A pathological liar who faked his way into an aircraft engineer's job with Qantas has been sentenced to a maximum three years and five months in prison.

Timothy Leslie McCormack posed as a licensed aircraft engineer for almost 10 months before his deception was discovered in July 2007.

By then, he had conducted 30 maintenance checks on planes leaving Sydney, putting the lives of some 12,000 passengers at risk.

On the day his trial was due to begin in September this year, the 27-year-old pleaded guilty to 42 charges, including forging a maintenance engineer's license.

Even after that, his deceptions continued — McCormack faked four character references he tendered to the court before his sentencing hearing.

His defense lawyer Sydney Jacobs has argued McCormack is a pathological liar.

Handing down sentence in the New South Wales District Court Tuesday, Judge Mark Marien said while he acknowledged McCormack had some impairment in controlling his lies, he must be sentenced severely as a deterrent for future offenders.

"Not only was the degree of the offender's course of conduct extreme, but the offender conceded ... that the potential consequences of that course of conduct might have been catastrophic," he said.

McCormack will be eligible for parole in two years.

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