The wheel and support structure underneath a vehicle like an airplane or car, or the landing gear of an aircraft.
From 'under-' plus 'carriage' (a vehicle or the framework that carries something). Originally used for the running gear of coaches and wagons.
For aircraft, the undercarriage is like the plane's legs—and pilots have to remember to put them down before landing! There are actual cases of pilots forgetting to lower the undercarriage, causing belly-landing disasters, which shows why it's such a critical system.
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