Engineered for today's aviation needs

ExxonMobil Fuels & Lubricants’ global chief engineer (retired), Ed Barnes and ExxonMobil Research and Engineering’s global technical advisor - aviation lubricants, Tom Hitchner discuss why good hygiene is critical during hydraulic fluids maintenance.

As the commercial aviation industry continues its recovery from the downturn brought on by Covid-19 pandemic-related travel restrictions and economic concerns, airlines are increasingly turning to maintenance, repair and overhaul (MRO) services to help safely and efficiently bring fleets back online. As these aircraft come back into service, one of the most important considerations for maintenance teams has been ensuring the integrity of the hydraulic systems that operate landing gear, slats, flaps, and other critical mechanical functions. The detection and prevention of hydraulic fluid contamination – challenging during the best of times – has been made even more difficult for MRO services hindered by a shortage of experienced personnel.

In aviation, hydraulic fluid’s primary purpose is to provide energy transmission that enables an aircraft’s hydraulic systems to function properly. Hydraulic fluids are also responsible for lubrication and for transferring heat during their operation. Unfortunately, hydraulic fluid contamination is a very real problem, as moisture, debris, and other foreign substances can find their way into an aircraft’s hydraulic system.

At a time like this, it’s worth revisiting not only the basics of hydraulic fluid contamination – including its causes and impact on aircraft – but also best practices for avoiding this problem in the first place. Foremost among these are good maintenance hygiene and routine fluid testing.

Contamination culprits

Hydraulic fluids are susceptible to contamination from liquids, solids, and airborne substances. Aviation hydraulic fluids are engineered to meet aerospace safety specifications, where microscopic particles are removed during manufacture. So you’re starting with an extremely clean product.

Most of the time, contamination happens before the fluid even enters the system, whether the problem is in the service cart or in bad handling practices, where maintenance people are not respecting the fluid.

In such cases, the result could be liquid contamination from a mechanic putting the wrong substance into the hydraulic system, the result of having the wrong oil in the fill cart or adding directly into a hydraulic reservoir from a container of the wrong fluid. It’s also possible that water might have gotten into the cart, or the cart was improperly hooked up to the hydraulic system. Another scenario, albeit not as common as the others, is contamination from an engine-driven hydraulic pump leaking into the engine accessory gearbox.

Contamination from solids could be the result of dust or dirt entering the hydraulic system through a poorly performed maintenance practice or a dirty tool. Hydraulic systems have filters to extract some of the larger particles, but if an internal part is wearing out and constantly adding contaminants to the fluid, the filters can do only so much to prevent problems. Such particles can plug filters, causing them to potentially go into bypass and triggering alarms that result in any number of unwanted situations – a system shutdown, flight diversion and precautionary landing, or a cancelled flight. This type of contamination also causes longer-term wear on some parts that have really tight tolerances.

Airborne contamination can be a problem because hydraulic fluids rapidly absorb moisture from the air, leading to hydrolysis that chemically degrades the fluid’s integrity. Commercial aircraft use a phosphate ester hydraulic fluid that tends to absorb moisture from the air whenever it can, causing the fluid to lose its viscosity and creating phosphoric acids that wreak havoc on hydraulic systems.

Another source of contamination is mixing of unapproved fluids such as turbine engine lubricant or mineral-based strut fluid, which can cause deposits, blocked filters, and corrosion of component surfaces, and lead to increased mechanical wear.

What to do

Fortunately, neither the causes of nor the solutions for hydraulic fluid contamination are a mystery.

Filtration and additives are important for system performance but have limited mitigation effect for contamination. For example, certain additives can delay the build-up of harmful acids in the hydraulic fluid, but those additives may eventually be overwhelmed by the effects of contamination.

That’s why training in maintenance hygiene is so important. MRO personnel must be taught the proper handling of hydraulic fluids. Contamination can easily make its way into a system if MRO personnel are not careful during repairs and maintenance. What makes this concern particularly relevant at this time are the challenges the MRO industry faces due to a shortage of experienced workers, rising workforce costs, and the reduced interest recent technical graduates seem to have in maintenance jobs.

Regular testing is also a must, as it’s the best way to determine the condition of an aircraft’s hydraulic fluid. Although this can be a costly process, it can be done on a schedule of 12-18 months to coincide with other maintenance and make it more cost effective.

Unprecedented challenges over the past few years have put aviation hydraulics systems at risk of contamination. As a result, proper handling of hydraulic fluids and regular testing are critical to the industry’s continued recovery.

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