History of Flight
On February 24, 2022, at 1245, a Cessna C208B Supervan 900 airplane, N10JA, was substantially damaged when it was involved in an accident in Oceanside, California. The pilot and passenger were seriously injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 skydiving flight.
The pilot stated that the pilot-rated passenger was spending the day observing the skydiving operation. He had made three consecutive flights climbing to around 13,000 feet mean sea level (msl) to unload the skydivers. On the fourth flight, the pilot departed runway 25 and made a gradual climb to 12,700 ft msl as he circled back to the airport. The skydivers departed the airplane and the pilot initiated a steep, turning descent with the power at idle. He stated that the airplane was in “beta mode” but that the power lever was above the flight idle gate and the speed lever was left in the high position. The pilot stated that, while the airplane was on the base leg of the traffic pattern to runway 25, at an altitude about 4,000 ft msl and a descent rate of 4,500 fpm, he attempted to arrest the descent by adding power.
The pilot further stated that despite his attempt to add power, the engine did not respond to movements of the power lever. He moved the propeller speed lever, which also had no effect. The pilot presumed that the engine had flamed out and attempted a restart. With the propeller still windmilling, he switched the ignition to “continuous” and turned the boost pump on. He observed the torque gauge increase from 0% to 20%, but the engine was still unresponsive to movements of the power lever. 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. With the airplane in a nose-low attitude, it collided with terrain about 1,400 ft short of the runway. The pilot did not feather the propeller.
The pilot-rated passenger stated that the pilot told him that during the skydiving operations they sometimes used a “beta” setting in flight. The pilot said that they would use beta to descend and race the skydivers to the ground. On the accident flight, after the skydivers were dropped, the pilot began a steep descent and retarded the thrust lever into beta. The fuel gauges showed zero, indicating that the fuel tanks were empty. The passenger notified the pilot of the fuel situation, and the pilot stated that the gauges were inaccurate. While on the base leg of the traffic pattern, he thought that the airplane was high and fast. The pilot stated that he was going to make a right turn without using any rudder.
The passenger further stated that the pilot initiated a steep right turn for a few seconds and then a steep left turn. He recalled that the turns were all uncoordinated and the auxiliary fuel pump light was illuminated. The pilot began to move the power and propeller speed levers and stated that they “lost the engine.” The airplane impacted terrain in a nose-low attitude. The passenger took several videos of the accident and previous flights from his cell phone. The clip from the accident flight showed that the fuel gauges indicated empty, the propeller rpm was about 65%, and the oil temperature and pressure gauges were in the green arcs (normal range).
ADS-B information indicated that the airplane reached a maximum rate of descent of about 6,400 fpm shortly after entering the descent, and was descending around 5,300 fpm while turning from the base leg to final approach about 3,350 ft above ground level.
Aircraft Information
The airplane was modified in 2020 by the Supervan Systems, Ltd. (Texas Turbines) TPE331 engine installation Supplemental Type Certificate (STC) SA10841SC and composite 4-blade propeller installation by BS Designs Inc. STC SA09872AC.
The airplane’s engine is managed by the pilot through the power lever and speed lever located in the cockpit center console. The engine power lever (black) connects to the propeller pitch control and the manual fuel valve. The engine speed lever (blue) connects to the propeller governor and to the underspeed fuel governor (USFG).
The propeller control system is designed to operate in either propeller governing range or beta range. When the engine power lever is forward of the flight idle gate, the engine is in the propeller governing mode. The gate is a mechanical detent that prevents inadvertent reduction of the power lever below the flight idle setting. To move the power lever below the flight idle gate, the pilot must lift a silver T-handle.
In the propeller governing range, the power lever controls engine power by adjusting fuel flow. Engine rpm is selected using the speed lever. In the beta range, the power lever varies the propeller blade angle using the propeller pitch control. The USFG maintains the speed lever and engine rpm settings.
The propeller is a Hartzell Lightweight Turbine Series, 4-bladed, single-acting, hydraulically operated constant-speed model with feathering and reverse pitch capability. Oil pressure from the propeller control system exerts force on one side of a piston to decrease blade pitch angle. A feather spring and counterweights drive the blades toward the feather position.
During normal flight at flight idle or above, the propeller governor regulates blade angle. In beta mode, the propeller pitch control takes over, and blade angle can decrease below the safe in-flight minimum low-pitch angle of 5.5°. If the power lever is placed below flight idle into beta mode during flight, the blade position may move to a fine blade angle. Depending on propeller rpm and flight speed, the aerodynamic moment on the blades may reverse, causing the blade to enter reverse and become “aerodynamically locked.” No power lever input can alter this, and the feather valve is ineffective.
Wreckage and Impact Information
A teardown examination of the engine revealed no evidence of mechanical failure or malfunction. There was metal spray throughout and several impeller blades bent in the opposite direction of rotation, consistent with the engine rotating, fuel flowing through the fuel nozzles, and flame in the combustor at impact.
A bench flow-test and teardown of the fuel control unit revealed no malfunction; trace amounts of an unidentifiable blue debris were found in the screen. A bench-flow test of the propeller governor revealed a few test points outside limits.
The propeller remained attached, with all four blades fractured at the shank. Blade counterweights were rotated beyond the mechanical reverse stop. Chordwise/rotational scoring was evident on some blades. The propeller hydraulic unit appeared undamaged. All four blades displayed leading edge nicks and trailing edge compression delamination. The beta tube rigging was equivalent to about 3.7° rather than 5°. Maintenance records had no entries related to propeller rigging.
The propeller manufacturer stated that all damage was consistent with impact while turning under low power at a low blade angle while creating drag; the propeller was not feathered.
Additional Information
The passenger recorded videos during the flights. A video about two minutes before the accident showed rpm at 64%. A video on the previous flight showed the “right fuel-low” and “beta” light illuminated.
Skydiving airplane operators have been known to use beta mode in flight. Texas Turbines stated that “beta mode” can occur with the power lever forward of the flight idle gate if airspeed is about 80 knots or slower. Hartzell defines “beta mode” as when the propeller pitch angle is finer than 5.5°, occurring only when the power lever is aft of the flight idle gate.
The Supervan Airplane Flight Manual Supplement states: “Operation below the flight idle gate is prohibited in flight.” Honeywell issued a Pilot Advisory Letter cautioning against using beta mode in flight.
The Safety Board Vehicle Performance division performed a study. Results showed that negative thrust levels during the accident descent and two previous descents were significantly higher than those during descents with the power lever at flight idle. During the accident descent, thrust did not increase following the pilot’s advancement of the power lever. It is likely that the propeller blade pitch angle was finer than 5.5° and the aerodynamic moment on the blades had reversed, locking the propeller in place.