N9613MMooney M20F

On June 27, 2021, about 1115 mountain daylight time, a Mooney M20F airplane, N9613M, was substantially damaged when it was involved in an accident near Marion, Montana. The pilot was not injured, and the passenger received minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The accident occurred as the pilot was taking off from the Cabin Creek Landing Airport (97MT). The pilot reported that he computed a takeoff performance based on a 5,000 ft field elevation, 80° F ambient temperature, and a takeoff weight of 2,740 pounds. The actual weight of the airplane at the time of departure was about 2,510 pounds. According to the pilot, the performance figures indicated a required ground run of 1,560 ft and takeoff distance of 2,395 ft to clear a 50 ft obstacle. The pilot reported that his engine run-up was normal and that he did not observe any anomalies with the engine prior to takeoff. He set the brakes and increased the throttle to full power to perform a short field takeoff. The engine rpm was climbing past 2,650 rpm when he released the brakes, and the airplane began the takeoff roll. The pilot recalled that the manifold pressure was above 23 inches Hg and the engine rpm was beyond 2,650 rpm. The takeoff roll was uneventful to this point. He rotated about midfield; 1,700 ft down the runway at 75 mph. He subsequently noticed that the airplane had not accelerated as expected, but realized that it was too late to abort the takeoff.

The pilot retracted the landing gear to gain airspeed, but once the landing gear had retracted, he immediately felt that the airplane was not on a proper trajectory to clear the advancing tree-lined hill at the end of the runway. With an obstacle to his right, he chose to bank left at which time he pushed the turbocharger control knob to the ON position, which closed the manual wastegate and engaged the turbocharger. At this time, he also advanced the mixture to the full rich position and verified the propeller lever was in the full forward position. There was a slight improvement in the airplane’s performance, but not as he had experienced during previous flights. The pilot immediately leveled the wings and maintained the airplane’s altitude before the airplane impacted trees, which substantially damaged the fuselage.

In a follow-up statement, the pilot reported that he experienced “general underperformance by engine both N/A [normally aspirated] and the Turbo engaged. Plane was making full RPM and had a manifold pressure over 23 inches Hg. The plane accelerated to 75 mph at a rate that seemed correct. Clear underperformance after liftoff that did not allow a climb over terrain.”

According to a witness located near the departure end of the runway, he heard the accident airplane’s power advance but did not observe the airplane until it was about midfield. Seconds later, and immediately after the airplane had passed midfield, he observed it transition into a high angle of attack as the nose landing gear lifted off the runway. The main landing gear lifted off the ground about one second later and all three-landing gear were then retracted. The airplane maintained a slow airspeed and high angle of attack as it overflew the departure end of the runway at a low altitude and eventually disappeared from the witness’ view. He heard the airplane impact trees about 15 to 30 seconds later. The witness reported that he did not detect any changes in engine rpm during the takeoff attempt.

The pilot further reported that “picking a more conservative abort point on the runway would have allowed me to abort the take off. I believed that I had rotated by the point I picked, but the airport manager said that a witness told him I rotated later than I think I did.”

The engine was equipped with a Rajay turbocharged engine system in accordance with Supplemental Type Certificate SA1411WE.

According to the flight manual supplement for the turbocharger system,

When taking off from high elevation airfields, the turbocharger may be utilized to obtain 28.5” MAP at 2,700 rpm for 3 minutes during the takeoff. After full throttle is reached, push turbocharger control toward “ON” until 28.5” MAP is attained.

The pilot reported that he had recently experienced intermittent drops in engine rpm engine had recently been serviced and appeared to function normally during the accident flight. He further reported that the propeller had recently been balanced and after the service he would occasionally observe a rapid drop in power of about 50 rpm. According to the pilot, at his home airport, near sea level with a 2,465 ft long runway, the airplane seemed underpowered when it was loaded to its full weight.

According to his mechanic, the pilot informed him that the airplane appeared underpowered at times during takeoff and had difficulty maintaining 2,700 rpm. The mechanic stated that he did not observe any performance concerns with the airplane when he last serviced it 6 days before the accident. He further remarked that he believed the propeller governor may have caused the decrease in engine rpm, but he did not have a way to test it.

The airplane was equipped with a Hartzell model D-1-6 propeller assembly. A review of the airplane logbooks showed that the propeller governor was last replaced during its last major engine overhaul in 1991.

The propeller governor was removed from the accident airplane and functionally tested at an overhaul facility. Initial observations showed that the propeller governor did not arrive with its coupler. The linkage control lever showed some impact damage, but the lever turned normally, and the spring-loaded mechanism worked as designed. The flyweights were normal in appearance and did not show any indication of sticking and the relief valve and spring were in good condition. Pitting was observed on the pilot valve.

The propeller governor was connected to a test stand designed to test the rpm, output, and internal leakage of the unit. The manufacturer’s recommended maximum rpm was 2,300 rpm, the maximum allowable internal leakage was 16 quarts/hr and required pressure output range to the propeller was 290 – 310 psi. The results of the functional checks/tests showed that the propeller governor’s maximum rpm was 2,270 rpm, the internal leakage was 50 quarts/hr and the output was 250 psi.

The propeller manufacturer stated that only about 140-160 psi would have been required to start the propeller off in low pitch during the takeoff roll and initial climb phases of the accident flight.

The density altitude at the time of the accident was about 6,400 ft. Computations of the airplane’s takeoff performance using the pilot’s operating handbook (POH) indicated that the airplane required a ground run of about 1,587 ft. According to landing data derived from the POH, the airplane required a ground roll of about 875 ft to stop, or approximately ¼ of the total runway distance.