A United Airlines Boeing 737-900ER arrives at Los Angeles International Airport in 2019. Mario Tama/Getty Images hide caption toggle caption Mario Tama/Getty Images A United Airlines Boeing 737-900ER arrives at Los Angeles International Airport in 2019. Mario Tama/Getty Images The Federal Aviation Administration is recommending that airlines visually inspect the door plugs of more Boeing planes after a similar panel blew off a jet in midair earlier this month. The safety alert issued late Sunday recommends that airlines operating Boeing’s 737-900ER jets inspect the door plugs “as soon as possible” to make sure they’re properly secured after some airlines reported unspecified issues with the bolts. The 737-900ER is not part of Boeing’s newer Max series, but it has the same optional door plug design as the Boeing 737 Max 9, according to the FAA. More than 170 of the newer jets have been grounded since Jan. 5, when a door plug blew off a 737 Max 9 plane operated by Alaska Airlines. That plane had only been flying for a few months, according to investigators …