When getting ready to test Aptern, Harm is told to do five traps on the America. USS America (CV-66) was decommissioned Aug. 9, 1996, about one year after this episode aired. On 14 May, 2005, ex-USS America was sunk after approximately four weeks of live fire testing to determine design features of future aircraft carriers to improve their survivability.
Negative G-force is the force experienced, mainly by pilots, when accelerating downward towards the surface of the Earth faster than the rate of free-fall, 32 feet per second squared or 9 meters per second squared. As solids and liquids have different inertia, the liquid in the body (blood) moves slower than the solid parts of the body causing a feeling of weightlessness; it also makes it difficult for blood to be pumped by the heart, which is why high G-forces are dangerous. But negative G-force has double the effect of positive G-force, which is why it is more dangerous, as it also pushes blood away from the brain. This is why pilots will sometimes fly inverted when experiencing negative G's as it helps blood stay in the brain.