The Air Force’s overall aviation mishap rate has hit a seven-year high, fueled by a growing number of non-fatal “Class C” mishaps, which experts say is an ominous warning sign.

While the major mishaps that result in deaths and cost millions in damages, known as “Class A” mishaps, are ticking downward for the Air Force, the fleet is reporting a rise in the less-severe accidents that cause more modest damage and injuries.

And that may be a potential “canary in a coal mine,” foreshadowing a future swelling of more serious mishaps, said Todd Harrison, director of the Aerospace Security Project at the Center for Strategic and International Studies.

“That’s your [early] warning that there’s a problem and you need to do something before something bad happens,” Harrison said.