Accident Overview
The airplane was destroyed following an attempted landing after a reported partial power loss of the left engine while en route. The wreckage distribution was consistent with an impact resulting from a Vmc (minimum control speed with the critical engine inoperative) roll to the left. No emergency was declared.
Flight Path and Decision Making
During the flight, the airplane was at an altitude of about 15,900 feet. The closest airport, located about 27 nautical miles to the southwest, was a controlled field equipped with airport rescue and fire fighting (ARFF) and had a longest runway of 9,005 feet. Instead of diverting there, the flight proceeded to the departure airport, located about 93 nautical miles to the north. That airport was an uncontrolled field without ARFF, with a longest runway of 5,000 feet. A witness reported that the airplane was too high and too fast to land on runway 34 at that airport. At the time, winds were from 140 degrees at 6 knots.
Pilot Experience and Checkout
The pilot received a checkout from the right seat of the accident airplane by the airplane owner. The checkout lasted about 20 minutes and did not include any single-engine flight maneuvers or emergency procedures. The owner did not hold a certified flight instructor certificate. The pilot had stopped flying for 12 years and resumed flying about a year prior to the accident, giving flight instruction and flying single-engine airplanes. The pilot's recent multiengine experience was limited to a couple of non-revenue flights within the past year, seated in the right seat of a King Air used for commercial charter work. One landing was made in the King Air the day before the accident. The accident pilot asked the King Air pilot to accompany him on the accident flight, but that pilot declined. A multiengine commercial rated pilot-rated passenger, known to the accident pilot, was seated in the right front seat.
Maintenance and Engine Issues
Examination of the airplane's supplemental type certificate (STC) revealed numerous inspections by different maintenance personnel. The left engine's variable absolute pressure controller had safety wire around its control arm, precluding normal operation, and a pressure relief valve not called for in the STC drawings. At the time of issuance, federal regulations did not require STC instructions for continued airworthiness. Relying on the airplane and engine maintenance manuals would not have provided sufficient information for continued airworthiness per the STC and could have yielded settings exceeding those for which STC parts were originally certificated, thus increasing Vmc speed. Examination of the left engine revealed a cylinder head separation on the number six cylinder assembly, which had accumulated an estimated 240 hours since installation. Visual inspection showed undecipherable characters in its parts numbering. A cylinder head separation from another airplane was also examined, with about 270 hours since installation. Both examinations revealed additional material on the cylinder barrel threads and fatigue fracture on the cylinder head.
Probable Cause (Official Findings)
The pilot's failure to maintain adequate airspeed (Vmc) resulted in a loss of control. Contributing factors were the improper in-flight planning/decision not to land at a closer airport, the lack of recent experience in multiengine airplanes by the pilot-in-command, the cylinder head separation, the inadequate manufacturing process, and the lack of continued airworthiness instructions relating to the Riley Super-8 STC.