TIRES MAKE THE WORLD GO ROUND
by H. Dean Chamberlain
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Poor pun. But without tires, the world would soon come to a screeching halt. The aviation world is no exception. Yes, as a seaplane rated pilot, I know some seaplanes don't need tires, but aviation has evolved a long way since the days when large Pan Am flying boats ruled the air and waves. Because tires are so important, I stopped to watch a demonstration given by an aircraft brake manufacturer's representative on the proper care and use of his company's wheel and brake assemblies at the 1996 Sun 'n Fun Fly-In in Lakeland, Florida. He would disassemble a brake and wheel and then reassemble both to show the proper installation and workings of the brake assembly. It was interesting to think about how such a simple looking system could stop an aircraft landing at a speed of a hundred or more miles per hour. Later, I picked up material from a major tire manufacturer on the proper care and use of aircraft tires to complete the brake-tire combination. My interest in tires and brakes started years ago when I was involved in the management of a military flying club. |
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I spent more than one Saturday morning helping to change tires on club aircraft after some student ruined the tire by locking the brakes after landing. Skidding can wear good tread down to the tire's cords in one landing. Such damage makes the tire unserviceable.
During that same period, I had an opportunity to watch a major aircraft company do a new brake certification test on a large, wide-body passenger jet. As part of the test, the aircraft was loaded to gross weight, accelerated to rotation speed, and then the flight test crew hit the brakes.
The proposed brake system did its job; the aircraft stopped within the requirements of the certification test. The test did have its cost. Most of the wheel and brake assemblies became red-hot and their fusible plugs melted and let the air out of most of the tires. Fusible plugs are a safety device designed into high-pressure wheel assemblies to reduce the danger of the tires becoming too hot and over inflated and possibly exploding. Because of their use in a gross-weight rejected takeoff, the new tires used for the test were no longer serviceable. Since there was the potential risk of plane's magnesium wheels catching fire because of the heat build up from the energy absorbed by the tires in stopping the aircraft, crash/rescue fire fighters were on scene in case any of the wheels caught on fire and threaten the safety of the aircraft.
This was an expensive test, but it was well worth the cost because it proved the new braking system could stop the wide-body jet up to rotation speed in case of an aborted takeoff. Rotation speed is an important speed in air carrier operations because once they are at or above rotation speed the normal operational procedure is to fly the aircraft off the runway rather than braking it to a stop. But sometimes the crew might decide to stop the aircraft rather than completing the takeoff as happened in New York several years ago. The fact some crews might decide to abort rather than continue the takeoff is why high speed brake tests are an important part of FAR Part 121 aircraft certification.
But what about the little guys? Although most general aviation pilots don't have to think about stopping something the size of a jumbo jet, the fact is they have to be able to stop their GA aircraft at any time on any type or size of runway. In fact, GA pilots may have to stop their aircraft in proportionally much less runway than a jumbo jet pilot. The reason is air carrier pilots have to meet specific accelerate-stop runway distances in planning their takeoffs that many GA pilots may never consider or even know about.
Simply stated, if an air carrier pilot doesn't have enough runway to accelerate to rotation speed and at that moment be able to stop based upon load and other conditions, the pilot can't use that runway. Single-engine GA pilots operating under FAR Part 91 don't have such a requirement. In fact, it's a fair bet to say most single-engine aircraft flight manuals don't even list such data.
As a result, many GA pilots of small, single-engine aircraft may be betting their lives and the lives of their families and friends on the stopping ability of their aircraft's brakes and tires when taking off on very short runways without knowing the accelerate-stop distance for their respective aircraft. Most GA pilots simply don't know how much runway they must have to stop on without going into the trees or whatever else is off the far end of the runway.
Since lives may be at stake in a rejected takeoff, that moment is not the time to wonder how and if your brakes will work or stop you before you hit the trees. So take a moment to review the following information on tires, brakes, and their wheel assemblies. Someday, you may be glad you did.
