Lufthansa Technik’s recently opened wheel shop Lufthansa Technik
Lufthansa Technik’s recently opened wheel shop in Frankfurt is seeing an increasing number of fatigue cracks at tie-bolt holes, inflation plug holes and spokes, which it attributes to weight- reduction designs.

New Alloys, Coatings, Design Considerations For Wheels

Balancing costs and benefits of new materials and corrosion-resistant coatings, along with new designs such as boltless wheels, involves making tradeoffs.

Printed headline: Wheel Revolution

Greater durability, more robust corrosion resistance, less maintenance-intensive, lighter weight: These are among the phrases that will be increasingly heard to describe the emerging trends in aircraft wheel design and protection. To achieve those goals, wheel OEMs are focusing on new alloys and corrosion-inhibiting coating, as well as fewer parts.

Aircraft wheels take a beating, especially from multiple corrosive elements, primarily carbon dust generated by the action of carbon brakes, along with runway deicing and hydraulic fluids, according to Dutch Kipp, engineering director for Landing Systems at UTC Aerospace Systems (UTAS) in Troy, Ohio. “At the same time, the wheels are susceptible to damage from high impact and fatigue loads,” he notes.

Kipp reports that the primary structure of an aircraft wheel is constructed with closed-die aluminum forgings. Over the past 50 years, aluminum “alloy 2014” has been the one most commonly used. While it is still one of the strongest aluminum alloys available, it offers “very poor” corrosion protection, he says. However, that is not the only problem with alloy 2014.

“The wheels are tightly integrated into a total wheel-and-brake system, with respect to thermal management and balance of components,” Kipp explains. “On application of the brakes, the temperature will often exceed 2,000F, especially during a rejected takeoff where the brake is intimately connected with steel and aluminum alloy structures. When you get above 400F, that’s when the strength of alloy 2014 drops quickly.”

UTAS, says Kipp, is pursuing a new silver-based aluminum alloy as a co-development between the OEM’s technical staff and its outside forging sources. Under current planning, the new alloy will be available in 2019, and applied to several new wheel programs in development. Among the benefits are a 10% increase in “room-temperature” strength, a 20% improvement in wheel strength at and after high-temperature exposure, and a 70% increase in damage tolerance and improved corrosion resistance, he says.

“It will also be lighter and much less maintenance-intensive,” Kipp points out, adding that a primary focus is on making the new alloy integral to a wheel that would be approved for forward fit under FAA Technical Standard Order approval. UTAS, he says, has several new production commercial aircraft types—which he did not reveal—in mind for application of the new wheel.

Another focus of research, reports Kipp, is on developing a composite wheel structure that would weigh less than conventional wheels and offer better corrosion resistance. UTAS does not estimate a time when that version could be introduced. “The heat generated by a braking event is still the most significant barrier to composite wheels,” he says.

World Aero

A technician performs eddy-current NDT testing on a widebody aircraft inboard wheel half at World Aero’s shop in West Sussex, UK, to find cracks in critical areas, such as around a fusible plug hole, shown here.

In tandem with developments in wheel structure at UTAS have been advancements in corrosion protection. Working with its suppliers, the company has made available a chromate-free, low-volatile organic compound (VOC) into primer and paint systems. “The benefits include better abrasion resistance, 3.5 times more resistance to fluids and a 1.5 times reduction in VOCs. For the repair shop, it provides quicker drying times and twice the shelf life of current products on the market,” Kipp states.

Up-to-date wheel technology has resulted in improved corrosion protection and prevention. Samuel Becquerelle, vice president for engineering for wheels and brakes at Safran Landing Systems cites, as an example, an “aluminum anodization, chromium-free process” developed by the French company. He also points to an “advanced primer coating,” co-developed with Mapaero, an aerospace coatings and finishes producer also located in France. 

“The combination of the primer coating and the chrome-free sulphuric anodization process has resulted in a very good barrier against the main wheel corrosion phenomenon,” says Becquerelle. “Also, wheel tie bolts are now using Inconel alloys, which decrease the weight of the bolts, and prevent corrosion. Torque bars linking the brake to the wheel are also using another corrosion-free Inconel alloy.”

Becquerelle explains that wheel removals generally take place as tires need replacement. But he notes that wheel fatigue will also result in removal. “Wheel fatigue-resistance is a requirement of the aircraft manufacturers, but typically, it is a minimum of 10,000 flights,” he says. “But this can be shortened if corrosion pits are not [overlooked], since even minor corrosion damage acts as a stress concentrator and significantly accelerates wheel fatigue.” Safran, he says, has also developed specific components to prevent corrosion abrasion on the heat shields that protect the wheel from high brake temperatures.

