Jan 8 Reuters Boeing shares fell 8.5 in U.S. premarket trading on Monday following the temporary grounding of some of its bestselling 737 MAX jets by the U.S. aviation regulator.

A piece of fuselage tore off an Alaska Airlines 737 MAX 9 jet on Friday following takeoff from Portland, Oregon forcing pilots to turn back. The U.S. Federal Aviation Administration FAA subsequently ordered the temporary grounding of some narrowbody MAX 9 jets.

Alaska Air39;s shares were down 4.3, while United Airlines, the other U.S. carrier that operates the jet, was down 2.4.

Spirit AeroSystems, which manufactured and initially installed the fuselage part on the brand new MAX 9 jet in question, was down 20.8, deepening the gloom around the supplier recently recovering from a string of quality problems.

Wall Street analysts viewed the accident as a temporary setback to Boeing, but some took a dim view of a series of quality problems related to the 737 MAX family of aircraft.

It highlights a history of quality escape problems, particularly at Spirit AeroSystems. Quality escapes are not acceptable in an industry in which single failures can have serious consequences, Bernstein analysts said.

Boeingrival Airbus39; shares were up 2 on Monday. The European planemaker has expanded its market share since two Boeing MAX crashes in 2018 and 2019 that killed nearly 350 people and led to the MAX39;s worldwide grounding for 20 months.

Airbus will announce that it delivered 735 planes last year,…