Airliner wheels are subjected to the daily punishment of multiple takeoffs and landings. Tires are exposed to temperatures below -40°C during cruise. At touchdown, rubber temperatures can momentarily exceed 200°C.
Wheels must handle the most extreme torture in aviation: a maximum weight, high-speed rejected takeoff: A fully loaded aircraft accelerates to takeoff speed, then stops on the remaining runway. Tires withstand extreme heat and stress until the aircraft is safely stopped.
Few aircraft components take more daily abuse than the tire and wheel assembly.
Tire vs Tyre
Readers outside of North America likely prefer the spelling “tyre” when referring to the rubber portion of an aircraft wheel. Please bear with me as I use the spelling common in my neighborhood.
Aircraft tires are too rigid to be forced onto a rim like automotive tires. Aircraft wheel hubs come in two parts. The inboard and outboard hubs are bolted together with the tire in the center, then pressurized with nitrogen.
Nitrogen Instead of Air
A gas station air pump is fine for filling car tires, but large airliner tires must be filled with an inert, dry gas. Nitrogen is inexpensive and perfect for the job.
Nitrogen-filled tires reduce the chance of fire or explosion (it’s an FAA regulation). Tire rubber is flammable and wheel brakes reach very high temperatures. A large tire with 200 psi of atmospheric air would provide a lot of oxidizing power to feed a fire. Nitrogen does not support combustion, greatly reducing the risk of a tire fire or explosion.
Other Benefits of Nitrogen
- Dry nitrogen contains no water vapor. The lack of moisture reduces tire pressure variations at temperature extremes (water density varies significantly at different temperatures). With the effects of moisture eliminated, change in tire pressure due to temperature is linear and predictable
- Oxygen and moisture in atmospheric air cause corrosion to aluminum and steel wheels. Dry nitrogen eliminates this problem.
- Air and moisture cause oxidation of a tire’s inner liner. Nitrogen won’t degrade the rubber.
- Due to their larger effective molecular size, nitrogen molecules permeate through tire rubber at a slightly slower rate than oxygen molecules. Using nitrogen may marginally contribute to reductions in tire inflation loss by permeation.
Should we use nitrogen in our automobile tires?
An article from Scientific American suggests that maintaining proper tire inflation weekly is far more important than spending extra money for nitrogen.
Keep your tires balanced, rotated, and at the proper pressure to save fuel and maximize tire life.
Large airliners are heavy (right?). A Boeing 767 has a max takeoff weight of over 400,000 pounds. A fully loaded 747-8 weighs nearly a million pounds. All that weight rides on a handful of tires.
Automobile tires are pressurized to around 30-40 psi. If large aircraft tires were filled with 35 psi, they would be flat under the weight.
Large aircraft tire pressures are ridiculously high. A Boeing 767-300 main wheel is inflated to 205 psi. The high pressure supports the tire’s maximum rated load of 51,000 lbs.
Tire Safety Devices
Aircraft wheels incorporate safety devices to protect the aircraft and personnel working nearby.
Fusible (or thermal) Plugs
Fusible plugs protect tires and wheels from exploding if the brakes get too hot. A fusible plug is a small hollow bolt filled with low melting-point metal (like solder used for electronics or plumbing).
In the event a wheel becomes too hot, the soft metal in the plug melts at a predetermined temperature to allow the tire to safely deflate.
Fusible plugs often come into play after heavy braking, as would happen during a high-speed rejected takeoff. After the aircraft stops, the hot brake assemblies continue to heat the wheels until the fuse cores reach their melting temperature and deflate the tires.
Fusible plugs are mounted inside the wheel hub. When the plugs deflate the tire, nitrogen is directed over the brakes to aid in cooling. Pretty clever!
Over Pressure Relief Valve (OPRV)
An over pressure relief valve is a hollow bolt with a rupture disk inside. The disk ruptures when nitrogen pressure exceeds the design limit.
OPRVs are installed on most wheel rims to protect tires from over-pressure or explosion that can occur during nitrogen servicing.
On a Boeing 767, the pressure relief valves release pressure at 375-450 psi.
