Boeing says it is working with engine maker CFM International and the fuel supply chain to resolve a rare but potentially serious thrust instability observed in the CFM56-7B-powered Boeing 737NG fleet.

While the problem typically affects one engine during climb and is resolved by slowly throttling back until the engine stabilizes, there have been two cases where both engines simultaneously experienced a thrust instability event (TIE) .

“In one case, both engines regained normal control and performance, and the airplane safely returned to base,” Boeing tells Aviation Week. “In the other case, one engine’s thrust did not respond properly, but the other engine fully recovered, and the airplane returned to base without further incident.”

The airfamer did not disclose which airlines experienced the dual-engine anomalies but says the dual-engine incidents took place in August and November of 2012.

In total, Boeing says operators of the CFM56-7B-powered twinjets have reported 32 TIEs since 2008, but the rate of incidents increased in 2011. CFM referred questions about the issue back to Boeing.

Boeing describes the problem as a “momentary oscillation” of the fuel control valve in high fuel flow situations, such as climbing to cruise altitude, for up to 10 sec., after which engine operation “typically returns to normal.” The company says pilots in some cases have continued to their destinations after determining the engine was operating normally.

“These events are rare and brief, and the level of overspeed is well within the demonstrated capability of the engine,” says Boeing, adding that the 737NG fleet operates about 16,000 flights per day and have accumulated more than 23 million flights since the first incident was reported in January 2008.

Boeing has issued a Flight Operations bulletin to airlines advising pilots to use the existing quick reference handbook (QRH) procedures for engine surge or compressor stall events.

The QRH instructs pilots to disengage the autothrottles and decrease the thrust levers until abnormal engine noises or engine performance indications (rpm and exhaust gas temperatures) return to normal levels, after which the power can be increased. If noises or performance do not stabilize as the power is decreased, pilots are instructed to fully retard the thrust lever, shut down the engine and land “at the nearest suitable airport,” using the one engine inoperative checklist.

At least one TIE occurred at Southwest Airlines, the largest U.S. operator of 737NG aircraft. According to a memo issued to crews in January, one crew experienced a TIE during cruise flight and was able to restart the engine after its performance stabilized at sub-idle speed as they shut it down.

Boeing says it has made a software change to the CFM56-7B electronic engine control module, part of the full authority digital engine controller, to lessen the chances of the problem occurring.

“We are doing a root cause analysis though because it’s not just [the] fuel control unit,” says Boeing. “We are working with CFM and with the fuel supply chain to understand the issue and build a fix. It might be fuel-based. It’s one of the areas we are looking at.”

Boeing says there have not been any new incidents since the software change.