Horizon Air (QX, Seattle Tacoma International) and SkyWest Airlines (OO, Salt Lake City) are set to be fined USD390,000 and USD450,000 respectively by the US Federal Aviation Administration (FAA) for operating aircraft deemed non-compliant with US Federal Aviation Regulations.

Concerning Horizon Air, the FAA alleges that between 2012 and 2013, the Alaska Airlines (AS, Seattle Tacoma International) subsidiary operated six Dash 8-400s on more than 2,600 flights with transponders that the airline had neither tested nor inspected within that timeframe.

"The agency alleges the aircraft were not in airworthy conditions during those flights," the regulator said.

Concerning Skywest, the FAA alleges the carrier failed to perform proper maintenance on air conditioning units in three Canadair regional jets and on the auxiliary generator in one E120 despite pilot reports of problems with those systems.

The FAA alleges that SkyWest operated the four aircraft on a total of 15 flights when they were not in an airworthy condition. In one instance, one of the CRJ200s depressurized in mid flight resulting in the deployment of passenger oxygen masks and an emergency descent, the agency alleges.

Both carriers have 30 days from the receipt of the FAA's enforcement letter to respond to the agency.