Special Report: Landing Gear - The Forgotten System
The rejected takeoff of a Boeing 777 at approximately 160 knots results in the landing gear absorbing more than one billion joules of energy in a few seconds, according to research recently done by Messier-Bugatti. To put this number in perspective, consider it as the equivalent of supplying energy to the average house (lamps, outdoor lights, refrigerator, television, computer and other appliances) for about 18 hours. While the 777 weighs in substantially heavier than almost all corporate aircraft, there is still a lesson to be learned: every airplane’s landing gear takes a beating.
Sam Jantzen, v-p of marketing and sales for Commuter Air Technology of Scottsdale, Ariz., waxed poetic and put it this way: “Of all the various and sundry systems, from the simple to the complex, that are incorporated in the design of an aircraft and are fully intended to perform their needed function to the highest designed level of performance, there is none that is so absolutely ordinary and common as the system on which the aircraft departs and arrives the landing surface. And, at the same time, there is none, no matter its level of sophistication, that takes the abuse that this blue-collar working system endures day after day and without due recognition. All too often it goes about its business without the comfort of a caring inspection and a polite touch before the commencement of a flight, or a comforting look and quiet thank you after a routine return.”
Landing gear systems take the most severe punishment of any system on the aircraft, bar none. They are in contact with outside elements, feeling firsthand the bombardment of water, ice, slush, snow, freezing temperatures and soaring heat. And these assaults can often come in rapid succession.
There is little anyone can do about the environmental issues, but Jantzen stresses that the key to landing gear health is maintenance. “When it comes to keeping your landing gear healthy, effective maintenance heads the list. Each tire, wheel, brake, strut and other system component, while performing the same overall function, is vastly different in care and adjustment,” he said. And the ground crew that handles the aircraft plays an important role in the health of the gear.
Landing gear doesn’t like tight turns because they are harmful to connecting hardware and extremely hard on tires. Tug-induced tight turns can cause severe tire sidewall strain on standing or slow-moving aircraft. When coupled with takeoff friction heat, the result can be premature breakdown in the tire sidewall strength.
Karl Detweiler, technical support for Duncan Aviation of Lincoln, Neb., explained how subtle potential problems can be. “Keep struts wiped down with a rag that has hydraulic fluid on it to clean and lubricate them,” he said. Further he explained a common occurrence: “Often the crew will see the wing drop as the aircraft is being fueled. In the same way, you may feel the aircraft drop after the gear has touched the ground. What’s probably happening is that the environmental part of the seals became dry, so instead of easily moving up and down the strut hangs up until there’s sufficient weight, and then it gives and causes the strut to compress rapidly. You really wear the seal out quickly that way. Wiping down the struts and lubricating them will extend seal life.”
Chips with That?
According to Detweiler, a chip on the chrome part of the strut extension is a major cause of hydraulic leaks. “Chips or scratches on the piston as it extends and contracts through seals and scrapers will cause deterioration in the O-rings, leading to hydraulic leaks. The best method of detecting them is simply to run your hand up and down the chrome part of the extended strut to feel for chips,” he said.
Flying from warm to cold weather airports will affect strut height. For instance, on the ramp in Tampa during December there may be four inches of strut extension; but in Fargo, N.D., there may only be 2.5 inches. The difference isn’t a hydraulic fluid problem. The nitrogen charge expands and contracts with temperature. As long as the strut isn’t bottomed out, there shouldn’t be a problem.
Incidentally, nitrogen is used in struts because strange things happen with oxygen or air. When either oxygen or air combines with moisture it can lead to internal corrosion. In addition, strut performance is calculated by engineers based on nitrogen. Air expands at a different rate and will not perform as designed.
One of the most common landing gear maintenance-related mistakes relates to painted gear. According to Detweiler, paint on landing gear is more than cosmetic–it is there for corrosion prevention. “Keep an eye out for missing paint,” he warned. “It allows corrosion to start. Corrosion is the number-one cause of damage to landing gear. Really watch out when you have the gear repainted. If it isn’t done right it’ll start to peel and lead to corrosion. And particularly watch for towbar use because it will chip paint. If you find a chip while you’re on the road, you can get some primer from the local FBO and just touch it up with a small brush.”
