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.

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.

Worries about the plane flared up again after a door plug blew off a Max operated by Alaska Airlines, leading regulators to cap Boeing’s production at 38 jets per month. The NTSB plans to meet next Tuesday to discuss what investigators found about that incident.