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 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.