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Aviation’s muscles can sometimes get fatigued

Hydraulics are at the heart of many aircraft systems and while their power and endurance are taken for granted, things can go wrong. Kevin Rozario reports.

Safety may well be the first word that springs to mind when thinking about hydraulics on aircraft. These systems provide the power and precision needed to operate critical components, from landing gear and braking activation to flight control surfaces (ailerons, elevators, and rudder) and flaps. Maintaining hydraulic systems is therefore critical.

In October, an Air India Express flight to Sharjah in the UAE with 141 passengers on board had to land shortly after take-off from Tiruchirappalli Airport in Tamil Nadu. India’s NDTV said the reason was a hydraulic failure, but an investigation had yet to report at the time of writing.

Two years earlier in April 2022, another incident involved a DHL Boeing 757-200F cargo flight from San José in Costa Rica to Guatemala City in Guatemala had to return to its origin for an emergency landing after a left-side hydraulics failure. According to ICAO’s archive website SKYbrary, the aircraft suddenly veered to the right with landing gear collapse and major fuselage damage. The investigation showed there had been an alert advising low hydraulic pressure.

From an MRO standpoint all hydraulic systems need examination, but some service arrangements must have fixed schedules and/or take priority. Andy Wheeler, divisional vice president and managing director of AEM/Ametek MRO explains: “Hydraulics are essential in aircraft systems and, from AEM/Ametek MRO’s point of view, landing gear is the most important. Depending on the type of aircraft, they must be overhauled every 10 to 12 years. Flight controls also have firm time-between-overhaul (TBO) periods.”

Outside of this, AEM normally only receives units if they develop a fault on-wing or are shelf-life expired; in other words sitting in storage. “However, some airlines have reliability programmes in place and demand overhaul rather than test, to ensure optimum performance,” Wheeler adds.

In October, AEM/Ametek MRO signed an agreement with Liebherr‑Aerospace for the provision of landing gear MRO services to Embraer E-Jet E1 operators across the EMEA region for the E190 platform. This underscored AEM/Ametek MRO’s capabilities in the hydraulics field.

Extreme environments

Hydraulic systems typically comprise hydraulic fluid plus the three mechanical components of pressure generators or hydraulic pumps; hydraulically powered ‘motors’ for specific components; and the channels through which the fluid flows around the aircraft. They are subject to extreme wear, but designed to be highly reliable in harsh environments.

Moreover, these systems are very energy-efficient, transmitting power over long distances with minimal energy loss. In addition, ‘redundant’ hydraulics are often used to ensure flight safety with multiple systems providing backup in case of failure. All this comes at a cost.

For example, fluids operate at pressures of 3,000 to 5,000 psi and, for aviation, they therefore consist of phosphate esters rather than being mineral oil-based. This ensures a high flash point to avoid ignition in case of a leak. The auto-ignition temperature of most aviation hydraulic fluids is about 475°C. They also need to be viscous over a wide temperature spectrum to retain good lubricating properties and have thermal capacity/conductivity to act as a system coolant.

Due to the constant up-and-down demands from a typical aircraft, hydraulic systems are subject to three major threats that MROs must watch for, according to ICAO, the specialised agency of the United Nations that coordinates global civil aviation regulations and policy. They are:

Overheating – where the system exceeds its allowable operating temperature and must be deenergised
Loss of pressure – which can occur through loss of fluid or failure of a hydraulic pump; and
Fluid contamination – either chemical or particulate in nature and can be caused by improper servicing or a component failure.

Robust maintenance practices are required to avoid all of the aforementioned. As the OEM for pumps and other hydraulic components, Eaton is a design authority on hydraulics so the company is well positioned to maintain these systems. Mike Nemeti, director of commercial aftermarket at Eaton’s Aerospace Group, says: “The pump acts as the heart of all the hydraulics systems. To ensure high reliability and safe functioning, operators must conduct inspections and maintenance as defined by the OEM.”

Unlike independent MROs, Eaton – which designs the product, repair procedures, and writes the Component Maintenance Manual (CMM) – benefits from having its design and repair teams co-located, enabling proactive fault‑finding and repair.

