Carbon brakes are now clearly taking the place of steel brakes in the global fleet of commercial aircraft, thanks to better durability and lighter weight. Suppliers Honeywell, Meggitt, Safran and UTC Aerospace Systems (UTAS) do not all agree, however, on whether it is worth retrofitting commercial aircraft using steel brakes with carbon technology. And runway deicing fluids, which damage carbon brakes, are increasingly problematic.
Some 60% of in-service commercial aircraft (including regionals) use carbon brakes, Cory May, UTAS’s vice president for airline sales, estimates. For models with more than 100 seats, the fleet using carbon brakes stands at 14,000, according to Pamela Groult, Safran’s marketing director for wheels and brakes. The number has more than doubled in 10 years; there were 6,500 in 2006.
The 5,000 or so aircraft with steel brakes is a number that “will decline as legacy aircraft retire,” says Kristopher Rich, Honeywell’s product marketing director for wheels and brakes. But there still will be some in use in 15-20 years, he predicts.
All newly designed aircraft use carbon brakes. For in-production airliners, the Boeing 737NG is a notable exception; it can still be ordered with steel brakes. “Thirty percent of new 737NGs coming off the line are with steel brakes,” Rich says. Customers with long-term fleets specify steel, he explains, to simplify logistics and maintenance by relying on one line of products.
UTAS offers both types on the 737NG. “There may be a business case for existing aircraft with steel brakes, in very short-haul operations, when you don’t mind carrying the extra weight,” May says.
In 2003, the 737NG was already the only commercial aircraft with more than 100 seats that could be purchased new with steel brakes. The 737 MAX will be available with carbon brakes only.
To decelerate an aircraft to rest, brakes use multiple stages of stators and rotors. A hydraulic or electric system squeezes them together. The amount of energy that has to be converted to heat is gigantic. “If you could harness the braking energy of a widebody aircraft, you could power a midsize aircraft carrier for a week,” Rich points out.
Carbon is a much better heat sink than steel. “The stopping performance can be strong with either type of heat-sink technology, carbon or steel, but at the extremes of the operating envelope, carbon will have greater tolerance for ‘beyond design’ conditions,” says Jim Valentic, Meggitt Aircraft Braking Systems senior vice president for sales and marketing. Carbon is also lighter. “On a Boeing 737NG, you save 700 lb. with carbon brakes from UTAS,” May says.
Greater durability is another major benefit. “Over the years, virtually all critical carbon friction material characteristics have improved two- or threefold,” Valentic asserts. These characteristics include
the friction coefficient (in terms of the absolute value, stability and repeatability), strength and life. As a result, Valentic is talking about 3,000 landings between refurbishments, compared to life in the range of 500-1000 landings when the technology first appeared.
UTAS’s May also referred to an average of 3,000 landings with his company’s carbon brakes. The average is closer to 750-1,000 landings with competing steel brakes, he says.
In terms of maintenance, Safran’s Groult says steel brakes are more time-consuming. But “it is possible for steel heat-sink technology to be overhauled by airlines and well-equipped MRO service providers, using parts provided by the brake manufacturer,” Valentic notes. The methods and equipment required to overhaul steel heat sinks are well known throughout the industry, he says.
It is quite different for refurbishing carbon heat sinks. “It requires specialized and proprietary overhaul techniques, materials and equipment and is performed by the brake manufacturer,” Valentic explains. Operators and MRO providers simply exchange the worn carbon heat sink for a new one.
Given the slower wear and lighter weight (which translates into reduced fuel consumption), it might seem obvious that the cost of a retrofit could easily be recouped. “It is highly recommended to [retrofit],” Safran’s Groult says. May concurs: “We are often contacted for carbon retrofits, and we have some success,” he says. An opportunity for an airline operating 737NGs to change types may appear when ordering 737 MAXs.
Honeywell’s Rich warns that for older aircraft, “you have to do the balance.” Moreover, the predominant model for contracts between airlines and brake suppliers is on a cost-per-landing basis. In that case, airlines are not directly interested in life of the brakes, he says.
Meggitt’s Valentic is rather negative about retrofits. “Typically, this is not attempted because of the considerable expense involved,” he says. Carbon requires a more sophisticated thermal design for the wheel and brake in order to manage the considerably higher temperatures generated by the carbon heat sink. In addition, brake control and antiskid performance may be affected by the different frictional characteristics of the carbon brake, “necessitating tuning or even hardware changes to the system, further driving up costs,” he says.
But carbon brakes are suffering from an unexpected problem—runway deicing fluids, which are becoming more corrosive for the brakes as they are turning friendlier for the environment. “Carbon brakes are susceptible to the phenomenon of ‘catalytic carbon oxidation,’ caused by the exposure to materials containing potassium, calcium or sodium,” Valentic says. During landings on runways where residual deicer is present, the spray from contact of the tire with the runway can enter the wheel and brake, depositing active ingredients onto the surface of the brake disks. Sodium and potassium are chemical “oxidizers” that erode the carbon disk material—specifically the carbon-fiber matrix—to the point where the disk strength is reduced. Eventually, the brake disk may fail during use.
The issue has become more prevalent over the past decade due to the introduction of new runway deicer materials, driven by environmental waste-water regulations in the U.S. and Europe. These rules essentially banned older-generation deicers, which were primarily urea-based. But they were less harmful to carbon brakes.
Brake manufacturers have developed “anti-oxidation” systems to protect carbon disks. “While improved [anti-oxidation] systems have reduced the occurrence and severity of the effects of catalytic carbon oxidation, the persistence and extent of exposure of carbon brakes to powerful oxidizers makes it virtually impossible to eliminate this phenomenon entirely,” Valentic says.
It is like an arms race, as every time there is a change in runway deicers, a brake manufacturer has to change its anti-oxidation product.
UTAS has developed an oxidation- protection system called HTx. “It is our state-of-the-art oxidation-protection system with a proven track record of protecting carbon brakes from thermal and catalytic oxidation,” May says. HTx has been in service for six years.
The carbon brake industry keeps investing in new friction materials for increased durability, performance retention and ease of manufacturing. “Manufacturing is very long; we measure it in months,” Honeywell’s Rich says. Could a supplier come up with something better than carbon? “Universities are looking at that,” he says.
Data-Enabled Services for Brakes
Honeywell is working on data-enabled services to better predict maintenance needs. The idea is to look at how the aircraft operates, such as using a brake-to-vacate system. “Based on how you have been using the product, we can predict [whether] you will need maintenance within the next 10 landings or at the next major check,” says Kristopher Rich, Honeywell’s product marketing director for wheels and brakes. Such services use the connected aircraft concept, but airlines have to accept sharing a lot of information on their operations.