How important are OPR valves? Over-pressurization accidents have dismembered and killed maintenance personnel. Aircraft tires are so strong that the wheel rim and bolts fail before the tire, launching shrapnel outward. OPR valves reduce this risk. Maintenance technicians receive special training before they can service wheels.
TPMS (Tire Pressure Monitoring System)
Some aircraft models have TPMS sensors in their wheels. The system is very similar to the TPMS in automobiles. Cockpit displays show tire pressures for all tires equipped with the sensors.
The TPMS triggers an alert in the cockpit if a tire has low pressure. The UPS fleet has two fleets with TPMS; the 747 and MD-11.
Brake Temperature Monitoring System
Many large aircraft have brake temperature monitoring systems. The photo below shows the system on a Boeing 767-300F.
Each of the 8 boxes represents a main gear wheel (there are no brakes on the nose wheels). Unlike the MD-11 in the previous photo, the Boeing system doesn’t display actual temperatures. The numbers 0-9 represent temperature ranges.
Temperatures 0-2 are cool to warm. The above photo was taken after landing on a long runway, using light braking.
The Normal temperature range is 3-4. It’s typical to see twos and threes after a normal landing. An occasional four after a heavy weight landing on a hot day is common.
High temperature range is 5-9. When brake temps reach the high range, a BRAKE TEMP warning light illuminates. At 5-6, wheel fuse plugs may melt and deflate the tires. If the brakes reach 7-9, the crew will exit the runway and stop the aircraft. Airport fire fighters are called to monitor the landing gear in case of fire. Tire, wheel, and brake replacement may be required. Temperatures this high are typically caused by an emergency landing or rejected takeoff.
Actual brake temperatures: 5 correlates to 371°C – 427°C depending on the type of brakes installed (steel vs. carbon). That’s smokin’ hot!
Big Airliner Tires Are Big
Like cars and trucks, aircraft tires come in many sizes. Tire size data is molded into the sidewall of every tire. A Boeing 757-200 uses H40x14.5-19 tires on the main landing gear. Decoded, the “H” means high deflection, 40 inch tire diameter, 14.5 inch tire width, and 19 inch wheel/rim diameter.
Main gear tire diameter and width for a few popular airliners:
|Boeing 737-700, 800, 900||44.5″||16.5″|
|McDonnell Douglas MD-11||54″||21″|
|Embraer ERJ 170/175||38″||13″|
|Embraer ERJ 190/195||41″||16″|
Big Wheels Are Heavy
747-8 main tires weighs 270 lbs each. A fully assembled -8 wheel with hardware is close to 550 lbs!
A Boeing 757 main tire weighs about 150 lbs. Main tires for an Embraer ERJ190 regional jet are about 145 lbs each.
Tire Chines (or deflectors)
Chines are sometimes found on the nose wheels of aircraft with fuselage mounted engines. The edges of the tire sidewalls have a curved protrusion (chine) that deflects standing water outward to reduce water ingestion into the engines.
The Boeing 727 uses chined nose wheel tires as well as several other aircraft with fuselage mounted engines. I’ve seen them on Embraer RJs and a few business aircraft. See if you can spot them!
Who makes aircraft tires?
You might recognize the names of aircraft tire companies. They also manufacture automobile tires! Goodyear, Michelin, Dunlop, and Bridgestone, to name a few.
Who owns the tires?
Airlines often purchase tires directly from the manufacturer and retain ownership for the life of the tire. When tires are sent back to the factory for retreading, the same tires are returned to the airline that owns them.
There are also tire leasing and tire service contracts available. Each airline makes their own deal with tire distributors and manufacturers.
Retread tires? On aircraft?!
An aircraft tire carcass/casing (tire without the tread) is constructed super-tough. A carcass that is eligible for retread is a desirable asset; it has demonstrated that it can stand up to the abuse of airline operations.
Retreading a tire is less expensive than buying a new one. Some tires can be retread as many as 16 times! Airlines often retread tires less than the manufacturer’s limit as an added measure of safety. Another benefit: retreads have more plies than new tires so they can handle more takeoffs & landings.
Don’t retreads fall apart?
