Overview
According to the pilot, the airplane took off with a flaps setting of 25 percent, as per the operator's procedures. Shortly after raising the landing gear, he heard an explosion and observed that all gauges for the left engine, a Pratt & Whitney Canada PT6A-34, indicated zero, signifying a loss of power. The left propeller had stopped. The pilot added full power to the right engine, left the flaps at 25 percent, and kept the landing gear up. Despite applying full right rudder, he could not prevent a left turning descent. He later noted that he could not maintain V speeds and that the stall warning horn activated continuously.
Engine Failure
Post-accident examination revealed fatigue fracturing of the first-stage sun gear in the left engine. The sun gear is the central gear around which the planet gear assembly rotates, reducing engine rpm to propeller rpm. Maintenance records indicated that during an October 1998 engine overhaul, the first-stage planet gear assembly was replaced due to frosted and pitted gear teeth. The mating sun gear was examined and deemed serviceable, so it was reinstalled—an accepted practice at that time. Subsequently, the engine manufacturer determined that if either the sun gear or planet gear assembly required replacement with a zero-time component, the corresponding mating component must also be replaced to avoid potential distress from mismatched wear patterns. The accident airplane's first-stage sun gear failed at 22,064.8 hours total time since new.
Maintenance Program
The engines were maintained under a Federal Aviation Administration (FAA)-approved "on-condition" maintenance program, which Business Air adopted in May 1995. In April 2002, the engine manufacturer issued Service Bulletin (SB) 1403 Revision 7, which mandated replacement of certain life-limited components, including the first-stage sun gear, at 12,000 hours total time since new. However, SB 1403R7 did not address previously approved on-condition maintenance programs; Business Air continued operating under its existing program without incorporating the updated standards. Three months after the accident, the engine manufacturer stated it would continue to endorse Business Air's on-condition program. The National Transportation Safety Board's review found numerous deficiencies in Business Air's maintenance program that went undetected by the Portland, Maine, Flight Standards District Office (FSDO), which was responsible for monitoring the operator's activities. Examples included failure to specify which parts were covered by on-condition versus hard-time schedules, missing or inaccurate engine condition trend monitoring data, lack of regular trend analyses, and delays in oil sample testing—a sample taken more than two weeks before the accident showed increased iron levels, indicating declining engine health, but results arrived days after the accident.
Oversight Issues
The FSDO's oversight failed to identify the operator's inadequate maintenance practices, which allowed an engine with a sun gear well beyond the manufacturer's recommended reliable operating timeframe to continue in service. It took over two and a half years after the accident for the FAA to present a consent order acknowledging the operator's maintenance inadequacies and required corrective actions.
Probable Cause
The pilot's misapplication of flight controls following an engine failure. Contributing to the accident was the failure of the sun gear, which resulted in the loss of engine power. Contributing to the sun gear failure were the engine manufacturer's grandfathering of previously recommended, but less reliable, maintenance standards, the Federal Aviation Administration's (FAA) acceptance of the engine manufacturer's grandfathering, the operator's inadequate maintenance practices, and the FAA's inadequate oversight of the operator.