WHAT TO KNOW ABOUT THE BLOWOUT ON A BOEING 737 MAX 9 JET AND WHY MOST OF THE PLANES ARE GROUNDED

An emergency landing by an Alaska Airlines jetliner last week prompted U.S. authorities to ground most of Boeing’s 737 Max 9 aircraft, another black mark in the troubled history of the company’s Max jets. Here’s what you need to know.

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WHAT WENT WRONG?

U.S. aviation authorities have begun an investigation focused on a paneled-over exit door — called a door plug — that blew off the passenger jet shortly after takeoff. Airlines have the option to install a door plug in place of an actual emergency exit door. While Boeing’s Max aircraft have been plagued with problems, no previous incidents have involved blowouts such as this one, which are exceedingly rare in air travel. None of the passengers or crew members on the flight were seriously injured.