MIA06FA120 · Mitsubishi MU-2B-60

Witnesses stated that they observed the twin-engine airplane roll into a steep right bank and enter a spin at a low altitude (less than 700 feet) during the initial climb. The airplane then descended and impacted terrain about 1.5 miles from the end of the departure runway. Some witnesses reported hearing an unusual engine noise just before the airplane began to roll and spin. Day visual meteorological conditions prevailed.

Examination of the right engine revealed that the ring gear support of the engine/propeller gearbox had fractured in flight due to high cycle fatigue originating from the corner radii of the high-speed pinion cutout. The reason for the fatigue could not be determined. The ring gear support disengaged from the ring gear due to this failure, resulting in a disconnection in power being transferred from the engine power section to the propeller.

In addition to the ability for a pilot to manually feather the propellers, and an automatic feathering feature, the engine (Honeywell TPE-331) design also includes a “Negative Torque Sensing” (NTS) system that would automatically respond to a typical failed engine condition involving a propeller that is driving the coupled engine. Feathering the propeller reduces drag and asymmetric yawing due to the failed engine. All Federal Aviation Administration (FAA) certification evaluations for one-engine inoperative handling qualities for the airplane type were conducted with the NTS system operational. According to the airplane manufacturer, the NTS system was designed to automatically reduce the drag on the affected engine to provide a margin of safety until the pilot is able to shut down the engine with the condition lever.

However, if a drive train disconnect occurs at the ring gear support, the NTS system is inoperable, and the propeller can come out of feather on its own, if the disconnect is followed by a pilot action to retard the power lever on the affected engine. In this scenario, once the fuel flow setting is reduced below the point required to run the power section at 100% (takeoff) rpm, the propeller governor would sense an “underspeed” condition and would attempt to increase engine rpm by unloading the propeller, subsequently driving the propeller out of feather toward the low pitch stop. This flat pitch condition would cause an increase in aerodynamic drag on one side of the airplane, and unanticipated airplane control difficulty could result due to the asymmetry.

The emergency procedure for an in flight power loss, regardless of the cause, published in the Airplane Flight Manual (AFM) required that the power lever for the failed engine be moved forward to the Takeoff position (following the step to immediately shut down the engine by moving the condition lever to the Emergency Stop position). Additionally, a “WARNING” follows this procedure to reiterate that the pilot must “…NOT RETARD FAILED ENGINE POWER LEVER.” The warning also states: “PLACE FAILED ENGINE POWER LEVER TO TAKEOFF POSITION DURING THE FEATHERING OF PROPELLER AND LEAVE THERE FOR REMAINDER OF THE FLIGHT."

Postcrash examination of the wreckage revealed evidence that both condition levers were in the “Takeoff/Land” position. The left engine power lever was in the “Reverse” position, and the right engine power lever was in the “Flight Idle” position.

Based on an analysis of evidence from the wreckage and technical data from the airframe and engine manufacturers, a likely scenario for the accident sequence is as follows: Shortly after takeoff, and after being instructed to change frequencies, the pilot may have perceived a loss of power in the right engine and an associated rise in rpm. The right propeller then went into a feathered position about 3 seconds later. The pilot then reduced the right engine power lever, contrary to the AFM procedure. At this point, the fuel flow decreased, leading to a decrease in power section rpm. The propeller governor then sensed an under-speed condition. As a result, oil was routed to the propeller by the propeller governor, causing the propeller to come out of feather toward a flat pitch (increased drag) position. The pilot may not have been aware that the propeller came out of feather. As a result of the increased drag condition on the right side of the airplane, the airplane yawed and rolled to the right and entered a spin. In an attempt to control the airplane, the pilot reduced power on the opposite (left) engine. However, at this point, the airplane was not at a sufficient altitude to recover.

The investigation revealed that a TPE331 engine gearbox uncoupling event is an unusual engine failure that results in substantially different engine indications to a pilot in comparison to a typical flameout event in which the NTS system in operable.

According to the engine manufacturer, there have been five incidents of similar TPE331 ring gear support cracks during about 29 million engine hours of service history. All of the cracks originated at the high-speed pinion cutout detail. Three of these incidents were shop findings, one incident resulted in an in-flight shutdown at altitude followed by a safe landing, and the other incident was this accident.

The accident pilot reportedly accumulated 11,000 hours of total pilot experience, 2,000 of which were in the same make and model as the accident airplane. About 300 hours were logged within the previous six months of the accident. The pilot received recurrent training the same make and model about 11 months before the accident.

On March 3, 2008, the airplane manufacturer published MU-2 Service News No. 110/00-017, entitled “Power Lever Position Warning for In-flight Engine Failure” which reiterated the warning that the failed engine power lever must not be retarded. In January 2009, the airplane manufacturer also published Service News No.114/00-020, entitled “Engine Failure Modes,” which provided additional detail for pilots regarding an uncoupled gearbox, and again reiterated the AFM procedure and warning. In addition, the engine manufacturer intends to issue a letter to provide a description of the engine symptoms and recommended actions in the event of an uncoupling event, or in the event of an engine failure for any reason.

Verbatim NTSB analysis and probable cause from the NTSB dataset