Tamarack Aerospace Group Inc., creator of what remains the only active winglet retrofit in service on general aviation aircraft, expanded its roster of Cessna Citation jets now equipped with the fuel-saving modifications by more than 50 percent since 2019, while navigating a bankruptcy proceeding that recently culminated in full repayment of creditors and exit from Chapter 11—in the midst of the coronavirus pandemic. The company may have more challenges ahead, depending on the outcome of an ongoing NTSB investigation of a fatal crash in 2018 that has focused almost exclusively on the winglets and systems that automate associated control surfaces. Tamarack, as a party to that investigation, concluded that its winglets were not a factor.
Customers and vendors chipped in $1.95 million to keep Tamarack afloat after company officials filed for Chapter 11 bankruptcy protection in June 2019. That filing came a few weeks after the FAA issued an airworthiness directive grounding all 91 Cessna Citation jets that were by then equipped with Tamarack’s retrofit.
The bankruptcy case proceeded on a slower track to a resolution, culminating with a final decree filed August 27. The company continued to operate through the Chapter 11 process, and the ensuing pandemic, installing winglet retrofits that come with an automatic flight control system that cannot be directly controlled by the pilot.
Tamarack President Jacob Klinginsmith said in a recent news release that the company is “relieved that these bankruptcy proceedings are in our review mirror. The final results, allowing us to support our fleet and to reorganize and repay creditors, validates our decision to voluntarily seek Chapter 11 protection. It’s very gratifying to see our investors, existing customers and new customers have the faith in our technology and business model to support us during this bankruptcy test and now Tamarack is set to keep growing.”
The company noted that new installation centers opened in 2020 in Aiken, South Carolina, and Oxford, England, augmenting the installation capacity at the headquarters in Sandpoint, Idaho. The workforce that was reduced soon after the grounding, and later because of the pandemic, has grown again: The company “has since expanded facilities and doubled in personnel in an effort to keep up with the continued demand for Active Winglets and the engineering consulting services provided to other aerospace engineering firms developing new technologies.”
Winglets have long been known to enhance aerodynamic efficiency and have been installed for many years on transport aircraft. Vertical winglets extend the wingspan, and the resulting aspect ratio increase reduces drag, but the higher aspect ratio of the wing can also significantly increase the force applied to the wing structure by wind gusts encountered in level flight, or during maneuvers. Additional structure can allow a flat wing to hold up under winglet loads without damage or failure, but that comes with a weight penalty. On smaller aircraft, winglets are not practical retrofits if they require bracing that is prohibitively expensive to design, install, test, and certify, and that adds weight that would offset much of the fuel-saving benefit.
The core technology to sidestep this tradeoff was developed and built by Tamarack, and originally given supplemental type certificate approval by European regulators in December 2015. The FAA issued its own STC in December 2016, and the retrofit proved popular. Through bankruptcy proceedings and the coronavirus pandemic, Tamarack has retrofitted another 50 Citations, the company reported.
Tamarack’s active winglet system improves aircraft performance without the need to make the flat wing stronger by shedding some of the load with a system that includes a new control surface. This “active camber surface” is mounted along the trailing edge, just inboard of the new winglet. It is controlled by a dedicated system that is independent of other aircraft controls. The “active” control surface is manipulated during flight automatically in response to accelerations measured by a control module and connected to control units in each wing that contain actuators to move the aileron-like control surface between 18 degrees above and 8 degrees below the trailing edge, as required, in response to aerodynamic loads.
If the system becomes disabled, a warning light alerts the pilot, who can attempt to reset the system. If that fails, airspeed must be limited to avoid overloading the wing.
AOPA documented the benefits of this automated efficiency enhancement in a real-world fly-off in January. A Citation with Tamarack’s active winglet retrofit flew from Maine to Florida, with an unmodified “flat wing” Citation flying the same mission, departing minutes apart on the same day for comparison. The Citation with Tamarack’s active winglets made the trip from Portland International Jetport to Palm Beach International Airport nonstop in four hours and 36 minutes, covering 1,386 miles and burning 2,610 pounds of fuel. The flat-wing Citation had to stop for fuel, adding 110 miles and an extra hour of flight time between departure and destination, and burning an additional 1,040 pounds (155 gallons) of fuel to make the trip.
