WPR23FA034

The pilot’s improper recovery following a departure from controlled flight after an intentional aerodynamic stall, which resulted in an exceedance of airspeed limitations, airframe overstress, and a subsequent inflight breakup.

The pilot and three other crew members were performing flight testing for a new Supplemental Type Certificate (STC) for the single-engine turboprop-powered airplane. After departure, the pilot performed several maneuvers from the test card, then configured the airplane with the flaps extended for an intentional accelerated stall in a 30° left bank with the engine torque set to 930 ft-lb. Analysis of ADS-B data combined with a simulation matching the recorded trajectory of the accident maneuver revealed that, after the stall, the airplane rapidly rolled to the left, reaching a roll angle of 120° while the pitch angle decreased to 60° nose down. The airspeed rapidly increased, exceeding both the maximum flaps-extended speed (Vfe) and the airplane’s maximum operating speed (Vmo). Recorded engine data indicated that, after the stall, the engine torque increased. ADS-B data was lost at an altitude about 7,000 ft above ground level; the final track data indicated an approximate 8,700 ft/min rate of descent. Witnesses observed the airplane break up in flight and subsequently spiral to the ground. The wreckage was found in a rural field distributed over a distance of about 1,800 ft. Analysis of the aerodynamic loads in an overspeed condition showed that the wing design stress limit loads would be exceeded at high speeds with full flaps. The simulation of the stall maneuver indicated that reducing engine power to idle after the nose dropped could have reduced the rate at which the airspeed and associated aerodynamic loads increased, and would have likely given the pilot more time to recover. The airplane was equipped with an Electronic Stability and Protection (ESP) system, which was designed to deter attitude and airspeed exceedances during hand-flying and maintain stable flight by applying an opposite force to the direction of predetermined travel. It was designed to provide a light force that can be overcome by the pilot. To deactivate the ESP, the pilot needed to navigate to a specific page in the primary function display (PFD). Although the accident pilot was an experienced test pilot and qualified to operate the airplane, his experience with the accident airplane’s avionics system could not be determined. Videos of his previous flights in the airplane suggested that he was unfamiliar with the ESP system, as he did not deactivate it before the flight nor discuss the forces it was applying during the flight. Onboard video recording from a test flight the day before the accident indicated that, while performing a turning stall at idle power and 30° of left bank with the wing flaps extended, the airplane rapidly entered a left roll to a maximum of 83° before the pilot recovered to a wings-level attitude. After recovery, the pilot pitched the airplane’s nose down about 25° in order to “get some airspeed back,” during which the ESP activated the autopilot to effect recovery to a level attitude. The airplane continued to gain airspeed, exceeding the Vmo of 175 knots and reaching 183 knots indicated airspeed, before pilot arrested the airplane’s acceleration and disconnected the autopilot. These two exceedances illustrated shortcomings in the test execution. First, although the 83° roll exceeded the allowable roll limit during this maneuver, the crew failed to identify this exceedance even though they discussed what angle had been reached and had a data acquisition system on board, which they could have consulted to determine the maximum roll angle reached during the maneuver. Correctly identifying the roll exceedance would have resulted in a “failed” test. In accordance with risk mitigation procedures for the test plan, the test buildup should have been stopped after roll limits were exceeded in order to determine the reasons for the exceedance and to implement corrective actions before proceeding with higher-risk conditions in the test plan. Secondly, after exceeding Vmo, the crew did not remark upon the exceedance, and even though the exceedance met the requirements for an overspeed inspection as described in the airplane’s maintenance manual, there was no indication that this inspection was completed. The accident flight simulation indicated that, during the stall immediately preceding the accident, it is likely that the ESP activated as the airplane pitched in excess of 19° nose-up. This would have required the pilot to apply more aft force on the control column in order to induce the stall. After the stall, the ESP would have activated at 45° bank, then deactivated as the airplane quickly exceeded 75°. The extent to which the control forces from the ESP, or the potential distraction due to the system’s engagement and disengagement, may have contributed to the pilot’s failure to recover from the nose-low attitude following the stall could not be determined. FAA guidance warns of the risks associated with upset events during stall maneuvers and advises against performing accelerated stalls with flaps deployed due to the increased risk of exceeding the airplane’s limitations in this configuration. Following a nose-low departure from controlled flight, reducing the power to idle immediately is crucial to avoid exceeding airspeed limitations and overstressing the airplane. The circumstances of the accident flight are consistent with the pilot’s improper recovery from a nose-low attitude following an intentional aerodynamic stall. Whether the increase in torque following the stall was the result of intentional application of power by the pilot could not be determined; however, the pilot’s failure to reduce engine power to idle following the airplane’s departure from controlled flight was contrary to published guidance as well as test flight hazard mitigation procedures. It is likely that this resulted in the airplane’s rapid exceedance of its airspeed limitations, and subsequently, a structural failure and inflight breakup.

