parabolic flights
The European Space Agencey has an old A300 down in Bordeaux that they use for parabloic flights & weightlesness training of astronauts. This aircraft has split control systems, whereby they seperate the ailerons and elevators for parbolic flight, one pilot operating each control respectively. The captain contorls pitch and the F/O has control of the ailerons. Once normal flight is resumed, the controls can be 'locked' back together again.
One of the issues with parabolic flying and sustained low / negative 'g' is the engine oil system. Many of the mainline bearings have very critical spray patterns and flow paths for the oil, and this can be severly disrupted under prolonged low / negative 'g' scenarios therby depriving the bearings of oil.