LIHU'E, Kaua'i — One of the helicopters destroyed in a fatal crash on Kaua'i within the past five days was 28 years old and the other was 20 years old. But aircraft experts say their effective age is much younger because parts are regularly replaced as they approach the end of the service life calculated by manufacturers.

"Parts have life cycles to them, and owners have to replace parts at certain intervals," said Ian Gregor, Federal Aviation Administration spokesman for the western-Pacific region.

What that means is that while an aircraft may originally have been built a long time ago, very few of its working parts are that old. Engines, electronics, rotors and thousands of other parts are replaced at regular intervals, generally based on how many hours the aircraft is flown. Replacements are made to get a part out of the aircraft long before engineers expect it to be subject to failure from wear or metal fatigue, he said.

"All these things are time-limited, and they go through regular changes," said National Transportation Safety Board investigator Brian Rayner, who is the lead investigator of Thursday's crash of a Heli USA A-Star aircraft.

Gregor said there are examples of antique aircraft that still fly safely because of the regular updating of parts.

"Out where I live in Long Beach, there is a company that operates a fleet of DC-3s that are 60 or 70 years old. The age of the aircraft has nothing to do with its airworthiness if an aircraft is properly maintained," Gregor said.

The Hughes 500 helicopter that crashed Sunday, killing one passenger, was built in 1987. The A-Star AS350BA that crashed Thursday was built in 1979.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.