MOONEY AIRCRAFT CORP. M20K — Woodland, WA

What happened

The pilot and two passengers boarded the airplane for a personal cross-country flight. After a normal engine run-up, the pilot began the takeoff roll on the 1,953-ft-long runway. According to the pilot, the airplane lifted off after a ground roll of about 1,250 ft, climbed to about 35 ft above ground level, but then stopped accelerating. The pilot then lowered the nose and discovered that the airplane was just above the ground and seconds from impacting a berm. However, video of the takeoff showed that the airplane became airborne after a ground roll of about 1,933-ft, about 20 ft from the end of the runway. Further, according to the witness who took the video, the airplane attained a maximum altitude of only about 4 ft before it touched down in the grass beyond the runway end. The airplane impacted the airport's perimeter fence located about 375 ft from the end of the runway and then collided with the 9-ft-tall berm.

A sound spectrum analysis of the video's audio channel showed that the propeller rpm was about 2,430 during takeoff, which was about 270 rpm below the expected maximum rpm of 2,700. Examination of the propeller governor showed that the unit was set to a maximum rpm of about 2,600, which was about 100 rpm below the manufacturer's specified setting. In addition, during engine test cell runs with a fixed-pitch club propeller, the maximum engine rpm was increased 100 rpm, from 2,640 to 2,740, by retiming the magnetos from an improper setting to the manufacturer's specified setting. Regardless, the propeller governor would have inhibited the engine from reaching rated power, thus it is likely that the propeller governor resulted in a takeoff rpm below maximum rpm.

The pilot reported that before departure, he calculated that the airplane's gross weight was about 2,864 pounds (lbs) and the takeoff ground roll would be about 1,250 ft. Postaccident calculations revealed that the airplane's gross weight was about 2,978 lbs, which exceeded the airplane's maximum gross weight by 78 lbs. The airplane's zero-wind ground roll at its maximum gross weight of 2,900 lbs was about 1,350 ft, and may have been longer due to the higher gross weight. Although the pilot's calculations indicated a safe takeoff was possible, the airplane did not perform as expected, likely due to the engine not achieving maximum rpm and the pilot's exceedance of the airplane's maximum gross weight. The pilot should have been sensitive to the short runway length and closely monitored the airplane's performance. If the pilot had recognized promptly that the airplane was not performing as expected, given the distance from the runway end to the fence and the berm that the airplane impacted, adequate distance would likely have been available to safely abort the takeoff. Additionally, if the pilot had designated a go/no-go runway liftoff point, this would have helped him detect the performance deficiency. However, the pilot did not recognize that the airplane was not performing as expected until the airplane was near the runway end and the impact with the berm could not be avoided.

The pilot seated in the left front seat and the passenger seated in the right front seat were restrained with lap and shoulder belts and survived with compression fractures of the lumbar spine and extremity injuries, demonstrating that this accident was survivable for occupants who were properly restrained. The compression fractures sustained by the front seat occupants are typically caused by vertical loading from below while in a seated position; in this case, likely from the airplane striking the berm in a nose-high attitude.

The passenger seated in the left rear seat was likely restrained only by a lap belt and sustained fatal injuries to his brain and spinal cord. It could not be determined if the rear seat passenger's cervical spine injuries were primarily from hyperflexion or hyperextension. The head rests had been removed from all seats in the airplane, and the rear passenger received a significant posterior scalp laceration that was consistent with hyperextension of his neck over the low back of his seat. This hyperextension could have been prevented by the presence of a head rest at an appropriate height on his seat but would not have been prevented by his use of the available shoulder harness. The rear passenger's cervical spine injuries may also have been caused by hyperflexion of his neck over the pilot's seat back and could have been prevented by his use of the available shoulder harness and/or the presence of a head rest on the pilot's seat. Thus, it is likely that the appropriate use of head rests and shoulder restraints would have mitigated the severity of the rear seat passenger's injuries.

An editorial "what led to it / how to avoid it" analysis for this accident is generated separately and will appear here.