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 have warned over a safety issue with a vibration-reduction feature on the CFM International Leap-1B turbofans that power the Boeing 737 Max which can cause smoke to enter the cockpit.

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