Later this year, CFM International will make available cleaning carts that will allow operators to remove build-ups of deposited carbon from the fuel nozzles of their Leap engines while the engines and fuel nozzles remain on-wing.

CFM is also studying whether modifying the Leap’s engine control software could reduce such carbon build-ups, CEO Gaël Méheust told AIN. These deposits—which according to CFM can affect any jet engine, in a phenomenon known throughout the aerospace industry as "coking"—can create performance issues that require pilots to shut down engines in flight. Coking “is not unique to the Leap engine: it’s a phenomenon as old as aircraft engines themselves,” said Méheust.

However, one reason CFM is paying so much attention at present to potential coking in Leap engines is that fuel-nozzle coking played a role in a highly publicized incident in which Southwest Airlines pilots flying a Boeing 737 Max 8 on March 26 from Orlando to Victorville for storage had to shut down one of the 737’s Leap-1B engines in flight and bring the aircraft back to Orlando, where it has remained throughout the worldwide 737 Max grounding. Southwest inspected that engine and 12 other Leap-1Bs installed on its 737 Maxes and replaced on-wing the engine involved in the incident and several others. The airline said later that the incident had happened because fuel-nozzle coking had created several “hot spots" in the engine, which in turn affected some of its turbine blades.

In parallel, CFM inspected 13 other Leap engines worldwide “out of an abundance of caution, to make sure there was not the potential for another issue,” said Méheust. It then began an investigation into fuel-nozzle coking that he said will continue until CFM feels it has fully understood the phenomenon as it applies to the Leap. Additionally, CFM is continuing to look into the Southwest incident, in which it believes factors other than coking may also have been involved. “We’re still working on the order of events, what happened first” to result in the Southwest in-flight shutdown, he said.

Causes of Coking

Although its Leap fuel-nozzle coking investigation continues, CFM knows what the root cause of coking is for its own and others’ jet engines. “The [carbon] build-up tends to occur post-shutdown,” after an engine is shut down upon completing a flight, said Méheust. Various factors and environmental conditions can contribute to coking happening, “including the temperature of the engine at shutdown.” The phenomenon is “not about the nozzle [design] itself”: it is caused by temperature-induced evaporation of unburned fuel, which leads to hard deposits of solid carbon being laid down in some parts of the engine, particularly the fuel nozzles spraying fuel into the combustor.

Méheust added that coking “occurs over many cycles,” a fact that is helping CFM to detect a gradual carbon build-up in any given engine and alert the operator before the coking results in an urgent problem. “Ongoing monitoring allows us to detect it early and recommend replacement or cleaning” of affected Leap fuel nozzles, he said. “It’s no big deal; we have all the capabilities to monitor it, detect it in advance, and provide spare parts” in a timely fashion, “via a rotable pool. Nozzles can be changed on-wing overnight. And we’ll have techniques to clean fuel nozzles available pretty soon.” At present, any fuel nozzles found with carbon build-up are swapped out on-wing overnight, sent to CFM to be cleaned ultrasonically, and subsequently returned to the relevant operators.

To prevent Leap fuel-nozzle coking from remaining an issue, “we just need to understand exactly the conditions that are happening” during engine shutdown that promote carbon build-ups, said Méheust. This research is underway and it will inform CFM in deciding if the company “might be able to modify or upgrade the [engine control] software for shutdown,” he said. In particular, CFM reckons it “might be able to motor the engine a little differently” during shutdown to eliminate conditions favoring carbon build-up.

Despite its investigation into Leap fuel-nozzle coking, CFM is pleased with the Leap engine’s overall reliability since it entered service. After 5 million flight hours of commercial service (a milestone the Leap family recently reached), so reliable are the in-service Leap-1A and Leap-1B versions proving that even early in its program life the new family is demonstrating a higher utilization rate than its direct predecessor, the CFM56 family, said Méheust.

When CFM launched the Leap program in 2008, “an internal goal was to bring in a brand-new engine to replace a very mature engine” with the new engine demonstrating from the outset reliability as high as that of the mature one, said Méheust. For the on-wing Leap population, “We see consistently a utilization rate of 96 percent, two or three percent better than CFM56-powered airplanes,” he said. “With a little bit of [additional] maintenance burden” that has arisen from early technical issues for which fixes are already or soon will be available, “we have been able to secure an even higher utilization rate than [the] CFM56. We have delivered that goal.”

The average Leap daily utilization rate is now “close to 10 hours a day,” and the additional maintenance burden arising from technical issues in Leap operations has “reduced by 50 percent from where we were a year ago,” said Méheust. One of three early technical teething problems affecting Leap engines has been the need to replace some fuel hoses prematurely, “after 1,200 hours,” according to Méheust. In response, CFM has developed a new fuel tube as a line replaceable unit and will make it available to all operators from “very early in the third quarter.”