The union representing United Airlines' aircraft mechanics said on Tuesday the airline outsourced more than the agreed amount of work to external maintenance workers.

The airline exceeded its contractual limit for aircraft maintenance outsourcing by almost USD$500 million over 2005 and 2006, according to the Aircraft Mechanics Fraternal Association (AMFA), which had commissioned an audit.

The union, which is in arbitration with United Airlines over the issue, said the excessive amounts violate a clause of United's agreement with its mechanics.