Transport category
Transport category is a category of airworthiness applicable to large civil airplanes and large civil helicopters. Any aircraft's airworthiness category is shown on its airworthiness certificate. The name "transport category" is used in the US, Canada, Europe and many other countries. A principle behind transport category design standards is that any element in an airplane or helicopter can fail, but the risk of such a failure causing an accident should be extremely low. Consequently, transport category airplanes and helicopters have duplicated elements wherever failure of one element is likely to cause an accident. For example, transport category  airplanes must have at least two engines and be flown by at least two pilots. The loads on the wings and tailplanes are usually carried by multiple load paths. If one element of the primary structure fails due to metal fatigue or corrosion the remaining sound elements of the structure must carry the loads until the failed structural element is discovered in routine maintenance. This "fail-safe" principle is not necessary in airplanes and helicopters that are not certificated in the transport category. If an element in a transport category airplane or helicopter is not highly critical, or if its  failure is relatively benign, it is possible for that element not to be duplicated. For example, most transport category airplanes do not have duplicated nosewheel assemblies; and some do not have duplicated wheels on each main undercarriage. A transport category helicopter is permitted to have only one main rotor head, and may have only one engine. If a transport category helicopter has only one engine it is only eligible to be a Class B (or Performance Group 2) helicopter. An example of a transport category helicopter with only one engine is the Bell 204/205. Standards. In the United States, Federal  Aviation Regulations Parts 25 and 29 are applied to transport category airplanes and helicopters. In Europe the European Aviation Safety Agency has similar certification specifications, also titled Parts 25 and 29, for application to large airplanes and helicopters. FAA. Airplane. Transport category airplanes are either: Easily recognizable examples of transport category airplanes are Boeing and Airbus aircraft, Learjet 30 series, de Havilland Canada Dash 8, Embraer EMB 120 Brasilia. Helicopter. Transport category helicopters typically have maximum takeoff weights greater than 7,000 lb (3 175 kg) although there is no lower weight limit. Easily recognisable examples of transport category helicopters are  Bell 412, Eurocopter AS332 Super Puma, MBB/Kawasaki BK 117, AgustaWestland AW139.