They weigh 220 kilograms and there's 10 of them on one plane. Roadside saviors -- you may need a bigger wrench By CNNGo staff 2 February, 2012 There are many things about air travel we take for granted: that drinking whiskey at 10 a.m. is OK; that moving your feet in small circles counts as exercise. But some things are worth explaining. Adam Nelson, manager line maintenance for Cathay Pacific Airways, told us how to change a tire on an Airbus A330. While you may swap out that ragged piece of rubber on your Ford only when the rim starts squealing on the asphalt, with eight main wheels and two nose wheels, an A330 goes through 25 tire changes a year. That's 1,150 tire changes for Cathays A330 fleet each year. A Boeing 747-400 is even needier. It has 16 main wheels and will get roughly 50 tire changes in a year. It takes two aircraft mechanics 45 minutes to an hour to replace a tire on an A330, each of which weighs 220 kilograms, using a jack (two for the center wheel), a wheel dolly, a spanner and a wrench. A tire is retreaded up to six times during its lifespan too, then finally replaced after 1,500 landings. The listed price for an A330 tire manufactured by Michelin is nearly US$7,000 Step 1 Before changing a wheel you need to ensure the landing gear safety locks are installed to prevent unwanted movement during the replacement Step 2 Check that all boarding gates and external equipment are clear and will not make contact with the aircraft as it is jacked. Once confirmed you use a pneumatic jack to raise the aircraft until the wheels are approximately five centimeters off the ground. Step 3 Remove the wheel hub cap and tire pressure indication sensor. With this done the tire pressure is then reduced from 200 PSI down to 30 PSI. This eliminates any chance of the wheel exploding, which can happen if the wheel tie bolts are broken. Once deflated the locking bolts and Step 4 With the wheel nut removed a protective sleeve is fitted over the axle threads and a lifting tool is used to remove the wheel Step 5 Check that the part number of the wheel is correct against the maintenance manual. The axle sleeve is then greased and the lifting tool is used again to install the serviceable wheel Step 6 Install and torque the wheel nut. The wheel is rotated in the same direction as you torque the nut to ensure the wheel is seated correctly. Then install the locking bolts, tire pressure indication system and refit the hub cap. You are now ready to inflate the tire to approx 200 PSI. Nitrogen is used to inflate the tire. If oxygen were used the moisture would freeze at the sub-zero temperatures during cruise. Step 7 With the wheel now inflated and pressure checked, the jack is lowered/disconnected, gear safety locks removed and the technical log is updated to record the maintenance activity.