N13973Piper PA-18-150 · 1972

On September 4, 2011, about 1500 Alaska daylight time, a Piper PA-18 airplane, N13973, sustained substantial damage following a loss of engine power during final approach to McGrath Airport, McGrath, Alaska. The airplane was operated by the pilot as a visual flight rules (VFR) personal cross-country flight, under the provisions of 14 Code of Federal Regulations, Part 91, when the accident occurred. Visual meteorological conditions prevailed at the time of the accident, and the solo commercial pilot received serious injuries. The flight departed Kingston Creek, about 85 miles east of McGrath.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on September 4, a rescuer that responded to the accident site said that the pilot told him that while on approach to land, he advanced the throttle to apply engine power to adjust his glide path, but the engine did not respond. The pilot reported that he thought the throttle cable had broken.

The airplane subsequently descended into trees at the approach end of the runway, sustaining substantial damage to the wings and fuselage.

Additionally, the pilot told the rescuer that he heard an audible snap, which made him think the throttle cable had snapped, and then the engine failed to respond to throttle application.

During a telephone conversation with the NTSB IIC in January 2012, the pilot reported that he had owned the airplane for many years, and that there were no mechanical anomalies with the airplane prior to the accident. He said that on the accident flight, while on short final for the airport, the airplane had been trimmed for the approach, and the engine was throttled back. He said the approach was going to be short of the runway, and he applied power, but the airplane engine remained at idle. He thought the throttle cable had broken. The airplane descended striking trees and the ground, and nosed over.

During recovery of the airplane, usable amounts of fuel were found in the wing tanks. The wings were removed and the airplane was transported to a maintenance facility in Anchorage, Alaska.

On November 10, the airplane was examined by the NTSB IIC, accompanied by an FAA air safety inspector. During the inspection the engine was run on the airframe. The engine was started, and run through various rpm ranges. The throttle cable was found intact and it worked as anticipated. No mechanical anomalies were found with the engine.

An FAA air safety inspector, a party to the investigation, said the pilot made the following statement during a telephone interview. On September 4, 2011, the accident pilot said that immediately prior to the loss of engine power, he had made elevator trim control adjustments and was in the process of making a throttle control adjustment when he heard a snap and the engine quit operating, “as if the magnetos had been shut off.“ The elevator trim adjustment handle is situated just below the magneto toggle switches.

During the engine run, the magneto switches were turned off while the engine was at cruise rpm. The switches being single-pole, single-throw mechanical switches, made an audible click/snap when turned off, which could be heard above the noise of the engine.

Due to the pilot's injuries, no NTSB form 6120.1 was submitted. Airplane and pilot log books were examined, and no anomalies were found.

Additional Information

In a similar accident also involving a Piper PA-18 airplane, which occurred on August 29, 2011, near Cape Yakataga, Alaska, (ANC11CA089) the pilot reported that, during the initial climb, he inadvertently switched off the engine magnetos with his coat sleeve while using the trim handle causing the airplane’s engine to shut down; the switches were located on the left side of the cabin. The pilot realized the problem, but, due to the airplane's low altitude, he decided to land instead of trying to restart the engine.

A postaccident 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.

As a result of the previous investigation the NTSB determined that the pilot inadvertently switched off the engine magnetos during the initial climb, resulting in a loss of engine power. Contributing to the accident was the manufacturer's placement of the magneto switches.

The airplane in the current accident had the same magneto switch configuration as the airplane listed in ANC11CA089, and the pilot had just trimmed the airplane for the approach.

Following the investigation of ANC11CA089, the FAA looked into instances of magnetos being inadvertently shut off in similarly configured airplanes. Their investigation led to publishing a notice of proposed rulemaking (NPRM), with the intent to publish an airworthiness directive (AD) for the reconfiguration of the magneto switches.