On the issues of wear, Nemeti says: “Hydraulic components have a lot of wearable surfaces that can degrade over time which is why aircraft-level maintenance in accordance with the Aircraft Maintenance Manual (AMM) is critical. The AMM will identify the actions to take on an aircraft level on an ‘as-needed’ basis.

“One of the benefits of having MRO activities performed by the OEM is that all serviceable units are guaranteed to be repaired to the latest approved CMM, with new OEM components to extend the life of the pump and increase time on wing.”

Wheeler adds: “Due to their nature and design, hydraulic systems can be subject to intense wear. For example, the main landing gear dampers from B737NG aircraft are a part that requires additional inspection for deterioration. Other items that experience a lot of wear include spool and sleeves (matched items) and working valves.”

This article continues after the below picture…

An AEM/Ametek MRO technician pushing a unit

Flexibility in service approach

Eaton collaborates with operators to identify the most valuable service packages for their needs. Various support arrangements are offered including cost-per-flight-hour agreements, which convert maintenance costs into predictable periodic charges.

For operators aiming beyond AMM targets, Eaton collaborates to develop additional service actions or in‑service modifications.

With OEMs tending to set their own criteria for hydraulic checks and TBO periods, MROs must fall in line with those and conduct overhauls in accordance with the CMMs. AEM/Ametek MRO’s Wheeler says: “This includes measurement inspection, visual inspection and, if applicable, non‑destructive test (NDT) inspection. NDTs detect internal cracks, weak points and other hidden wear. Fluid is replaced as standard when a unit is stripped and inspected, for example landing gear shock struts are cycled and the fluid is replenished as part of an inspection and test.”

As for the overarching AMM requirements, Nemeti comments: “The AMM typically organises major pre-planned activities into A-, B- and C-level maintenance events, while some pre-planned activities are based on flight cycles or hours. Because there are multiple redundancies, Eaton’s hydraulic products are generally designed to be ‘on condition’, meaning they are used until an issue arises and then repaired, without a specified service interval.”

Evolving technologies also continue to be an enabler for providing and analysing data from product returns to identify areas of improvements for future enhancements or possible changes to work scopes.

Components manufacturer Liebherr has experience in flight control and landing gear systems. The company’s Liebherr-Aerospace business says it is “aware that the aviation MRO is undergoing major transformations towards digitalisation” and that it is “investing heavily in smart, integrated technologies”.

In the case of business aviation MRO, the company is taking a data-centric, TRAINING & DEVELOPMENT modular approach to address business jets, helicopters and specifically the emerging advanced air mobility (AAM) sector using what Liebherr describes as “small installation envelopes”.

As well as the recent deal with AEM/Ametek MRO, in November Liebherr- Aerospace and Air France Industries KLM Engineering & Maintenance (AFI KLM E&M) signed a five-year heat exchanger maintenance services agreement. It covers Air France KLM Group’s A320ceo/neo and A220 fleet as well as the aircraft fleet supported by the French/Dutch MRO provider.

Heat exchangers transfer heat between fluids and are commonly used in hydraulic systems to cool the fluid and prevent overheating, optimising performance.

Regulatory standards

The industry is always striving to evolve the performance and reliability of hydraulics. In Eaton’s case, this is especially true for newer-generation pumps that are coming to market. The company also introduces product improvements through service bulletins and work scope recommendations.

Next-gen pumps can provide higher performance while reducing weight and footprint within an aircraft thanks to new material and manufacturing capabilities which can minimise failure rates to improve a product’s on-wing performance, efficiency and reliability.

MROs enable airlines to comply with regulations in terms of their hydraulic systems performance and efficiency. However, Wheeler cautions: “While MRO companies play a crucial role in helping airlines meet regulatory standards, it is ultimately the airline’s responsibility to make sure they remove items in line with OEM requirements.

“AEM/Ametek MRO advises of any airworthiness directives, service bulletins or service letters that are applicable when components are received. We also liaise with the OEM if we find any discrepancies with the technical data (which) can then lead to a CMM revision.”

This feature was first published in MRO Management – November/December 2024. To read the magazine in full, click here.

 

The post Aviation’s muscles can sometimes get fatigued appeared first on Aviation Business News.

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