Accident Overview
On June 27, 2021, about 1115 mountain daylight time, a Mooney M20F airplane, registration N9613M, was substantially damaged when it impacted trees during takeoff from Cabin Creek Landing Airport (97MT) near Marion, Montana. The pilot was not injured, and the passenger received minor injuries. The flight was operated as a personal flight under Title 14 Code of Federal Regulations Part 91.
Pilot Account
The pilot reported computing takeoff performance based on a 5,000 ft field elevation, 80°F ambient temperature, and a takeoff weight of 2,740 lbs, though the actual weight at departure was about 2,510 lbs. Performance figures indicated a required ground run of 1,560 ft and a takeoff distance of 2,395 ft to clear a 50-ft obstacle. The engine run-up was normal, and the pilot observed no anomalies before takeoff. He set the brakes, applied full power for a short field takeoff, and released the brakes when engine rpm exceeded 2,650 rpm, with manifold pressure above 23 inches Hg. The takeoff roll was uneventful until rotation at about midfield (1,700 ft down the runway at 75 mph). The pilot noticed the airplane had not accelerated as expected but determined it was too late to abort. He retracted the landing gear to gain airspeed but felt the airplane would not clear the tree-lined hill at the end of the runway. After banking left due to an obstacle on the right, he engaged the turbocharger control, advanced the mixture to full rich, and verified the propeller lever in full forward. There was a slight improvement in performance, but not as experienced previously. The pilot leveled the wings and maintained altitude before the airplane impacted trees, substantially damaging the fuselage. In a follow-up, the pilot stated he experienced "general underperformance by engine both N/A and the Turbo engaged."
Witness Account
A witness near the departure end of the runway reported hearing the power advance but did not see the airplane until it was about midfield. He observed it transition into a high angle of attack as the nose gear lifted off, followed by the main gear about one second later, then all gear retracted. The airplane maintained slow airspeed and high angle of attack as it overflew the departure end at low altitude and disappeared from view. He heard the airplane impact trees about 15 to 30 seconds later and did not detect any changes in engine rpm during the takeoff.
Post-Accident Examination
The engine was equipped with a Rajay turbocharged engine system per Supplemental Type Certificate SA1411WE. The flight manual supplement states that for takeoff from high elevation airfields, the turbocharger may be used to obtain 28.5" MAP at 2,700 rpm for up to 3 minutes. The pilot reported intermittent drops in rpm and that at his home airport (near sea level, 2,465-ft runway), the airplane seemed underpowered when loaded to full weight. The mechanic stated the pilot reported difficulty maintaining 2,700 rpm and believed the propeller governor might be the cause, but he could not test it. The airplane logbooks showed the propeller governor was last replaced during an engine overhaul in 1991.
Propeller Governor Testing
The propeller governor, a Hartzell model D-1-6, was removed and functionally tested. Initial observations noted the coupler missing and linkage control lever impact damage, though the lever turned normally. The flyweights appeared normal, and the relief valve and spring were in good condition, but pitting was observed on the pilot valve. Functional checks on a test stand revealed the maximum rpm was 2,270 rpm (manufacturer spec: 2,300 rpm), internal leakage was 50 quarts/hr (spec: 16 quarts/hr max), and output pressure was 250 psi (spec: 290–310 psi). The propeller manufacturer stated only about 140–160 psi would have been required to start the propeller off in low pitch during takeoff roll and initial climb.
Performance Data
The density altitude at the time was about 6,400 ft. Calculations using the pilot's operating handbook indicated a required ground run of about 1,587 ft. Landing data from the POH indicated a ground roll of about 875 ft to stop, or approximately one-quarter of the total runway distance.