Brakes and tires may be the two most neglected components on an aircraft even though most flights start and end with aircraft tire tread on the ground. Although aircraft preflight checklists have a reminder for pilots to check the brakes and tires for wear, some pilots may only give them a cursory look. This may be especially true of pilots of low-wing aircraft when the aircraft are sitting on the ramp and rain or snow is falling. It takes a dedicated pilot to crawl under a low-wing aircraft at such times to check the wheel assemblies, especially if the pilot is wearing good clothes. (Maybe that is why high-wing aircraft were invented.) The good news is once a pilot goes under a low-wing aircraft to check the tires and brakes, they are also in a great position to check the fuel sumps. So it is a win-win adventure for the truly disciplined pilot. It need not be a challenge of the spirit and body. Just don't try to kick the tires while you are down under that low-wing.
Generally speaking for most of us who fly aircraft with disk brakes, we may only give a quick look at our brakes to see if there is some material on the pads, to check for any obvious hydraulic fluid leaks, and to see if a tire is attached. But according to one manufacturer's aircraft disk brake service manual there are some other checks that the average pilot may want to consider.
Although the following are some of the listed on-aircraft maintenance instructions written for mechanics inspecting an aircraft brake assembly still on an aircraft, the instructions provide additional information for all. No, we are not suggesting that pilots carry a torque wrench in their flight bags to check the bolts on their brakes since this check list is designed for mechanics, but it does give you something to think about when preflighting your aircraft.
1. Visually inspect the brakes for corrosion, cracks, or other visible damage. Check inlet fitting bosses and anchor bolt lugs for cracks and fluid leakage.
2. Check back plate attachment bolts to insure they are properly torqued and have not worked loose. Gaps between the back plate and cylinder would be evidence of this.
3. Check fit of brake cylinder anchor bolts in torque plate bushings for sloppiness. This can be accomplished by grasping the cylinder and moving it; slight movement is normal. Excessive movement is cause for removal and detailed inspection.
4. Linings should be visually checked for extreme chipping on the edges. Lining worn to minimum thickness of an 1/8 inch (2.54 mm) must be replaced. If you wear off more lining material you will also take off the heads of the brass rivets that hold the lining into the back plate and the rest of the lining will fall off.
5. Visually check torque plate for corrosion, cracks, loose anchor bolt bushings, or other visible damage. Anchor bolt bushings must be flat against torque plate surface.
Although this list is only representative of a typical disk brake assembly, it does provide more items to check and what to look for than the typical pilot preflight checklist. If you find something that doesn't look right, contact your aircraft maintenance technician for help.
The manual also says that it is important to visually check wheel assemblies for "corrosion, cracks, or other visible damage." Wheel nuts should be checked to make sure the nuts are installed on the bolts and have not worked loose. The book said that bolt threads should be flush to 1-1/2 threads beyond the nut.
Pilots should also check their brake disks for "rust, excessive grooves, large cracks, or other visible damage."
For most of us, the real check for all brakes is when the aircraft is first moved. The aircraft should be started moving in a safe direction, and then the pilot should apply the brakes to see if the brakes stop the aircraft. Since most aircraft have differential braking, you are looking for even braking on both sides of the aircraft with the same amount of braking. If not, you should shut down the aircraft and have the brakes checked by a maintenance technician.
The key to doing the first brake check after startup is to always point the aircraft in a safe direction. One that would NOT damage you or your aircraft or anything else if you suddenly discover you don't have brakes. Or, as your insurance company would remind you, "If you must hit something, please make sure it is cheap to repair or replace."
Finally, the manual pointed out that after new brake linings are installed on an aircraft, they must be properly conditioned to ensure they provide the estimated service life. Depending upon which type of brake lining is installed, metallic or organic, there is a specific conditioning procedure designed for each. The procedures are not interchangeable. So if you want the most life from your brakes, and if your mechanic does not condition them, you need to follow his or her instructions to the letter. Failure to follow instructions could result in premature wearout of your brake linings.
Brakes and wheels are only part of the stopping story. To paraphrase an old expression from the automotive world, tires are where the rubber meets the road. Another manufacturer summed up the importance of aircraft tires and the design challenge of aircraft tires by stating in one of its brochures, "Aircraft operating conditions require a wide variety of tire sizes and construction. The modern aircraft tire is a highly engineered composite structure designed to carry heavy loads at high speeds in the smallest, lightest configuration practical. In many cases, retreadability is also a design requirement."
