Rate of Climb

In aeronautics, the rate of climb (RoC) is an aircraft's vertical speed - the rate of change in altitude. In most ICAO member countries (even in otherwise metric countries), this is usually expressed in feet per minute and can be abbreviated as ft/min. Elsewhere, it is commonly expressed in metres per second, abbreviated as m/s. The rate of climb in an aircraft is indicated with a vertical speed indicator (VSI) or instantaneous vertical speed indicator (IVSI).

The rate of decrease in altitude is referred to as the rate of descent or sink rate. A decrease in altitude corresponds with a negative rate of climb.

There are a number of designated airspeeds relating to optimum rates of ascent, the two most important of these are V_x and V_y.

V_x is the indicated airspeed for best angle of climb. V_y is the indicated airspeed for best rate of climb. V_x is slower than V_y.

Climbing at V_x allows pilots to maximize the altitude gain per unit ground distance. That is, V_x allows pilots to maximize their climb while sacrificing the least amount of ground distance. This occurs at the speed for which the difference between thrust and drag is the greatest (maximum excess thrust). In a jet airplane, this is approximately minimum drag speed, or the bottom of the drag vs. speed curve.

Climbing at V_y allows pilots to maximize the altitude gain per unit time. That is, V_y, allows pilots to maximize their climb while sacrificing the least amount of time. This occurs at the speed for which the difference between engine power and the power required to overcome the aircraft's drag is the greatest (maximum excess power). Climb rate is proportional to excess power.

V_x increases with altitude and V_y decreases with altitude. V_x = V_y at the airplane's absolute ceiling, the altitude above which it cannot climb using just its own lift.

The initial rate of climb record for pilot aircraft is held by MiG-29 at 330 m/s (65,000 ft/min). The average rate of climb for a MiG-29 is 109 m/s from sea level to 6000 m.