Westchester County Airport
Aircraft Accident/Incident Report

Armonk, New York 10604
Saturday, June 18, 2011 13:06 EDT

Witnesses observed the pilot perform multiple engine run-ups both on the parking ramp and at the end of the runway prior to departure. Both witnesses reported that the engine initially idled rough. Shortly after takeoff from runway 34, the pilot informed the air traffic controller that he needed to return to the airport and requested runway 16. The pilot declared an emergency but did not state the reason why. The airplane impacted trees and terrain about 1 mile from the approach end of runway 16. The cockpit and cabin sections were found inverted and mostly consumed by a post-crash fire. The wreckage path through the trees indicated that the airplane was established on a shallow glide angle when the impact occurred, with the landing gear and flaps retracted. A disassembly and examination of the engine revealed that the No. 2 cylinder piston head exhibited severe thermal deterioration consistent with a pre-ignition or detonation event. The electrodes of the No. 2 cylinder spark plugs were fully imbedded with aluminum and appeared incapable of producing a spark. The other five cylinders appeared to be functional and were undamaged. The left magneto was set to 30 degrees before top dead center (BTDC); the manufacturer's specification was 22 degrees BTDC. The right magneto was broken at its mount and its timing could not be verified. Examination of the engine maintenance records revealed that the magnetos were last retimed during an annual inspection about 27 months before the accident. The airplane was flown infrequently since the engine was installed in 2007. The airport was equipped with noise abatement monitoring microphones that captured the airplane on the outbound and inbound (return) legs during the accident sequence. Although the engine appeared to be running just prior to impact, the engine sound pressure recorded on the outbound leg was significantly greater than on the inbound leg, suggesting that the pilot was approaching the airport at a reduced power setting. Although the amount of engine power available could not be determined, it is likely that additional power was available had the throttle been advanced.

The pilot's decision to depart on the flight with a suspected mechanical deficiency and his subsequent decision to fly the final approach at a reduced power setting. Contributing to the accident was the improper timing of the magneto(s) that resulted in a severe detonation event.