Similarly, aircraft owners often suggest to flight crews that they should clean up the gear so it’s spotless. In an effort to make the boss happy, crews will use a pressure washer to clean the gear. “One of the most damaging things you can do to gear is to use a pressure washer on it,” Jantzen stressed. “You can wash the lube out of the bushings. Even if you go back and relube it there’s no guarantee that you can get the water out, and water leads to corrosion. The best method is to do it by hand with a brush, solvent and rag. You might not get the gear as clean, but you will extend its life.”
Jantzen suggested that when preflighting the aircraft it’s important to look inside the wheel itself to visually inspect the brakes, if possible. “Make sure everything is tightened up and in good condition. Particularly watch out for brake-line chaffing to head off a potential problem,” he said. “That’s also a good time to take a look at wear on the tires. If you consistently see uneven wear between tire changes, you probably have a gear-alignment problem–either toe in or toe out. Some aircraft, such as the Citations, are adjustable by torque links, but others can’t be adjusted. Gear can warp and twist over time, especially with hard or sideways landings.”
Hard landings that exceed the shock absorber’s ability to handle the load will damage the gear. Depending on the severity of the impact, the damage may range from wheel shimmy or vibration to total collapse of the landing gear. It is worth noting that normal wear on the gear can result in loose bolts and torque links that may also cause vibration. The mere presence of vibration in the gear does not necessarily betray a hard landing. While landing gear damage as a result of a hard landing is probably going to be obvious, permanent internal damage to a wing can be done with less force. Oil canning is an example.
When an aircraft lands hard on the gear it causes the wings to flex excessively, which stretches the skin. When the pressure is released the skin has been stretched and never completely returns to its original state. If you can push on the skin on the underside of the wing and make it flex, that’s probably oil canning; think wing damage, and particularly spar damage. This is something that should be carefully checked for on aircraft that have been used for training purposes.
The entire subject of landing gear is far more complex than it might appear at first blush. Goodrich’s aircraft wheels and brakes division, for example, designs and qualifies wheels for a specific aircraft, with the tires specified by the airframe manufacturer. Changing a tire has implications for the type of wheel on the aircraft, but even what constitutes change is not intuitively obvious.
Goodrich defines a “tire change” as any one of the following:
• New tire supplier, size or ply rating.
• Significant modifications of the original tire, including change in the manufacturing process.
• Changes in the tire manufacturing site.
• Changes in tire rating, retread level, tire life, tire weight or stiffness.
“This is a critical safety issue as we develop lighter and longer-life wheels,” said Steve McCrillis, director of engineering for Goodrich’s regional, business and military business unit. “Many of our automotive wheels are specified for a service life of more than 50,000 miles to reduce the operating costs. The fitment and load transfer from the tire to the wheel are keys to the safety and reliability necessary to achieve that service life. Wheel stress tests have shown that just as the tire life may vary from one manufacturer to another, the load imparted to the wheel will vary among tire suppliers and may significantly affect the wheel’s fatigue life and resulting inspection requirements.”
As an example, McCrillis cited the case of a tire manufacturer’s moving production from one factory to another. No tire change was reported, yet wheel stress tests showed a 60-percent reduction in the wheel’s beadseat life. Ultimately, the wheel had to be redesigned and operators were forced to retrofit their wheels to accommodate the revised tire.
“Once a wheel and the assembly are qualified, any changes to the original tire should be evaluated by Goodrich to ensure that the change has not jeopardized the wheel’s structural integrity, service life or maintenance requirements,” McCrillis said. “The tire change may necessitate a stress test to assess the effect on the wheel. Without this evaluation, the safety, reliability and warranty coverage provided by Goodrich may be affected.”
