ski

A ski is a long, curved or flat runner mounted under an aircraft fuselage or landing gear to enable landing and takeoff on unprepared surfaces, particularly snow, ice, water, or soft terrain where conventional wheels would sink or fail to grip. Fixed-wing aircraft operating in remote northern regions, bush operations, and amphibious roles rely on skis as primary or alternative landing gear. The ski distributes the aircraft's weight over a much larger surface area than a wheel, reducing ground pressure from typically 40 to 100 psi down to 2 to 5 psi, allowing operation from frozen lakes, glacier approaches, tundra, and bog.

Skis come in two main configurations. Conventional skis are fixed rigidly to the landing gear struts, with no articulation; they are common on tailwheel and conventional aircraft. Retractable skis are hydraulically or electrically raised into the fuselage or wheel fairings once airborne to reduce drag and improve cruise performance. Retractable systems are typically fitted to twin-engine or larger aircraft operating regularly between prepared airstrips and remote sites. Material varies between laminated wood cores with steel or aluminum runners (older aircraft), fiberglass shells with aluminum wear strips, or full aluminum construction.

In practice, skis are mounted in pairs, one under each main landing gear leg, and often paired with a nose skid or retractable nosewheel ski. An aircraft on skis requires significantly longer takeoff distances than the same airframe on wheels, particularly from snow with a thick crust or in warm spring conditions when surface conditions deteriorate. Pilots must assess snow hardness and density before landing; a ski designed for firm spring snow may punch through unconsolidated powder. Crosswind limits are often reduced compared to wheeled operations because sideways load transfer is less efficient on skis.

The undersurface ski runner wears through abrasion and impact, especially when landing on gravel-contaminated snow or glacial ice. Replacement runners are bonded or bolted on; most operational skis carry a replaceable wear strip. Corrosion of aluminum skis exposed to salt spray requires regular inspection and protection. Skis must be checked for cracks in the core, especially near the attachment points to the landing gear, where bending stress is highest during landing.

The term 'ski' carries over directly from winter sports, though aviation skis evolved independently during the 1920s as bush flying expanded into Arctic and subarctic regions. Today skis remain essential equipment for operators in Alaska, Canada, Greenland, Norway, and Siberia, and they remain standard retrofit option on many general aviation aircraft types designed for backcountry work.