Accident Overview
A company pilot and two employees were conducting an aerial imagery survey flight over forest vegetation. The airplane climbed and leveled off at approximately 16,100 feet mean sea level (msl), accelerating to a maximum recorded groundspeed of 209 knots. Less than two minutes later, the groundspeed dropped to about 93 knots, and the airplane descended roughly 500 feet while maintaining a steady heading.
Events Leading to the Accident
Shortly after the descent, the airplane entered a rapid descent combined with a right turn. The pilot transmitted “mayday, mayday, mayday” and “we’re in a spin” to air traffic control. A witness near the accident site observed the aircraft descending nose-down at high speed and then spinning rapidly around its longitudinal axis.
Wreckage and Examination
The wreckage was located in remote wetlands and wooded terrain. The airplane impacted the ground in a nose-low vertical attitude at high speed. All major components were accounted for at the accident site. Postaccident examination of the airframe, engines, and propellers found no preimpact mechanical malfunctions or failures that would have precluded normal operation.
Performance Analysis
An aircraft performance study indicated that when the airplane pitched down, the normal load factor decreased rapidly from about 1.6 G to less than 1 G. Such a rapid decrease in load factor is consistent with a stall occurring when the wing exceeds its critical angle of attack. At that point, airflow separates from the wing, preventing it from generating sufficient lift. If the airplane is in uncoordinated flight during a stall, a spin can result.
Pilot Actions and Manual Guidance
The analysis concluded that the pilot likely did not maintain adequate airspeed, causing the airplane to exceed its critical angle of attack and enter an inadvertent stall and spin. However, an important but unknown factor is the pilot’s behavior regarding flight control inputs, including any possible attempts to recover. The airplane’s Pilot Operating Handbook states that spins are not authorized and does not include a procedure for inadvertent spin recovery.