Alex Dumm, general manager of aviation tires for Goodyear Tire & Rubber, carried that thought a step further. “You have to understand, an inflated tire/wheel assembly is a potentially explosive device. Aircraft wheels made today, for both tube and tubeless tires, are the split wheel or demountable flange variety,” he explained. “While this makes the job of mounting and demounting physically easy, strict attention to detail is required. Mounting and demounting of aircraft tires is a specialized job that is best done with the correct equipment and properly trained personnel.”
Another area of major concern is brakes. On the subject of carbon disc brakes, Detweiler said, “You do not want to get hydraulic fluid on brakes because it will damage the carbon. Look for that regularly during preflight inspections to be sure there are no leaks. Hydraulic fluid will harm the rubber of your tires over time, too.”
Sandi Schickel, service center team leader for Parker Hannifin corporate aircraft wheel and brake division, offered some preflight advice. (Parker Hannifin manufactures wheels and brakes for various Cessna Citations.) “One thing the crew should pay attention to during a preflight inspection are the fuse plugs. Make sure they are intact on the wheel assemblies. Any disturbance indicates the wheel has been overheated at some point. Proper tire inflation is also important, and so is proper tire alignment,” she said.
Schickel also suggested that the retract studs in the brake assembly are also wear indicators. “When the broached end of the three retracts are flush with the brass friction sleeve, the brake assembly should be overhauled. This should be checked so the pilot knows if brakes are getting close to the overhaul point. Excessively worn brakes compromise performance.” It is also worth noting that for any aircraft, a rejected takeoff or hard, hot landing can also compromise the future performance of wheels and brakes.
Carbon Brakes Require Special Care
Mike Patterson, Goodrich service engineer, offered some insight into extending the life of carbon brakes. “Minimize the number of brake applications during taxi, especially while the brakes are relatively cold during the taxi out,” he suggested. “You might be surprised to learn that repeated brake applications during taxiing while the brakes are cold can have a greater effect on brake life than the landing stops. This commonly occurs as the pilot applies the brakes over and over to maintain a constant speed on taxiways and around gate areas. To increase carbon brake life, simply reduce the number of times the brakes are applied, focusing primarily on the taxi-out stops when the brakes are cooler. Rather than repeatedly applying the brakes to maintain a constant speed, allow the speed to build up between brake applications.”
Patterson recognized that this advice might go against a pilot’s instincts. However, unlike steel brakes, which wear faster as temperature increases, carbon brake wear decreases as brake temperature increases. After the landing stop, taxi stops on the way to the ramp cause very little wear. Goodrich also cautions against spraying de-icing agents on landing gear. “De-icing fluids can be harmful to carbon brakes. During normal usage, the brakes generate enough heat to keep themselves dry and thus ice-free. However, during exposure to freezing temperatures both carbon and steel brakes should be kept dry to prevent freezing,” said Jeremy Wick, a Goodrich service engineer. “This may be done by using wheel covers while spraying the aircraft, avoiding taxiing through standing water and drying the brakes through extra braking before a flight.”
Anatomy of a Tire
Bill Wilkerson Sr., president of Wilkerson Co. of Crewe, Va., shared his simple explanation of a tire. (Wilkerson is one of four licensed high-speed aircraft tire retreaders in the U.S. It ships approximately 80,000 aircraft tires annually, including 50,000 retreads and 30,000 new Goodyear and Dunlop aircraft tires.)
“To understand tires,” Wilkerson said, “you have to understand how they’re built. First, take two coat hangers, bend each of them round and you have just made bead wires. Then take a pair of nylon hosiery and stretch it across your bead wires for your nylon ply. If you want a bias ply, place them across the beads at about a 45-degree angle or, for a radial, place them so they go straight across. Now you have a basic tire, but the nylon needs protection from the sun and moisture.
“For protection of the plies we’ll stretch a rubber snow boot over the nylon hose. Now you have a tire with bead wires, plies and a rubber coating, but it’s not really durable,” Wilkerson explained. “For that, we’ll add a pair of shoes and place them on top of the snow boots. Now you have tread, completing the tire.”
