Fort Pierce, FL, USA
N316PR
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.
HISTORY OF FLIGHT On June 25, 2006, about 1224 eastern daylight time, a Mitsubishi MU-2B-60, N316PR, registered to and operated by Flyin Cloud LLC., as a Title 14 CFR Part 91 ferry flight, crashed shortly after takeoff from Saint Lucie County International Airport, Fort Pierce, Florida. Visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed. The airline transport-rated pilot, the sole occupant of the airplane, received fatal injuries, and the airplane was destroyed. The flight was originating at the time of the accident. The accident pilot had flown to the Bahamas, discharged passengers, and had returned to the U.S., landing at Saint Lucie County Airport (Fort Pierce Airport), where he cleared U.S Customs and refueled. At the time of the accident the pilot was on a flight to Murfreesboro, Tennessee, to have scheduled 100-hour maintenance performed on the airplane. A review of the FAA Port Pierce Air Traffic Control Tower (ATCT) recorded communications revealed that the pilot had initially contacted the Fort Pierce tower controller, stating “tower 65PR ready to go runway 14.” At 1222:39 the tower controller responded stating, “on departure turn left heading zero niner zero runway one four cleared for takeoff.” The accident pilot responded, “left to zero niner zero, one four cleared to go, papa romeo thanks.” At 1223:44 the controller stated, “Mitsubishi six five romeo heading zero nine zero contact miami center”, and the pilot responded “zero niner zero going to miami, have a good day, papa romeo.” The controller responded, “see ya.” There was no record of further communications between the Fort Pierce tower controller or the Miami Air Route Traffic Control Center (ARTCC) controller and the pilot of N316PR. Subsequent attempts by either controller to establish communications with the flight yielded negative results. The airplane impacted the terrain in a wooded area, about 80 yards west of U. S. Highway 1 (US-1) about 1.5 miles away from the departure end of runway 14. Several witnesses saw the accident and reported that the airplane was at a very low altitude, after having just taken off from the Saint Lucie County International Airport. One witness stated that he was traveling north on highway US-1, at Juanita Street, and he witnessed a black twin-engine airplane “in trouble.” He said the airplane was in a nose-up attitude, pointing east, and then it yawed to the right, pointing south in a wings level position. It then continued to slowly spin in a clockwise direction until striking the ground and bursting into flames. According to the witness, the right propeller was rotating very slowly, such that he could count its revolutions. Another witness, an experienced commercial pilot who had accumulated about 6,000 hours of flight experience, and who was also former law enforcement officer/investigator, stated that he was in a car proceeding northbound on State Road 5 (SR-5), also known as US-1, and was at the intersection of highway A1A and highway US-1. He said the weather was broken to overcast, the visibility was 7 miles or more, and there was no rain. He said his attention was drawn to an aircraft departing the Saint Lucie County International Airport to the east, possibly from runway 14. He said the airplane cleared the airport boundary area, and appeared to fly over SR-5, which runs north and south, on the east side of the airport. He said the rate of climb was not consistent with a turboprop, and it did not appear to climb above 500 to 700 feet above ground level. He said the airplane started a wing wobble motion, and that the wings banked slightly to the left, and then back to the right. He further stated that the motion could have been a slight yaw, but added that he was not sure. Seconds after the wing wobble the airplane started a right turn until the wings were 90 degrees to the ground circling to a westerly direction. Within seconds after achieving a 90-degree bank the airplane pitched slightly upward and then executed a “wing-over” maneuver (to the right) appearing to enter a spin. He said the nose pitched down, and the airplane impacted the ground in nearly a vertical nose down position. The witness further stated that to the best of his recollection it appeared that both propellers were turning until the crash, and added that he could not hear any engine noise because he had his car windows rolled up and the radio playing. He did not see any abnormal smoke coming from the engines or debris from the airplane, nor did he recall if the gear was up or down. In addition he did not observe the flap positions. When asked, the witness said that the engines did not appear to be feathered, and both propellers were definitely rotating. Three witnesses located in the area of a marine store noticed the accident airplane. The first witness at the store said that he was parked at the store and he noticed a black or dark colored twin-engine airplane about 200 feet above the ground. He said the airplane began to bank to the side and it continued until upside down. It then nosed down behind the tree-line. Another witness, also at the store, said he was exiting the store and walking to his truck in the parking lot when he heard an airplane and glanced to his left. He said he then heard the airplane’s engine “throttle up”, after which it then spun to the left, and proceeded upside down two times before impacting the ground. The third