The test pilot was seated in the front left seat. He was contracted by Raisbeck through the company’s Organization Designation Authorization program, which allows Raisbeck to perform specific functions on the FAA’s behalf. On August 1, 2022, he completed the FAA recurrent training for a Designated Engineering Representative/Flight Test Designee. The pilot's personal flight records were not recovered, but information provided to the insurance company before the accident indicated 11,720 total hours of flight experience, of which 232 hours were in the accident airplane make and model. The right seat pilot had accrued 10,900 total hours of flight experience, of which 5,000 hours were in the accident airplane make and model. The Textron Aviation (formerly Cessna) 208B EX Grand Caravan is a single-engine, propeller-driven, single-pilot airplane originally designed as the 208 for the first production model certified in 1984. The 208B incorporates a fuselage extended by 4 feet and was certified as an 11-seat passenger airplane in 1989. The high-wing airplane is equipped with wing struts, a conventional tail, fixed tricycle landing gear, and an underbelly cargo pod. The airplane is powered by a single Pratt & Whitney Canada PT6A-140 turboprop engine driving a McCauley 4-blade, constant speed, full feathering, reversible pitch propeller. The airplane is certificated in the normal category, which includes maneuvers incidental to normal flying such as stalls (except whip stalls), lazy eights, chandelles, and turns with bank angles not more than 60°. Aerobatic maneuvers, including spins, are not approved. On November 1, 2022, AeroAcoustics Aircraft Systems STC SA01213SE, Aircraft Payload Extender (APE) III, was installed on the airplane. The STC adds wing stall fences to both wings outboard of the landing lights, new main landing gear axles, and 29-inch main landing gear tires to increase the maximum takeoff and landing weights and increases the payload and range. On November 11, 2022, AeroAcoustics Aircraft Systems STC SA01805SE, Aircraft Payload Extender STOL, was installed on the airplane. The STC installs a scalloped Gurney flap on the trailing edge of each flap to improve the low-speed aerodynamics of the wing. On November 14, 2022, the FAA issued a Special Airworthiness Certificate in the Experimental category for the purpose of research and development for the airplane. The certificate was requested by Raisbeck Engineering to perform company flight testing for the development of an STC for the airplane. The installed STCs on the airplane imposed limitations beyond those published in the Pilot’s Operating Handbook (POH): Limit Speeds (IAS): Maneuvering Speed at 9,062 lbs = 143 kt Maximum Weights: Takeoff: 9,062 lbs Limit Factor: Flaps up: +3.36, -1.34 Limits (g’s): Flaps down (all settings): +2.00 Max Operating Speed (Vmo): 175 KIAS Maneuvering Speed (VA): 148 KIAS (at 8,807 lbs) Max Flap Extended Speed (Vfe): 125 KIAS (Land) ; 150 KIAS (Takeoff) Flap Operating Range: 50-125 KIAS The airplane’s approximate weight at the time of the accident was 7,965 lbs with the CG at 203.5 inches. The airplane’s aft CG limit is 204.35 inches aft of datum at all weights up to 8,807 lbs. The datum is located 100 inches forward of the face of the firewall. The airplane was equipped with a Garmin G1000NXi integrated glass cockpit. Included is an electronic flight instrument system (EFIS) composed of 2 primary flight displays (PFDs) and a multi-function display (MFD). Incorporated is the GFC 700 Automatic Flight Control System (AFCS), a fully digital, three-axis, dual channel, fail passive autopilot. The airplane was also equipped with an Electronic Stability and Protection (ESP) system, which operates through the air data computers, the attitude and heading reference system (AHRS), and autopilot servos in the integrated avionic systems independently of the autopilot. According to