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The Rutan Voyager, the first aircraft to fly around the World without refueling, is a notable example of an experimental aircraft

In generic use, an experimental aircraft is an aircraft that has not yet been fully proven in flight. Often, this implies that new aerospace technologies are being tested on the aircraft, though the label is more broad. Experimental aircraft is also a specific term referring to an aircraft flown with an experimental category Airworthiness Certificate. The term experimental aircraft is often erroneously used to mean homebuilt aircraft. While most homebuilt aircraft are registered as experimental category aircraft in the U.S., there are many types of experimental aircraft that are not homebuilt.

The U.S. and Australia have much more flexible rules than most countries for experimental aircraft, and this supports a large fleet of homebuilt, imported, and ex-military aircraft flying today.

The United States Federal Aviation Administration requires an Airworthiness Certificate to be maintained as part of the official paperwork associated with each aircraft. Aircraft produced by certified aircraft manufacturers will go through an extensive period of testing to prove that they are airworthy. These tests cover everything from engineering and construction to the flying characteristics of the aircraft in question. Once the manufacturer has satisfactorily completed these tests, the aircraft design is given a type certificate and the aircraft produced under this design are given a Standard Airworthiness Certificate. Aircraft that do not meet these requirements must meet the requirements for a Special Airworthiness Certificate, which includes the experimental category.

The FAA issues experimental airworthiness certificates for eight defined purposes:

• Research and Development -- Aircraft whose purpose is to test new design concepts, equipment, or operating techniques.
• Showing Compliance with Regulations -- An aircraft that is built for the purposes of demonstrating the airworthiness of a design. For example, any model of aircraft that is today built with a standard airworthiness certificate, will have initially flown as a prototype with an experimental certificate.
• Crew Training -- An aircraft used solely for training that, for some reason, does not have a standard certificate. For example, NASA operates a highly modified Gulfstream II to train pilots as a simulation of the landing behavior of the Space Shuttle (Shuttle Training Aircraft).
• Exhibition
• Air Racing
• Market Survey -- A sales demonstration aircraft.
• Operating Amateur (Homebuilt) Aircraft
• Operating Kit-built Aircraft

In the eyes of the FAA,and CASA in Australia, an Experimental Homebuilt Aircraft is not constructed by a licensed aircraft manufacturer. Instead, at least 51% of the aircraft is constructed by a private individual; the remaining 49% percent can be purchased from a kit manufacturer. In the past, far less than the allowed 49% of the kit was assembled by the manufacturer of the kit. Now, the majority of experimental aircraft builders purchase kits that are assembled by the kit manufacturer up to the 49% limit. These are often called "quick build" kits.

This category of aircraft can be built and flown by any licenced pilot, although an examiner must certify the aircraft for flight. Most nations' aviation regulations require new designs and amateur-built aircraft to be physically marked as experimental, and extra flight testing is usually required before passengers (who are not pilots themselves) can be carried. At least 25,000 of these homebuilt aircraft exist in the U.S. alone, though many are based on conventional designs and experimental only by name.

Homebuilt aircraft are built from materials in one of four categories: rag and tube, metal, wood, or composite materials (fiberglass, carbon fiber, etc.). The first category, describes a building method where the aircraft's superstructure is built using welded steel or bolted aluminum tube covered with Dacron fabric. This fabric can be painted to stiffen it. The second category, metal, describes the common all aluminum aircraft. Examples of this kind of experimental aircraft include Murphy Aircraft's kits and the Vans RV series of 1,2 & 4 seater kits. Finally, the most recent addition to the fleet is made using Sandwich structured composite methods. This category is notable for its designs employing body curvature and light weight which would be impossible with the other materials.

The FAA's Experimental Aircraft designation is supported by the Experimental Aircraft Association. The largest airshow in the world is the EAA's annual EAA AirVenture Oshkosh airshow in Oshkosh, Wisconsin, which takes place in late July and early August. Other annual events are the Sun N' Fun Fly-In, which occurs in the early spring in Lakeland, Florida, and the Northwest EAA Fly-In in Arlington, Washington. These events are called a "Fly-In" as many people fly their homebuilts and other aircraft into the airport hosting the show, often camping there for the duration. Both events last a week. Takeoffs and landings at these shows number in the thousands.

• List of experimental aircraft
• X-plane
• Foot-launched Powered Hang Glider

• Experimental Aircraft Association
• XLTA.org
• Experimental Aircraft Information