ERA21FA317 · Rotorway Exec 162F

A witness reported observing the helicopter flying southbound at an altitude of about 50 to 75 ft above ground level. The witness stated that the helicopter was “having issues” and “could not stay up in the air.” The witness further stated that the helicopter seemed as if it were “bouncing” as it flew across a field. Just after the helicopter flew over a tree line, the witness heard a “loud sound” and “watched the [rotor] just stop.” The helicopter then “fell straight down.”

Examination of the wreckage revealed that the lower inboard ends of the main rotor blade grips were bent upward and that both had semicircular indentations with the same diameter as the rotor mast. This damage was consistent with the blade grips contacting the mast, commonly referred to as mast bumping, which occurs due to excessive flapping motion of rotor blades (specifically, up-and-down motion of the blade tips).

The aft tailboom was separated from the helicopter and found about halfway along the 400-ft-long wreckage path. An impact mark consistent with the shape of the main rotor blade was found at the forward end of the separated section, consistent with a main rotor blade severing the tailboom.

Teetering rotor systems, such as the one on the accident helicopter, are particularly susceptible to mast bumping during low-G conditions, which can result from abrupt forward (nose-down) cyclic inputs. The witness’s description of the helicopter appearing to bounce as it was flying is consistent with repeated abrupt control inputs resulting in oscillating low-G conditions. These inputs likely led to the mast bumping and excessive blade flapping, which resulted in a main rotor blade contacting the tailboom. The reason for the pilot’s abrupt control inputs could not be determined.

Toxicology testing of the pilot’s specimens detected ethanol and npropanol, diphenhydramine), trazadone, and a metabolite of clonazepam. The ethanol was detected in the pilot’s blood at low levels (below those considered to be impairing) and was not detected in his urine. That information, along with the presence of n-propanol in the pilot’s blood, suggested that the ethanol was from sources other than ingestion and that it likely was not a factor in the accident. The diphenhydramine results were also below the therapeutic level; therefore, that medication was not likely a factor.

The pilot had been denied a Federal Aviation Administration medical certificate due to his medical history, which included bipolar disorder and depression. The detection of trazadone (which was well below therapeutic levels) and a metabolite of clonazepam indicated that he had been taking these medications to treat diagnosed conditions. However, the toxicological results suggested that the pilot’s use of these medications was likely not a factor in the accident.

Toxicology testing also detected Delta-9-tetrahydocannabinol (THC), which showed that the pilot had used cannabis. Although the effects of THC can last a few hours, THC can be detected in the body for days or weeks. Thus, the investigation could not determine if the pilot’s use of cannabis could have been impairing and influenced his behavior during the accident flight.

Verbatim NTSB analysis and probable cause from the NTSB dataset