# 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 (ft/min). Elsewhere, it is commonly expressed in metre per second (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.

## Speed and 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.^{[1]} 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.

## Glide path

On descent to landing, the sink rate is determined by the flap configuration and the power setting. Normally a 3° glide path is followed, with a sink rate that is proportional to the approach speed. All aircraft have a best glide configuration, a flap setting and speed (V_{BG}) that will result in the longest unpowered glide distance.^{[2]}