What You Do Not See CAN HURT YOU


Any in-flight engine loss is concerning, but when you lose them all…

In 1989, KLM Flight 867, a Boeing 747 jumbo jet, took off from Anchorage, AK, enroute to Narita, Japan. During the climb out, it encountered a cloud of volcanic ash from the active eruptions of Mount Redoubt along Alaska’s Cook Inlet. Shortly after passing flight level (FL) 250, all four engines shut down. The crew attempted several restarts and were able to light engines 1 and 2 descending through FL130 and the remaining two at FL110. The aircraft limped back to Anchorage, and the post-landing inspection confirmed that all four engines required replacement. When combined with additional damage to the aircraft, the total repair cost reached $80 million.

Volcanic ash has disrupted air mobility operations many times, including the eruptions of Mount St. Helens, Washington State (1980); Mount Pinatubo, Philippines (1991); and, significantly, Eyjafjallajökull, Iceland, in 2010. This most recent eruption affected Operation Enduring Freedom in Afghanistan and Operation Iraqi Freedom (OEF/OIF) after encroaching ash clouds forced a shift from Northern European bases to Southern European bases. Eventually, the entire European airspace shut down, and AMC was forced to open a western airbridge via the Pacific Ocean to support Central Command. In all, nearly 800 missions were affected, with an average delivery delay of 40 hours.

These events served as a catalyst for advanced research on impacts from various environmental particulates (EPs), including salts, sand/dust, and smoke/pollutants, emphasizing their effect on engines. Under a long-standing cooperative research program, Air Force Research Laboratory (AFRL) and North Atlantic Treaty Organization researchers conducted a 3-year study to catalog EPs’ effects on aircraft systems, existing and future mitigation technologies, and resultant operational changes. The study was completed in 2019 and was approved for release in October 2021.1 One promising mitigation technology identified in the report, with significant implications for civilian and military aviation, is compressor blade coatings (CBC) for engine compressor sections.

CBCs are not new, and the first rudimentary coatings appeared in the 1980s. It could be argued, however, that coatings can trace their origin to early aviation, when metal strips were placed on the leading edge of early propellers to prevent erosion of the wood. That same concept applies to compressor stages that are vulnerable to EP erosion and corrosion. Today, the technology has become so advanced that innovative protective coatings of less than 15 microns, or approximately one-third the thickness of a human hair, are applied element by element to specific portions or entire compressor blades in high-vacuum plasma chambers. This process results in coated compressor blades of previously unimagined strength that resist EPs, allowing engine performance retention, reduced maintenance, greater fuel efficiency, and reduced emissions during an engine’s time on wing.

Before any in-depth discussion of CBC benefits, it is essential to understand the basic geometry and physics of compressor blades or, more accurately, “compressor airfoils.” Simply put, each compressor blade is an airfoil like an aircraft wing. The blade’s camber, thickness, and chord define the blade’s performance, and each compressor section consists of numerous blades that manipulate the airflow for optimum performance. Unfortunately, the shape of the blades (chord, thickness, or camber) is degraded by EPs over time, and the airflow becomes sub-optimized. That change in design flow leads to insidious performance reductions, loss of fuel efficiency and increased emissions and eventually necessitates maintenance to replace the compressor blades.

The U.S. Marine Corps CH-53 fleet experienced a combat example of EP erosion impacts during OEF/ OIF. Compressor blade erosion due to sand and dust (see Figure 2) was so great that engine changes were required after a fleet-wide average of approximately 100 hours. The tremendous maintenance burden and shortage of spares threatened CH-53 combat operations. To stem the attrition, a CBC from MDS Coating Technology was expedited into service. The impact was immediate and dramatic as engine time on wing soared and, in some cases, reached more than 20 times the initial fleet average, with numerous engines exceeding 2,000 hours of engine time on wing. Fleet-wide, coated engine time on wing averaged a 10-fold increase to more than 1,000 hours in the harsh Iraqi combat environment. This success story set in motion additional research that resulted in today’s next-generation coatings with broad applications across the entire fleet of military aircraft—from fighters to heavy-lift aircraft.

Despite the operational success of the U.S. Marines, timely adoption of advanced CBCs by sister services lagged. Although the Department of Defense was slow to adopt the advanced coatings, the commercial sector was not. Spurred by rising fuel costs and high maintenance costs, investments in blade coatings provided a rapidly implemented and inexpensive solution with a short return-on-investment timeline. The business case was simple—if coated blades retain their shape longer, performance and fuel efficiency will also improve compared with non-coated engines. This point was proven by a commercial carrier when they adopted an advanced coating from MDS Coating Technology for their Boeing 737 fleet. A side-by-side 38-month comparison of an uncoated and a coated CFM-56 (the commercial version of the KC-135R’s CFM) revealed a performance and fuel efficiency divergence at the 19th to 20th month of monitoring. By month 34, the fuel efficiency difference was a staggering 1.3 percent, and it was approximately 0.7 percent average during the 34 months of operations in favor of the coated engine. It is important to remember that time on wing for modern turbofan engines can exceed 10 years, and maintenance and fuel savings are significant. For this commercial carrier, the fuel savings alone more than justified the investment.

For years, AFRL has researched and investigated CBCs. Their insightful work kept pace with recent advancements and, combined with strong support from the Air Force Operational Energy Office, generated a renewed emphasis on fielding the next generation of compressor blade coatings. That emphasis has led to the competitive selection of MDS Coating Technology’s next-generation “BlackGold” coating as the CBC for the Air Force. The final certification process of BlackGold is underway with the support of original equipment manufacturers (OEMs) of the KC-135’s CFM-56 and the C-17’s F-117 engines. This selection is beneficial for many in the Air Force, particularly the Air Mobility community.

The numerous future benefits of advanced CBCs will be a valuable improvement for everyone involved.

The operator will gain performance retention, a guarantee that, when you need it, the engine thrust will be at its peak. The maintainer will see significant reductions in maintenance generated by compressor blade replacements due to EP erosion. The accountants will benefit from reduced sustainment costs, freeing up funds to support other needed programs. It will enhance OEM’s outstanding engines and strengthen their reputation of supporting the warfighter. In addition, it will benefit society by reducing emissions, thereby reducing atmospheric carbon and other pollutants.

1 The report is titled Gas Turbine Engine Environmental Particulate Foreign Object Damage [EP-FOD]. Report reference number: STO-TR-AVT-250. Available at https://www.sto.nato.int/publications/.