Snohomish, WA, USA
N2069B
TEXTRON AVIATION INC 208B
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.
HISTORY OF FLIGHTOn November 18, 2022, at 1019 Pacific standard time, a Textron Aviation 208B, N2069B, was substantially damaged when it was involved in an accident near Snohomish, Washington. The two airline transport pilots and two flight test crewmembers were fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 test flight. The operator, Raisbeck Engineering, held the STC for an aerodynamic drag reduction system (DRS) on the Cessna 208B. The accident flight was part of the testing for Raisbeck to expand the applicability of that DRS to the Cessna 208B EX model modified with the Aircraft Payload Extender (APE III) STC, developed by AeroAcoustics Aircraft Systems Inc. QuickSilver Aero was contracted to provide instrumentation support for Raisbeck’s flight test program. At the time of the accident, the Raisbeck DRS STC was not installed. The airplane began flights to support the flight test three days before the accident. The three flights on the first day comprised a total of 1.1 hours and included a pilot familiarity flight and a ferry flight to have the airplane’s weight and balance measured. Two days before the accident, the flight test data collection flights began. Those included two flights, totaling 4.6 hours, to gather baseline data for both mid center-of-gravity (CG) cruise flight and forward CG stall speeds. The day before the accident, two test flights were performed with the accident test pilot and the accident aft-seated testing personnel. The purpose of the first flight, totaling 1.2 hours, was to test aft CG static stability. The last flight that day, totaling 1.4 hours, was terminated prematurely with only about half of the test plan (card) completed, because an aft crewmember was feeling airsick. The purpose of the accident flight was to complete the test card from the previous day, which consisted of baseline testing of the aft CG stall characteristics of the airplane modified with the APE III STC. A review of ADS-B and radar track data revealed that the airplane departed Renton, Washington, around 0925, and continued to the north in a gradual climb to about 9,500 ft mean sea level (msl) and began a series of turns/maneuvers. The airplane proceeded for about 45 minutes, its altitude varying between about 6,500 ft to 10,275 ft msl. The combination of ADS-B and winds aloft information indicated that, at 1017:00, the airplane was in a nearly level, 10° left-banked turn at 10,000 ft msl and about 110 knots calibrated airspeed (KCAS). Around 1017:20, the airplane climbed about 100 ft while slowing to between 90 and 105 KCAS. Shortly thereafter, a gradual left roll increased to 30° and the altitude began steadily decreasing at a rate of about 400 ft per minute (fpm), with the airplane descending from 9,700 ft to 9,350 ft msl by 1018:46. The airspeed remained constant at approximately 105 KCAS between 1018:20 and 1018:44. Engine torque was consistent about 930 ft-lb between 1016:50 and 1019:05. At 1018:43, the computed airspeed dropped rapidly, reaching a minimum of 48 KCAS at 1019:01 before sharply increasing. Engine torque also abruptly increased, reaching 2,200 ft-lb at 1019:20. Indicated airspeed recorded by the Pratt & Whitney engine monitoring system (FAST) measured 35 knots indicated airspeed (KIAS) at 1019:00 and 37 KIAS at 1019:03 before quickly rising to a maximum of 223 KIAS at 1019:21, then dropping to approximately 80 KIAS as the airplane descended. During this period, the altitude increased from 9,320 ft to 9,680 ft msl between 1018:47 and 1018:59 as airspeed decreased. Vertical speed decreased from +2,560 fpm at 1018:54 to -14,000 fpm at 1019:13. Between 1019:25 and the end of the FAST data at 1019:52, the computed average vertical speed reached about -12,000 fpm. At 1019:05, the ADS-B data indicated a sudden and tight course reversal from east to west, corresponding with the minimum airspeed and a dramatic increase in the descent rate. The computed roll angle stayed consistent at about 30° left until the course reversal. The ADS-B data stopped at 1019:17 about 7,025 ft msl with a recorded descent rate of 8,700 fpm (see Figure 1 below). The location of the wreckage, witness statements, and video footage were all consistent with the airplane breaking apart inflight shortly thereafter. The main wreckage was located about 2,145 ft east of the last recorded return. Figure 1: Radar data of maneuvers during flight (left) and last turn (right) Witnesses reported that they observed the airplane break up in flight and watched pieces floating down. The airplane then descended in a nose-low, near-vertical corkscrew maneuver toward the ground. Several witnesses reported seeing a white plume of smoke when they observed the airplane break into pieces. A security camera recorded a low-quality image of the airplane rotating about its longitudinal axis in nose-low attitude (see figure 2 below). Figure 2: Picture from witness (left) and excerpts of video still images (right) The airplane’s flight test data acquisition system, used as part of the flight test program, was destroyed in the accident and no flight test data for the accident flight was recovered. The right seat pilot who flew the test flights the day before the accident reviewed the track data for the accident flight. He believed that just before the accident, the crew were likely performing second-to-last maneuver on the card, which specified: airspeed 96 KIAS; flaps in landing configuration; 930 ft-lbs of torque; propeller rpm fully forward; and accelerated 30° bank to the left. The airplane's maneuvers and speeds leading up to the minimum airspeed recorded at 1019:01 suggests the performance of an intentional stall in a 30° left roll with the engine power above idle (at about 930 ft-lb of torque), consistent with the stated intent of the flight and the items remaining on the flight test card for that flight (including a power-on stall in a 30° bank). PERSONNEL INFORMATIONThe 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. AIRCRAFT INFORMATIONThe 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° 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. This force is intended to encourage movement in the pitch axis in the direction of the normal pitch attitude range for the aircraft. According to Garmin, the maximum column force ESP can apply is 15 lbs. When ESP has been engaged for more than 10 cumulative (not necessarily consecutive) seconds of a 20-second interval, the autopilot is automatically engaged with the flight director in Level Mode, bringing the aircraft into level flight. An aural “Engaging Autopilot” alert is played, and the flight director mode annunciation will indicate LVL for vertical and lateral modes. The airplane was equipped with a Garmin GFC700 autopilot. According to the Garmin Manual: When the autopilot is engaged, a small amount of pressure or force on the pitch controls can cause the autopilot's automatic trim to run to an out-of-trim condition. Therefore, any application of pressure or force on the controls should be avoided when the autopilot is engaged. Overpowering the autopilot during flight will cause the autopilot's automatic trim to run, resulting in an out-of-trim condition or causing the trim to hit the stop if the action is prolonged. Unanticipated control forces are required after the autopilot is disengaged. AIRPORT INFORMATIONThe 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
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.
Source: NTSB Aviation Accident Database
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