Let’s talk about commercial truck retreads for a moment… Big chunks of disintegrated tires litter the sides of busy highways. Is it fair to blame retread tires for the debris?
NHTSA Truck Tire Study
The U.S. National Highway Traffic Safety Administration published a commercial vehicle tire debris study. Researchers analyzed hundreds of tire debris samples to figure out why the tires failed. The results show that retread and Original Equipment (OE) tires fail at about the same rate.
The majority of truck tire failures (retread or OE) are not caused by problems with manufacturing. The number one cause of tire failure is “road hazard” — potholes, nails, car parts, and other hazards on the roadway.
The study lists the second highest cause of tire failure as “maintenance and operational issues” — overloaded trucks, improper tire inflation, and worn out tires. In other words, operators aren’t taking care of their tires.
Back to Aircraft…
Airport crews check runways for debris and damage regularly (far more often than highway crews). This significantly reduces the “road hazard” risk.
To reduce “maintenance and operational” risk, airlines load aircraft within manufacturer limits. Tire pressures are checked and adjusted religiously, so over/under inflation is not a concern.
Maintenance personnel and flight crews check tire condition before every departure. Technicians change tires when they reach the manufacturer’s wear limit.
All this attention to detail makes aircraft tire failures extremely rare.
Who does the retreading?
Most (if not all) aircraft tire manufacturers have retread plants located world-wide (Bridgestone has 5 retread facilities).
Goodyear retreads bias-type airline tires of any manufacturer. This often creates “Frankenstein tires” with logos of the original manufacturer on the sidewall and the Goodyear logo on the retread. They’re fun to spot when doing a preflight walk-around.
How much do tires cost?
Retail tire prices range from a few hundred dollars for regional aircraft to as much as $5000 for a wide-body main tire. Airlines negotiate purchase prices or service contracts with tire manufacturers and distributors.
Mixing Tire Brands
You would never dream of mixing new Goodyear and Michelin tires on a car. Aircraft tires are all manufactured to the same specifications so it’s common to see two different brands on the same landing gear bogie.
Aircraft tire treads have several circumferential grooves molded into the tread that help channel water away from the tire surface. Complex patterns that improve traction on automobile tires are not necessary on aircraft because the wheels rotate freely.
Large aircraft land on straight, well prepared runways. Modern runways are “crowned” — the runway gently slopes away from the centerline — to drain water. To further improve drainage and tire traction, runways often have grooves cut perpendicular to the direction of travel.
With all the attention paid to runway design, fancy tread patterns aren’t necessary. More grooves and patterns in an aircraft tire reduce the amount of rubber contact with the runway, increasing landing distances and hurting rejected takeoff braking performance.
Rotate & Balance
Airlines don’t rotate tires. A tire’s lifespan is too short to worry about uneven wear.
Large aircraft wheels are not balanced. Tires take a lot of punishment and each landing leaves rubber on the runway. Keeping them balanced would be a losing battle as every landing changes the weight distribution of the tires.
Tires Have Speed Ratings
Most airliners have tires rated for around 220-235 mph. This is way faster than the aircraft is typically traveling on the runway. Takeoff and landing speeds vary between 140-200 mph so there is a good margin of safety in the event an aircraft needs to land at a high speed (due to emergency or equipment malfunction).
The Great Pre-Spin Debate
Tires take a beating every time they touchdown on the runway. Why not have a mechanism to spin-up the tires prior to landing? If tire speeds match the touchdown ground speed, it would keep tire temperatures lower and save a little rubber… right?
It seems like a good idea. If fact, it’s been studied and tried several times throughout aviation history. So, why aren’t tires pre-spun before landing?
One of the earliest ideas was to place small vanes on the wheel hub to catch the airflow. The wheels would spin like a waterwheel. This concept won’t spin the wheels fast enough to match runway touchdown speed. It also adds extra drag which wastes fuel.
Another proposal uses an electric motor on each wheel to pre-spin before touchdown. This adds considerable weight and complexity to an already complicated system. Weight increases fuel burn and reduces payload capacity. Added complexity costs money for initial installation and on-going maintenance.
Other problems with pre-spin:
- Accurately matching wheel speed to the ground speed of touchdown is complicated (adds complexity and cost).
