Amateur-built II N287BM accident near Broomfield, Colorado

Probable cause

The pilot's inability to maintain airplane control following an encounter with mountain wave turbulence.

NTSB analysis

The private pilot and his passenger were returning from a local flight. According to GPS data, the airplane overflew the airport and entered a midfield left downwind leg for the landing runway before turning to the base leg. Multiple witnesses observed the airplane fly at a high speed and a low altitude before it banked sharply, descended toward the ground, and impacted terrain.

The damage to the airplane was consistent with the airplane impacting the ground in a nose-low, wings-level attitude. Postaccident examination of the airframe and engine revealed no mechanical anomalies that would have precluded normal operations.

Two AIRMETs that were valid for the accident location and time warned of moderate turbulence and low-level wind shear conditions. Various weather products, including computer model simulations and satellite images, depicted conditions consistent with moderate-to-severe turbulence that were conducive for mountain wave activity near the airport. In addition, multiple pilot reports in the area indicated moderate-to-severe turbulence. The observed wind speeds at the airport directly before and after the accident time were weaker than what was encountered by the accident flight due to the inversion likely staying in place longer at the airport. A weather simulation indicated that the accident airplane was in an area of downdrafts with magnitudes between 200 and 1,000 fpm and updrafts with magnitudes between 300 and 1,500 fpm. The airplane was in an area with horizontal wind magnitudes between 20 and 30 knots.

No evidence indicated that the pilot obtained a weather briefing for the intended route of flight; however, even if the pilot had obtained a weather briefing, no weather products at the time of the accident would have specifically mentioned the potential for mountain wave activity.

The accident flight likely encountered mountain wave and rotor conditions while on final approach to the airport with wind gusting between 30 to 50 knots and updrafts and downdrafts with magnitudes between 500 and 1,500 ft per minute. The pilot was most likely not able to maintain airplane control due to this encounter.

Text from the NTSB. Open the NTSB docket for CEN19FA022 for the full record.