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A twinjet is a member of the jet aircraft family that has two separate engines for power sources. This aircraft design creates a failsafe in the event one engine fails, as the other engine can provide enough thrust to keep the plane in the air while it is diverted to an emergency landing. Numerous aircraft manufacturers make twinjet aircraft, including both commercial airliners and private planes, and it is possible to have aircraft customized by the manufacturer for specific needs.
The classic twinjet design includes an engine under each wing. Some military planes mount the engines in the fuselage. In all cases, the engines are designed to operate entirely independently. They are not treated as a yoked pair when it comes to maintenance and engine checks. The goal is to make sure each engine can act on its own in the event of an engine failure. Both engines are routinely evaluated by personnel responsible for maintenance to make sure they are in working order, and maintenance logs are generated to document replacement of parts and routine maintenance.
In the air, if one of the engines fails or develops a problem, the other can take over to power the plane. While many twinjet aircraft can technically fly indefinitely on one engine as long as the plane has fuel, the protocol is to land the plane as quickly as possible. The second engine could also fail if the plane has a systemic problem and freak accidents can occur, rendering the engine useless and endangering the aircraft and its occupants. If a problem is identified with one engine during a check of the aircraft, it will be grounded until the problem can be addressed.
Twinjets usually fly on established routes with a number of airports along the way in case of an emergency. While emergency landings of twinjet aircraft are relatively rare, especially when one considers the number in the air, they do happen. On commercial aircraft with passengers, the crew usually works to minimize concern and panic while an alternate flight path is mapped out for an emergency landing, and the airline takes steps to get the passengers to their end destination quickly and safely, such as having them met by a replacement aircraft.
Costs of twinjet planes vary considerably, depending on the make and model. People in search of private planes may have a range of options including lease or rental of aircraft and opportunities to purchase used aircraft. Airlines in search of new planes for their fleets typically incur high costs at purchase, and rotate out their fleets slowly over time to avoid having to replace a large number of planes at once.