Tamarack Responds to AAIB Report About 2019 CJ1 Incident

Accident and incident chains usually follow a similar pattern when a number of innocent errors stack to create an event. Despite 4,400 hours of total flight time, nearly three quarters of which had been logged in Cessna Citation CJs, a US-registered aircraft N680KH nearly got away from its pilot during an April 2019 incident near Bournemouth, England. A follow-up investigation showed the pilot simply did not read a critical AFM supplement, despite a close call just a month before.

The incident reported on by the UK’s Air Accidents Investigation Branch (AAIB), nearly cost the 73-year old ATP-rated pilot and his three passengers their lives, though he eventually managed to land the aircraft safely. The pilot purchased N680KH just a month before the incident and successfully completed factory-sponsored training.

The AAIB said, “The aircraft had been modified with an active-camber winglet system intended to enhance its performance, which included supplementary control surfaces designed to deflect symmetrically and automatically to alleviate gust loads.” The performance-enhancing winglet system, known as Atlas, was manufactured by Sandpoint, Idaho-based Tamarack Aerospace.

Not long after departing Bournemouth on April 19, with the CJ level at 3,000 feet and 258 kias, the pilot later told investigators he felt a slight vibration in the aircraft just as a red “Atlas Inop Limit 140 kias” warning illuminated on the instrument panel. Unknown to the pilot, an internal electrical system of the active winglet had failed which caused the aircraft to quickly roll to the left as it also began to descend. Full right aileron and right rudder had no effect, and neither did popping the Atlas circuit breaker. The pilot reduced power to idle and said he used two hands on the control column to try and right the aircraft.

The AAIB report said, “Recorded data showed that a bank angle alert was generated at around [a] 60 degree roll, and there was a sharp increase in normal acceleration, which reached [positive] 2.65 g. The aircraft’s roll angle peaked at 75 degree left wing down, with 9 degree nose down pitch, 19 seconds after the onset of the roll. Its rate of descent peaked soon after at 4,500 ft/min, corresponding with an airspeed of 235 kias, reaching a minimum altitude of 2,300 ft.”

The pilot said it took all his strength to lift the aircraft’s nose, reduce its airspeed, and reduce the bank angle to 30 degrees left wing down. He managed to climb the aircraft back up to 3,200 feet as the airspeed slowed to 144 kias. It was then that the airplane entered another left descending turn. Luckily, the weather was good VFR at Bournemouth where the pilot entered a left downwind for Runway 8. The aircraft descended as low as 300 feet agl on downwind while the pilot used full right aileron and some right rudder all the way to touchdown.

The AAIB’s investigation discovered the TACS computer control unit in the left winglet had failed due to a screw that have worked itself loose and shorted a circuit board. The TACS control unit on the right winglet tested OK. The Board also learned that a Tamarack supplement to the CJ’s aircraft flight manual contained a procedure to address this specific type of failure, a document the pilot admitted he’d never read. However, the AAIB said despite the AFM supplement’s existence, it “did not adequately characterize the significance of the system failure, nor address the failure in all anticipated flight conditions. Certification flight tests of the system [also] did not reveal the severity of possible outcomes.”

While inspecting the aircraft, an AAIB investigator located the AFM in the back of the aircraft where it would have been unavailable to the pilot in flight had he wanted to refer to it. The AFM supplement included the first critical emergency steps required to deal with an electrical failure of the Tamarack winglets; “reduce power and deploy the speed brakes to slow the aircraft below 140 knots.” The incident aircraft’s speed exceeded that called for in the AFM supplement by more than 100 knots. Tamarack explained these two steps were more important that trying to right the airplane. Initial certification of the Tamarack winglets also reported the importance of slowing the aircraft in order to maintain control. The pilot knew none of this.

EASA issued an Emergency AD on the same day as the April incident while the FAA took a little longer. The FAA issued an AD grounding all Cessna Citation CJ 525, 525A and 525B with the Atlas system installed warning pilots a malfunction could lead to a total loss of aircraft control. In October 2019, Tamarack filed for Chapter 11 bankruptcy at the same time announcing plans to soon reemerge. In July 2019, the company’s products were approved for an alternative means of compliance that effectively removed the grounding directive from all aircraft.

The AAIB investigation also discovered something shocking, that the pilot had a month earlier experienced another Atlas failure, but never reported the incident. He said during the earlier event, the Atlas fail button illuminated as the aircraft banked about 50 degrees (the report did not indicate in which direction). After five seconds the light went out and the aircraft again flew normally. Since the aircraft was again operating normally, the pilot did not investigate any further, nor did he bother to acquaint himself with the emergency procedures in the AFM, if he knew they existed.

Before purchasing the incident airplane, however, the pilot received a pre-buy report indicating the Atlas system had been installed as well as a mention of the AFM supplement. A previously issued service bulletin calling for replacement of the TACS computers was never complied with and was never mentioned in the report because scope of the pre-buy apparently did not include a search for any outstanding SBs.

The AAIB issued four safety recommendations following this incident, primary of which was to specifically warn pilots of how severe a failure might become if the specified procedures were not followed. It also asked both EASA and the FAA to review the additional training pilots operating a Tamarack aircraft should receive before they were allowed to act as PIC because this pilot’s instinctive reaction, while considered understandable, was much different from the way the system test pilot had experienced things.

On December 3, 2020, Tamarack published a number of rebuttal arguments on its website related to the Bournemouth incident, including that the pilot’s initial incident report contained inaccuracies that ultimately grounded the fleet of Tamarack-modified aircraft for a short time. “The pilot reported his Citation rolled 90 degrees in one second. Data captured from the incident showed the roll rate was approximately four degrees per second which aligns with what Tamarack reported in its failure recovery procedures. The Citation’s initial roll also went uncorrected by the pilot for some 19 seconds, an issue the pilot later could not explain.” Tamarack also pointed to the service bulletin it had issued on the TACS computers a year prior to the incident—one the company offered to operators at no cost—that the company said would have prevented the 2019 incidents aboard N680KH.

This incident serves as yet another reminder of the need to learn for aircraft buyers to learn the entire story behind their specific aircraft before they accept delivery, as well as the need to look deeper into system failures when they first appear.