Saturday, September 1, 2007
General Aviation: Wheel and Brake Update
They’re small, lightweight and they are one of the most highly stressed, ofttimes abused part of any aircraft. What are "they"? Aircraft wheels and brakes of course. And if you don’t believe that they lead very stressful lives, just take a look at any runway. See all those 100-foot-long skid marks? The wheels and brakes are at the center of all that stopping power. Sure the tires take their share of the licking, but that’s another story.
The truly amazing thing is, whether you’re looking at a Cessna 172 or a Boeing 777, the technology found in today’s wheels and brakes hasn’t really changed much. "Technology from a design standpoint has not really changed a lot over the past 15 to 30 years," explained John Bakos, manager, sales and marketing, Parker Hannifin, Cleveland Wheels and Brakes. "The calculations are all the same, but what everyone is looking for today is lighter and stronger."
Bakos explained that the use of computers has made the design process easier. Design calculation software, solid modeling, thermal analysis and FMEA (Failure Mode and Effects Analysis) are some of the computerized tools used in the design process of today’s wheels and brake systems.
"Weight is paramount, so we are being forced to look at ways to develop designs and materials that provide the same performance at lighter weights," he said. "The more rigorous, robust design requirements of corporate, military and commercial aircraft pretty much dictate the use of forgings, which are more expensive to manufacturer. The typical GA aircraft uses lighter, less expensive magnesium castings. The trade-off is magnesium’s susceptibility to corrosion if nicked or scratched."
But just because wheel and brake manufacturers are spending most of their time fine-tuning today’s designs, that doesn’t mean they are not working towards the future. According to Bakos, there has been a trend towards electric brakes. Right now they are focused on the new commercial aircraft like the Boeing 787, which is slated to be the first to have them. And that’s primarily due to their higher cost and size, which will probably be a barrier to their entering the business and general aviation markets for a long time to come.
Brake Maintenance 101
Wheel and brake maintenance hasn’t changed much through the years. "Pretty much the [maintenance] procedures have been in use since Charles Taylor’s days," Bakos said. "We are bound by the FARs and regulations."
"When you look at today’s maintenance procedures you don’t have to re-invent the wheel here," he continued. "The processes really do work pretty well, providing the mechanics follow them." And therein lies the proverbial "rub." Technicians, (none reading this story of course) become so familiar with doing the same type of work over and over again that they tend to create their own "shortcuts."
Bakos stressed that if the technicians would take the time to read and follow the manuals there would be a lot fewer problems with wheel and brake maintenance. All the major wheel and brake manufacturers have established certain procedures that they recommend for proper inspection and maintenance. "That’s why we write the manuals," he said.
Speaking of manuals, Cleveland makes it really easy for technicians to get the latest information. They have a dedicated Web site:
If your cave doesn’t have Internet access, then Cleveland makes all the same information available in printed form and on CD-ROM. "There is absolutely no excuse for people not having the information they need," Bakos said.
So what if you’re stuck with a problem that you can’t figure out by reading the manuals? Not a problem. Cleveland has a dedicated e-mail directly to their technical service department. "We monitor that continuously," he said. "If that’s not fast enough, technicians can call and talk to a technical support specialist. The information you need is readily available no matter where you are and how you want to get it."
But, aside from the simple error of just not reading the manuals, when asked what the most common mistake technicians make when working on wheels and brakes he was quick to come back with an answer. "Not torquing bolts properly, both on the wheels and brakes," Bakos replied. "We hear ‘the bolt broke.’ Why did it break? Well, they weren’t torqued properly."
As an example of this mistake, Bakos said that when technicians install new discs or linings, they tighten the caliper bolts down and find they cannot turn the wheel. If that’s the problem, you need to look for a depression in the metal around the bolt holes on the face of the cylinder. It’s a magnesium or aluminum casting with a steel back plate, so the housing is going to give. "Typically they are over-torquing the back plate bolts," he explained. "It crushes the cylinder. They use their ‘calibrated elbow’ to put 100-inch pounds on it instead of the specified 65."
Like most problems, this one is easy to avoid. Just make sure you use a calibrated torque wrench and follow manufacturer specifications. Simple enough!
Bakos said that another big mistake is not properly lubricating the grease seals. "On some of our wheels have felt grease seals that need to be oiled or greased before they are installed to help keep moisture out of the wheel." That’s critical because moisture leads to internal corrosion, which can kill a wheel pretty fast.
Of course, there are limits to the amount of corrosion that can be removed from a wheel. Removing too much material can affect the design integrity of the part and can lead to further damage or even wheel failure. So keep those wheels as clean and dry as you can.
Keeping moisture out is the primary reason you never want to use a pressure washer to clean wheels and brakes. That will just force moisture up through cracks and seals. It not only encourages corrosion, but also contaminates the lubricants that are in there. Corrosion and loss of bearing lubrication are two of the major causes of wheel failures.
So you not only want to avoid using a hose on the wheel, but you also want to make it a standard practice to lubricate the bearings at every tire change or more often if specified by the wheel manufacturer’s maintenance procedures.
