Considering the contribution of helicopter engine intake filters
By Tracy Martin
Military helicopters are required to operate in all kinds of environments, including those with excessive sand and dust. The risk of foreign particle ingestion often is caused — or at least substantially increased — by an operational situation known as brownout, in which a helicopter flies through the dust cloud generated by its own rotor wake when landing or taking off. In fact, this problem is so serious that the U.S. Air Force Central Command published the statement: “Helicopter Brownout is a $100 million per year problem, leading to significant hardware loss, injuries, and fatalities.”
Because the engines may be operating close to their maximum power output during these conditions, as much as a kilogram of dirt can enter the engine for every cubic meter of air. This can, and does, cause irrevocable engine damage to compressor blades — such as blunted or eroded leading edges, sharpened trailing edges, and increased pressure surface roughness. Smaller particles of contamination also can damage turbine blades and combustor walls. This type of damage reduces overall airflow through the engine and results in a loss of power. In addition to potentially dangerous in-flight performance issues, damage to engines significantly increases downtime and maintenance costs. According to Matthew Fortuna, General Manager of Donaldson Aerospace and Defense, “The Middle East’s routine high temperatures and intense dust and sand conditions demand highly effective means of protecting helicopter engines from severe failure and performance degradation.” Helicopters operating in these dirty environments are protected from engine contamination in two primary ways: using mechanical particle separators or, more commonly, intake barrier filters.
Creating a Vortex
Vortex-type particle separators are designed to keep contaminants from entering turbine engines. These devices remove dirt by inducing a vortex airflow into the incoming engine intake air. Because dust and dirt are heavier than air, these particles are thrown outward and discharged from the aircraft, leaving the center of the airflow pattern relatively clean before it passes into the engine. A good example of this type of filtration system is the PUREair Vortex Tube, made by Pall Aerospace and used in Sikorsky SH-3 and S-61 Sea King helicopters. The advantage of this system is that there is no filter to clean or change, since the system is self-cleaning. In addition, a non-bypass system means that there is no degradation in filtration performance and no risk of in-flight filter blockage; it also provides protection against ice, snow, heavy rain, and salt spray conditions.
A possible downside to using vortex airflow is that the filter system may not remove some finer dust particles, though it should be noted that all filtration methods fail to deflect or capture some level of particulate matter. Vortex systems also use engine bleed air to purge the system, which can cause minor pumping losses that reduce engine power. (Power loss is a problem common, to a larger or lesser degree, to filtration in general.) Finally, vortex filtration systems are not entirely maintenance-free. Grass, straw, and other organic matter can become lodged in the vortex tube, reducing the efficiency of the system, and must be removed.
The Barrier Approach
Intake barrier filters (IBFs) take a different approach. An IBF separates incoming air from particles by trapping debris in its air-permeable surface. The filter media is made from engineered fibers that can withstand the harsh environments they are subject to. IBFs can reach filtration efficiencies as high as 99 percent when measured using Air Cleaner Fine Test Dust (ACFTD) for calibration and testing purposes. Use of barrier filters substantially reduces engine wear, extends overhaul intervals, decreases unscheduled maintenance, and enables military rotorcraft to increase operational time in dirty environments. In addition, because particulate laden air does not have to be mechanically removed from the aircraft, no bleed air is required, and the absence of a vortex at the engine inlet maintains smoother airflow into the engine. There are two types of filter media used in barrier filters: oil-wetted and dry. Dry filters significantly decrease maintenance burdens associated with cleaning, drying, and re-oiling conventional oil-wetted barrier filters. Dry filter media is generally preferred for high-tempo operations, while oil-wetted is ideal for aircraft flying in a more normal, less extreme environments. Overall, the dry media is preferred by the military, having been battle tested for years in such missions as U.S. Black Hawk helicopters serving in Iraq.
Early IBF filter shapes were flat and difficult to adapt to the rounded contours of a typical helicopter cowling. Because a flat filter element’s orientation directly affects its flow capacity, the flat design also resulted in less than optimum airflow. Another problem was that a filter that is oriented to maximize airflow when the aircraft is moving forward does not have the same capacity when the flight direction of the helicopter is sideways. These issues have been largely solved, as IBFs shapes now match the contours of the helicopter they are designed for. Where flat filters are used, they often are installed in multiple locations near the engine’s intake ducting—some pointing forward and others facing to the sides.
Barrier Filter Maintenance
While it was thought that IBFs would be a high-maintenance item when they were first introduced, Engineering Director Tom Newman, of Donaldson Aerospace and Defense, related, “Filters can last much longer in a severely erosive environment than people think. In the earlier days of barrier filters, it was anticipated that cleaning would have to occur after a very short period of operation. But we have shown that in the worst of environments, barrier filters can sometimes operate in the vicinity of 200 hours between cleanings.” Still, IBFs do require maintenance when a certain amount of dirt and dust accumulates in the filter media. As the filters become clogged with debris, the pressure will drop at the engine’s intake. Thus pressure drop monitoring is required to ensure that the benefit of protection to the engine is not outweighed by the loss in engine performance. And, after some number of cleanings, IBFs eventually have to be replaced.
IBF filter maintenance is straightforward. In general, filter elements are serviced at practical intervals that can be tailored to individual maintenance cycles, typically 100 to 300 flight hours or as required. Element life is generally “on-condition,” not limited to time or cleaning cycles. At each service, filters assemblies are visually inspected for filter media tears, punctures, and uneven or damaged pleats; seals are checked for tears/damage; and frame components are checked for corrosion, cracks, and missing or damaged fasteners. Usually, the filter is first brushed cleaned, using a soft bristle brush similar to a paintbrush, to remove as much dirt as possible. Some IBFs then have to be cleaned with a special solution; most have to be cleaned with an approved spray cleaner. Next, the filter is rinsed in the opposite direction of airflow (clean side to dirty side), using low-pressure water, until all of the debris and oil is removed. After air drying, dry filters can be reinstalled. Oil-wetted filters have to be soaked with a specified filter oil that is a unique blend of mineral and organic oil base stocks with added polymers. When the right amount of oil is applied, the lubricant forms an efficient “tack barrier” to trap dirt and dust.
With substantial concentrations of dust during desert landings and other grit-laden conditions, maintainers face a substantial challenge in keeping helicopter engines and systems up and running. While not all particulates can be prevented from entering the aircraft, properly applied and maintained filtration can make a big difference in keeping it mission-ready.