Variometer is also known as a Rate of Climb and Descent Indicator (RCDI), Rate-of-Climb Indicator, Vertical Speed Indicator (VSI), or Vertical Velocity Indicator (VVI). Variometer is one of the flight instruments in an aircraft used to inform the pilot of the near instantaneous (rather than averaged) vertical speed (rate of descent or climb) of the aircraft. It can be calibrated in knots, feet per minute (101.333 ft/min = 1 kn) or metres per second, depending on country and type of aircraft. Rate of Climb and Descent Indicator (RCDI) is usually called as Vertical Speed Indicator (VSI) in powered flights, and Variometer in gliders.

Operation of Variometer & Vertical Speed Indicator

Variometers measure the rate of change of altitude by detecting the change in air pressure (static pressure) as altitude changes. A simple variometer can be constructed by adding a large reservoir (a thermos bottle) to augment the storage capacity of a common aircraft rate-of-climb instrument. Electronic variometer designs directly measure the static pressure of the atmosphere using a pressure sensor and detect changes in altitude directly from the change in air pressure instead of by measuring air flow. These designs tend to be smaller as they do not need the air bottle. They are more reliable as there is no bottle to be affected by changes in temperature and less chances for leaks to occur in the connecting tubes.

Advanced electronic variometers in gliders can present other information to the pilot from GPS receivers. The display can thus show the bearing, distance and height required to reach an objective. In cruise mode (used in straight flight), the vario can also give an audible indication of the correct speed to fly depending on whether the air is rising or sinking.

In powered flight the pilot makes frequent use of the Vertical Speed Indicator (VSI) to ascertain that level flight is being maintained, especially during turning manoeuvres. In gliding, the instrument is used almost continuously during normal flight, often with an audible output, to inform the pilot of rising or sinking air. It is usual for gliders to be equipped with more than one type of variometer. The simpler type does not need an external source of power and can therefore be relied upon to function regardless of whether a battery or power source has been fitted. The electronic type with audio needs a power source to be operative during the flight. The instrument is of little interest during launching and landing, with the exception of aerotow, where the pilot will usually want to avoid releasing in sink.