The average age of small piston-engine planes flying in Australia is now more than 30 years, making them harder to maintain and raising questions about safety.

Research by air safety investigators found that the average age of smaller piston-engine aircraft in Australia had risen 10 years in the decade from 1995 and warned that this could mean some planes were no longer supported by the companies that made them.

On Feb. 4, the pilot of a twin-engine Commanche aircraft was killed when it plunged into the sea off northern New South Wales after what was reportedly a double engine failure. And two men were seriously injured when their Cirrus SR22 light aircraft crashed in a paddock next to the M7 motorway in western Sydney after sending an emergency message on Feb. 6.

Aviation authorities have worried about the ageing small aircraft fleet for some time.

On Feb. 6, Australian Transport Safety Bureau research released identified smaller multi-engine aircraft used in charter and public transport operations as Australia's oldest.

The average age in this category in 2005 was 31 years - an increase of 10 years from 1995 - indicating operators were buying very few new aircraft.

"These airlines often operate with low profit margins that limit their capacity to acquire new or newer aircraft," the report said.

"Operators are therefore left with the option of maintaining ageing aircraft with only limited continuing airworthiness support from the manufacturer."

A Civil Aviation Safety Authority spokeswoman said the regulator continually monitored the airworthiness of all aircraft.

"We take proactive steps such as researching defects, issuing directives and auditing maintenance programs to ensure the ongoing safety of all aircraft," she said.