Sunday, November 18, 2007


The vertical speed indicator (VSI), which is sometimes called a vertical velocity indicator (VVI), indicates whether the airplane is climbing, descending, or in level flight. The rate of climb or descent is indicated in feet per minute. If properly calibrated, the VSI indicates zero in level flight.

Although the vertical speed indicator operates solely from static pressure, it is a differential pressure instrument. It contains a diaphragm with connecting linkage and gearing to the indicator pointer inside an airtight case. The inside of the diaphragm is connected directly to the static line of the pitot-static system.

The area outside the diaphragm, which is inside the instrument case, is also connected to the static line, but through a restricted orifice (calibrated leak). Both the diaphragm and the case receive air from the static line at existing atmospheric pressure. When the airplane is on the ground or in level flight, the pressures inside the diaphragm and the instrument case remain the same and the pointer is at the zero indication. When the airplane climbs or descends, the pressure inside the diaphragm changes immediately, but due to the metering action of the restricted passage, the case pressure remains higher or lower for a short time, causing the diaphragm to contract or expand. This causes a pressure differential that is indicated on the instrument needle as a climb or descent. When the pressure differential stabilizes at a definite ratio, the needle indicates the rate of altitude change.

The vertical speed indicator is capable of displaying two different types of information:
  • Trend information shows an immediate indication of an increase or decrease in the airplane's rate of climb or descent.
  • Rate information shows a stabilized rate of change in altitude.

For example, if maintaining a steady 500-foot per minute (f.p.m.) climb, and the nose is lowered slightly, the VSI immediately senses this change and indicates a decrease in the rate of climb. This first indication is called the trend. After a short time, the VSI needle stabilizes on the new rate of climb, which in this example, is something less than 500 f.p.m. The time from the initial change in the rate of climb, until the VSI displays an accurate indication of the new rate, is called the lag. Rough control technique and turbulence can extend the lag period and cause erratic and unstable rate indications. Some airplanes are equipped with an instantaneous vertical speed indicator (IVSI), which incorporates accelerometers to compensate for the lag in the typical VSI. Instrument Check—To verify proper operation, make sure the VSI is indicating near zero prior to takeoff. After takeoff, it should indicate a positive rate of climb.


  1. may i know.. what are the purpose of the ivsi?
    how to find the ivsi meaning?
    one more question.
    what are the function of square law compensator?
    what are the meaning?

    It's a VSI with a special mechanism which holds a suspended mass.
    The purpose of this system is to "anticipate" (by detecting de vertical acceleration) the response of the VSI.
    A "normal" VSI senses the vertical speed with a noticeable delay. As long as the vertical speed is constant, the delay occurs only during the first seconds after initiated the vertical movement, but with continuous changes in vertical speed (i.e. climbing and sinking)the indication of the VSI can be meanigless.

    So, with the inertial sensors, it´s posible to partially compensate the delay. We need advanced mathematics (i.e. Control Theory) to explain how this is accomplished

  3. Is there such a thing as a "Pitot-Static System" ? The pitot system is self-contained as is the static system; the only thing they have in common is that they both hook up to the Airspeed Indicator.

    Contrary to what is stated, the Vertical Speed Indicator is NOT hooked up to the P-S System, it is only hooked to the Static System.

    The term "Pitot-Static System" has been used for a long time but it is inaccurate and confusing for the student since it implies that the Pitot is involved — when it is not.

  4. very nicely written:)

  5. Pitot-static system is the term used for a system that utilises a pitot-static probe. Some aircraft use a probe which incorporates both, other aircraft use individual pitot probes and static ports.

    I agree a vertical speed indicator only uses inputs from the static pressure system, BUT there are instruments which use a pitot-static input, for instance an airspeed indicator. The ASI has a pitot and static inputs (also termed total pressure) to the aneroid capsule and a static input to the instrument casing.

    Student shouldnt be so quick to doubt teacher.

  6. why vertical speed indicator is not affected by position error?

  7. Why is the Vertical Speed Indicator not affected by position error??

  8. The vertical speed indicator is actually affected by position error..How ? If the static vent is subjected to a changing pressure it may briefly indicate a wrong rate of climb or decent.