Aviation regulators on Thursday proposed their second fine in a week against American Airlines, a unit of AMR Corp
The latest civil penalty sought by the Federal Aviation Administration (FAA) totaled $300,000.
The agency alleged on Thursday that American mechanics misdiagnosed a problem with the heater on an MD-82 air speed sensor in 2009 that should have restricted when the plane could fly.
Instead, the work was deferred and the aircraft flew five regularly scheduled flights before the mistake was corrected.
Last week, the FAA proposed a $787,500 fine against American for three alleged maintenance violations that occurred in 2008 and 2009.
The case involved six planes, two of which the FAA said were operated on flights even though the airline said it would not do so until outstanding work was completed.
American said it was reviewing both fines and planned to meet with FAA officials. Airlines can appeal proposed civil penalties.
(Reporting by John Crawley, editing by Maureen Bavdek)