While Wilkerson offered this example tongue-in-cheek, he makes the point that a tire is a complex combination of dissimilar materials. “In production, high-tensile steel wire is wound to make the bead wire; rubber-coated nylon fabric is cut and placed at the correct angles to give the tire its shape, size and strength; the fabric is covered with rubber for protection; and then the tread rubber is applied for durability,” he said. “Essentially what you have is a fancy balloon.”
Wilkerson used a 6.00-6 tire as an example. “If you take a brand-new 6.00-6 that has never been inflated and measure the diameter, you will find the diameter to be probably around 16 inches. Inflate it to 20 psi, measure it and you’ll find it’s around 16.25 inches. When you inflate it to the average operational setting of about 30 psi, the tire will grow to its designed operating diameter of 17 inches,” he said. “Now, if you decide to overinflate the tire, it gets bigger just as a balloon would get bigger. Generally, a tire can withstand four times the operational pressure before it blows, but in our example it will probably be around 22 inches in diameter. The point is simply that tire size is dictated by air pressure.”
He said it is common for the same size tire to have different ply ratings because aircraft designers can match the tire ply rating with the load requirement for a given airplane. “As the load rating increases, the ply rating increases,” he explained. “The main difference is that as the ply rating increases, the air pressure increases so that no matter what the ply rating of 6.00-6 is, for example, the operational size will be 17 inches in diameter. Think of it like this: it will take more pressure to make an eight-ply tire grow to 17 inches than it will a four-ply tire because the eight-ply is a thicker balloon.”
The reason it is important to understand how a tire works is because air pressure is important not only in controlling the size of the tire; it also allows the tire to bend (deflect) in the correct location of the sidewall. Correct pressure allows the tire to run cooler, and it allows the tread area to provide the footprint for which it was designed.
Dumm underscored the issue: “Keeping aircraft tires at their correct inflation pressure is the most important factor in any preventive maintenance program. The problems caused by underinflation can be particularly severe. Underinflation produces uneven tread wear, and the heat generated by excessive flex shortens tire life. Overinflation can cause uneven tread wear, reduce traction, make the tread more susceptible to cutting and increase stress on aircraft wheels.”
Goodyear recommends that only dry nitrogen be used to inflate tires because it will sustain neither combustion nor oxidation of the liner material and casing plies. Dumm emphasized that tire pressures should be checked daily using an accurate gauge.
“Ideally, pressure on high-performance aircraft tires should be checked before each flight,” he said. “Check cool tires at least two to three hours after a flight using a calibrated gauge. Inaccurate gauges are a major source of improper inflation pressures. Gauges should be checked periodically and recalibrated as necessary.”
“Why check your tires?” Wilkerson asked. “First of all, your tire performance, or tread life, will probably increase around 10 percent when the tires are properly maintained. Second, and probably more important, your risk of tire failure will be greatly reduced.”
Aircraft Tire versus Car Tire
“The major design philosophy of an aircraft tire, as compared with a passenger car or truck tire, is that it is designed for intermittent operation,” Dumm explained. “Because of this design feature, and to allow the lowest possible ground bearing pressure, the aircraft tire operates at much higher deflections than other tire types. [Tire deflection refers to the amount a tire deflects (or deforms from circular) when rolling under load.] Aircraft tires are designed to operate at 32-percent deflection, with some as high as 35 percent. In comparison, car and truck tires operate in the 17-percent range.”
Compare similarly sized aircraft and automotive tires: the aircraft 27X7.75-15 and the passenger vehicle P205/75R15. Both are nominally 27 inches in external diameter with roughly the same section widths (7.75 inches for the 27X and 7.99 inches for the P205). The aircraft tire is designed to carry 9,650 pounds per tire–approximately six times the load of the automotive tire, rated at 1,598 pounds. The airplane tire is also rated to operate at up to 225 mph compared with the automotive tire’s maximum rated speed of 112 mph. The 27X has an operating pressure of 200 psi, nearly six times that of the 35-psi P205, and it operates at a deflection of 32 percent compared with 11 percent for the auto tire. The dual demands of heavy load and high speed make for extremely severe operating conditions for aircraft tires.
