What physical aspect is constant for the smaller plane before the collision?

The correct answer is that the altitude of the smaller plane remains constant before the collision. In the context of flight dynamics, altitude typically refers to the height of the aircraft above a reference point, often sea level. If the smaller plane is flying at a steady altitude, this means it is maintaining a consistent vertical position relative to the ground. While the plane may experience changes in velocity, heading, or other flight parameters while still being at a constant altitude, the altitude itself does not change unless the pilot intentionally ascends or descends.

In many scenarios, especially during flight operations, various factors such as wind and air pressure can cause changes in velocity or heading. However, if the aircraft's altitude is specified as constant, it indicates that the plane is flying level without climbing or descending. This aspect is crucial in collision analysis, as both aircraft need to be within the same altitude range for a potential collision to occur.