Oceanside, CA, USA
N10JA
CESSNA 208B
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.
HISTORY OF FLIGHTOn 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 (see Figures 1 and 2 below). The pilot did not feather the propeller. Figure 1: Flight Path on Accident Flight Figure 2: Flight Path during Descent 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 descent while turning from the base leg to final approach about 3,350 ft above ground level. AIRCRAFT INFORMATIONThe 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. As airspeed decays, the propeller governor outputs higher oil pressure until “beta pressure” is available, turning on the beta light in the cockpit. The presence of beta pressure enables the engine to operate in the beta range once the power lever is pulled aft of the flight idle gate. The gate is a mechanical detent in the engine control console that prevents inadvertent reduction of the power lever below the flight idle setting. Before the power lever can be reduced below the flight idle setting, the pilot must lift a silver T-handle located under the power lever over the detent gate/lock, which allows the power lever to be moved further aft. In the propeller governing range, the power lever controls the engine power by adjusting the fuel flow from the manual fuel valve. Engine rpm is selected using the speed lever, which varies the propeller blade angle using the propeller governor. In the beta range, the power lever varies the propeller blade angle using the propeller pitch control. To maintain the speed lever and engine rpm settings, the USFG varies the engine power by scheduling the fuel flow. The USFG controls the engine rpm settings between 68% and 96%. Propeller Operation The Honeywell TPE331 turboprop engine was equipped with a Hartzell Lightweight Turbine Series, 4-bladed, single-acting, hydraulically operated constant-speed model propeller with feathering and reverse pitch capability. The propeller features an aluminum hub and composite blades. The propeller pitch is adjusted by using engine oil pressure to displace an internal piston axially along the propeller assembly. Pitch change knobs connecting the piston movement to each blade root cause changes in blade pitch angle with any axial movement of the piston. Oil pressure from the propeller control system exerts force on only one side of a piston, leading to a reduction (or fine adjustment) in the propeller blade pitch angle. Conversely, oil pressure cannot drive the blades in the coarse direction. A feather spring situated on the opposing side of the piston continually drives the propeller blade pitch toward the feather position (or coarse adjustment). Counterweights on each blade also provide a twisting moment toward the feather position. The feather position represents the maximum coarse position of the blades. During operation at flight idle or above and with sufficient airspeed, the propeller governor (PG) automatically regulates the propeller blade pitch angle based on engine speed and power requirements. The propeller pitch control (PPC) unit is inactive during this flight mode. The engine propeller controls ensure that the propeller blade angle remains above the established safe in-flight minimum blade low-pitch angle, which is set at the time of installation to 5.5°. In the beta mode, the PPC takes over controlling the propeller blade pitch angle, and any movement of the power lever by the pilot directly adjusts the propeller blade angle. The PG is not controlling the blade angle in this mode; however, it is still supplying high-pressure oil to the PPC. The high-pressure oil is what triggers the beta light. Beta mode permits the blade angle to decrease below the safe in-flight minimum blade low-pitch angle. During normal flight conditions, when the power lever is at flight idle or above, the combination of inherent moments on blades always acts in the coarsening or feathering direction, allowing the PG oil pressure to fine-tune the blade position until a propeller rpm of 100% is achieved. However, if the power lever is placed below flight idle into beta mode during flight, 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. Eventually, the sum of the fining moments can exceed the sum of the coarsening moments, causing the blade to enter reverse and remain “aerodynamically locked” in that position. No power lever input from the pilot can alter this and using the feather valve is ineffective because the dome pressure is already at zero. A pressure switch in the propeller control system is used to power an indicator light in the cockpit. The light is illuminated when the propellor governor is supplying max output pressure (beta pressure) and the pilot has the ability to control the propeller blade angle using beta mode. Fuel The Federal Aviation Administration (FAA) inspectors who responded to the accident stated that the fuel tanks remained intact with no apparent perforations. Recovery personnel drained about 20 gallons of fuel from the airplane, all of which was in the right wing (the airplane came to rest in a right-wing-low attitude). AIRPORT INFORMATIONThe 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. As airspeed decays, the propeller governor outputs higher oil pressure until “beta pressure” is available, turning on the beta light in the cockpit. The presence of beta pressure enables the engine to operate in the beta range once the power lever is pulled aft of the flight idle gate. The gate is a mechanical detent in the engine control console that prevents inadvertent reduction of the power lever below the flight idle setting. Before the power lever can be reduced below the flight idle