Hydraulic and pneumatic systems: Priming the pump for specialist maintenance

Maintaining hydraulic and pneumatic systems presents a range of challenges due to the materials involved. Bernie Baldwin reports on how some of those are being met

[This article first appeared in MRO Management September 2021, which you can read in full here.]

Mention the term fluid mechanics to many people and their eyes are likely to glaze over. But explain that, at its most basic, it is simply the study of the way fluids behave when pressure is exerted on them, and there is usually some recognition.

On aircraft, hydraulics – where the fluid in question is usually a special liquid – and pneumatics – where the fluid is air – are used in a variety of systems that perform a function to move and actuate parts of landing gears (in both deployment and retraction), brakes, flight control surfaces (flaps and slats), steering (particularly for the nose wheel) and thrust reversers. They are also used on doors, including large cargo doors on freighters.

As with any systems involving moving parts, wear and tear occurs on them and they require monitoring and maintenance. This is especially true on systems such as landing gears, which are mission critical.

Liebherr-Aerospace is a name synonymous with such systems, providing a range of equipment from landing gears to high-lift systems, mechanical and electromechanical actuators, nose wheel steering, rudder control systems and more across the Airbus A220, A320, A330, A350 XWB and A380 families, Boeing 777 (including actuators for the folding wingtips on the new 777X) and 787, MHI RJ Aviation (formerly Bombardier) CRJ700/900/1000 family, Comac ARJ21, and the Embraer E-Jet (both generations) and ERJ families.

As well as constantly developing systems as the original equipment manufacturer (OEM), Liebherr has its maintenance divisions which develop repairs which can lengthen the life of the part/component and thus delay the need for new parts. Repairs such as these can have their development driven for a variety of reasons, such as regulatory requirements, new technologies and customer input via regular communication and feedback.

According to Sven Dicke, vice-president MRO operation & strategic investments, customer support & services, Liebherr-Aerospace & Transportation, Liebherr-Aerospace continuously exchanges information and opinion with its customers to better understand their needs, “especially in times like these, where the entire aviation industry is in a restructuring process”.

He continues: “Our interpretation of how to improve the benefits of our MRO services lies in a combination of further developing salvage repairs for piece parts through our Design Organisation Approval (DOA) agreement and extending our portfolio to component health management with Liebherr-Aerospace Analytics.

“With the parallel approach, our engineers are able to carefully select and develop solutions with a single objective to increase aircraft availability, reduce unscheduled maintenance burden and help to optimise assets and parts maintenance.”

An independent MRO provider once again, after six years as part of the GKN Aerospace group, Fokker Services is pushing to add to its status in the aftermarket. The company is now owned by Dutch investment group Panta Holdings, which has its own long history with Fokker including Netherlands Aircraft Company (previously known as Rekkof, the backwards spelling of Fokker), which has long sought – though unsuccessfully thus far – to restart aircraft production of the Fokker 100 family. Panta has also previously owned airlines which operate Fokker aircraft, namely Denim Air and VLM Airlines.

As for its MRO work on hydraulic and pneumatic systems, Mehmet Cetin, product owner, mechanical systems at Fokker Services, explains the main factors to consider when repairing these systems. “In general, pneumatic components are susceptible to dirt and corrosion, which can lead to bleed air components getting stuck in a certain position,” he says. “Typical faults that we see for hydraulic components are leakages, which result in pressure losses within the system, or the contamination of fluid, which can cause wear and tear within (sub)components.”

Moving on to how the repair of these systems has developed and will continue to develop, Cetin remarks: “Owing to the Covid-19 pandemic, aircraft around the world were placed into storage. While in storage, the environmental conditions impacted the pneumatic bleed air system of some of these aircraft. Operators are now faced with valves that are stuck or that have seized up due to corrosion. This is more likely to be seen in aircraft stored outside with high temperatures or humidity. In extreme cases, operators could face more intense corrosion.

“For hydraulic components, proper sealing is essential to prevent pressure losses within the system. When seals are pressurised with hydraulic fluid, material tends to expand. However, due to inactivity, the seals could be shrinking and may not be properly fitted at the surface. This could lead to unexpected early failures of hydraulic components with leakages.”

