page_title: Variometer
section_title: Description
hierarchical_section_title: Variometer / Description
caption_attribution_description: English: Diagram of how a vertical airspeed indicator works
context_page_description: A variometer – also known as a rate of climb and descent indicator, rate-of-climb indicator, vertical speed indicator, or vertical velocity indicator (VVI) – is one of the flight instruments in an aircraft used to inform the pilot of the rate of descent or climb. It can be calibrated in metres per second, feet per minute or knots, depending on country and type of aircraft. It is typically connected to the aircraft's external static pressure source.
  In powered flight the pilot makes frequent use of the VSI to ascertain that level flight is being maintained, especially during turning maneuvers. 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.
context_section_description: 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. In its simplest electronic form, the instrument consists of an air bottle connected to the external atmosphere through a sensitive air flow meter.  As the aircraft changes altitude, the atmospheric pressure outside the aircraft changes and air flows into or out of the air bottle to equalise the pressure inside the bottle and outside the aircraft.  The rate and direction of flowing air is measured by the cooling of one of two self-heating thermistors and the difference between the thermistor resistances will cause a voltage difference; this is amplified and displayed to the pilot.  The faster the aircraft is ascending (or descending), the faster the air flows.  Air flowing out of the bottle indicates that the altitude of the aircraft is increasing.  Air flowing into the bottle indicates that the aircraft is descending.
  Newer 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.
  The designs described above, which measure the rate of change of altitude by automatically detecting the change in static pressure as the aircraft changes altitude are referred to as "uncompensated" variometers.  The term "vertical speed indicator" or "VSI" is most often used for the instrument when it is installed in a powered aircraft.  The term "variometer" is most often used when the instrument is installed in a glider or sailplane.
  An "Inertial-lead" or "Instantaneous" VSI (IVSI) uses accelerometers to provide a quicker response to changes in vertical speed.
caption: Schematic drawing of the internals of a classic aircraft vertical speed indicator
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