The NTSB issued an “urgent safety recommendation” related to the engine on the 737 MAX after two incidents where smoke entered the cabin.

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 has raised serious concerns over CFM International LEAP-1B engines used to power Boeing 737 MAXs after incidents where smoke entered the cockpit.