NH Industries is working to deliver a solution to an issue with the main rotor head on NH90 military helicopters that means the blades can impact the tail structure when starting or stopping the rotors during high winds.
Although the problem emerged in the media over the past week following tests with Australia’s MRH90 Taipan fleet, this phenomenon has also been experienced by other operators.
The NH90, in common with other rotorcraft, is fitted with stops on the rotor hub which prevent excessive blade flapping during low-RPM operations.
As the disc spins up, centrifugal force causes the stops to automatically move to a position allowing the blades to move freely for flight.
However, it appears that in some conditions, the stops remain jammed in their initial position at rotor start and are consequently damaged by the vertical movement of the blades.
NHI says although the instances of damage are a problem, “they don't by themselves represent an immediate airworthiness issue”.
Australia’s defence department says that its MRH90s have twice experienced the problem, which resulted in “main rotor head damage”. In both cases the rotorcraft were embarked on Royal Australian Navy vessels – the HMAS Success in 2014 and in March this year during flight trials aboard the service’s new helicopter landing dock, HMAS Canberra.
“The second incident resulted in minor aircraft damage, which is currently under investigation,” it says, while noting that the 11t-class MRH90 continues to operate from other navy ships.
NHI says it is “redesigning the upper stops” with the modification currently due to be introduced during 2017, although it hopes to accelerate the process.
It adds: “NHI is supporting the operators to try to define a limited operational envelope allowing reasonably secured rotor start and stop.
“We are striving to define a more robust preventive measures to limit the occurrences as much as practicable.”
It additionally recommends inspections of the upper stops and main rotor head before the first flight of the day.
NHI is a three-way consortium comprising AgustaWestland, Airbus Helicopters and Fokker.