Ed Wasilewski, director of product marketing for wheels and brakes at Honeywell Aerospace, cautions that wheel service life between removals is also heavily dependent on individual airline operations. “As airlines continue to make changes in their procedures—driven either by safety, regulations or cost-savings initiatives—there can be a positive or negative impact on the service life of a wheel,” he explains. “In addition, various constituents have a cumulative effect on wheel fatigue, with tire performance the biggest contributor at 50-60%. Brakes are the next significant contributor.”

For greater corrosion protection, says Wasilewski, Honeywell has applied an aluminum anodization process, which has demonstrated “superior performance” in the field.

“This can be seen in wheel features such as radiuses and other higher-stress areas where other wheel manufacturers have seen shorter field life and significant machining repairs required to remove the corrosion,” he says. “Also, while some manufacturers require primer and paint, Honeywell’s anodize protection eliminates the need for painting, to help with corrosion protection.”

Wasilewski predicts that the next generation of wheel designs will focus on low-weight/high-strength characteristics. In that regard, he cites Honeywell’s boltless wheels.

“While this type of solution is prevalent on smaller military platforms, new alloys can allow this to be scaled up to larger commercial applications without negatively impacting weight,” he says. “That would enable differentiating benefits to airlines through improved overhaul processing efficiency, as well as increased brake performance through a larger available volume for carbon heat sinks.”

According to Alex Lara, director of wheels and brakes for AAR Corp., MROs have been “challenged with the increase in heat generated by carbon brakes,” coupled with the airlines’ desire to reduce maintenance costs. “As a result, some manufacturers have introduced a rim-retained wheel design that eliminates the use of, and maintenance associated with, the tie bolts found on most wheels,” he explains.

A boltless wheel—also referred to as a lock-ring or rim-retained wheel—employs a lock-ring device, instead of tie bolts, to attach the wheel flange to the wheel base. According to UTAS’  Kipp, the OEM has shipped 16,000 boltless wheels, encompassing 10 designs, to military and business aircraft OEMs over the past 20 years. However, he says commercial airliner manufacturers have been hesitant to adopt it. “I think its application to a large commercial transport is something that is still new to them,” Kipp notes. Yet, he adds, the advantages to the airline customer are clearly there.

Eliminating the tie bolts and hence speeding up tire changes are the a key benefit, says Kipp: “In fact, we have conducted time studies and found that with boltless wheels, there is about a 20% time savings with each tire change.” Among the reasons he cites is the fact that there is no disassembly of bolts, no bolt hardware cleaning, no anti-seize compound application and no tie-bolt torques process.

Phil Randell, CEO of World Aero, a wheel and brake MRO in the UK, refers to boltless wheels as “a large potential change for the wheel service industry” from an engineering and design standpoint.

“You first saw boltless wheels on business aircraft, but we’re now seeing some interest in boltless-wheel technology in the commercial air transport sector,” says Randell. “Within the next 10 years, we could see more boltless wheel options on commercial aircraft offered by the OEMs.”

For example, a typical Airbus A320’s main landing gear wheel uses 18 bolts, 18 nuts and 36 washers to hold the two halves together, for a total of 72 parts and a weight of around 20 lb. —just for this hardware, says Randell. In contrast, a similar boltless wheel would typically rely on just two or three fastening components—a single locking ring and retaining devices—which would weigh only 3-5 lb. per wheel.

In addition, the weight savings, shop visit labor-hours are also reduced since, as Randell points out, there is less to inspect. “With a conventional wheel, all of those nuts, bolts and washers have to be individually inspected during a tire change,” he says.

However, Lufthansa Technik, which opened a new wheel and brake shop in Frankfurt in September 2017, says new weight-saving wheel designs, while beneficial, can present issues of their own. A statement from the company explains that weight-reducing designs for wheels and brakes have trended toward high-strength materials that are presenting challenges for MROs. Titanium brake torque tubes require different treatments than steel torque tubes, and there are additional requirements for surface treatment of high-strength aluminum wheel rims. 

“On newer models, we are seeing an increasing number of fatigue cracks, starting at tie-bolt holes, inflation plug holes and spokes,” says a Lufthansa Technik representative. “From our point of view, that is a result of weight- reduction design.” 


Boltless Wheels

A key benefit of boltless wheels is the elimination of all maintenance tasks associated with tie-bolts.

  • At Tire Changes
  • No disassembly of bolts
  • No bolt hardware cleaning
  • No bolt application of anti-seize compound
  • No tie-bolt torque process
  • At wheel overhaul, similar benefits, plus:
  • No tie-bolt non-destructive testing
  •  No inspection of tie-bolt holes in the wheel
  •  No masking of tie-bolt holes for paint
  • Significant maintenance labor-cost reductions
  • Other direct maintenance cost-reduction benefits:
  • Reduced components scrap rate (tie-bolts, nuts, washers)
  •  Reduced wheel half replacement caused by bolt-hole misalignment
  •  Fewer parts to store in inventory and forecast replacement
  •  No bolt-related in-service problems
  •  Does not require torque wrenches or complicated assembly equipment

 (Source:  United Technologies Aerospace Systems)

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