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 safety recommendation” related to the engine on the 737 MAX after two incidents where smoke entered the cabin.

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.