Boeing is urging airlines to inspect 737 Max planes to look for a “possible loose bolt” in the rudder control system, the latest quality issue to affect the manufacturer’s bestselling jetliner.

The company recommended the inspections after “an international operator discovered a bolt with a missing nut while performing routine maintenance on a mechanism in the rudder-control linkage,” the Federal Aviation Administration said in a statement Thursday. “The company discovered an additional undelivered aircraft with a nut that was not properly tightened.”

The inspections will take about two hours per plane, and all new 737 Maxes will undergo the check before they’re handed over to customers, Boeing said.

“The issue identified on the particular airplane has been remedied,” Boeing said in a statement. “Out of an abundance of caution, we are recommending operators inspect their 737 MAX airplanes and inform us of any findings.”

Shares of Boeing were down more than 1% in afternoon trading.

Alaska Airlines plans to start the inspections on Thursday. A spokeswoman said the carrier anticipates completing them in the first half of January. “We don’t expect any operational impact as a result,” she said.

A spokeswoman for United Airlines, one of the biggest 737 Max customers, said the carrier doesn’t expect any impact to its operations as a result of the issue.

American Airlines said in a statement that it will complete the inspections and that it also doesn’t anticipate its operations to be impacted by them.