witness sitting in the store parking lot said that he witnessed an airplane that was heading east. He stated that that he looked back out the window a minute or two later and saw the same airplane rise above the trees, “give full throttle (or so it sounded), then roll over on the left wing, and dove instantly. A witness stated to an officer with the Saint Lucie County Sheriff’s Office, that he was in a mobile home park working on his van, and he heard a sound like a “broken” airplane engine, coming from the airport. He said that he had some experience working on airplane engines before, and that the noise from the airplane sounded like one of the twin engines had a failure. He said he looked up and saw an airplane about 60 feet above banking right. The airplane banked right in a 90-degree to about a 180-degree angle, as if to avoid the mobile home park, and it went head-on into the bushes. PERSONNEL INFORMATION Records on file with the FAA cite the accident pilot as Chief Pilot/Manager of Flyin Cloud LLC. The pilot held an FAA airline transport-rated pilot certificate with an airplane multiengine land rating. He also held commercial pilot airplane single engine land and sea ratings, issued on February 5, 2004. In addition, the pilot held an FAA third class medical certificate, with the stated limitation that the pilot must wear correcting lenses when flying, which had been issued on March 24, 2006. At the time the pilot made application for his medical certificate, he reported having accumulated 11,000 hours total pilot experience, and 300 within the last 6 months. In addition, he had reported having accumulated about 2,000 hours of flight experience in the same make and model airplane as the accident airplane. The pilot last received recurrent Mitsubishi MU-2 training at Howell Enterprises Inc., Smyrna, Tennessee, from July 19 to 25, 2005, and was scheduled to return to Howell Enterprises Inc., for his next recurrent training class on July 24, 2006. AIRCRAFT INFORMATION Airframe Information N316PR, serial number 761SA, was a MU-2B-60 “Marquise”, manufactured in 1980, and was a high performance, twin-engine, high-wing turboprop powered airplane. FAA airplane records indicate that on January 26, 2001, Flyin Cloud LLC., purchased the accident airplane from Headrick Properties Inc., Laurel, Mississippi. FAA records did not show Flyin Cloud LLC., as possessing a Title 14 CFR Part 135 (air-taxi) certificate. At the time of the accident the airplane had accumulated about 4,073 flight hours total time. Engine and Propeller Information The accident airplane was powered by two Garrett Turbine Engine Company, now Honeywell Aerospace, TPE 331-10-511M turboprop engines, serial number 36117C (left engine), and serial number 36126 (right engine), 940 shaft horsepower engines, de-rated to 715 shaft horsepower. The airplane was equipped with two Hartzell Propeller Inc. 4-bladed, constant speed, full feathering, reversible-pitch propellers, each propeller of which was 98 inches in diameter. The propellers were model number HC-B4TN-5JL, and their blades are model LT10282NSB-5.3R. The left propeller hub’s serial number was CDA3514M2, and the serial number on the right hub was CDA3340M2. Negative Torque Sensing (NTS) System Information 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 windmilling propeller. According to the aircraft manufacturer, when this condition is sensed, the NTS system automatically vents oil pressure from the propeller, allowing the balance springs to drive the propeller toward feather. Feathering the propeller reduces drag and asymmetric yawing member due to the failed engine. All certification one-engine inoperative handling qualities evaluations are conducted with the NTS system operational and the aircraft meets all FAA requirements. Airplane Flight Manual procedures for engine failure instruct the pilot to control the airplane, identify the failed engine by power asymmetry and instrument panel indications then move the condition lever to Emergency Stop to shut down the engine and completed the propeller feathering process. Additionally, the procedures instruct the pilot to move the Power Lever to the Takeoff position to prevent the propeller blades moving toward flat pitch. An engine flameout will result in NTS activation, with decreasing torque, EGT or ITT, and rpm. The manufacturer stated that the NTS system is the drag reduction component, providing a margin of safety until the pilot is able to shut down the engine with the condition lever and advance the power lever to the takeoff position. However, if component in the engine gearbox fails in such a manner that the propeller uncouples from the power section, the NTS system becomes inoperable, because the system cannot sense torque in this failure mode. Additional information regarding failure modes of this system, and emergency procedures published in the Airplane Flight Manual (AFM), are provided later in this report. Maintenance History A review of the maintenance records revealed that both engines were manufactured by Honeywell in 1979, and entered service on the accident airplane in 1982, in Argentina. Engine logbooks indicate the engines/airplane accumulated about 1,100 hours while in Argentina, with the last entry occurring in 1984. There were no other entries in the engine logbooks until 1998 when the airplane came to the United States. On October 11, 2000, while in flight, records show that the No. 2 engine had an oil leak, and the pilot secured