A National Transportation Safety Board investigation into the 2013 emergency landing of a Mokulele Airlines passenger plane on Piilani Highway on Maui has concluded that the incident was likely caused by damage to the plane’s engine as a result of improper cleaning.
The probable cause report was released on Wednesday.
On Oct. 21, 2013, Mokulele Flight 1770 lost engine power while while flying from Kahului to Waimea on the Big Island, prompting head pilot Robert Fields to turn the plane around in an attempt to return to Kahului Airport. Upon determining that the plane would not be able to reach the airport, Fields, 29, brought the Cessna 208B down in the northbound lanes of Piilani Highway.
The flight crew reported hearing a loud bang and a grinding sounds just before the loss of power. First Officer Mike Lisman also reported seeing sparks coming from the exhaust.
In a statement to the NTSB, Fields reported: "I chose the area with the least cars and Mike pointed out obstructions during the descent on both sides of the highway. We narrowly missed power lines, light poles and steep embankments on both sides of the highway. We touched down on top of the hill, braked hard and came to a stop after missing a car."
The plane sustained substantial damage upon striking two highway traffic signs but none of the 10 people aboard — Fields, Lisman and eight passengers — was injured.
An inspection by the plane’s manufacturer found significant damage to the power turbine blades, and the compressor turbine hub was observed to have a "frosted" appearance, consistent with glass blasting. Glass-like beads and fragments were also found, "consistent with the disc assembly having been cleaned by glass media blasting in the assembled condition."
Glass media blasting is a common method for cleaning parts. However, the manufacturer’s manual specifies that such cleaning must be performed with the disc and blades disassembled and cleaned before assembly.
All 58 compressor turbine blades were found to have been fractured, and the remaining stubs were "gouged and battered," according to the report.
The report noted that the areas of the compressor turbine disc assembly are highly precise and extremely smooth, and that hard contaminants can cause the assembly to function irregularly.
The post-accident examination of the engine found evidence of "glass bead contamination" in the compressor turbine portion of the engine.
The report stated that the plane’s compressor turbine disc assembly had been repaired two months before the accident by Southwest Airmotive Corp. and that the assembly had been removed during the repair.