Following a petition for reconsideration by Tamarack Aerospace, the National Transportation Safety Board (NTSB) has issued a revised final report for the Nov. 30, 2018 crash of N525EG, a Cessna 525 CitationJet equipped with a Tamarack active winglet system. The new probable cause is: “The pilot’s inability to regain airplane control after a left roll that began for reasons that could not be determined based on the available evidence.”
The NTSB issued the original final report on the accident on Nov. 21, 2021. Tamarack submitted the petition for reconsideration in January 2022.
In a statement about the revision, Tamarack president Jacob Klinginsmith said, “Tamarack is very pleased that the NTSB has decided to grant our petition for reconsideration concerning this 2018 accident and taken steps to correct multiple technical errors in the original investigation. This reversal shows the NTSB has the courage, professionalism, and proper process to make these corrections, and for that, we applaud the NTSB. Our growing fleet of nearly 200 CitationJet customers see the safety, performance, and fuel-saving sustainability benefits of our technology on every flight. Our customers know that the active winglet upgrade is safe and reliable, and consistently brings a lot of value to Cessna CitationJets and other upgraded aircraft.
“Tamarack Aerospace Group extends its deepest condolences to the families and friends of those who lost their lives in the 2018 mishap and hopes the new conclusions can bring closure to this prolonged investigation. In early 2023 Tamarack Aerospace Group reached a confidential resolution of all claims related to this mishap. Tamarack remains dedicated to enhancing aviation safety.”
In its petition, Tamarack pointed out that the NTSB published the final report just four days after Tamarack, a party to the investigation, submitted a supplementary party submission. According to the petition, “There is nothing in the public docket or the final accident report that addresses any information contained in the Tamarack supplemental submission. Given that the accident occurred 35 months prior to the date of publication of the final accident report, it is difficult to comprehend why the NTSB published its final report a mere four business days after receiving the new information contained in Tamarack’s supplemental party submission without even acknowledging the existence of that supplemental submission.”
In its February 26 response to the Tamarack petition, the NTSB acknowledged the Tamarack party submission. “The NTSB notes that the supplemental party submission was evaluated during the original investigation by the investigative team members, their supervisors, and senior management. They determined that the concerns in Tamarack’s supplemental submission had already been considered by the investigative team. Nevertheless, the NTSB considered this petition for reconsideration given the petitioner’s claims of erroneous findings.”
The accident airplane was modified with Tamarack’s winglets, which add 6 feet 1 inch to the CitationJet’s wingspan along with active technology load alleviation system (Atlas) active-camber surfaces (TACS) that actuate to alleviate the extra wing loading caused by the addition of the winglets.
The original NTSB final report stated that the probable cause was “the asymmetric deployment of the left-wing load alleviation system for undetermined reasons, which resulted in an in-flight upset from which the pilot was not able to recover.”
The accident airplane took off from Clark Regional Airport in Jeffersonville, Indiana, and was climbing through 3,000 feet at 230 to 240 knots with the autopilot on. According to the NTSB, “The airplane then began to bank to the left at a rate of about 5 degrees per second. About 3 seconds later, the autopilot disconnected, and the airplane continued to climb. The airplane reached a maximum altitude of about 6,100 feet before it began to rapidly descend; during this time, the left bank angle reached about 90 degrees. About 23 seconds after the autopilot disconnected, the pilot made a mayday call, stating that the airplane was ‘in an emergency descent’ and that he was ‘unable to gain control of the aircraft.’ The airplane subsequently impacted a wooded area about 8.5 miles northwest of the departure airport.”
In its petition for reconsideration, Tamarack cited “key erroneous findings” and “factual errors” in the original NTSB final report. Tamarack wrote that “each error is significant."
Tamarack’s issue with the original report had to do with witness marks on the left TACS bellcrank and hinges, six bent pins on the TACS control unit, when and how the autopilot automatically disconnected, and the report’s citing of a UK incident that had nothing to do with the Indiana accident.
