Since 1997 the Airbus A300 Zero-G of Novespace, a private company of CNES (french space agency) and which program is also supported by ESA (European Space Agency), has been doing parabolic flights that allow their passengers to stay during 22 seconds in a state of complete weightlessness in each parabola. In one flight, the A300 Zero-G does 30 parabolas which means about 11 minutes with zero gravity. However, the aim of these trips are not the same as current flights. They are mostly thought for scholars conduct experiments (behavior of materials, fluids, life sciences, etc.) in the absence of gravity, since those researches cannot be carried out on Earth.
To understand how this 0g effects works we are going to distinguish 3 phases in each parabola: an horizontal phase in which the gravitational force is the same as on Earth, 1g. A second phase which goes from 0º to 45º where the passengers weight increase up to 1.8g. Finally, for angles greater than 45º together with the adequate speed, the 0g effect is achieved.
The A300 after 17 years and more than 13000 parabolas did its last flight in October 2014 and from now on an A310 will substitute it. This aircraft will be capable to transport up to 40 passengers in a volume of more than 200 cubic meters and will start to help scientists in April 2015.