Rate of climb
In aeronautics, the rate of climb (RoC) is an aircraft's vertical speed – the rate of altitude change with respect to time. 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 the indicated speeds Vx and Vy.
Vx is the indicated forward airspeed for best angle of climb. This is the largest angle the aircraft can climb at without overloading and stalling the engine. Such a climb may be instigated to avoid a collision with a stationary object. Vy is the indicated airspeed for best rate of climb. This climb mode allows the aircraft to reach a specified altitude in the minimum possible amount of time. In contrast, Vx minimizes the horizontal distance covered to reach the specified altitude. The vertical ascent rate is altered so that the desired Vx or Vy is maintained on the airspeed indicator. The engine's power setting then determines the positive vertical speed, as it also does during descent. While indicated V speeds are independent of the engine's power setting and altitude, most small propellered aircraft will only maintain a climb at nearly full power. Jet liners usually require 70% power or more to maintain climbing. For the best rate of climb the engine is not close to being overloaded. Invariably Vx is lower than Vy. Reaction engines generally can not be overloaded at low speeds. Maximum power and climb rates are typically used as the normal operating procedures for very small aircraft, such as a Cessna 150, but only during emergencies for large jet liners.
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.
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.
Typical Vy values for a small four-seat aircraft are 80KIAS to give a climb of 800ft/min, without flaps. A large passenger jet, such as an A320, may typically yield Vx=220KIAS and Vy=270KIAS. With flaps set at five-degrees Vy is reduced to 200KIAS.
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 (VBG) that will result in the longest unpowered glide distance.