the engine, and landed the airplane. Post incident examination revealed that the leak was coming from the propeller pitch interconnect tube. On November 2002, about 2484 hours, records indicate that maintenance was performed as part of a gearbox repair on the No 2 engine. On February 27, 2004, about 3,082 hours, the No. 2 engine underwent an engine shutdown due to the backing off of a nut securing the sun gear to the bull gear, which resulted in an uncoupling of the propeller to the turbine. On October 28, 2004, about 3387 hours, records showed that the No. 2 engine experienced low oil pressure, the absence of a beta light, and it was noted to be difficult to come off the locks. A small sliver of metal was found in the check valve in the beta block. On May 3, 2006, about 3282 hours, the No. 2 engine experienced a sudden loss of oil pressure, and was removed from the airplane and repaired. On May 13, 2006, about 4,020 hours total time, the No. 2 engine experienced an oil leak, and it was repaired. Additional airplane maintenance information was obtained from witness statements. An FAA inspector stated that a gentleman telephoned to report that he and the deceased pilot had been friends for many years, and because of the frequency with which he had flown with the pilot, he could have been on the accident flight under different circumstances. The witness said that the pilot had confided in him after an incident had occurred that the accident airplane had a problem with the right engine, which caused the propeller to go into reverse. According to the FAA inspector, the gentleman stated that the pilot had explained that the accident airplane’s right engine had experienced the failure of a particular “pin” during takeoff from Greensboro, North Carolina, over a year previously. A person at Howell Enterprises Inc, who regularly conducts recurrent training on the MU-2, and who had also given the accident pilot his recurrent training, stated that the pilot told him of three instances of incidents having occurred to the right engine. The first incident was in December 2003, and involved an aborted takeoff in Greensboro, North Carolina. The second, date unknown, was during a planned flight to the Bahamas for vacation, in which during climb-out the right engine oil pressure and torque fluctuated. The third event was during a flight in the Washington DC area, when a problem occurred. The mechanic traveled to where the airplane was located, and he replaced the oil line between the propeller governor and the propeller pitch control. METEOROLOGICAL INFORMATION Visual meteorological conditions prevailed at the time of the accident. The Ft. Pierce Airport surface weather observation at 1220, was winds from 240 at 5 knots, visibility 10 statute miles; sky condition, few clouds at 2,600 feet, broken at 4,300 feet, broken at 5,500 feet; temperature 29 degrees C, dewpoint temperature 23 degrees C; altimeter setting 29.88 inHg. WRECKAGE AND IMPACT N316PR impacted the terrain in a wooded sloping area consisting of trees, shrubs, and brush, just south of a mobile home park, about 80 yards west of US-1 at the following geographic coordinates: 27 degrees 28.447 minutes North latitude, 080 degrees 20.229 minutes West longitude. The accident site was located in a rural area of Fort Pierce, in St. Lucie County, about 1.5 miles southeast of the departure runway (runway 14). The airplane came to rest on a magnetic heading of about 210 degrees, and when it impacted a fire ensued. The fire consumed the vegetation in an area from the main wreckage and forward for a radius of about 150 feet, and after the fire was extinguished a strong odor of fuel remained throughout the burned-out area. The crash site encompassed a small debris field and showed evidence of a steep to near vertical descent, consistent with low forward movement associated with an airplane having impacted terrain in a near nose level attitude. The soil in the immediate area was disturbed only in the area of the main wreckage and had signatures consistent with the fuselage having rotated in a clockwise direction. No elongated debris field or ground scarring was noted Evidence of all components of the airplane was in the immediate area of the main wreckage, and examination of the airframe and flight controls revealed no anomalies. The mostly burnt fuselage, when viewed from the side, displayed fractures and compression damage consistent with a high vertical rate of descent. The unconsumed burned sections of left wing had separated from the fuselage, but remained close to its normal position with the engine and propeller under-slung. The remaining burnt, compressed, and fractured section of wing outboard of the engine drooped downward. Pieces of the left wing tip tank had detached, and were found a short distance outboard and forward of the left wing. The right wing was bent slightly backwards and had incurred extensive damage
the pilot’s loss of aircraft control during the initial climb which was precipitated by the sudden loss of thrust and increase in drag from the right engine, and the pilot’s failure to adhere to the published emergency procedures regarding the position of the failed engine power lever. Contributing to the accident was the fatigue failure of the right engine’s ring gear support for undetermined reasons, which rendered the propeller’s automatic drag reducing system inoperative.
Source: NTSB Aviation Accident Database
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