What defines a hard landing according to the performance standards?

A hard landing is defined in aviation performance standards by the amount of vertical acceleration experienced upon touchdown. This is typically measured in terms of "g's," which indicate the force of gravity experienced relative to normal conditions. The correct choice, which indicates 3.7 g's at a vertical descent rate of 600 ft/min, represents the established threshold where any landing exceeding this value is considered hard.

This criterion is important because hard landings can result in structural stress or damage to the aircraft, which is critical for maintaining safety and operational integrity. The values provided in the other options represent different thresholds but do not meet the standard for classifying a hard landing as per the performance measures. Therefore, the choice indicating 3.7 g's is recognized as the point where the landing is categorized as hard based on the parameters set forth in aviation guidelines.