In aeronautics, the rate of climb (RoC) is the speed at which an aircraft increases its altitude. In the 'west' (US, UK, Can, Aus, NZ etc), this is most often 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 measured 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 two airspeeds relating to optimum rates of ascent, referred to as Vx and Vy.
Vx is the indicated airspeed for best angle of climb. Vy is the indicated airspeed for best rate of climb.^{[1]} Vx is slower than Vy.
Climbing at Vx allows pilots to maximize the altitude gain per unit ground distance. That is, Vx 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. Climb angle is proportional to excess thrust.
Climbing at Vy allows pilots to maximize the altitude gain per unit time. That is, Vy, 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.
Vx increases with altitude and Vy decreases with altitude. Vx = Vy 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 MiG29 at 330 m/s (65,000 ft/min). The average rate of climb for a MiG29 is only 109 m/s from sea level to 6000 m. ^{[2]}
