When an engine problem causes an AOG (aircraft-on-ground) situation, the operator faces the cost of rebooking passengers or cargo on other flights while the aircraft sits idle, awaiting repairs. 

“Many field repair events, in our experience, can be traced back to the engine’s last shop visit—particularly with the internal components such as high-pressure compressor (HPC) blades, high-pressure turbine (HPT) blades and nozzle guide vanes,” observes Fergal Whelan-Porter, CEO of Aeolus Engine Services, a global engine line services and technical support company in Dublin. “On the CFM56-3, for example, we have seen cases of insufficient workscopes because it is late in its life cycle, and the operator sees a short time horizon.”

To guarantee “good time on-wing,” operators need to perform sufficient performance restoration repairs in the HPC and HPT modules during off-wing maintenance to match life-limited parts’ allowance, Whelan-Porter explains. He also reports that line replaceable units (LRU) such as fan blades, sensors, main engine controls and fuel nozzles are among the more common items prone to failure, often the result of high time on wing and/or faulty installations.

Fortunately, new developments with materials and predictive data technologies are helping to reduce field repair events, which can keep an aircraft out of service for days at a time.

“At Standard Aero, we are just beginning to incorporate digital inspection tooling with a focus on laser measurement inspections of critical airfoil components,” says Brent Ostermann, the MRO’s vice president of engineering, in Winnipeg. “Using digital technology to create an image of a compressor or fan blade reveals an image of its in-service condition compared to that of a brand-new unit, providing a baseline as to when to refurbish the airfoil. And by applying automated blending, we can restore the airfoil back to its designed condition to assure the engine will have excellent performance in the field.” 

Automated blending, he explains, uses robotics and computer numerical control (CNC) programming to perform precision machining or blending of airfoils to replicate new conditions following in-service erosion. “It removes the variance of humans performing the same task,” says Ostermann.

Pratt & Whitney also is expanding its digital capabilities with the goal of minimizing unexpected MRO events, according to William Cermignani, the engine OEM’s executive director of Global Services Engineering. 

“We are working to make engine maintenance events highly predictive and move the focus from reactive to proactive maintenance,” he says. “The tools we have developed proactively look at these types of events and give us an early warning on potential fleet trends. We expect further growth and integration of our predictive analytics capabilities.”

In that regard, Cermignani reports Pratt & Whitney is using state-of-the-art data acquisition systems, analytics and real-time intelligence through its EngineWise suite of services to monitor the health of its engines in order to predict and prevent engine disruptions before they occur. As an example, he cites the geared turbofan (GTF) engine.

“The GTF incorporates 40% more sensors than the [legacy] V2500 engine and can generate approximately 4 million data points per engine, per flight,” he explains. “That provides significant improvements in maintenance through our EngineWise program to mitigate disruption to our customers.”

Pratt & Whitney, Cermignani points out, also is leveraging data generated by advanced diagnostics and engine management (ADEM) and enhanced flight data acquisition storage and transmission (eFAST) to “reduce operator maintenance burden,” such as borescoping, and connect the trend analytics to remote on-wing and near-wing maintenance, enabling faster and more cost-effective maintenance options.

 He says “the expansion of component inspection limits is an effective way” to extend time on wing and reduce engine operating costs. “Our latest initiatives to collect feature-specific component distress data allow us to understand progression rates and effectively extend on-wing limits,” he adds. 

Sylvia Stuenkel, director of on-site services at MTU Maintenance in Germany, says the MRO uses an engine trend monitoring system, which culls engine data from flight operations to help prevent engine failures. “We currently work with smaller, more specific data sets directly from operations, rather than large volumes of big data,” she explains. “However, we are working on the integration of continuous engine operational data.”

MTU Maintenance’s engine trend monitoring system data includes remaining on-wing time prediction, based on critical performance parameters such as exhaust gas temperature margins, automatic diagnostics that identify the root cause of any trend shift and a quick fleet analysis tool to review on-wing deterioration per engine serial number and shop visit effects.