These comments are from a Goodyear Tire and Rubber Company publication "Aircraft Tire Care and Maintenance, What You Should Know About Aircraft Tires." The title says it all. The manual was written to help aircraft owners and maintenance technicians get the maximum life from their aircraft's tires.
Like automobiles, aircraft can use either bias or radial tires. Check your aircraft manual for the specified type of tire recommended for your aircraft. If changing type tires, owners need to make sure the type of tire being considered is approved for use on the aircraft because bias and radial tires may have different operating characteristics and limitations. Like cars, types of aircraft tires should not be mixed unless approved by the aircraft maintenance manual. For example, although the nosewheel tire construction may be different than the main tires, the mains should be of the same construction whether radial or bias.
Since it is common to retread aircraft tires, buyers need to know and understand the markings used on tires. Of particular importance is the Technical Standard Order (TSO) used to certificate the tire. Like automobile tires, aircraft tires have specific speed and load ratings. It is important that those ratings not be exceeded. The correct tire rating data for each aircraft is listed in the aircraft's manuals.
Goodyear states that proper tire preventive maintenance is an operator's best way to reduce tire expenses to the lowest level. It also says that proper tire inflation is the single most important part of a good tire preventive maintenance program.
Unlike coins that are either heads or tails, tires can be over inflated, under inflated, or within approved pressure. Of the three states, under inflation is the most damaging and potentially dangerous. Under inflation causes a tire to flex too much, which shortens tire life by uneven tread wear on the edges and excessive heating. Over inflation also can cause uneven tread wear in the middle of the tread, reduce traction, make the tread more susceptible to cutting, and increase stress on aircraft wheels. Goodyear also recommends "...that only dry nitrogen be used for tire inflation as nitrogen will not sustain combustion and will reduce degradation of the inner-liner material due to oxidation."
Without a pressure gauge, pilots can only look to see if the tires are inflated about "right." The only way to tell if a tire is properly inflated is with an accurate (tested) tire pressure gauge used on a daily basis. Once the pressure is checked, pilots should check the tire for wear, cuts, or other damage.
Tires used on training aircraft need to be checked carefully for hard braking or skid wear. Skidding is when someone locks the brakes after landing and skids the locked wheel and tire assembly down the runway. The result can be a ruined tire with one spot worn down to the core threads on an otherwise perfectly good tire. If a tire has a side wall crack or cord is showing, such a tire is no longer serviceable.
The need to check tires for wear and damage is why many pilots don't like wheel covers on aircraft. It is easier to inspect a wheel assembly when everything is in view. The extra knot or two of airspeed the wheel covers provide may not be worth not being able to check the tire without having to move the aircraft.
Goodyear had several important suggestions about how to check aircraft tires.
Check tire pressure only on cool tires. Allow at least two or three hours after landing.
Use only the airframe manufacturer's recommended inflation pressure. You must determine if the pressure listed is the "loaded or unloaded" pressure. Unloaded pressure is no load on the tire. Because of the weight of the aircraft, tires installed on an aircraft are "loaded." Your maintenance manual should have the conversion factor to convert between unloaded and loaded pressures.
When flying from a cold region to a warmer region, you must adjust your tire pressure for the "worst" condition which is the colder area. When flying into a colder area, tire pressure should be adjusted for the anticipated colder requirement before departure. "An ambient temperature change of 5 degrees F (3 degrees C) can produce approximately 1% pressure change."
Excessive inflation pressure should never be bled off from hot tires. All adjustments to inflation pressure should be performed on tires cooled to ambient temperature.
It is normal for mounted tubeless tires to lose a slight amount of pressure because of gas diffusion through the tire casing. Because tires can "stretch or grow" after being installed, it is important that they not be mounted until they have been inflated for a minimum of 12 hours and reinflated if necessary to recommended inflation pressure.
Nylon tires may develop a "flat spot" after sitting for a period of time. Factors such as load, temperature, and tire deflection can effect the development of the flat spot. Goodyear recommends that aircraft with nylon tires being parked for long periods (30 days or more) be jacked up to reduce the weight on the tires. Normally, flat spots should roll themselves out during taxiing.
