After an Alaska Airlines Boeing 737 Max 9 jetliner’s fuselage tore off midair on Friday and caused an emergency landing in Portland, Ore., the Federal Aviation Administration ordered the temporary grounding of some Max 9 planes until they are thoroughly inspected.

Hundreds of flights operated by Alaska Airlines and United Airlines were delayed or canceled on Saturday. It’s unclear how the grounding and ongoing inspections will impact flights in the coming days as these and other airlines grapple with concerns over a workhorse aircraft.

The Max, which comes in four variants, numbered seven through 10, is the most popular plane in Boeing’s history, accounting for a fifth of all orders placed since 1955, company data shows.

Here’s what passengers should know about the Max 9 airplane, how airlines are responding to the grounding, and how to navigate any upcoming flight delays or cancellations.

Which airlines fly the Max 9 aircraft?

About 215 Boeing Max 9 airplanes are currently in service globally, according to Cirium, an aviation data provider. United Airlines operates 79, the most of any airline, and Alaska has 65 — their combined fleets represent about 70 percent of the jets in service.

Other operators relying on the Max 9 include Panama’s Copa Airlines, Aeromexico, Turkish Airlines, FlyDubai and Iceland Air.