While the NTSB’s original investigation “found witness marks on the TCU ram guide at the intermediate- and full-extension positions, the NTSB’s re-examination also found witness marks that corresponded to a TACS neutral position and periodic witness marks on the ram guide. As a result, the available evidence for this accident is insufficient to confirm the petitioner’s assertion that the actuator was in an intermediate position at the time of initial impact. However, the NTSB agrees with the petitioner that the evidence is also insufficient to conclude that (1) the left actuator was in an extended position at the time of initial impact and (2) the Atlas caused the left rolling moment; the NTSB has revised the report accordingly.”
The bent electrical contact pins in the TACS control unit were the subject of a disagreement between the NTSB and Tamarack about how and when they were bent, with Tamarack claiming that the bending occurred during the accident impact. The NTSB disagreed and wrote: “The NTSB found that the additional information from the impact load testing and dimensional analysis supported the possibility that the pins became bent during impact. However, the additional information does not eliminate the possibility that the pins had become bent during manufacture or maintenance given that the pins’ pre-accident status is unknown.”
In the revised final report, however, the NTSB acknowledged the issue: “As a result, the final report will not be revised to indicate how or when the pins were bent. However, the final report will be revised to reflect the evidence obtained during this petition evaluation that supported the petitioner’s position that the pins could have become bent during impact.”
Regarding the autopilot disconnect, according to the Tamarack petition, “The report states that the autopilot prematurely disconnected at a 30-degree bank angle. The autopilot system disconnect threshold is 45 degrees, which is 15 degrees more than the bank angle at which the accident aircraft's autopilot disconnected. Therefore, the autopilot clearly did not disconnect because of excessive bank angle.
“The aircraft rolled at 5 degrees per second, but the autopilot disconnect roll rate threshold is 10 degrees per second. Therefore, the autopilot clearly did not disconnect due to excessive roll rate. Atlas has no connection whatsoever to the autopilot, meaning the only way for an Atlas failure to disconnect the autopilot is via bank angle or roll rate. If the autopilot did not disconnect due to bank angle or roll rate, Atlas could not have caused the autopilot to disconnect.”
Without more precise data that would have been available if a flight data recorder had been installed in the accident CitationJet, the NTSB could only estimate the roll rate and bank angle by analyzing ADS-B position data and audio from the cockpit voice recorder. The autopilot did disconnect, but there remains disagreement between the NTSB and Tamarack about when and how this occurred. According to the NTSB, “Thus, given the inherent noise and associated uncertainty in the calculated roll rate, the roll rate could have reached and then surpassed the autopilot disconnect threshold of 10 degrees per second. Thus, although the petitioner stated that the autopilot did not disconnect due to an excessive roll rate, the data are insufficient to support that assertion.”
One other incident was reflected in the original final accident report and removed in the revised report, an uncommanded roll event in N680KH in the UK on April 13, 2019, caused by a TACS failure that was the result of a short circuit from a loose screw. Tamarack’s petition noted that “There was no evidence whatsoever of any such screw assembly issue in the [U.S.] accident aircraft.”
The NTSB agreed and wrote: “The analysis section of the NTSB’s final report stated that five previous uncommanded roll events had occurred on Atlas-equipped airplanes (including the uncommanded roll event involving N680KH). Although this statement is accurate, the NTSB has decided to remove the statement to ensure that our analysis does not inadvertently imply that the cause of the uncommanded left roll in the UK event was a factor in the subject accident or that a screw assembly issue affected N525EG.”
In granting the petition for reconsideration, the NTSB concluded: “After review of the evidence, the petition for reconsideration of the NTSB’s probable cause in connection with the aircraft accident involving a Cessna 525 airplane, N525EG, on Nov. 30, 2018, in Memphis, Indiana, is granted in part because the available evidence for this accident does not sufficiently show that the Atlas was the cause of the in-flight upset from which the pilot was unable to recover. In addition, the factual and analysis sections of the report and the findings have been revised to reflect the information presented in the petition response sections addressing witness marks, TCU bent pins, and the UK uncommanded roll event.”