Anchorage, AK, USA
N400KL
Gardner SR3500
The airline transport pilot was concluding a personal flight under Title 14, CFR Part 91. He reported that as he turned final approach to land, he added power, and the airplane decelerated and violently pitched nose down. He said he applied additional power to raise the nose and arrest the rate of descent, but the nose pitched down further. He indicated the airplane was low on final approach, in a level attitude, and descending rapidly. He applied power again, but the airplane pitched down again. The airplane continued to descend until it struck a curb, dirt berm, and an unoccupied vehicle in a parking lot near the approach end of the runway. The pilot said there were no known mechanical anomalies with the airplane prior to the accident. The airplane is equipped with a propeller that is capable of changing its pitch angle to provide reverse thrust for ground operations. The airframe mounted anti-reverse safety devices, intended to prevent the propeller from reversing in-flight, were tested on the ground and found to function properly. A third anti-reverse device, fly-weights in the propeller governor, could not be tested. The propeller governor was removed and disassembled. The electric propeller pitch change solenoid functioned properly, however the reversing valve, which is located in a valve body to which the solenoid is attached, was found stuck in what, according to schematics provided by the manufacturer, was consistent with the reverse pitch position. The reversing valve is intended to move freely within the valve body, but had to be forcefully removed. The interior of the reversing valve body had a coating of carbon-like material around the interior.
On August 22, 2005, about 1500 Alaska daylight time, a Gardner Murphy Moose SR3500 experimental amateur-built airplane, N400KL, sustained substantial damage during an uncontrolled descent and impact with terrain and a vehicle, while on final approach to land at Merrill Field, Anchorage, Alaska. The airplane was being operated by the pilot as a visual flight rules (VFR) local personal flight under Title 14, CFR Part 91, when the accident occurred. The airline transport pilot and sole pilot-rated passenger were not injured. Visual meteorological conditions prevailed, and no flight plan was filed. The flight originated at Merrill Field, about 1400. On August 22, an on-site examination of the wreckage was made by the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) accompanied by an FAA aviation safety inspector. Ground scars disclosed that the airplane had touched down in a parking lot, where the main landing gear struck a concrete curb. The debris path continued across a parking lot entrance road, where the airplane struck a dirt berm. The wreckage came to rest in a shopping mall parking lot. All the major airframe components were located at the site. Flight control continuity was established between the flight controls and their respective control surfaces. During an interview with the IIC on August 23, the pilot said the flight departed Merrill Field and flew to Goose Bay, Alaska, to practice touch-and-go landings. After about an hour of practice, he said he returned to Merrill Field for landing on runway 25. He said as he turned onto the final approach leg, about 500 feet above ground level, he added power, and the airplane decelerated and pitched violently nose down. He said he applied power to raise the nose and arrest the rate of descent, but the nose pitched down further, and the rate of descent increased. He said both he and the pilot-rated passenger held the elevator control stick back as he reduced power. He said the airplane was low on final approach, in a level attitude, and descending rapidly. He said he applied power again to arrest the rate of descent, but again the airplane pitched down. He said he pulled the power off, and leveled the airplane as best he could. The airplane impacted a curb and dirt berm in a parking lot near the approach end of the runway, struck an unoccupied sport utility vehicle, and came to rest facing in the opposite direction of its approach. The pilot said there were no known mechanical anomalies with the airplane prior to the accident. On August 23, the wreckage was re-inspected by the IIC, accompanied by an FAA aviation safety inspector. The airplane was equipped with a reversing propeller. The pitch of the propeller could be changed to allow the propeller blades to move from a positive angle of attack, to a negative angle of attack, and thus produce reverse thrust. Reverse thrust is only for use while on the ground. The propeller governor was removed for further examination. There are three safety devices designed to prevent the propeller from changing to a "beta" or reverse thrust pitch in-flight, that were also examined. Two of these devices are a guarded manual switch on the instrument panel, and an airspeed switch connected to the pitot static system. These switches functioned properly when tested on the ground. The third safety device is a pair of fly-weights in the propeller governor, which are intended to prevent activation of reverse thrust until engine speed is reduced below 1,500 revolutions per minute. This safety mechanism could not be tested. The propeller governor was taken to an FAA certified propeller repair station and disassembled. The electric propeller pitch change solenoid functioned properly. The propeller pitch reversing valve is located in a valve body to which the solenoid is attached. The reversing valve was found stuck in what, according to schematics provided by the manufacturer, was consistent with the reverse pitch position. The reversing valve, which should move freely within the valve body, had to be forcefully removed. The interior of the valve body had a coating of carbon-like material around the interior. The propeller governor was returned to an adjustor for the pilot's insurance company, and no pieces or part of the airplane were retained by the NTSB.
The malfunction of the propeller reversing system during landing, which allowed the propeller to reverse pitch in-flight, resulting in an uncontrolled descent and impact with terrain.
Source: NTSB Aviation Accident Database
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