Accident Details
On May 30, 2013, at about 2307 eastern daylight time, a Bell 407 helicopter, registration N407HC, sustained substantial damage during an autorotation following a total loss of engine power at Columbia, Maryland. The commercial pilot and three crewmembers were not injured. The helicopter was registered to the State of Maryland and operated by the Howard County Police Department as a public use flight. Night visual meteorological conditions prevailed, and no flight plan was filed. The local flight originated at Tipton Airport (FME) in Fort Meade, Maryland, about 2210.
According to the operator, the helicopter was on a routine patrol flight on a heading of 130 degrees at a speed of 120 knots. About 600 feet above the ground, the helicopter suddenly yawed to the left and the red "ENGINE OUT" warning light illuminated. The pilot commenced an autorotation for landing on a nearby high school football field. To avoid trees and a goal post, the pilot extended the glide path by raising the collective control. The helicopter landed hard on the football field and came to rest in an upright position. The pilot secured the rotor blades and engine, and the crew exited the helicopter without injuries.
Investigation
The wreckage was recovered and secured in a local hangar. On June 4 and 5, 2013, Federal Aviation Administration inspectors and investigators from the airframe and engine manufacturers examined the wreckage. Damage was substantial, including a severed tail boom, damaged main rotor blades, and damaged landing gear. The on-scene examination of the engine revealed no damage or anomalies. The engine's electronic control unit (ECU) was removed and shipped to the manufacturer's facility in West Hartford, Connecticut, for further evaluation and testing.
Examination of the ECU revealed that the engine was operating normally until an unexpected activation of the Full Authority Digital Electronic Control (FADEC) Overspeed Protection System (OSPS) fuel reduction solenoid, internal to the ECU, occurred, resulting in an uncommanded in-flight shutdown of the engine. The unit's non-volatile memory showed evidence of an OSPS power supply malfunction shortly after the solenoid activated. An ECU functional review confirmed that a short circuit failure of component C49, a 22 microfarad tantalum capacitor (part number M39003/09-0254), was the source of the ECU malfunction.
The ECU (Rolls-Royce part number 23080490) was affected by Rolls-Royce Commercial Engine Bulletin (CEB) 73-6052, dated December 3, 2012. Compliance with this bulletin would replace the old ECU with an improved design (part number 23088856), which incorporated improvements to the OSPS. The compliance date per Rolls-Royce was March 31, 2015. Compliance was not mandatory through an FAA Airworthiness Directive. On January 23, 2014, Bell Helicopter Alert Service Bulletin (ASB) 407-14-102 and Rolls-Royce CEB A-73-6059 were issued. These bulletins addressed installing an overspeed adapter to reduce the likelihood of a false overspeed activation, introduced a recurring functional check of the overspeed protection circuits within the ECU, provided instructions to apply Stabilant 22 to the connector and adapters, and introduced a recurring 50-hour requirement to functionally check the overspeed protection system by performing overspeed test shutdowns.
Cause
The engine shutdown was caused by an uncommanded activation of the FADEC Overspeed Protection System (OSPS) fuel reduction solenoid, internal to the ECU, due to a short circuit failure of capacitor C49 within the ECU.