WPR22LA114 · Cessna 208B Supervan

The pilot was performing skydiving operations in the turboprop-equipped airplane. During the fourth flight of the day, after the skydivers departed the airplane, the pilot initiated a steep, turning descent with the power at idle, during which the airplane reached a maximum descent rate of 6,400 ft per min (fpm). While the airplane was on the base leg of the traffic pattern, the pilot attempted to arrest the descent by adding power; however, the engine did not respond to his movements of the power lever. He moved the propeller speed lever, which was also had no effect. He applied full nose-up trim and pulled the control yoke full aft against the stop, but the airplane would not maintain a level attitude, and subsequently collided with terrain short of the runway in a nose-low attitude.

There were numerous publications from the STC holder and the engine and propeller manufacturers warning pilots to not use beta mode during inflight operations unless it is certified for those operations. If the power lever is placed below the flight idle gate into beta mode while inflight, the blade position will move to a fine blade angle. Depending on propeller rpm and flight speed, this adjustment may cause the moment from the aerodynamic load on the blade to shift from coarsening to fining, and may result in the propeller blades entering reverse and remaining “aerodynamically locked” in that position. No power lever input from the pilot can alter this, and using the feather valve is ineffective. It is likely that the only viable option to unlock the propeller is to shut down the engine to reduce the moments locking the propeller in place, given it could overcome windmilling speed loads on the blades.

Postaccident examinations revealed no evidence of a mechanical malfunction with the engine, propeller, or airframe. A bench-flow test and subsequent teardown examination of the propeller governor revealed a few test points outside of limits that would not have precluded normal governor operation. The propeller rigging was incorrect and it is unknown if the rigging was changed on purpose to aid with faster descent rates; if this was the rigging at the time of the accident flight, it would have contributed to entering an aerodynamic lock sooner.

A performance study of the flight based on Automatic Dependent Surveillance – Broadcast (ADS-B) data revealed high negative thrust values that can only be achieved with the power lever below the flight idle gate. It is likely that the propeller blade pitch angle was in beta and that the aerodynamic moment on the blades had reversed, locking the propeller in place and preventing the pilot from recovering control of the propeller.

Verbatim NTSB analysis and probable cause from the NTSB dataset