Cirrus Design Corp SR22 N401SC accident near San Antonio, Texas

Probable cause

The pilot's uncoordinated flight control inputs and subsequent inadvertent cross-control aerodynamic stall in the airport traffic pattern that resulted in a loss of control and uncontrolled descent with insufficient altitude for recovery.

NTSB analysis

The pilot was maneuvering in the airport traffic pattern at the time of the accident. The pilot entered a continuous right turn from downwind toward the final approach course when he abruptly lost control. A witness stated that the airplane wings were "totally vertical" before it nosed over and descended toward the ground. A second witness also reported that the airplane wings were nearly vertical before it descended below the tree line. He added that the engine sounded "fine."

An NTSB performance study revealed that after the airplane entered the airport traffic pattern, it began a continuous right turn from downwind toward the final approach course suggesting that the pilot did not fly a traditional rectangular traffic pattern, but instead flew a circling base to final pattern. The airplane approached the extended runway centerline in a 48° right bank, at 103 kts and about 220 ft agl. Lateral accelerations began to increase shortly before the accident and varied between 0.37g and 0.62g for the final portion of the flight. The lateral accelerations were consistent with sideslip angles of 15° to 20° during the final turn. The calculated angle-of-attack (AOA) of the wing subsequently exceeded the critical AOA and the airplane entered a descent which ultimately reached 1,800 fpm. Although the pilot's control inputs were not directly recorded, the large lateral accelerations are consistent with left rudder input and an uncoordinated flight condition for the airplane.

The accident site was located in a wooded area about 1/2 mile southeast from the landing runway threshold. Airframe and engine examinations did not reveal evidence of any anomalies consistent with a preimpact failure or malfunction.

The Pilot's Operating Handbook noted that extreme care must be taken to avoid uncoordinated or accelerated control inputs when close to the stall, especially when close to the ground. If, at the stall, the flight controls are misapplied and accelerated inputs are made to the elevator, rudder, and/or ailerons, an abrupt wing drop may be felt and a spiral or spin may be entered.

The FAA Airplane Flying Handbook (FAA-H-8083-3B) noted that coordinated flight is important to maintaining control of the airplane. Situations can develop when a pilot is flying in uncoordinated flight and depending on the flight control deflections, may support pro-spin flight control inputs. This is especially hazardous when operating at low altitudes, such as in the airport traffic pattern. A cross-control stall occurs when the critical AOA is exceeded with aileron pressure applied in one direction and rudder pressure in the opposite direction, causing uncoordinated flight. The aerodynamic effects of an uncoordinated, cross-control stall can occur with very little warning and can be deadly if it occurs close to the ground. The nose may pitch down, the bank angle may suddenly change, and the airplane may continue to roll to an inverted position, which is usually the beginning of a spin.

Text from the NTSB. Open the NTSB docket for CEN17FA084 for the full record.