The Federal Aviation Administration (FAA) issued an emergency inspection order for all operators with 737’s, Thursday.

The emergency inspection follows four recent cases of engine shutdown issues. The FAA says the problem is so severe it could result in a “forced off-airport landing.”

The order requires operators who have stored 737 aircraft for more than a week to inspect critical engine valves, stating:

“Corrosion of these valves on both engines could result in a dual-engine power loss without the ability to restart. This condition, if not addressed, could result in compressor stalls and dual-engine power loss without the ability to restart, which could result in a forced off-airport landing.”

The 737 is one of the most popular aircraft in service around the world.