- Crosswind landings – Landing in a crab is an approved and recommended technique on many airliners. This wears a fair amount of rubber off the tread layer that pre-spinning would not prevent.
- A malfunctioning pre-spin system would cause maintenance delays.
- A fair amount of tire wear occurs during taxi.
Tires are relatively inexpensive and considered normal consumable items (like oil, hydraulic fluid, filters, etc) for aircraft operation. Technicians can replace a tire with little or no delay. A pre-spin system would be costly in the long run.
When are tires changed?
Maintenance techs inspect tires after every landing. The grooves molded in the tread are used as wear indicators. Tires are replaced when the tread is worn to the base of a groove. Cuts, sidewall damage, or bulges may require an early tire change.
If a replacement tire isn’t available, the tire can stay in service, even with the first layer of fabric (cord) visible, until it reaches a maintenance base.
I know what you’re thinking:
You fly around with bald tires? That’s CRAZY! 😮
On the family car, that would be crazy. Aircraft tires are very different than automobile tires. Aircraft tires don’t need tread grooves for max performance (similar to a smooth Formula 1 racing tire). They’re designed to meet full performance specs, even with the first layer of reinforcement fabric showing.
If you see a tire that looks bald on an aircraft, don’t freak out. Tires are inspected after every flight. They are replaced when they reach the manufacturer’s service limit.
How many takeoffs & landings?
Tire change cycles vary based on runway conditions, weather, and aircraft operating weights. A rough average is about 100 cycles for a main tire on a large aircraft (one takeoff and landing = one cycle). Nose wheel tires last a few more cycles than main tires.
Are tires changed in sets?
Main landing gear and nose tires on large aircraft are usually changed only when they reach wear limits. It’s common to see new and old tires next to each other. Certain types of wear or damage will require tires to be changed in sets.
There are always exceptions. On some aircraft types, technicians change nose wheel pairs together.
How to Change a Tire
Tire changes are actually wheel changes. The whole wheel is removed and replaced, just like changing a flat tire on a car. Wheel changes can be accomplished quickly, often without delaying the next departure.
767 Main Wheel Change
During a 50 minute turn-around, our maintenance technician identified a tire that reached its service limit. The tread layer was worn through to the first layer of fabric. Remember, this is not an automobile tire. Aircraft tires are designed to be flown until the tread grooves are gone. Again, they can be safely flown with fabric showing in order to reach a maintenance base.
Remove the Old Wheel
The worn tire is first deflated for safety. In the photo, a hose can be seen connecting the wheel to a pneumatic jack. The 200 psi of nitrogen in the tire can be used to raise the jack. Might as well put all that pressure to work!
Our replacement wheel assembly with a new Goodyear Flight Leader tire is standing by. The 767-300F uses an H46X18.0-20 tire for the main landing gear. It’s a Big Wheel.
The tire load rating for our main tires is 51,100 lbs (it’s stamped on the tire sidewall). The 767-300F has 8 main wheels. 51,100 x 8 = 408,800 lbs. Our maximum takeoff weight is 408,000 lbs. It’s fun to see how the math works out.
After raising the gear bogie with the jack, the technician removes safety wire and a bolt, then spins off a large axle nut. The old wheel assembly is pulled onto a dolly and moved out of the way.
Installing the New Wheel
The new wheel assembly is moved into position with the dolly. The photos below show the brake assembly: a sandwich of brake rotors and stators along with hydraulic actuating pistons that compress the brake sandwich. Brake rotors are keyed to the inside of the wheel and rotate with the wheel; stators don’t move. Rotors are positioned precisely before the wheel is pushed into place over the brake assembly.
After securing the new wheel, technicians lower and remove the pneumatic jack. Tire pressure is checked and topped off with nitrogen if necessary. Here’s the truly amazing thing: from start to finish, this wheel change took less than 20 minutes. I can’t find my car’s spare tire that fast!
What happens to old wheels?
Airlines don’t discard worn out wheel assemblies. Rims, tires, and hardware have a lot of life left in them. Stay tuned… In the next article we’re going on a field trip to an airline Wheel & Brake Shop!
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