The National Transportation Safety Board issued an urgent safety recommendation Wednesday to address the possibility of smoke entering the cockpit or cabin of Boeing 737 MAX airplanes equipped with CFM International LEAP-1B engines.

US safety investigators called for urgent action to be taken in response to an engine issue on Boeing Co. 737 Max aircraft that can lead to smoke in the cockpit or cabin.The National Transportation Safety Board’s recommendation comes after at least two incidents in which planes were filled with smoke after large birds struck their engines.

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.