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K-Max servo flap failure before fatal accident


A recent investigation by the U.S. National Transportation Safety Board (NTSB) determined that a K-Max helicopter’s servo flaps malfunctioned before a catastrophic breakup in flight, but both manufacturer Kaman and the U.S. Federal Aviation Administration (FAA) No information has been released to provide guidance on how to prevent such failures in the future.

After being certified by the Civil Aviation Administration of China, K-MAX has begun operations in China.Carman PhotosAfter being certified by the Civil Aviation Administration of China, K-MAX has begun operations in China.Carman Photos
The Kaman K-Max has a unique meshed rotor system that relies on servo flaps to change the pitch of the rotor blades.Carman Photos

Pilot Tom Duffy died on August 24, 2020, when his helicopter broke apart during a firefighting operation in Pine Grove, Oregon. The pilot, who flew for the Montana-based family company Central Copters, had more than 2,000 hours of flying experience and was aboard a K-Max helicopter when the crash occurred.

First certified by the FAA in 1994, the K-Max features a unique meshing rotor system with twin counter-rotating, two-bladed rotors mounted side-by-side on individual towers. A single transmission typically keeps the blades of each rotor system 90 degrees out of phase with the blades of the other systems, and the tower is tilted to allow the blades to clear the opposing rotor hub.

Control of the blades is achieved through servo flaps, which are small airfoils mounted about three-quarters of the way to the tip of each blade’s trailing edge. The pilot’s flight control inputs are transmitted via levers to servo flaps, which deflect the airflow to twist the rotor blades and change their pitch.

Since the pilot only moves the servo flaps, the amount of force required is relatively small. This allows the aircraft, which has a maximum gross weight of 12,000 pounds (approximately 5,440 kilograms) with external loads, to fly without hydraulic assistance.

Pilot Tom Duffy was filling a bucket on a long line when the Central Helicopters K-Max, registration N314, crashed. The long rope was not damaged, indicating that the rotor system and pylons were separated from the fuselage while the helicopter was flying.Central Helicopter/NTSB photoPilot Tom Duffy was filling a bucket on a long line when the Central Helicopters K-Max, registration N314, crashed. The long rope was not damaged, indicating that the rotor system and pylons had separated from the fuselage while the helicopter was flying.Central Helicopter/NTSB photo
The Central Helicopters K-Max, registration N314, crashed while pilot Tom Duffy was on a long line filling a bucket. The long rope was not damaged, indicating that the rotor system and pylons were separated from the fuselage while the helicopter was flying.Central Helicopter/NTSB photo

In the Oregon crash, National Transportation Safety Board investigators determined that a crack in one of the servo flaps of the left rotor system gradually widened, eventually causing the tail portion of the flap (the panel attached to the wing spars shaped like the airfoil) to separate. . This caused the associated left blade to lose control.

The NTSB concluded that the left rotor blade then struck the right rotor blade, causing the left rotor blade to separate and shatter in flight. At the time, Duffy was hovering about 140 feet above the river, filling a bucket on his long rope. As the helicopter descended, it rolled to the left and struck the river.

Investigators were unable to determine what caused a servo-flap failure that ultimately caused the collision between the left and right rotor systems. They said flight control inputs, including the pilot’s response to abnormal vibrations in the rotor system, were likely a factor in the catastrophic outcome, but a lack of flight data prevented an analysis of the control inputs that led to the collision.

The NTSB noted that in 2009, the same helicopter (then operating under a different registration) experienced an incident in which the entire servo flap detached from its attachment bracket in flight, but the pilot was able to land the aircraft safely.

However, the agency also drew attention to another K-Max accident involving a Woody Contracting operation that occurred on June 16, 2010 in Donnelly, Idaho. At this time, the counter-rotating blades of two of the helicopters collided with each other, causing a crash and killing Keitel.

Investigators determined that the servo flap on the left rotor system of the Central Helicopter K-Max malfunctioned before the collision between the rotor systems.National Transportation Safety Board photoInvestigators determined that the servo flap on the left rotor system of the Central Helicopter K-Max malfunctioned before the collision between the rotor systems.National Transportation Safety Board photo
Investigators determined that the servo flap on the left rotor system of the Central Helicopter K-Max malfunctioned before the collision between the rotor systems.National Transportation Safety Board photo

Investigators of this accident were also unable to determine the cause of the collision, but found that the tail section separated from the servo flap of one of the left rotor blades.

Industry speculation is that a similar situation may have occurred when the Black Tusk helicopter K-Max crashed in Killam Bay, British Columbia, on October 4, 2021. , died unfortunately. The accident is still being investigated by the Transportation Safety Board of Canada (TSB), but documents from the Central Helicopter Crash docket indicate that TSB representatives inspected the wreckage last June in connection with the Black Tusk crash. comparative analysis”.

vertical Several questions were submitted to Kaman, including whether the company has issued any new guidance to operators as a result of the crash or plans to do so in the future. A Kaman spokesman said the company could not comment because of the ongoing investigation. Asked whether the FAA planned to issue an airworthiness directive for K-Max helicopter blades, a spokesperson said via email, “We have nothing to report at this time.”

Meanwhile, the family of Tom Duffy is suing Kaman in U.S. District Court, alleging that “defective design, manufacturing and marketing of the servo flaps on the blades of his K-Max helicopter caused the crash,” and 2020 crash ‘essentially the same’ Carman was involved in one before.

In a 2010 accident, the servo-flap tail also separated along a straight spanwise line behind its pivot point.National Transportation Safety Board photoIn a 2010 accident, the servo-flap tail also separated along a straight spanwise line behind its pivot point.National Transportation Safety Board photo
In a 2010 accident, the servo-flap tail also separated along a straight spanwise line behind its pivot point.National Transportation Safety Board photo

Kaman denied the accusations, initially claiming that Duffy “negligently operated the K-Max helicopter” and that Central Helicopters failed to adequately maintain the aircraft and/or failed to adequately train and supervise Duffy. However, both parties agreed on May 19 to dismiss Kaman’s counterclaim.

The K-Max is a niche helicopter designed for repetitive heavy transport operations, with only 60 units produced in two production runs. After halting initial production in 2003, Kamann decided to restart the production line in 2015. “High-volume” capital inventory requirements.

Kaman said it would continue to support operations of the existing K-Max fleet, but further investment in product improvements seemed unlikely. In 2019, the company announced a plan to design and certify all-composite rotor blades for the K-Max, a project that still relies on rotor blade spars made of Sitka spruce. However, an all-composite blade never materialized, and Kaman declined to comment on the progress of the program.



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