Fokker Services’ advice for operators is to conduct health checks to identify any inactivity within the pneumatic system. “If faults are detected, operators can send their accessories to one of the Fokker Services in-house repair facilities in the Netherlands or USA,” Cetin says.

Expanding on some of the processes the company uses to assist a client, he adds: “Our teams perform detailed inspections according to the latest approved maintenance data. Surface corrosion is categorised in one of three categories: light, mild or heavy. Depending on its classification, a suitable maintenance practice is applied to bring the part back to a serviceable condition. In addition to corrosion maintenance practices, thorough inspections are performed especially at narrow flow passages within systems. Any contamination found is removed with ultrasonic cleaning.

“If operators face leakages with hydraulic components, our recommendation is to send the accessory to a shop for a bench test. By pressurising the inlet and discharge ports and putting the actuators through a couple of cycles, parts are encouraged to settle and properly fit into position, which improves the sealing capacity of certain connections. Without tearing down a unit or fully overhauling the part, this may already be extremely helpful to ensure activity and extend its serviceable condition.”

Looking at the latest repairs developed and introduced by his company and the factors which drove these enhancements, the Fokker Services executive comments: “We have recently invested in technology that aims to prevent the replacement of expensive subparts. Not only does this technology improve our turnaround times, it also reduces the total cost of ownership and improves reliability of the corresponding part.” He adds that more will be revealed to the market about “this great new development” in the coming months.

As Cetin noted, there are different challenges which arise in the maintenance of these systems due to the fluids involved – compressed air for pneumatics and special liquids for hydraulics. Liebherr’s Sven Dicke outlines his company’s work to deal with the varying demands that this maintenance work brings, particularly to the environment.

“Being ISO 14001 certified, we at Liebherr-Aerospace are committed to continuously reduce any environment impacts, and to manage closely environmental risks related to our global MRO operation,” he elaborates. “Our teams are constantly working on optimising the consumption of energy and raw materials, on improving industrial procedures, as well as on reduction and treatment of waste. All these actions and requirements are shared with our worldwide supply chain.”

The final validation of any repair development, of course, is how a customer benefits in terms of cost savings, increased reliability, increased safety and better operational flexibility or a positive combination of any of those.

“As an OEM, Liebherr-Aerospace has a vast history of not only designing and delivering systems, subsystems or components to the final assembly lines of the big airframe producers, but also to collect and assess data to optimise performance of our systems and/or equipment in service,” Dicke comments.

“To ensure a close proximity to our customers we have established a global network of field service representatives and MRO service centres. We believe that this combination of first-hand information from the field, our strong engineering skills, expressed also in our DOA certification, and our ‘Health Monitoring Services’, enable us to create solutions, that extend on-wing life, manage obsolescence, optimise repairs and reduce noise emissions and consumption while enhancing flight performances.”

Fokker Services’ Cetin returns to the extra challenges he sees arising from the maintenance of pneumatics and hydraulics due to the difference of the fluids involved. “Within hydraulic and pneumatic components there is a significant shift in what we call electrification of the component,” he explains. “In the past, the majority of these parts were mechanically actuated. Today, fly-by-wire systems are the common technology within aircraft.

“For any mechanical facility, including our hydraulic and pneumatic shops, it is crucial to become even more knowledgeable in electrical signal interpretation to properly conduct fault and isolation analysis. To tackle this, our in-house avionics and mechanical shops are joining forces to share best practices within maintenance activities.

“We have all seen how the Covid-19 pandemic has significantly impacted our industry, with orders being cancelled, production rates being downsized, and more. When the sector recovers, it may be challenging for organisations to keep up with demand due to potential backlogs at manufacturers. By adding new technologies, expanding our capabilities and optimising our portfolios, we are becoming a global partner – responding to our customers’ needs and enlarging our component MRO footprint.”

The nature of hydraulic and pneumatic systems means that their airworthiness is vital. Getting the mechanics of their maintenance right will make scheduling of the aircraft to which they are fitted far more fluid for the airlines.

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