"They're under no illusions about the severity of the situation," AerCap CEO Aengus Kelly told Reuters on the sidelines of an Airline Economics conference in Dublin Monday. The door plug that blew out is present on most 737 Max 9 planes, in place of an exit airlines could have added if they wanted a greater number of seats. Trade publication The Air Current last week reported that the door plug on the affected Max 9 reached Boeing's factory from Spirit AeroSystems with bolts installed, but that the planemaker re-opened it to give access to nearby rivets that had been mis-installed by Spirit. Spirit and Boeing referred queries on the probe to the NTSB. The agency's chair Jennifer Homendy said on Jan. 18 it was too early to say if the root cause of the blowout was missing or mis-installed bolts.