Manteo, NC, USA
N31062
PIPER PA46-500TP
After takeoff, the pilot retracted the landing gear and the turbopropeller-powered single-engine airplane’s engine lost partial power. The pilot attempted to regain engine power using a manual override but was unsuccessful. A forced landing was made at the departure airport, during which the airplane touched down at intersecting runways and traveled through the grass before coming to a stop, resulting in substantial damage to the fuselage. with the nose gear sheared off. Examination of the airplane after the accident revealed substantial damage to the fuselage. Examination of the engine revealed that the front of the compressor discharge air (P3) pneumatic line was secured to the gas generator case. The P3 line’s rear coupling nut was lockwired to the adjacent bolt. The lockwire was removed and the coupling nut was loosely fitted (no torque) with the adjacent nipple. The exterior surface of the nut exhibited fretting wear from contact with the twists of the lockwire. The fretting wear was observed on the conical surface of the nipple from contact with the mating surface of the tube. Examination at higher magnification of the oval-shaped fretting wear along the outer surface of the “B”-nut mating with the nipple showed the fretting damage occurred due to the nut being in contact with the lockwire while the nut was attempting to move in the counterclockwise direction (in the direction loosening the nut). Since the nut would not move significantly once the lockwire was in place, it suggested that the torque at installation was low, which would have allowed the nut to loosen, thus compromising the seal of the connection. Subsequently, when the pneumatic line seal became loose, it likely resulted in the loss of P3 air pressure and the subsequent partial loss of engine power like what the pilot described. A review of the maintenance records could not determine when the P3 pneumatic line was most recently secured.
On July 6, 2021, about 1507 eastern daylight time, a Piper PA-46-500TP, N31062, was substantially damaged when it was involved in an accident near Manteo, North Carolina. The airline transport pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that the fuel tanks had been topped off with Jet-A fuel before the flight. He departed Dare County Regional Airport (MQI), Manteo, North Carolina from runway 23; however, just after he retracted the landing gear, he noted a loss of thrust from the engine. He verified that the throttle had not moved, unlocked the Manual Override (MOR), and moved it “just under halfway to full” forward in an attempt to increase engine power. Engine power did not increase, and the pilot began to prepare for a forced landing. Suddenly, the engine surged, and the pilot initiated a climb. There was a brief interruption of engine power, but the airplane continued to climb, and the pilot started a turn back to the airport. As the pilot maneuvered the airplane back toward the airport, the engine again lost power. He subsequently landed about midfield, at the intersection of runway 35 and runway 23, and was aligned about 35° off the runway heading for runway 35. The airplane continued off the paved surface and into the grass, where the nose landing gear collapsed, resulting in substantial damage to the fuselage. Examination of the propeller revealed that all blades were bent aft, opposite rotation and twisted toward low pitch with chordwise/rotational scoring in the tip region. There were no visible discrepancies noted that would prevent or degrade normal propeller operation prior to impact. All visible damage was consistent with high impact forces while rotating at low power. Examination of the electric fuel pumps on the airframe revealed they were operating within limits. Examination of the engine revealed that the reduction gearbox and the exhaust duct were unremarkable. All six of the inlet case struts were fractured. The accessory gearbox was unremarkable. The propeller shaft was manually rotated, and the power section rotated freely. The gas generator case could not be rotated manually. The compressor blades and the centrifugal impeller exhibited rotational scoring. The flow divider, fuel-to-oil heat exchanger, fuel pump, and the torque limiter were functionally tested and no anomalies were noted. The fuel control unit, overspeed governor, and propeller governor were tested and deviations that were within field adjustments were noted. The front of the compressor discharge air (P3) pneumatic line was secured to the gas generator case (See Figure 1). The rear coupling “B” nut was lockwired to the adjacent bolt. The lockwire was removed and the coupling nut was found loosely fitted (no torque) with the adjacent nipple. Figure 1. Photograph of the compressor discharge air (P3) pneumatic line. The red arrow (right) points to the front of the P3 line that was secured to the gas generator case and the red circle (left) highlights the lockwired rear coupling “B” nut. The exterior surface of the nut exhibited fretting wear from contact with the twists of the lockwire (See Figure 2). The fretting wear was observed on the conical surface of the nipple from contact with the mating surface of the tube. The fittings on the rear section of the line between the filter housing and the fuel control were lockwired and secured/torqued to their respective nipple/elbow. Both fittings were tight in the filter housing. Figure 2. Close up photograph of the P3 pneumatic line rear coupling nut with fretting wear. The red arrow indicates the fretting. Examination at higher magnification of the oval-shaped dents along the outer surface of the “B”-nut mating with the nipple showed that the damage occurred as the result of material loss and not as the result of outward material displacement, indicating that damage occurred as the result of a wear mechanism. Some evidence of directionality indicated a relative movement between the nut and the lockwire. Blackish residues were observed within the wear grooves consistent with fretting wear damage. In addition, the wear residues appeared to have accumulated predominantly towards one extremity of the oval grooves indicating that the fretting damage occurred with the nut contacting the wire and the nut attempting to move in the counterclockwise direction (in the direction loosening the nut). A review of the maintenance records could not determine when the P3 pneumatic line was most recently secured. According to the airplane flight manual, the manual override lever (MOR) “is an emergency device that may allow the crew to regain power and continue safe flight and landing following fuel control unit (FCU) malfunction or power lever control loss. The MOR is used to control fuel flow to the engine in the event a pneumatic malfunction occurs in the engine fuel control unit. A malfunction of the pneumatic signal (Py) input to the FCU will result in the fuel flow decreasing to minimum idle…. Additional effects of a Py malfunction are a loss of torque/Ng limiting functions and, Nf governor operation…To operate the MOR, lift up on the lever and slowly move it forward to take up the dead-band until the engine responds. If possible, allow engine to stabilize before advancing further.”
The improperly secured pneumatic compressor discharge air (P3) line coupling nut on the engine, which ultimately allowed the nut to back off resulting in a partial loss of engine power.
Source: NTSB Aviation Accident Database
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