The National Transportation Safety Board issued a rare, urgent safety bulletin warning of potential problems with the jet engines on Boeing 737 Max aircraft. The recommendation was inspired by two incidents on Southwest Airlines flights that hit birds.

The NTSB issued an urgent warning on Wednesday about the potential for certain jet engines that, when damaged, allow smoke to enter the cockpit or cabin. The engines in question were used on Boeing 737 Max jets and on certain Airbus planes.

Safety experts recommend that the engines on Boeing’s troubled 737 Max airplanes and some Airbus models be modified quickly to prevent smoke from filling the cockpit or cabin after a safety feature is activated following a bird strike.