A parabolic flight is the following of a special parabolic trajectory by a suitably fitted airplane in order to reproduce the conditions of weightlessness for a short period. Such flights are used as a routine part of astronaut training.
On the upward arc, the thrust of the airplane is adjusted so that there is no lift: the plane is in free fall since nothing reduces the force of gravity. The plane remains in free fall over the top of the parabola and part of the downward arc. Microgravity conditions are achieved for time periods of 15 to 25 seconds.
The four-engine Boeing KC-135, similar to the commercial Boeing 707 aircraft, has been modified by NASA to support parabolic flights. The parabolas are performed in a restricted zone, about 8,000 and 12,000 meters in altitude.