Cessna 172N N6610D accident near Moss Beach, California

Probable cause

The pilot's loss of airplane control following an encounter with low level wind shear and turbulence during final approach for landing. Contributing to the loss of control was the pilot's decision to retract the wing flaps on final approach, which resulted in a sudden loss of lift.

NTSB analysis

The private pilot and a passenger were approaching the airport to land with a 70° right crosswind at 10 knots (kts) with gusts to 14 kts. The pilot reported that, on final approach, the airplane was configured with full flaps (30°) at an airspeed of 60 kts, and an altitude of 500 ft mean sea level (msl) when it began to encounter turbulence. The pilot then elected to conduct the landing approach at a higher airspeed and subsequently retracted the flaps from 30° to less than 20°. Immediately after reconfiguring the flaps, the pilot reported that he experienced strong turbulence that violently rocked the airplane and simultaneously felt a "strong downdraft" as the airplane entered a right bank. The pilot was unable to correct the airplane's attitude, and the airplane subsequently descended into terrain, where it impacted a paved road, several vehicles, and two houses before coming to rest.

A weather study revealed that, at the time of the accident, a weak temperature inversion was present between 400 and 600 ft msl. The stable layer of air produced by the temperature inversion trapped any updrafts or downdrafts created by the wind flowing over nearby terrain. This created an environment favorable for the development of low level wind shear and turbulence below 600 ft msl. Although airplane performance data revealed that the pilot maintained an airspeed above the airplane's stall speed throughout the landing approach, his decision to retract the flaps likely resulted in a sudden loss of lift. This loss of lift, combined with the low level wind shear and turbulent conditions, most likely resulted in the pilot's loss of control during the approach for landing.

Text from the NTSB. Open the NTSB docket for WPR17FA023 for the full record.