History of Flight
On November 18, 2022, at 1019 Pacific standard time, a Textron Aviation 208B, registration N2069B, was substantially damaged when it crashed 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 by Raisbeck Engineering, which held the STC for an aerodynamic drag reduction system (DRS) on the Cessna 208B. The accident flight was part of testing to expand DRS applicability to the Cessna 208B EX model modified with the Aircraft Payload Extender (APE III) STC developed by AeroAcoustics Aircraft Systems Inc. QuickSilver Aero provided instrumentation support. At the time of the accident, the Raisbeck DRS STC was not installed.
The airplane began flights three days before the accident. The three flights on the first day totaled 1.1 hours, including a pilot familiarity flight and a ferry flight for weight and balance measurement. Two days before the accident, two flights totaling 4.6 hours gathered baseline data for mid center-of-gravity (CG) cruise and forward CG stall speeds. The day before the accident, two test flights were performed with the accident test pilot and aft-seated testing personnel. The first flight (1.2 hours) tested aft CG static stability. The second flight (1.4 hours) was terminated prematurely with about half the test card completed because an aft crewmember felt airsick.
The accident flight aimed to complete the previous day's test card for baseline testing of aft CG stall characteristics of the airplane modified with the APE III STC. ADS-B and radar data showed the airplane departed Renton, Washington, around 0925, climbed north to about 9,500 ft mean sea level (msl), and began maneuvers. After about 45 minutes, altitude varied between 6,500 ft and 10,275 ft msl.
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, it climbed about 100 ft while slowing to 90–105 KCAS. Shortly after, a gradual left roll increased to 30° and altitude decreased at about 400 ft per minute, descending from 9,700 ft to 9,350 ft msl by 1018:46. Airspeed remained at approximately 105 KCAS between 1018:20 and 1018:44. Engine torque was consistent at about 930 ft-lb between 1016:50 and 1019:05. At 1018:43, computed airspeed dropped rapidly to a minimum of 48 KCAS at 1019:01, then sharply increased. Engine torque abruptly increased to 2,200 ft-lb at 1019:20.
Indicated airspeed from the Pratt & Whitney engine monitoring system measured 35 KIAS at 1019:00 and 37 KIAS at 1019:03, rising to 223 KIAS at 1019:21, then dropping to about 80 KIAS as the airplane descended. Altitude increased from 9,320 ft to 9,680 ft msl between 1018:47 and 1018:59 as airspeed decreased. Vertical speed changed from +2,560 fpm at 1018:54 to -14,000 fpm at 1019:13. Between 1019:25 and the end of FAST data at 1019:52, average vertical speed was about -12,000 fpm.
At 1019:05, ADS-B data indicated a sudden course reversal from east to west, corresponding with minimum airspeed and increased descent rate. Computed roll angle stayed about 30° left until the reversal. ADS-B data stopped at 1019:17 at about 7,025 ft msl with a descent rate of 8,700 fpm. Witness statements and video footage were consistent with the airplane breaking apart in flight shortly after. Main wreckage was located about 2,145 ft east of the last recorded return.
Witnesses reported observing the airplane break up in flight and pieces floating down. The airplane descended in a nose-low, near-vertical corkscrew maneuver. Several witnesses saw a white plume of smoke. A security camera recorded a low-quality image of the airplane rotating about its longitudinal axis in a nose-low attitude. The flight test data acquisition system was destroyed; no accident flight data was recovered.
The right-seat pilot who flew test flights the previous day believed that just before the accident, the crew were likely performing the second-to-last maneuver on the card: airspeed 96 KIAS; flaps in landing configuration; 930 ft-lbs torque; propeller rpm fully forward; and accelerated 30° bank to the left. The airplane's maneuvers and speeds leading to the minimum airspeed suggest the performance of an intentional stall in a 30° left roll with engine power above idle (about 930 ft-lb torque), consistent with the flight's intent and remaining test card items (including a power-on stall in a 30° bank).
Personnel Information
The test pilot was seated in the front left seat. He was contracted by Raisbeck through its Organization Designation Authorization program. On August 1, 2022, he completed FAA recurrent training for a Designated Engineering Representative/Flight Test Designee. His personal flight records were not recovered, but information provided to the insurance company indicated 11,720 total flight hours, of which 232 were in the accident airplane make and model. The right-seat pilot had accrued 10,900 total flight hours, with 5,000 hours in the accident airplane make and model.
Aircraft Information
The Textron Aviation (formerly Cessna) 208B EX Grand Caravan is a single-engine, propeller-driven, single-pilot airplane. It is high-wing with wing struts, a conventional tail, fixed tricycle landing gear, and an underbelly cargo pod. Powered by a single Pratt & Whitney Canada PT6A-140 turboprop engine driving a McCauley 4-blade constant-speed propeller. Certified in the normal category, it allows maneuvers 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 (APE III) was installed, adding wing stall fences, new main landing gear axles, and 29-inch tires to increase weights and payload. On November 11, 2022, STC SA01805SE (Payload Extender STOL) installed a scalloped Gurney flap on each flap trailing edge. On November 14, 2022, the FAA issued a Special Airworthiness Certificate in the Experimental category for research and development, requested by Raisbeck Engineering.
Installed STCs imposed limitations beyond the Pilot's Operating Handbook: maneuvering speed at 9,062 lbs = 143 kt; maximum takeoff weight 9,062 lbs; limit factors: flaps up +3.36/-1.34 g, flaps down +2.00 g; Vmo 175 KIAS; VA 148 KIAS at 8,807 lbs; Vfe 125 KIAS (Land), 150 KIAS (Takeoff); flap operating range 50–125 KIAS. The airplane's approximate weight at accident was 7,965 lbs with CG at 203.5 inches; aft CG limit is 204.35 inches aft of datum at all weights up to 8,807 lbs.
The airplane was equipped with a Garmin G1000NXi integrated glass cockpit with two primary flight displays (PFDs) and a multi-function display (MFD), plus a GFC 700 Automatic Flight Control System. It also had an Electronic Stability and Protection (ESP) system operating independently of the autopilot. ESP engages when the aircraft exceeds normal pitch, roll, or Vmo parameters, applying control forces to return to envelope. ESP can be enabled/disabled via MFD and interrupted by pressing CWS or AP DISC switches. Roll Limit Indicator bars on the attitude indicator show ESP engagement at 45° bank and disengagement at 30° or 75°. ESP applies force up to 15 lbs in roll and pitch. There is no minimum airspeed protection. When ESP engages for more than 10 cumulative seconds in a 20-second interval, the autopilot automatically engages in Level Mode with an aural alert. According to the Garmin GFC700 manual, overpowering the autopilot may cause automatic trim to run, leading to an out-of-trim condition.
Additional Information
The accident flight was conducted under a Special Airworthiness Certificate for research and development. The airplane's maneuvers were consistent with intentional stall testing. The National Transportation Safety Board investigated.