Which unit is typically NOT used for expressing speed at higher altitudes?

When discussing speed at higher altitudes, the most commonly used units are knots, Mach number, and sometimes feet per second. Knots, which measure nautical miles per hour, are standard for aviation because they correlate well with wind speed and other navigational elements. The Mach number is particularly relevant at higher altitudes where speeds approach or exceed the speed of sound; it is a dimensionless unit that compares an aircraft's speed to the speed of sound, adjusting for altitude and temperature variations.

Conversely, kilometers per hour is more typically associated with ground vehicles and is not commonly used in aviation, especially at higher altitudes. While it can describe speed, it lacks the specific relevance and practicality that knots and Mach number provide in an aviation context, particularly concerning aerodynamic performance and fuel efficiency. Feet per second may be used in some contexts but is generally less common for expressing speed compared to knots or Mach number. Thus, kilometers per hour stands out as the unit least used for expressing speed at higher altitudes in aviation.