Boeing alerted airlines Tuesday that 737 Next Generation aircraft flying on the West Coast have experienced intermittent and relatively rare instances of engine instability while climbing after takeoff.
The airline most affected was Alaska Airlines with 17 incidents, Boeing said Wednesday in response to questions.
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.
In one case, both engines on a jet were affected simultaneously. One engine recovered; the other had to be shut down. The plane turned back and landed safely.
Boeing's message said 32 such engine-instability events have happened in the past five years on the widely used single-aisle jets.
In addition to Alaska Airlines, another airline had 14 incidents and a third airline had one. Boeing said Alaska did not experience any engine shutdowns.
According to a source with knowledge of the problem, fuel dispensed at Seattle-Tacoma International Airport has been one focus of the investigation.
Boeing declined to identify the two other affected airlines and said the incidents included flights out of several different airports.
Alaska Airlines declined to comment and referred questions to Boeing.
In each of the 32 events, according to the Boeing message, the pilots observed sudden oscillations in the engine speed and in the fuel meter valve that controls the flow of jet fuel to the engine.
In most cases, a fault code displaying on the instrument panel indicated a malfunction of the fuel meter valve.
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."
On a flight last August and on another in November, both engines on a single jet were affected.
In one case, the engine instability in the two engines occurred one after the other and each engine recovered normal thrust within moments.
Nevertheless, the pilot returned to the airport.
In the second case, when both engines were affected simultaneously and one engine had to be shut down, again the pilot turned back.
Those incidents heightened the level of concern within Boeing, said spokesman Mike Tull.
"We've increased the focus on it since last year," he said.
Under scrutiny is not only the fuel, but also the fuel control unit, supplied by Honeywell, and the engine itself, supplied by CFM International.
"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."
Boeing already has taken some interim steps as it seeks the root cause of the problem.
Engine supplier CFM in December recommended a software update to enhance throttle control. There have been no incidents since that update, Tull said.
Boeing in December also issued a bulletin to flight crews flying in the Western U.S. that re-emphasized existing instructions to pilots to reduce the throttle smoothly and continuously after takeoff.
Tull said Next Generation 737s make about 16,000 flights per day worldwide and have flown 23 million flights since the first of these engine-instability incidents was recorded in 2008.
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.
If the engine recovers and operates normally after one of these engine-instability events, the pilot can decide whether to turn back or to continue to the destination.
Boeing said that in approximately half of the 32 incidents, the pilots turned back.
After each of these events, the airplanes are not flown again until the engine is inspected and the fuel control unit replaced.
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.