Uneven deployment of its active winglets likely caused the inflight upset and fatal crash of a Cessna Citation 525A business jet in 2018, the NTSB has determined in a finding disputed by winglet manufacturer Tamarack Aerospace Group.

The pilot and two passengers were killed when the Citation Jet crashed in a wooded area near Memphis, Indiana, on Nov. 30, 2018, within minutes of departing Clark Regional Airport (KJVY).

In a probable-cause finding dated Nov. 1, the NTSB said “asymmetric deployment of the left wing load alleviation system for undetermined reasons” caused an uncommanded roll from which the pilot was not able to recover. The finding refers to the Tamarack Active Technology Load Alleviation System (ATLAS), a retrofit wing extension and active winglet system that dynamically responds to changing wing loads and conditions to improve aerodynamic efficiency and stability.

NTSB accident investigators found the left-wing Tamarack Active Camber Surface (TACS)—the trailing edge control surface of the wing extension—“in a position consistent with trailing edge up and the right TACS likely in a position consistent with neutral, which would have induced a left rolling moment in the airplane,” the agency said.

Post-accident investigation revealed that the 40-pin connector of the left TACS control unit (TCU) “had six pins that were curled, with two of the pins not continuous,” which could indicate an intermittent electrical connection that could have interrupted power to the TACS, the NTSB said.

After climbing past 3,000 ft. with an airspeed between 230-240 kt., the jet began to bank to the left at a rate of about 5 deg. per second. When it reached about 30 deg. of left bank, the autopilot disconnected, accompanied by an aural alert. “About 1 sec. later, the cockpit voice recorder recorded a statement by the pilot consistent with surprise, likely made in response to the autopilot disconnect and/or the bank angle,” the NTSB said. “Based on the pilot’s statement of surprise, it is unlikely that the pilot commanded the left bank.”

The investigation determined that the roll angle changed from 90 deg. left wing down to about 53 deg. before the jet crashed.

“If an asymmetric TACS deflection caused the left roll, it is possible the pilot was able to roll the airplane back to the right but not enough to fully recover and arrest the descent,” the NTSB said. “However, because the airplane was not equipped with a flight recorder, control surface deflections and pilot input are unknown. Further, the ATLAS is independent of other airplane systems, and it does not record any information about TCU actuation or TACS deflection.”

After multiple reported incidents of the ATLAS system malfunctioning, the European Union Aviation Safety Agency (EASA) in April 2019 issued an emergency airworthiness directive requiring additional preflight inspection procedures and flight-envelope limitations for aircraft equipped with the system. The FAA followed with an airworthiness directive in May 2019 that grounded 91 ATLAS-equipped Cessna 525, 525A and 525B twinjets until an approved modification was performed.

The grounding forced Sandpoint, Idaho-based Tamarack Aerospace Group to enter into a Chapter 11 bankruptcy reorganization from which it exited in August 2021.

Responding to the NTSB report, Tamarack said it “strongly disputes” the agency’s probable-cause explanation.

“The forensic evidence collected in the investigation indicates that the load alleviation system was indeed operational, and deployed symmetrically, upon impact,” the company said. “There are inconsistencies within the report that do not support the conclusion published by the NTSB.”

“Of particular note,” Tamarack added, “the NTSB final report acknowledges that the aircraft was rolling at 5 degrees per second when the autopilot automatically disconnected at 30 degrees of bank, not at 45 degrees as would be the case for an excessive bank condition caused by an uncommanded roll. The investigation fails to explain or address the fact that the autopilot disconnected for other reasons.”