Heavy loads and high speeds, which cause the heat generation in aircraft tires to exceed that of all other tires, can have a detrimental effect. Rubber, the major material used in a tire, is a good insulator and therefore dissipates heat slowly. For this reason aircraft tires can be used only intermittently. The internal heat generation is significantly affected by taxi speed, inflation pressure or deflection and taxi distance. High taxi speeds and improper inflation substantially reduce tire life.
According to Goodyear, both heavy loads and high speed contribute to the severe centrifugal forces that act on an aircraft tire. High centrifugal forces can cause traction waves to develop, with the onset highly dependent on inflation pressure, which controls deflection. Traction waves can lead to groove cracking, rib undercutting and tread separations. The footprint of the tire on the ground causes that portion of the tire to deflect because it is bearing the weight of the aircraft, but the footprint changes constantly.
As a given portion of the tire leaves the deflected area it attempts to return to its normal shape. Due to centrifugal force and inertia, the tread surface doesn’t stop at its normal periphery but overshoots, thus distorting the tire from its natural shape. This sets up a traction wave in the tread surface, and the greater the underinflation the more pronounced the traction wave. The solution is simply to have the tire properly inflated. In general, tread longevity is shortened by improper tire pressure, high operating temperatures, excessive high-speed braking, sharp and fast turns and long rollouts.
Marion DeWitt, regional manager/product engineer for Michelin Aircraft Tire, said his company stresses tire training for both pilots and mechanics. “Maintaining pressure is the key thing. We make an issue about doing daily pressure checks. You always want to be sure that you have correct operating pressure before takeoff. Aircraft tires are designed to operate at a specific pressure, and deviation from it causes problems.”
Overinflation shifts wear to the centerline of the tire, according to DeWitt. “You get a smaller footprint, which has a slight impact on braking distance but the real issue is wear: it moves from the whole footprint to the center, reducing the life of the tire. Underinflation causes the wear to move to the outside of the tire. The point is that you must operate within the tire’s designed deflection, which for a standard tire is 32 percent. For an H-type tire, such as that used on the Premier I, it is 35 percent,” he said.
“Designed deflection is the percentage of the section height of the tire that deflects under load. That’s where your tire generates heat. An underinflated tire could be at 40-percent deflection, and that is going to generate a lot of extra heat and degrade the tire’s handling characteristics. Reduced handling might not be a big deal because you’re only going straight, but the heat generates fatigue within a tire and it degrades the tire’s internal performance characteristics. The bottom line is always check your pressure.”
While careful adherence to good maintenance and operating procedures has a powerful effect on tire life, sometimes it isn’t enough. When an Air France Concorde’s tires apparently hit a metal foreign object on takeoff from Paris in July 2000, the subsequent tire damage initiated a catastrophic sequence of events. According to DeWitt, that accident played a significant role in the development of Michelin’s NZG (near-zero growth) tire.
“After the Concorde accident the industry began exploring ways to produce a tire that would have better FOD resistance,” DeWitt said. “The NZG tire has a higher modulus [stiffness] chord made from a high-tensile textile. It makes the chord more stable and less temperature sensitive, so the tire maintains its geometry as it heats up.” What that means is that the NZG tire doesn’t exhibit the elongation of a standard nylon tire, so there is less stress on the rubber–and that reduces its propensity to tear.
“If you take a rubber band and stretch it tight, the rubber wants to tear. The slightest nick in the rubber will cause it to rip apart at that spot,” DeWitt explained. “But if the rubber band is not stretched, if there is little tension, then it’s harder to cut through and there is little propensity to tear. NZG reduces that tension.”
DeWitt said radial tires have approximately 8-percent total growth (pressure and centrifugal force), biased tires about 12 percent and the NZG about 3 percent. “The more growth you have, the more stress you have because you’re increasing the circumference of the rubber just as you are with the rubber band,” he said.
In the corporate aviation sector, NZG tires are currently available on the Falcon 20, 50 and 2000. They will also be available for the Cessna Sovereign. Dumm said Goodyear is working on a similar concept but the company is not yet ready to release an availability date.