By Joanna Plucinska, Abhijith Ganapavaram and Tim Hepher

(Reuters) -Boeing shares plunged to close 8% lower on Monday following the temporary grounding of some of its best-selling 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 171 narrowbody MAX 9 jets with a similar configuration.

Wall Street analysts on Monday viewed the accident as a temporary setback to Boeing, but some took a dim view of the incident as another in a series of quality problems related to the 737 MAX family of aircraft. The fuselage is produced by Spirit AeroSystems, which separated from Boeing years ago.

"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 wrote.

Spirit manufactured and initially installed the fuselage part on the brand new MAX 9 jet in question. Its shares settled 11.1% lower at $28.20.

Airline shares initially fell, but had since recovered, with Alaska Airlines settling down marginally, while United Airlines, the other U.S. carrier that operates the jet, ended 2.7% higher.

Airbus' shares closed 2.5% up 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 MAX's worldwide grounding for 20 months.

Airbus will announce that it delivered 735 planes last year, beating Boeing to remain the world's largest planemaker for the fifth year in a row, industry sources said. Reuters last week reported that the number would reach the mid-730s, beating the company's target of 720.

MANUFACTURING OR DESIGN ISSUE?

Some analysts said the problem appeared to be a one-off manufacturing issue, rather than a design issue that is more costly to fix. They also noted the number of aircraft affected was small.

"A serial manufacturing issue could require a design or manufacturing change for Boeing or the responsible supplier, but we would not expect an outsized cost," Melius Research analyst Robert Spingarn said.

Boeing has delivered 214 of the 737 MAX 9 jets, or 16% of the more than 1,300 MAX aircraft in service, most of which can still fly, including 737 MAX 9 jets with ordinary doors instead of the replacement panels.

Some investors saw Monday's dip as a buying opportunity.