|This article does not cite any references or sources. (December 2009)|
Landing is the final phase in flight where the vehicle returns to the ground. A hard landing occurs when the vehicle impacts the ground with a greater vertical speed and force than in a normal landing. The average vertical speed in a landing is around 2 metres per second. Anything above is classed as 'hard'. Hard landings can be caused by weather conditions, mechanical problems, over-weight aircraft, pilot decision and/or pilot error. The term hard landing usually implies that the pilot still has total or partial control over the vehicle, as opposed to an uncontrolled descent into terrain (a crash), which usually results in the destruction of the vehicle. Hard landings can vary in seriousness from simply causing mild passenger discomfort to situations resulting in serious vehicle damage, structural failure, injuries, and/or loss of life. When an aircraft has experienced a hard landing, it has to be inspected for damage before its next flight.
The term hard landing can also be applied to helicopter landings after mechanical failure and/or damage caused by hostile fire. A helicopter can often return to the ground after engine failure more safely than a fixed-wing aircraft provided that the rotor(s) are still intact and free to turn. Unpowered helicopter rotors exhibit autorotation, in which airflow over the rotors keeps them turning and provides buoyancy and limited pilot control during descent. As an unpowered descent, it requires considerable pilot skill and experience to safely execute. Autorotations can also be impeded by obstructions on the ground.
A hard landing of a spacecraft such as a rocket stage usually ends with its destruction and can be intentional or unintentional. When a high velocity impact is planned (when its purpose is to study of consequences of impact), the spacecraft is called an impactor.