


Fuel contamination is the leading suspect in the problems, which Boeing's message described as "uncommanded engine oscillations."

In most cases the instability disappeared after "a few seconds to a couple of minutes," but two incidents last year suggest the potential for a more serious problem.






Alaska Airlines declined to comment and referred questions to Boeing.




Engineers haven't yet pinned down the cause, although Boeing suspects it may be something in the jet fuel.

"These events are highly localized phenomena. All events have occurred on airplanes that were recently routed through western United States airports," Boeing's message to airlines said. "The investigation has narrowed the leading root cause to intermittent fuel contamination."



Nevertheless, the pilot returned to the airport.



"We've increased the focus on it since last year," he said.


"The engine, the fuel control unit, the fuel, it's all part of a system," said Tull. "We're looking at the whole system."

"These are extremely rare events, but clearly we don't want one to happen again," Tull added. "We're focused on finding a fix."





During the period when Alaska Airlines experienced its 17 incidents--between Jan. 1, 2008, and Nov. 30, 2012 -- the airline flew 737s on 378,674 flights.

That's less than one incident per 23,000 flights.




Boeing's message said it is working closely on the problem with "the fuel supply chain" and has notified the Federal Aviation Administration.

In such situations where a large number of airplanes are potentially affected, the FAA typically follows up with an airworthiness directive to make any Boeing recommendations mandatory.