power on the light.

The switch was identified as Part Number 8906K1268, manufactured by Eaton in week 52 of 1986. The switch is a two-pole three-position switch, designed to meet MS25201 specifications and can be used with alternating current or direct current power. In this application the switch was subjected to 115 volts alternating current. Per the specification, the switch must demonstrate an electrical endurance test life of 10,000 cycles and a mechanical endurance test life of 20,000 cycles.

A Systems Group was formed and the 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 of the switch was missing and the remaining edges appeared 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 this area and the B1 contactor pad on the arm had no thickness remaining. A detailed examination report with accompanying pictures is contained in the Public Docket for this accident.

Eaton performed a search for quality issues related to this part number between the dates of March 2004 and November 2010. No issues related to this failure mode were discovered. Eaton also performed a search for issues related to the lot number 8652 with no issues related to this failure mode identified.

Delta Airlines performed a search of log book entries related to ATA codes beginning with 331 (Lights-Flight Compartment Lighting) and 334 (Lights-Exterior Lighting) dating back to 1/1/08 for the event aircraft. This search revealed that a broken wire for the right wing landing light was repaired on 9/2/2008. No other right wing landing light discrepancies for this aircraft were identified in this search.

NTSB Probable Cause

The cause of the right wing landing light switch fire is undetermined.