The company also has a special recommendation for tires involved in a rejected takeoff (RTO). "Tires subjected to above normal braking energies during an RTO should be removed and scrapped. Even though visual inspection may show no apparent damage, tires may have sustained internal structural damage that could result in premature failure. Also, all wheels must be checked in accordance with the applicable Wheel Overhaul or Maintenance Manual after an RTO."
1. Dual tires mounted on the same land gear axle have limitations within which their diameters must match to ensure equal load bearing.
2. Chemical contaminants such as oil, grease, and tar should be removed to protect the tire. A recommended way is to wipe the tire off with denatured alcohol followed by washing with soap and water.
3. Since sunlight and weather can have some affect on rubber tires, tires on aircraft tied down outside can be protected more by covering them with protective, sunlight reflective coverings.
4. Runway defects, chuck holes, runway cracks, stones, and other items such as nuts and bolts and other sharp items laying around tie-down areas and hangars can all damage tires. Pilots need to watch out for such items and report runway and taxiway damage as soon as possible so that repairs can be made.
5. It is important that tires and wheels be properly balanced. "Vibration, shimmy, or out of balance is a major complaint. However, in most cases, tire balance is not the cause. Other items affecting balance and vibration are: installation of wheel assembly before full tire growth; improperly torqued axle nut; improperly installed tube; improperly assembled tubeless tire; out of balance wheel halves; poor gear alignment; bent wheel; worn or loose gear components; or flat spotted tire. In addition, pressure differences in dual mounted tires and incorrectly matched diameters of tires mounted on the same axle may cause vibrations or shimmy."
Inflated aircraft tires, especially high-pressure tires, are all potentially dangerous both on and off an aircraft. High-pressure tires can explode with deadly force. Everyone handling, working with or around aircraft tires need to know and follow all recommended safety advice and procedures, such as deflating the tire before proceeding with any maintenance.
In discussing braking and tire damage, something new pilots may not think about is how bad locking an aircraft's brakes is on stopping an aircraft. A locked, skidding wheel assembly is not efficient in stopping an aircraft. Properly applied rolling friction on a properly inflated tire is what stops a tire in the shortest distance. The importance of rolling friction and maintaining directional control is why all of the anti-lock braking systems on today's aircraft and automobiles are designed to keep tires from locking up. When the system's anti-lock sensors detect a wheel locking up, the system automatically releases brake pressure to that wheel to unlock the wheel. Once the wheel is again turning, the braking action is reapplied. Maximum braking is developed just before lockup.
When you are skidding, especially if you are hydroplaning on a water covered runway or street, you are literally out of control. If hydroplaning, you are floating on a thin layer of water with minimal braking action because you have little or no rolling friction with the hard surface. When you are skidding on a dry runway, you again are reducing your braking and control effectiveness while creating excessive tire wear. Flat spots of excessive wear on an otherwise good tire are a good indictor of a heavy foot on the brakes.
If you have anti-locking brakes you need to operate them in accordance with their operating instructions. Improper operation can result in loss of efficiency and anti-locking benefits. Anti-lock brakes normally require that full braking pressure be applied to the brake system. The system then automatically manages the braking action.
Since most small general aviation (GA) aircraft don't have anti-lock brakes with their special sensors, computers, and equipment to maximize braking, GA pilots must become their own anti-skid computer and brake operator. The pilot must recognize when he or she is starting to lock up the brakes, release brake pressure to regain rolling friction, and then carefully reapply brake pressure to maximize braking action. For GA pilots accustomed to driving their automobiles and trucks with anti-lock brakes, they may have to take special care in their aircraft to avoid using the wrong braking technique.
If you have any questions about the proper braking technique for your aircraft, check your aircraft's pilot operating handbook, check with a knowledgeable certificated flight instructor or certificated aviation maintenance technician who knows your aircraft, or contact your aircraft's manufacturer.
As we found in researching material for this article, your aircraft's tire and brake manufacturers are also a great source of information.
Finally, always remember to keep the blue side up and tread on the tires.
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