Flying on a Boeing 737 Max 9? Here's what to know.

For the first time since Boeing 737 Max 9 jetliners were grounded after a mid-air blowout earlier this month, the aircraft are again carrying passengers — a prospect that might prompt questions among some travelers.

Alaska Airlines resumed a limited number of flights with its Max 9s on Friday. United aims to follow suit on Sunday, but a spokeswoman said the airline might use them as spare planes Friday or Saturday.

Those are the only two U.S. airlines that operate this particular model of the Boeing 737, which gained widespread attention earlier this month when a door plug blew out minutes after takeoff, exposing passengers to a gaping hole.