Accident Overview
On September 4, 2011, about 1500 Alaska daylight time, a Piper PA-18 airplane, registration N13973, sustained substantial damage following a loss of engine power during final approach to McGrath Airport in McGrath, Alaska. The aircraft was being operated as a personal cross-country flight under visual flight rules (VFR) in accordance with 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed at the time. The solo commercial pilot received serious injuries. The flight had originated from Kingston Creek, approximately 85 miles east of McGrath.
Pilot Account
In a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on the day of the accident, a rescuer reported that the pilot stated he advanced the throttle to adjust his glide path during approach, but the engine did not respond. The pilot believed the throttle cable had broken. In a later interview in January 2012, the pilot elaborated that on short final, the engine was throttled back and the airplane was trimmed for the approach. When he realized the approach would be short, he applied power, but the engine remained at idle. The airplane then descended, striking trees and the ground, and nosed over. The pilot also mentioned hearing an audible snap, which he associated with a throttle cable failure. The pilot reported no preexisting mechanical issues with the airplane.
Post-Accident Examination
During the recovery of the airplane, usable fuel was found in the wing tanks. The wings were removed, and the aircraft was transported to a maintenance facility in Anchorage, Alaska. On November 10, 2011, the NTSB IIC, accompanied by an FAA air safety inspector, examined the airplane. The engine was run on the airframe through various RPM ranges. The throttle cable was found intact and functioning correctly, and no mechanical anomalies were detected with the engine.
Magnetos and Similar Accident
During the post-accident engine run, the magneto switches—single-pole, single-throw mechanical switches—were turned off while the engine was at cruise RPM, producing an audible click/snap that could be heard above the engine noise. The pilot had previously stated that he heard a snap and the engine quit “as if the magnetos had been shut off.” The elevator trim adjustment handle is located just below the magneto toggle switches. The pilot had just made elevator trim adjustments before the power loss.
A similar accident involving a Piper PA-18 (ANC11CA089) occurred on August 29, 2011, near Cape Yakataga, Alaska, where the pilot inadvertently switched off the magnetos with his coat sleeve while using the trim handle. Examination of the magneto switches on the accident airplane and other like models revealed that a pilot's coat sleeve can contact and move the guarded magneto switches to the off position. In that previous investigation, the NTSB determined that the pilot inadvertently switched off the engine magnetos, contributing to the loss of power, and noted the manufacturer's placement of the magneto switches as a contributing factor. The airplane in the current accident had the same magneto switch configuration, and the pilot had just trimmed the airplane for the approach.
Subsequent Regulatory Action
Following the investigation of the similar accident, the FAA examined instances of inadvertent magneto shutoff in similarly configured airplanes. This led to the publication of a notice of proposed rulemaking (NPRM) with the intent to issue an airworthiness directive (AD) for reconfiguration of the magneto switches.
Miscellaneous
Due to the pilot's injuries, no NTSB Form 6120.1 was submitted. Airplane and pilot logbooks were examined and no anomalies were found.