Aircraft spend most of their life in the air, but they cannot take off or land without the undercarriage. Landing gear carries the aircraft's weight on the ground, absorbs touchdown loads, and provides braking and steering.

Most gear retracts after takeoff to cut drag, which is why the wheels disappear into the belly or wings. Retraction systems must work reliably; a gear that fails to extend turns an otherwise normal flight into an emergency.

Common configurations

• Tricycle gear with nose wheel and two main wheels
• Conventional gear with tail wheel, common on older aircraft
• Tandem gear for gliders and some fighters
• Multi-bogey gear for heavy airliners

Brakes and tires wear quickly. A single landing can consume a measurable fraction of a brake pad, and tires on widebody jets cost thousands of dollars each. Ground crews inspect them after every landing.