Vertical Speed Indicator

Instruments in an aircraft are priceless for pilots. They are specially trained under the hood to get used to the instruments without looking outside the aircraft and understanding how the aircraft responds while flying only on instruments.After a student pilot has got used to the attitudes that he needs to set while flying visually with the horizon, he gets introduced to basic instrument flying.

In order to safely fly any aircraft, a pilot must understand how to interpret and operate the flight instruments. The pilot also needs to be able to recognize associated errors and malfunctions of these instruments.When a pilot understands how each instrument works and recognizes when an instrument is malfunctioning, he or she can safely utilize the instruments to their fullest potential.

Pressure Instruments

Instruments in an airplane can be categorized in several different ways,those are, pressure, gyro, vacuum and radio instruments. After that there is the magnetic compass which works on a different principle. The vertical speed indicator works on the principle of rate of change of differential static pressure.

Static Pressure

By definition, static pressure is the pressure exerted by a column of air of the atmosphere on a unit area.It can also be called the ambient pressure and is always present if the aircraft is moving or is at rest.Static pressure is simply the barometric pressure of the local area. If you fly at any altitude, the atmospheric pressure at that altitude can be called static pressure.Static vents on either side of the fuselage help in the measurement of static pressure.

The reason for two static vents is for redundancy purposes as a static vent might get blocked and in cases of crosswind and side slip maneuvers.In the latter cases, there would be a difference in the measurement of static pressure from the vents and hence to avoid incorrect indications both the vents are connected to each other from the inside to average out the result.

Vertical Speed Indicator

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


The VSI display two types of information :

• Trend information shows an immediate indication of an increase or decrease in the aircraft’s rate of climb or descent.
• Rate information shows a stabilized rate of change in altitude.

The VSI indicator in the aircraft uses a logarithmic scale. The reason for using a logarithmic scale is that the lower values have more spacing and the higher values have less spacing. Therefore, it becomes easy to identify even a slight change in the VSI needle from zero position and can be easily recorded.


  • The VSI as we mentioned above works on principle of rate of change of differential static pressure.Before we get to understand how this difference in static pressure is achieved, it is helpful to know the components of a vertical speed indicator.
  • The vertical speed indicator is made up of:
    • CAPSULE:The capsule is connected directly to the static line to receive air of existing atmosphere on one side and through linkages to the VSI pointer from the other side.
    • CASING:The capsule is placed in an airtight casing. The casing also receives static pressure from the static line, however, there is a lag in which it gets its pressure.This helps to create the difference in static pressure which is required for the VSI operation.
    • METERING UNIT/ CHOKE:Metering unit is used to achieve the time delay of static pressure between what is fed to the capsule and the case.The metering unit has a lot of names such as choke, restricted orifice or calibrated leak but the use of it remains the same, that is, to prove the necessary lag to feed the area outside the capsule.
  • When the aircraft is on the ground or in level flight, the pressure inside the capsule and casing is the same and there is no difference in static pressure. This will result in the VSI needle indicating zero.
  • When the aircraft is climbing, the atmospheric (static pressure) reduces as we climb and this is fed to the capsule. The same pressure is fed to the casing through a metering unit that will cause a delay and hence the pressure in the capsule will be less than in the casing which will cause the capsule to compress and indicates a RATE OF CLIMB.
  • When the aircraft is descending, the atmospheric (static pressure) increases as we descend and this is fed to the capsule. The same pressure is fed to the casing through a metering unit that will cause a delay and hence the pressure in the capsule will be more than in the casing which will cause the capsule to expand and indicates a RATE OF DESCEND.


To understand what the VSI needle will indicate in case the static vent gets blocked, I would recommend viewing the diagram used in operations.

  • CASE 1: The aircraft flying level and the static vent is blocked. As flying level, VSI indicates zero and would continue to indicate zero even if the static vent is blocked.
  • CASE 2:During climb, static pressure vent is blocked. Due to the delay in the casing, the Rate of Climb indication will progressively reduce and settle to zero.


  • POSITION ERROR: The position error is on account of the incorrect location of the static vents.Due to this error, the VSI will wrongly indicate a climb or descent when speed is suddenly changed and is most noticeable during take off acceleration.
  • INSTRUMENT ERROR: It is on account of manufacturing imperfections.
  • LAG ERROR: The pointer would take sometime to indicate the change from the time it senses it. This error is most noticeable during prolonged climb or descents at a high rate.
  • MANOEUVRE INDUCED ERROR: Different changes in attitude and configurations of the aircraft will lead to this type of error.This leads to false indications of rate of climb or descent.
  • HYSTERISIS ERROR: When an aircraft is flying at a flight level for a considerable period of time, it will result in the VSI unwilling to respond to changes in static pressure values.

Instrument Check

As part of a preflight check, proper operation of the VSI must be established. Make sure the VSI indicates a near zero reading prior to leaving the ramp area and again just before takeoff. If the VSI indicates anything other than zero, that indication can be referenced as the zero mark. Normally, if the needle is not exactly zero, it is only slightly above or below the zero line. After takeoff, the VSI should trend upward to indicate a positive rate of climb and then, once a stabilized climb is established, a rate of climb can be referenced.

Instantaneous Vertical Speed Indicator (IVSI)

  • To overcome the problem of lag, the Instantaneous Vertical Speed Indicator (IVSI) implements the use of accelerometer (an electromechanical device used to measure acceleration).The IVSI uses a dashpot or a vane type accelerometer.
  • The main advantage of using such an accelerometer is that it responds very quickly to changes in altitude.
  • The sensitivity of a dash pot IVSI is very high and this results in the instrument over reacting in turbulent flying conditions and resulting in false indications and the errors are termed as turning errors. At the time of initiating a level turn, the IVSI momentarily indicates a climbing turn.
picture credit:dutchops

Fact of the Week

Qamdo Bangda Airport, also called Qamdo Bamda Airport, is a plateau airport located in Bangda Prairie, Hengduan Mountains.The Qamdo Bangda Airport started its construction on December 2, 1992. The Air Force only spent 83 days on fixing the 5,500m long and 45 m wide airport runway – the longest one in the world and the airport is 4,334m (14,219 ft) above sea level, which makes it the second highest airport in the world.

 Climate environment of the Qamdo Bangda Airport is quite hostile. Wind speeds up to 30m per second in winter. Besides, the temperature often drops to 20 degrees centigrade below zero in winter and spring, which is difficult for flight operation. Due to its high elevation, the airport oxygen level is only 50% of that of sea level.he airport did a reconstruction and expansion to repaire the runway, building a new terminal of 5,018 square meters. From June 22 to July 15, 2013, the airport was shut down for a further maintenance of the old runway. A second runway is under construction from 2015.

picture credit: Tibet Discovery

We are done for this weeks post. I hope you gained some new knowledge from the world of aviation.If you did, please don’t forget to like this post and share it with your fellow aviation enthusiasts. Until next week, stay safe and stay healthy.



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