Extreme testing: how sand and dust forge tougher jet engines

In order to continually improve durability and reliability, engineers must devise the harshest, most rigorous testing conditions for aircraft engines. How do the ultimate laboratory trials make a difference?

Aircraft engines endure some of the most punishing conditions imaginable. From the fine, windblown sand of Middle Eastern deserts to frozen expanses of Arctic airspace, modern jet engines must perform reliably across extremes that would cripple lesser machines. A single engine can experience temperature swings from -60C at cruise altitude to more than 1,600C in its combustion chamber, all while ingesting whatever particles happen to be suspended in the air around it.

“What is more, aircraft operate an average of seven or eight cycles [flights] per day,” says Gaël Méheust, President and Chief Executive of aircraft engine-maker CFM International. “And many of those flights have 30-minute turn times, defined as the time from the aircraft arriving at the gate to the time it pushes back for its next flight.” Testing, in which manufacturers recreate extreme conditions, informs engine designs that keep these sophisticated machines in the air on demanding commercial schedules.

Commercial aircraft routinely operate in environments that test their limits. Desert routes expose engines to abrasive particles, while tropical regions experience torrential rain and high humidity. Northern latitudes bring ice and hail, and coastal operations introduce salt-laden air that corrodes components over time. 

“Before aircraft engines can enter service, manufacturers must verify that they can withstand extreme challenges,” says Mark Ricklick, Associate Professor of Aerospace Engineering at Embry-Riddle Aeronautical University. “Specialized testing facilities around the world simulate nature’s extremes.” These range from monsoon-level rainstorms on the ground to icing tests at altitude on purpose-fitted flying test beds.

Wind tunnels, some drawing up to 88MW of power, can generate conditions across the entire range of speed that an aircraft or engine will encounter, from low-speed take-off to supersonic flight. For example, the Boeing Transonic Wind Tunnel in Seattle operates at supersonic speeds. ONERA’s wind tunnels in the French Alps are capable of speeds up to the hypersonic regime (five times the speed of sound). At the other extreme, German-Dutch Wind Tunnels’ facilities at Marknesse in the Netherlands specialize in the low-speed take-off and landing phases of flight. 

Airborne particles and pixie dust

It’s not just airflow that gets tested in such facilities – they also look at how engines interact with particles in the air. Methods for testing the interaction of dust with jet engines have advanced considerably over time. Early experiments, during the 2000s, used Arizona road dust placed directly in front of engines. It was a crude approximation that succeeded in damaging fan blades but failed to replicate actual operating conditions. That is because the biggest problem turned out not to be ground-level debris but airborne particles, some as fine as a 10th of the width of a human hair.

To recreate conditions more accurately, engine manufacturers can work with geologists to create a mixture with the consistency of talcum powder that rises into the air like smoke rather than dropping immediately to the ground like grains of sand. The compound, informally referred to by test engineers as pixie dust, enters engines undergoing testing via nozzles that shoot particles at precisely the right velocity and concentration.

Méheust says: “The result is a proprietary new dust ingestion test process that replicates the kind of wear operators are experiencing in the field. This means we’ve been able to redesign hardware to reduce the wear on these parts and accurately validate performance in a test cell.”

Along with analogue innovations, modern sensors, digital twins and machine learning now provide insights that extend from the test cell to aircraft in flight. For example, CFM has developed a health monitoring system for its LEAP engines, now in service, that uses probabilistic diagnostic and prognostic machine learning tools to reduce unplanned maintenance.

Digital tools enhance testing

Sensors installed in key parts of the engine monitor various parameters and alert engineers to potential issues before they cause problems. The system models data generated during the take-off, climb and cruise phases of flight, then provides targeted alerts based on known engine operating signatures. “With the combination of this health monitoring system and the expertise of the global CFM fleet support team, the company achieved 60 per cent earlier lead times in identifying preventative maintenance recommendations, and cut the number of false alerts in half,” Méheust says.

LEAP engines have logged more than 90mn flight hours, collecting data all the while. And all that test and flight data also helps inform the next generation of engines, pointing engineers to potential design challenges and helping them to develop improvements.

Méheust says his company is applying lessons learned from the LEAP engine to potential future designs, such as the open fan architecture being developed as part of its CFM RISE (Revolutionary Innovation for Sustainable Engines) technology demonstration program. Engineers are already conducting dust ingestion testing on the core blades that will go into the engines. “This is the earliest CFM has ever conducted this type of testing,” Méheust says.

Every data point gathered from sophisticated sensors in flight and every cycle run in test facilities should add up to more resilient future engines serving airlines around the world. “As the operating environments for these engines have changed, the test methods have evolved to ensure the same level of veracity for these different conditions,” Méheust says. “As a result, operators can expect that the updates being introduced will provide better durability for longer time on wing.” In other words, today’s testing could result in more resilient and longer-flying aircraft tomorrow.

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