Manufacturers of aircraft wheels and brakes are striving to improve durability and ease of maintenance. Meggitt and Honeywell, for example, are improving such components early in the design phase and adding radio-frequency identification (RFID) tags to aid logistics. MRO providers such as London Gatwick Airport-based World Aero are also profiting from the emergence of electric braking systems.
“Durability is built in to the basic structure through extensive finite-element analysis,” Jim Valentic, Meggitt Aircraft Braking Systems’ senior vice president for sales and marketing, tells Aviation Week. This helps ensure that the design has adequate margin to tolerate the very harsh operating environment for wheels, brakes and some control-system components.
“To this durable, basic structure, we add features gathered from many years of experience and lessons learned,” he adds. Such features include cold working metals to enhance fatigue life as well as coatings that help eliminate corrosion, fretting and galling. Extensive thermal countermeasures to deal with the high temperatures generated by braking are part of the treatments.
Increasing durability is an imperative to remain competitive. Meggitt has invested in developing an environmental test capability. “We perform tests under a wide range of extreme temperatures while subjecting our products to vibration as well as moisture, to wring out new designs and provide information to improve future designs,” Valentic explains. In many cases, test conditions exceed those actually encountered by the aircraft.
Meggitt’s “highly accelerated life testing” provides laboratory feedback in a compressed time frame. “The testing allows us to de-risk durability enhancements and bring them to market much quicker,” Valentic says.
Programs based on a per-landing cost give predictability to airline customers while incentivizing the brake supplier to improve durability. “We are continuing to improve the life of the friction material,” emphasizes Hans Laudon, Honeywell Aerospace’s vice president for marketing and product management, wheels and brakes. “Our engineering evaluates carbon heat sinks when [components are] returned. We [synthesize] that feedback into the design process,” Laudon says. The data on landing life is maintained in a database. “We do platform-specific trending,” Laudon adds.
One ongoing challenge for durability has been damage from runway deicing fluids. Carbon brakes are susceptible to the phenomenon of catalytic carbon oxidation, caused by exposure to materials containing potassium, calcium or sodium. During landings on runways where residual deicer is present, the spray from the tire’s contact with the runway can enter the wheel and brake. Active ingredients are deposited onto the surface of the brake disks. Sodium and potassium are chemical oxidizers that erode the carbon disk material to the point where strength is reduced, and eventually the brake disk may fail during use.
“Oxidation by deicing fluids makes the heat sink go soft and, in some extreme instances, you can put a fingernail into it—[where] nothing should penetrate,” World Aero founder and Managing Director Phil Randell says. The component, which otherwise would have a lot of life left, has to be removed. “There is no way to repair it; it [has to be] scrapped,” Randell explains.
“When it comes to carbon brakes, which can remain on an aircraft for five years or more, the capability of our industry-leading antioxidation coatings is a key factor in achieving durability of the carbon heat sink material,” Valentic says.
Honeywell, for the Boeing 767, has a new heat pack that improves antioxidation, Randell notes. Life is extended from 1,000 to 2,000-2,400 cycles.
Wheel design can influence tire durability. On the Boeing 737 NG family, the main wheel is only suitable for a bias tire. This translates into a life of perhaps a mere 185 cycles for the tire—as opposed to the 500 typical for a radial tire on the Airbus A320, Randell says. To use radial tires on the Boeing 737 NG, “a new wheel would have to be designed, and maybe even a new landing gear, because weights are transmitted in a different way,” Randell says.
In terms of maintainability, the early design phase of wheels and brakes is paramount. “Designing for low stress levels using finite-element analysis provides the ability to tolerate repair in high wear-and-tear areas,” Meggitt’s Valentic says. Rework provisions may be included. These can be directives to repair bushings or an allowance for operators to re-machine areas where corrosion may appear. Generous repair provisions greatly reduce the total cost of ownership for the operator, since parts can be repaired and returned to service rather than being replaced.
“We seek to minimize the use of special tooling so that our products can be maintained with commercially available tools,” Valentic notes.
One feature that can reduce the labor associated with tire changes is incorporating a ring to secure the wheel halves in lieu of tie bolts. “A failed bolt can do a lot of damage to wheel and tire,” Randell comments. With a boltless wheel design, the tire-changing process is much more straightforward, and a lot less inspection is required. Replacing 16-18 bolts with a ring also saves weight.
Such design features should be carefully thought out, as they may introduce unique load paths and potential areas of stress and fatigue that require extensive analysis and testing, Valentic points out.
Honeywell’s Laudon adds that such a retained ring wheel design improves brake life through a larger design envelope for the carbon heat sinks. Common on fighters and also on some large business jets, the improved arrangement is offered on a few commercial aircraft, but is not in widespread service.
Another enhancement in wheel design has been tire-pressure monitoring. The system provides cockpit readouts of tire pressure, which can eliminate the need for manual tire-pressure checks. This helps in maintaining proper tire pressure, improving both safety and tire life.
Suppliers of wheels and brakes are endeavoring to further improve ease of maintenance. “All inspections and tests are keyed to either tire change or brake removal for wear,” Meggitt’s Valentic says. Materials and processes are selected to eliminate life-limited components so that no scheduled maintenance is required. New structural alloys are expected to boost the long-term serviceability of Meggitt’s products.
It is also important to make operators aware of the features and how best to use them. Meggitt maintains a global network of field support engineers who provide on-site support for the operators, regardless of the fleet size. “We offer periodic training programs at our facilities to provide both instruction and consultation on the maintenance features in the design,” Valentic says. The MSG-3 methodology for scheduled maintenance is also said to be a contributor.
Improved logistics may also help. “Airlines have to keep accurate inventory count records for spare provisioning; we have developed an automated [radio-frequency identification (RFID)] tag solution,” Laudon says. Honeywell uses a secured, ruggedized RFID tag on the wheel to determine its identification and repair scope when it rolls off the truck at the MRO depot. The internal system is in the process of being offered to customers to allow for more efficient spares tracking.
World Aero’s Randell sees maintainability improving, with better corrosion-resistance, fewer parts and components that are easier for technicians to handle. The phasing out of aircraft fitted with steel brakes will also eliminate a frequent mistake. “Many defects on steel brakes are due to incorrect fitting of wear pads; they come loose on stators and damage wheels,” he says.
Electric brakes, on the Boeing 787 and more recently on the Bombardier C Series are synonymous with progress for maintenance technicians. “You can do a lot more on wing, so sending the entire braking system to the shop is less needed,” Randell points out. The electric brake actuator can simply be removed.
The benefits of an electric braking system appear in health monitoring, fault tolerance and dispatch reliability, according to Valentic. Once in the shop, automated testing accelerates the return to service.