At that rate, fuel and time savings add up quickly, benefits that may help explain why Tamarack’s customers (and, by extension, its vendors) opted to provide nearly $2 million to Tamarack when the company that had retrofitted their airplanes faced financial ruin following the FAA grounding order issued in May 2019. The FAA based its decision on a series of uncommanded roll events reported to aviation authorities, or documented by investigators, involving Cessna Citation jets fitted with Tamarack’s active winglets.
In every case reported by pilots to date, the pilot recovered the aircraft and landed safely after coping with an uncommanded roll in flight.
On November 30, 2018, an airline transport pilot and two passengers died when a Cessna Citation rolled out of control while climbing through 6,000 feet. At 10:26 a.m., the cockpit voice recorder captured a click sound, followed a fraction of a second later by an electronic voice announcing “autopilot,” followed less than a second later by a human exclamation transcribed as “whoooaaaaa.” A series of “bank angle” advisories quickly commenced at two-second intervals, joined by a pulsing beep described in the NTSB transcript as similar to an overspeed warning alert.
About 24 seconds after the “autopilot” alert, according to the transcript, the pilot shouted into the radio: “mayday mayday mayday citation five two five echo golf is in an emergency descent unable to gain control of the aircraft.”
Less than 10 seconds later, the microphone recorded the sound of the aircraft’s impact with trees and terrain near Memphis, Indiana, and the recording stopped.
The NTSB preliminary report of the Indiana accident does not mention Tamarack or its product, but documents in the investigation docket made public in May show that Tamarack’s Active Technology Load Alleviation System was the primary focus of the investigation from the outset. Radiography and electron microscopy studies of recovered components of the system, including the modules that control the aerodynamic surfaces, are detailed in dozens of pages of descriptions and images, though much of the data that might have pointed definitively to what role ATLAS did or did not play in the crash was either not recorded by various aircraft systems, or was lost in the high-energy impact. The accident aircraft had several data recording devices, but none were sufficiently crash-hardened to preserve key pieces of evidence that would show investigators exactly how the pilot responded, and when.
The Systems Group Chairman’s Factual Report focuses mostly on ATLAS, including microscopic and radiographic examinations of the control units attached to each active camber surface, and other components of the system. (The primary flight controls and autopilot are covered in full on about two of the 56 pages; most of the rest of the report details the forensic examination of ATLAS components.)
Mounted near the winglets inside the left and right wing, Tamarack Active Camber Surface Control Units, or TCUs, are wired to a center-mounted system controller that contains redundant accelerometers and is designed to command the left and right control surfaces to actuate symmetrically in response to increasing load factors. Whether or not they were symmetrically deployed in the case of the Indiana crash, and if they deployed before the upset began or later, in response to increasing wing loads as G force increased, appears to be a matter of interpretation of witness marks and other analysis of crash-damaged circuits and components.
Tamarack, having become a party to the NTSB investigation early on by virtue of having designed, manufactured, and installed the active winglets, submitted its own report, 39 pages detailing the company’s examination of physical evidence and investigatory conclusions:
“First, there is not sufficient data currently available to determine the reason for the onset of the roll event; however, there is evidence that ATLAS was functioning normally at the time of impact. Markings within the actuators in each TCU on the upper and lower ball nut guides were caused by the impact. The relative positions of the markings indicate that the actuators were deployed symmetrically at the time of impact, at a position consistent with an elevated positive load factor. Other damage to the TCUs, particularly bent pins within the left hand TCU, was also caused by impact. There is no evidence available to Tamarack or indicated in NTSB factual data to indicate that Tamarack equipment failed in flight.”
The NTSB was not so quick to dismiss bent electrical connection pins found on one TCU as a possible pre-crash condition, though investigators stopped short of a confident conclusion that a fault occurred in the absence of other data that was either not recorded or destroyed by impact.
Tamarack further concluded in its report to fellow NTSB investigation team members that the initial 90-degree roll “most likely escalated into an accelerating descending steep turn due to the nineteen second delay between the onset of the roll event and the first indication of control response, coupled with the lack of throttle reduction during the response. These factors allowed the airplane to develop a flight condition which was not recoverable given the airplane’s initial altitude.”
