Aircraft Wheels

Fascinating facts about aircraft wheels

Underside of an MD-11 aircraft showing the main landing gear. 2 main bogies with four wheels each and a center bogie with 2 wheels.
MD-11 main landing gear. That’s a lot of wheels!

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.

Wheel Construction

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.

Graphic showing a tire in center and two wheel hub halves on each side. The wheel halves are bolted together with the tire in the middle.
Wheel halves are bolted together with the tire in place

Nitrogen Instead of Air

Graphic representation of a Nitrogen molecule. Two nitrogen atoms connected by 3 lines.

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.

Tire Pressure

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.

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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.

My open hand holding a small fusible plug and over  pressure relief valve.
Fusible Plug and Overpressure Relief Valve

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!

Graphic showing cutaway view of an over pressure relief valve: hollow bolt with a disc embedded inside that will rupture under pressure.

Fusible plug cutaway image showing hollow bolt with soft metal core.

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.

767 Nose wheel tire with pressure relief valve installed.
Over Pressure Relief Valve on a 767 nose wheel.

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.

MD-11 wheel with small TPMS sensor mounted in a small hole on the side. The sensor is a small cylinder about the diameter of a Sharpie marker, 2" long.
A new TPMS sensor installed on an MD-11 main wheel

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.

Center CRT display on MD-11 instrument panel. Display shows arrangement of 12 aircraft wheels. Wheels are shown as ovals. Each wheel shows a pressure inside it. 190 pounds for 2 nose wheels and 2 center wheels. 217-219 pounds for 8 main wheels. Smaller numbers below each wheel show brake temperatures (52 to 61 degrees Celsius).
Tire Pressure Monitoring System on the McDonnell-Douglas MD-11. Large numbers are tire pressures in pounds. Smaller numbers are brake temps in °C.

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.

Lower, center CRT on the 767 instrument panel. Lower-right side of the screen shows 8 rectangles arranged like the main landing gear, 4 rectangles on each side. Inside each rectangle is a number. In the photo, left gear numbers are 2, 2, 2, and 1. Right gear are 1, 1, 1, and 2.  Label below the rectangles says "BRAKE TEMP"
767-300F Brake Temperatures

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.

Tire size molded into side of a 757 tire:. H40X14.5-19
Tire size molded on a 757 tire sidewall

Main gear tire diameter and width for a few popular airliners:

AircraftDiameterWidth
Boeing 737-700, 800, 90044.5″16.5″
Boeing 747-852″21″
Boeing 767-30046″18″
Boeing 777-30052″21″
Boeing 787-850″20″
Airbus A32046″17″
Airbus A33054″21″
Airbus A350/A38055″21″
McDonnell Douglas MD-1154″21″
Embraer ERJ 170/175 38″13″
Embraer ERJ 190/19541″16″
Canadair CRJ700/90036″12″

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.

Graphic showing how nose wheel chines deflect water outward.

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.

767 main wheel with Bridgestone logo on the sidewall. Goodyear logo is on the edge near the tread.
Newly retread 767-300 main tire. Original tire was manufactured by Bridgestone and retread by Goodyear. Look closely at the sidewall to see the faded Bridgestone logo.

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.

A 757 main gear bogie with four wheels. 2 wheels are Michelin, the other other are Goodyear
Mixing Michelin Air® and Goodyear Flight Leader® tires on the same bogie? Sure! They are manufactured to the same specs and are functionally equivalent.

Tread Patterns

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.

Graphic showing an intricate automobile all-weather tread pattern and a simple aircraft tread pattern of 4 grooves going around the tire.

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.

Low angle photo of a runway surface. Grooves can be seen. They are about the width of a quarter and spaced about an inch apart.
Workers add grooves to a runway.
License: CC3.0

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 

Side wall of a 767 nose wheel tire.  Embossed on the side wall is 235 MPH.
767 nose wheel tire with a speed rating of 235 mph.

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.

767 Main gear bogie with 4 tires. 3 look new, the 4th has grooves almost worn down even with tread. Grooves are still visible so tire is OK for continued service.
The grooves are still visible on this tire. It’s safe for a few more landings.

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.

Nose gear of a 767. One nose wheel is new, the other is older. Grooves are almost gone indicating the tire will need to be replaced soon.
It’s common to see new and old tires together. The tire on the right side of the photo (left nosewheel) is close to its service limit and was changed shortly after the photo was taken.

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

Close up of a tire tread. Grooves are worn down to tread. In 2 spots, the fabric under the tread is starting to show through. Time for a wheel change!
When fabric appears, it’s time to change the tire. This is normal for aircraft tires.

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!

A pneumatic jack with rolled under the main gear bogie. Hose is connected from jack to tire's fill valve. New wheel is in the background on a dolly, ready to be installed.
Using tire pressure to power the pneumatic jack to raise the gear bogie.

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.

Photo of the brand new main gear tire.
Tire Math


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.

A technician is removing the large axle nut from the center of the wheel. Nut is about the diameter of an adult palm. A second technician stands nearby with a 3 foot long socket wrench.
Technician removing the axle nut. Look at the size of the wrench the other tech is holding!

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!

A nitrogen cylinder on a 4 wheel cart. Controls and pressure dials on the end of the cart. Long, thin hose attached and coiled on the ground.
Portable compressed nitrogen cylinder used to pressurize tires to 200 psi. Cylinder is charged to about 1500 psi. A regulator steps down the pressure to safely pump up the tire.

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. In the next article we’re going on a field trip to an airline Wheel & Brake Shop!

Additional Resources

If you’d like to read more about aircraft tires, check out these resources:

17 Comments

  1. Captain Hoke, once again a brilliantly laid out dissertation of the workings of an airliner. As an unfulfilled pilot (the closest I can get is to be an avid flight simmer) your information is a revelation to those of us relegated to flying “in the back”. Or in my case at the keyboard.

  2. Cool picture looks like MD-11? Tires seem to be something we don’t think of much! I sometimes sit and watch our planes land and amazing the cloud of tire/rubber smoke they produce sometimes!

    • Hi Tom,
      Yep! We take the wheels and tires for granted and they’re pretty amazing. I updated the caption on the title photo – You are correct, it is an MD-11.

  3. I was delighted to see you’ve written a post about one of my favorite parts of an airplane. I think part of it is because the tires are so abused and yet also underappreciated by both the general public and those of us who fly them. I mean, who doesn’t love an underdog, right?

    I re-read one of my own posts on aircraft tires and some of the stats are amazing. The tires are subjected to as much as 6,000 Gs and the traction wave they experience upon landing causes them to oscillate at a rate of up to 16,000 times per minute.

    As I recall some of the stats on space shuttle tires were even more extreme. Imagine the stress the F104 or other jets with high landing speeds must have been designed to endure.

  4. Great Article.. one of very few articles, in anything, I read from start to finish..
    Most articles are boring, this one wasnt..
    cant wait for the Wheel & Brake Shop article..

  5. Thanks for your illustrative article, Ken! One question: Do aircraft tires have some kind of inner tube? Or do the two halves of the wheel provide an airtight seal?

  6. Great article Ken! As you know I’ve been flying these transport category aircraft for over thirty years and learned a lot from this. Keep up the good work!

  7. After a bit long time period after your last post. But, like every time i enjoyed the post. Great one again!! Can’t wait that long to read the next post …. Hope you’ll post it sooner

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