Clarification: In an earlier version of this article concerning a Federal Aviation Administration's proposed penalty against Horizon Air, CNN used the term "pop rivets" as a generic description of the type of rivets used on the cockpit doors. POP is a registered trademark for a certain type of rivet and other products. The FAA did not specify a brand name for the rivets.
Washington (CNN) -- The Federal Aviation Administration has proposed a $1 million civil penalty against a regional airline for allegedly using improper rivets on cockpit doors, saying the rivets could have damaged wiring and other aircraft components.
The agency said Horizon Air operated 22 aircraft for more than three years with the improper rivets, making at least 186,189 flights while the planes were "unairworthy."
The FAA discovered the violations last year when Horizon modified a 23rd aircraft with blind rivets, and the plane experienced an incident during a flight because of wire damage, the agency said.
Horizon Air was required to use solid rivets on the doors, but instead used blind rivets, which can be fastened from one side of a panel or structure, "blind" to the opposite side.
"We expect airlines to comply with all of our safety regulations and to correct safety defects promptly," U.S. Transportation Secretary Ray LaHood said in a statement.
The FAA said the airline has replaced the blind rivets on its aircraft.
Horizon Air, which serves cities in the western U.S. and Canada, has 30 days to respond to the FAA's proposed penalty.
Bobbie Egan, a spokeswoman for Horizon Air, said the airline opted to use an "alternative and stronger" rivet, which Horizon believed was acceptable to the FAA.
"We believed at the time the alternative rivets were an acceptable substitution. At no time was passenger safety compromised or the integrity of the cockpit door affected," she said.