Tamarack based its assessment of the “first indication of control response” on the ADS-B data study from which aircraft attitude, altitude, and speed were calculated, singling out the point at which the roll was arrested and began to reverse seconds before the crash.
The NTSB Performance Study compared the roll rate calculated during the accident flight path analysis with the roll rates measured during certification tests, when Tamarack control surfaces had been deliberately deflected in the wrong direction generating a roll rate up to 20 degrees per second in a worst-case scenario.
“The accident roll rate of 5°/s was significantly less than the measured roll rate induced by full asymmetric deflection of the ATLAS TACS as tested in the critical failure condition. 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. However, since the airplane was not equipped with a flight recorder and control surface deflections and pilot input were unknown, there was not enough information to determine what initiated the left roll.”
On April 13, 2019, another incident involving an ATLAS-equipped Cessna Citation flown in the United Kingdom would lead to a 44-page Air Accidents Investigation Branch report that is also found in the NTSB docket of the Indiana crash investigation.
The 2019 upset incident is the most recent of five similar incidents noted in the NTSB Systems Group Chairman’s Factual Report, each involving uncommanded roll events in ATLAS-equipped Citations. Five incidents, excluding the crash not yet officially attributed to any specific failure, among a fleet of 140 aircraft (fewer than 100 at the time of each documented incident) on a system first certified in 2015 does not compare well to the overall tendency of Cessna 525s to catch pilots by surprise with an uncommanded roll, NTSB investigators noted:
“Utilizing manufacturer and FAA records, a review was conducted to note any uncommanded roll events for the fleet of Cessna CitationJet 525 aircraft without the ATLAS system installed. For the history of the aircraft, there have not been any reported events of uncommanded rolls.”
The Air Accidents Investigation Branch account of the 2019 incident in the United Kingdom prompted emergency ADs in Europe and the United States. That report details a scenario with striking similarities to the Indiana crash. Both upsets occurred shortly after takeoff during climbs. In both cases (based on available evidence in the crash) the autopilot disengaged soon after the roll began—in the British incident, it happened as the aircraft rolled through 45 degrees of bank while the aircraft was climbing through 3,000 feet at 258 knots.
“The pilot reported applying full right aileron and full right rudder, but these actions were insufficient to control the aircraft,” the AAIB report states, then adds a possibly crucial detail: “He moved the throttles to idle and used both hands on the control column, but the aircraft continued descending.”
Tamarack noted that ADS-B data from the accident flight in Indiana strongly suggest the pilot of that Citation did not reduce power.
The AAIB synopsis describes a pace of events in the 2019 incident that is similar to what the NTSB has detailed in its investigation of the 2018 crash: In the 2019 incident that culminated in a successful recovery, "the aircraft’s roll angle peaked at 75 [degrees] left wing down, with 9 [degrees] 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.”
In contrast, the accident airplane in Indiana maintained airspeed and continued to climb for 12 seconds after the roll began, according to the NTSB Performance Study, “indicating that engine power was not reduced in response to the roll onset.”
The NTSB has yet to determine the probable cause of the November 2018 accident. Participants in NTSB investigations are prohibited from publicly discussing the matter until after the board determines probable cause.
The AAIB investigation of the April 2019 incident in the United Kingdom traced the problem to a loose screw inside of the control unit, and the potential for a resulting short-circuit that explained the aircraft behavior:
“The manufacturer [Tamarack] conducted a failure assessment of the actuator to establish what might cause the left TACS to deploy trailing-edge up (actuator extended), and determined that a short between two pins within the connector head could drive the TCU arm to the extend hard stop.”
The applicable AD requires replacement of TCUs and incorporation of all prior service bulletins, including one issued in early 2018 to resolve the potential loose screw issue. That work had been done on the Indiana accident aircraft, and investigators noted the screws in question were found still in place, secured as prescribed. The probable cause of that accident has not yet been determined by the NTSB.
Tamarack noted, in an email following initial publication of this story, that there have been no accidents or incidents reported involving aircraft equipped with the active winglets since the 2019 AD was issued, and the fleet of nearly 150 aircraft has collectively logged more than 100,000 flight hours.