Stop! Time to Inspect Those Wheels and Brakes
The only way to spot cracks, nicks, corrosion and other issues that will eventually lead to catastrophic wheel failure is through routine visual inspection of the wheel and brake system. (Yes, it’s spelled out in the manufacturer’s maintenance manual.)
When it comes down to inspections it’s really pretty straightforward. The goal is to try and find issues that are detrimental to the life and performance of the wheel and brake. On the wheels you are looking for nicks, cracks and corrosion. If you find a cracked wheel you replace it. If you find corrosion, you dress it out and paint it — and keep watch for signs of more corrosion down the road.
For the brakes, inspection is focused on the condition of the mechanism and how much lining is left on the brakes. Inspection practices are different though depending on the types of aircraft you are working on. "Corporate jets and larger aircraft use what are called ‘internal brakes,’" Bakos said, "where the brake assembly is housed inside the wheel and uses a combination of rotating and stationary discs, similar to a bicycle coaster brake."
"Typical general aviation airplanes use an ‘external brake’ system. The disc is bolted to the wheel and the brake is essentially a ‘C’ clamp that is very similar to an automotive caliper," he continued. "The advantage is the disc is outside the wheel in the air stream for better cooling while taxiing. The better the cooling, the less chance you have of fading brakes." External design wheels and brakes are also less expensive and typically provide lower maintenance costs.
Another advantage of the external design is you can change the linings without having to jack up the airplane. Just unbolt the calipers and pull them off, change the linings, bolt it back together (with the proper torque values) and you’re done — well, almost. Bakos stressed the need to properly condition the linings after they’re changed.
Danger! Contents Under Pressure
While you’re checking the wheels and brakes it’s also a great time to give those tires a close look. Maintaining the proper tire pressure can go a long, long way to helping the aircraft’s wheels and brakes perform their roles trouble free.
"We recommend following the tire manufacturer’s guidelines," Bakos said. "Because changes in tire pressure will dramatically affect the life of the wheel." While airlines and most corporate operators routinely check tire pressure, the typical GA pilot/owner doesn’t. They’ll go fly once a week and if the wheel isn’t touching the tarmac it’s good to go. That’s why the majority of tires found on these aircraft are tube-type. They typically hold air pressure better than tubeless tires.
Another critical thing for technicians to keep in mind about tires and pressure is the potential hazard of working on a wheel with a fully inflated tire. As a good safety practice, any time you are going to remove the wheel and tire from an airplane remove all of the nitrogen or dry air from the tire. Sheered bolts or cracked wheels can lead to a catastrophic wheel failure. Way too many technicians have been seriously injured or even killed by an exploding wheel.
"Properly inspected and maintained, this stuff can seemingly last forever," Bakos said. "I’ve seen wheels and brakes that have been in service since the mid-1950s. You just have to take proper care of them."
Breaking in Those Brakes
Along with routine inspections and proper installation of the brakes, Bakos said that another part of ensuring you get all the stopping power and life that the brakes and linings are designed to give is to make sure the brakes are properly "conditioned" when they are first installed.
The first thing to remember is that metallic and organic linings are not conditioned in the same way because they have different operating characteristics. Mr. Bakos was kind enough to provide the following conditioning tips right out of the Cleveland maintenance manual.
Conditioning Non-Asbestos Organic Linings
Taxi aircraft for 1,500 feet with engines at 1,700 rpm applying brake pedal force as required to develop a 5 to 10 mph taxi speed.
Allow brakes to cool for 10 to 15 minutes.
Apply brakes and check for restraint at high static throttle. If brakes hold, conditioning is complete.
If brakes cannot hold aircraft during static run-up, allow brakes to completely cool and repeat steps 1 through 3.
Conditioning Metallic Linings
Perform two (2) consecutive full-stop braking actions from 30 to 35 knots. Do not allow the brake discs to cool substantially between the stops.
Allow the brakes to cool for 10 to 15 minutes.
Apply brakes and check for restraint a high static throttle. If brakes hold, conditioning is complete.
If brakes cannot hold aircraft during static run-up, allow brakes to cool completely and repeat steps 1 through 3.
According to the guidelines, these conditioning procedures will wear off any high spots and generate sufficient heat to create a thin layer of glazed material at the lining friction surface. Normal brake usage should generate enough heat to maintain the glazing throughout the life of the lining.
Properly conditioned linings will provide many hours of maintenance -free service. A visual inspection of the brake disc will indicate the lining condition. A smooth surface, one without grooves, indicates the linings are properly glazed. If the disc is rough (grooved), the linings must be re-glazed. The conditioning process should be preformed whenever a rough disc condition is observed. Light use, such as taxiing, will cause the glaze to be worn off rapidly.
It’s also a good idea to measure the brake disc thickness during tire changes and other maintenance intervals. Cleveland specifies a minimum replacement thickness for each part number disc. To ensure proper operation and the designed performance levels, it is imperative to replace worn (below limits) brake discs.