“Those features improve our ability to proactively manage fleets and recommend courses of action to customers that prolong time on wing and prevent secondary and serious damage from occurring when trend shifts are spotted. This all saves cost and reduces potential for AOGs for customers,” she says.

At the same time, she explains, engine trend monitoring itself is undergoing changes. The “traditional approach” to engine trend monitoring, she says, is to identify long-term trend changes that enable the aircraft to transmit “single snapshot reports” in different flight modes, producing only a small amount of data—about 10 kilobytes per report. “But newer technology developments use continuous data from the whole flight that includes snapshots from each second,” says Stuenkel. “That data is still below 1 gigabyte but are already exceeding the current inflight data transmission capabilities and are therefore [downloading the data] via a wireless groundlink quick access recorder [WQAR] after arrival.”

While data volume will likely continue to increase, Stuenkel notes that it is too expensive to transmit data for the whole flight in real time via the aircraft communications addressing and reporting system (ACARS). “But new technologies might help to reduce the costs to a level that makes real-time transmission a reality,” she remarks.

Stuenkel says data format, transmission capabilities and frequencies for each aircraft are not universal, as products from hardware and software producers and update levels vary. “Amassing the data to feed the databases and create enough data points to establish patterns across engines, regions and operations is also a challenge,” she says..” 

Alyson Thomas, director of fleet support at GE Aviation, says the engine OEM’s fleet support team continues to build and evolve detection analytics to improve detection rates of anomalies in the snapshot data.

“Customers send engine performance data that is referred to as ‘snapshot data’—discreet data points transmitted in real time during set points of the flight, such as takeoff, climb and cruise,” Thomas explains. “The outputs of these analytics go into ‘customer notification reports,’ in which we make recommendations to the customer regarding preventative inspection or maintenance actions.”

Thierry Chabroux, engine product director for Air France Industries KLM Engineering & Maintenance (AFI KLM E&M) says that since putting its Prognos solution in place in 2017, the MRO has implemented “very powerful analytics” with the current available data. “With even more data tomorrow, it is clear that even stronger analytics will be built,” he predicts.

Prognos is a multipurpose predictive maintenance solution service applicable to all engines as well as auxiliary power units. The system, which transmits data in real time from the aircraft via Wi-Fi or 4G, provides short- and long-term trend monitoring, hosted and controlled by AFI KLM E&M. The goal is to detect an unscheduled maintenance event at least several flights before the operator might get an alert from the OEM. AFI KLM E&M says this yields greater forecasting accuracy, as the service is tailored to the individual airline’s operations and fleet dynamics rather than worldwide fleet models used by the OEMs for data analysis.

But preventing engine failures is not just a matter of interpreting big data. It also comes down to materials and repair methods. “There is a lot of focus now on addressing hot section problems, mainly through the use of better coatings technologies and stronger materials,” says Standard Aero’s Ostermann. “For example, there are more applications of ceramic coatings because they last longer than vapor-phased aluminized coatings.”

Ostermann says Standard Aero is adopting “cold spray,” a repair method using metal powders that are accelerated onto the part using high-velocity inert gas without damaging the part. Standard Aero will start using this technology in the second quarter of this year. “Cold spray maintains the same strength characteristics as welding but without the high heat that leads to distortion of the metal surface,” he explains.

Stuenkel notes that MTU uses a CMAS (calcium-magnesium-aluminum silicates) resistant thermal barrier coating and its ERCoateco (erosion-resistant coating for HPC airfoils), which reduces scrap rates, improves the durability of hardware and reduces the specific fuel consumption of the engines. “Such repairs are a very cost-effective way to help operators combat high material costs and considerably increase engine on-wing times. These repairs are particularly beneficial to customers flying in harsh environments.” 

AFI KLM E&M’s Chabroux cites ceramic matrix composites among the technologies that have improved engine reliability. “They can withstand very high temperatures without any cooling system,” he says. “But it is premature to say how the new technologies will [work out] over time, since we only have very limited hindsight.”