the Garmin G1000NXi Pilot’s Guide for the Cessna Caravan: Electronic Stability and Protection (ESP™) is an optional feature that is intended to discourage the exceedance of attitude and established airspeed parameters. This feature will only function when the aircraft is above 200 feet AGL and the autopilot is not engaged. ESP engages when the aircraft exceeds one or more conditions (pitch, roll, and/or Vmo) beyond the normal flight parameters. Enhanced stability for each condition is provided by applying a force to the appropriate control surface to return the aircraft to the normal flight envelope. This is perceived by the pilot as resistance to control movement in the undesired direction when the aircraft approaches a steep attitude or high airspeed. As the aircraft deviates further from the normal attitude and/or airspeed, the force increases (up to an established maximum) to encourage control movement in the direction necessary to return to the normal attitude and/or airspeed range. For all conditions except for high airspeed, once maximum force is reached, force remains constant up to the maximum engagement limit. Above the maximum engagement limit, forces are no longer applied. There is no maximum engagement associated with high airspeed. The ESP can be enabled or disabled in a System Setup page within the MFD. It can also be interrupted by the pilot by pushing and holding either Control Wheel Steering (CWS) or Autopilot Disconnect (AP DISC) switch on the control yoke. Upon releasing either switch, ESP will again apply control force, provided aircraft attitude and/or airspeed are within engagement limits. Roll Limit Indicator bars are displayed on the attitude indicator where the 45° left and right bank hash marks are located. This indicates that the ESP will engage as the roll exceeds 45° and the Roll Limit Indicator bars will move to the 30° and 75° degree positions, which shows where ESP will disengage as roll attitude increases/decreases (i.e., ESP will disengage once roll is returned to 30° or beyond 75°). Once engaged, ESP force varies. The force increases as bank angle increases with maximum servo torque attained at 60°. For the Cessna Caravan, the ESP system engages at 19° nose up and 20° nose down. Once engaged, it applies opposing force between 17° and 50° nose up and between 18° and 50° nose down. Maximum opposing force is applied between 24° and 50° nose up and between 25° and 50° nose down. The opposing force increases or decreases depending on the pitch angle and the direction of pitch travel. For pitch recovery, ESP applies a maximum force of 15 lbs and is limited to about 1.3G or 1.5G, depending on manufacturer specifications. The system disengages when engagement parameters, such as pitch attitude or airspeed, are no longer met, but leaves the airplane’s trim unchanged. There are no indications marking the pitch ESP engage and disengage limits in these nose-up/nose-down conditions, nor is there an indication when it is activated. If Vmo (175 kts) is exceeded, the ESP activates and applies force to raise the nose of the aircraft until the overspeed condition is resolved. There is no minimum airspeed protection. In the roll axis, the only indication of ESP engagement other than the roll indices on the PFD moving to 30° is the additional control wheel force perceived by the pilot. The wheel force applied varies from 0 lbs at 30° bank to a typical maximum of 15 lbs at 60° bank. At 45°, ESP engages at 50% of the maximum force to ensure the pilot notices activation. Even if the pilot applies counteracting force, the system does not disengage. In the pitch axis, the only indication of ESP engagement is the additional column force perceived by the pilot. The Pilot’s Guide states that: Once ESP is engaged, it will apply opposing force between 17° and 50° nose-up and between 18° and 50° nose-down. … Maximum opposing force is applied between 24°