setting, the pilot must lift a silver T-handle located under the power lever over the detent gate/lock, which allows the power lever to be moved further aft. In the propeller governing range, the power lever controls the engine power by adjusting the fuel flow from the manual fuel valve. Engine rpm is selected using the speed lever, which varies the propeller blade angle using the propeller governor. In the beta range, the power lever varies the propeller blade angle using the propeller pitch control. To maintain the speed lever and engine rpm settings, the USFG varies the engine power by scheduling the fuel flow. The USFG controls the engine rpm settings between 68% and 96%. Propeller Operation The Honeywell TPE331 turboprop engine was equipped with a Hartzell Lightweight Turbine Series, 4-bladed, single-acting, hydraulically operated constant-speed model propeller with feathering and reverse pitch capability. The propeller features an aluminum hub and composite blades. The propeller pitch is adjusted by using engine oil pressure to displace an internal piston axially along the propeller assembly. Pitch change knobs connecting the piston movement to each blade root cause changes in blade pitch angle with any axial movement of the piston. Oil pressure from the propeller control system exerts force on only one side of a piston, leading to a reduction (or fine adjustment) in the propeller blade pitch angle. Conversely, oil pressure cannot drive the blades in the coarse direction. A feather spring situated on the opposing side of the piston continually drives the propeller blade pitch toward the feather position (or coarse adjustment). Counterweights on each blade also provide a twisting moment toward the feather position. The feather position represents the maximum coarse position of the blades. During operation at flight idle or above and with sufficient airspeed, the propeller governor (PG) automatically regulates the propeller blade pitch angle based on engine speed and power requirements. The propeller pitch control (PPC) unit is inactive during this flight mode. The engine propeller controls ensure that the propeller blade angle remains above the established safe in-flight minimum blade low-pitch angle, which is set at the time of installation to 5.5°. In the beta mode, the PPC takes over controlling the propeller blade pitch angle, and any movement of the power lever by the pilot directly adjusts the propeller blade angle. The PG is not controlling the blade angle in this mode; however, it is still supplying high-pressure oil to the PPC. The high-pressure oil is what triggers the beta light. Beta mode permits the blade angle to decrease below the safe in-flight minimum blade low-pitch angle. During normal flight conditions, when the power lever is at flight idle or above, the combination of inherent moments on blades always acts in the coarsening or feathering direction, allowing the PG oil pressure to fine-tune the blade position until a propeller rpm of 100% is achieved. However, if the power lever is placed below flight idle into beta mode during flight, 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. Eventually, the sum of the fining moments can exceed the sum of the coarsening moments, causing the blade to enter reverse and remain “aerodynamically locked” in that position. No power lever input from the pilot can alter this and using the feather valve is ineffective because the dome pressure is already at zero. A pressure switch in the propeller control system is used to power an indicator light in the cockpit. The light is illuminated when the propellor governor is supplying max output pressure (beta pressure) and the pilot has the ability to control the propeller blade angle using beta mode. Fuel The Federal Aviation Administration (FAA) inspectors who responded to the accident stated that the fuel tanks remained intact with no apparent perforations. Recovery personnel drained about 20 gallons of fuel from the airplane, all of which was in the right wing (the airplane came to rest in a right-wing-low attitude). WRECKAGE AND IMPACT INFORMATIONA teardown examination of the engine revealed no evidence of mechanical failure or malfunction that would have precluded normal operation. There was metal spray found throughout the engine 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 the time of impact. A bench flow-test and subsequent teardown examination of the fuel control unit revealed no evidence of malfunction; there were trace amounts of an unidentifiable blue debris found in the screen. A bench-flow test and subsequent teardown examination of the propeller governor revealed a few test points outside of limits. The propeller remained attached to the engine propeller shaft flange, with all four blades fractured at the shank. Three blades were fully recovered and the tip fragment of the fourth blade was with the wreckage; the entirety of that blade was not recovered. Blade Nos. 3 and 4 counterweights were rotated beyond the mechanical reverse stop, indicating damage to pitch change components. Chordwise/rotational scoring was evident on blade Nos. 3 and No. 4, similar to blade No. 1. The propeller hydraulic unit appeared undamaged, with one start lock pin fractured. The hub unit showed no visible fractures, but rotational scoring marks were observed. All four blades displayed leading edge nicks and trailing edge c
The pilot’s intentional inflight operation of the propeller in beta, which led to an aerodynamic lock, his inability to restore forward thrust, and subsequent collision with terrain.
Source: NTSB Aviation Accident Database
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