Accident Flight Likely Encountered Moderate Mountain Wave Turbulence With Downdrafts As Strong As 500 Ft Per Minute

Location: Victor, Colorado Accident Number: CEN23FA174
Date & Time: May 6, 2023, 09:35 Local Registration: N15188
Aircraft: Pool Cessna R172 Aircraft Damage: Destroyed
Defining Event: Loss of control in flight Injuries: 4 Fatal
Flight Conducted Under: Part 91: General aviation - Personal

Analysis: After takeoff, the flight proceeded toward the intended destination airport. Position data initially depicted the airplane about 0.75 miles from the departure airport at 6,000 ft mean sea level. The final data point was recorded about 10 minutes later when the airplane was about 15 miles from the departure airport at an altitude of 9,400 ft msl. This was about 0.20 miles from the accident site.

The pilot’s family informed local authorities that the flight was overdue, and the accident site was located the following morning. The accident site was in mountainous terrain an elevation of about 9,170 ft. Terrain in the immediate vicinity of the accident site rose to nearly 9,300 ft. The airplane came to rest inverted after impacting trees and terrain. A postimpact fire consumed most of the fuselage. Postaccident examinations did not reveal any anomalies attributable to a preimpact failure or malfunction.

The accident flight likely encountered moderate mountain wave turbulence with downdrafts as strong as 500 ft per minute (fpm) and updrafts as strong as 1,000 fpm at the time of the accident. The weather conditions encountered were consistent with the National Weather Service G-AIRMET forecast. It is unknown what weather information the pilot received before or during the flight.

The expected climb performance at maximum gross weight and a pressure altitude of 10,000 ft was 450 fpm. This was about one-half of the expected climb rate at sea level. The estimated density altitude at the accident site was about 11,300 ft. Advisory information regarding mountain flying provided by the Federal Aviation Administration suggested maintaining at least 1,000 ft terrain clearance when navigating a mountain pass or crossing a ridge line. However, if the wind is above 20 knots, the suggested terrain clearance increased to 2,000 ft to avoid most of the turbulence and downdrafts. 

As the airplane approached a mountain ridge line with about 100 ft altitude clearance, the pilot likely encountered previously forecasted moderate turbulence that exceeded the airplane’s climb capability at the elevated density altitude, resulting in an impact with trees and terrain. 

Probable Cause and Findings: The National Transportation Safety Board determines the probable cause(s) of this accident to be -- The pilot’s failure to follow established mountain flying guidance, which resulted in a loss of control and impact with trees and mountainous terrain. Contributing to the accident was the airplane’s reduced climb performance at the elevated density altitude and the presence of moderate turbulence as forecasted.

FMI: www.ntsb.gov