The warning follows the July 4 crash of an R44 in Broome, Western Australia.
Pilots of Robinson R44s should land as soon as possible if they sense unusual vibrations through the tail rotor pedals, the Australian Transport Safety Bureau (ATSB) says.
U.S. government-industry coalition says new efforts are needed to prevent crashes that follow inadvertent entry into IMC.
The NTSB says flight into IMC led to the California accident that killed 9.
The U.S. NTSB is recommending that all turbine helicopters should be equipped with crash-resistant flight recording systems.
