The Australian Transport Safety Bureau (ATSB) has raised questions about the use of the Robinson R22 for aerial cattle-mustering operations.

An ATSB study found that large and sudden power changes involved in aerial mustering apply very high loads on the helicopter's drive system, which may exceed the limits set during the type's certification process.

The R22 is the most common model of rotary-wing aircraft on the Australian register and is widely used for aerial stock mustering, which is a uniquely Australian application, with the R22 flying over 67% of all the aerial mustering hours in the country.

Although the type has a relatively good safety record compared with other light piston-engine helicopters in Australia based on activity levels, the spectrum of manoeuvres involved in aerial mustering did not form part of the flight profile used in the helicopter's certification process, says the ATSB, which was concerned that use for aerial mustering could have adverse effects on the structural integrity of the helicopter.

In particular, the ATSB is seeking assurance from the manufacturer that no components in the rotor drive train are adversely affected or life limited under the high amplitude torsional loading measured during aerial mustering, which involves rapid power changes at low airspeeds.

The manufacturer says that although aerial mustering causes more damage to the tail rotor drive shaft (TRDS) than had been allowed for in the original certification usage spectrum, the additional damage was not severe enough to require the TRDS be listed as a life-limited component.

The bureau commissioned Australian engineering company AeroStructures to study the forces acting on a R22 engaged in aerial mustering. AeroStructures installed Altair Avionics' MaxLife aircraft usage monitoring system on a single R22, supplied by Kununurra, Western Australia-based Heliwork, and monitored the helicopter through a cattle-mustering season.

The helicopter operated for 26 weeks, with 350h of data recorded. MaxLife was then fitted to a strain-gauged R22 belonging to the helicopter manufacturer and typical aerial mustering manoeuvres were performed.

AeroStructures concluded that helicopter usage in cattle mustering is significantly different to certification usage, both in frequency and type of manoeuvres. Aerial mustering involves a significantly higher percentage of low speed manoeuvres than specified for the helicopter's certification.