A Vertical Speed Indicator (VSI), called a Rate of Climb and Descent Indicator or variometer, uses the aircraft’s pitot-static system to provide information about the rate of climb or descent in hundreds of feet per minute. It displays two types of information - trend and rate - showing positive readings for climbs and negative readings for descents.
The presentation may be a conventional needle moving over a circular scale on a standalone instrument, or the result may be depicted on a ribbon at the side of an Electronic Flight Instrument System EADI. To reduce the lag time inherent in simple VSIs, the Instantaneous Vertical Speed Indicator incorporates an accelerometer - actuated air pumps, giving pilots an immediate sense of vertical motion.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What does VSI stand for in aviation?
VSI stands for Vertical Speed Indicator, which is an instrument indicating the rate of climb or descent of an aircraft. It is alternatively known as the Rate of Climb and Descent Indicator, the Vertical Velocity Indicator (VVI), or variometer, and it uses the aircraft’s pitot-static system.
The VSI is a basic flight-deck instrument that tells the pilot how fast the airplane is climbing or descending. The Vertical Speed Indicator compares current static pressure to a small sample of previous static pressure stored inside its sealed case. The resulting differential pressure moves a conventional needle around a circular scale to display vertical speed in feet per minute. Early mechanical VSIs date from the 1930s, while today's glass cockpits present the same data on ribbon readouts driven by Inertial Reference Units.
What is the purpose of the Vertical Speed Indicator (VSI)?
The Vertical Speed Indicator measures rate of change in altitude. It measures rate of climb or descent and displays vertical speed on a digital or analogue face. Inside the instrument, the VSI gauges the difference in internal pressure between two specific areas - it measures changes in static pressure as the aircraft climbs or descends. Static pressure drops as the aircraft climbs; the aneroid compresses and the diaphragm contracts, indicating a climb. Static pressure increases as the aircraft descends, the diaphragm expands and the indication shows a descent.
The VSI provides trend information showing immediate increase or decrease in rate of climb or descent, and it provides rate information showing stabilized rate of change in altitude. Trend information appears as direction of needle movement, while rate information appears once the needle is stabilized. The instrument smooths out short-term variations to give more stable readings, so the pilot receives real-time data on ascent or descent rate.
During IFR flight, the VSI helps maintain controlled descents and climbs by providing an average rate of climb or descent over a short period. Markings on the dial are typically in hundreds of feet per minute, and altitude is displayed in increments of 20 feet (6.1 meters). If the static source becomes blocked, an alternate static source can be used, otherwise the indication is off more than 75 feet (22.86 meters) from surveyed field elevation and the Airspeed indicator reads greater than normal.
How does a Vertical Speed Indicator work?
The Vertical Speed Indicator works using the pitot-static system. Inside the instrument case a diaphragm is the heart of the system. Altitude changes cause the diaphragm to expand or contract because pressure changes cause the diaphragm to expand or contract. When the aircraft climbs, static pressure decreases, the diaphragm expands, and the expanding diaphragm moves the needle upward. Conversely, as pressure increases the diaphragm contracts, indicating a descent.
Mechanically, the VSI uses a calibrated leak that allows pressure to leave the instrument case slower than it enters, so the gauge difference between internal pressures is measured continuously. The calibrated leak measures pressure changes, and trend information shows immediate indication of increase or decrease in rate, giving the pilot instantaneous vertical-rate data. This design yields typical accuracy within 61 m/min below 610 m/min, with tolerance widening to 10% at higher rates. The instrument therefore meets standard certification limits while presenting the vertical trend with minimal lag.
A sealed diaphragm inside the instrument expands or contracts the instant static pressure changes. Because the VS sensed compression alterations I could not perceive, it worked as an advance alert scheme, showing the speed of ascent or descent.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What are the components of a Vertical Speed Indicator?
The components of a Vertical Speed Indicator are detailed below.
- Static Pressure Port: The static pressure port is a flush mounted opening placed on the outside of the aircraft in an area of relatively undisturbed airflow. It collects the ambient air from outside the aircraft and channels it into the static line.
- Static Line: The static line is the hollow tube that connects the static pressure port to the case and diaphragm.
- Calibrated Leak: The calibrated leak is a special type of connection between the case and the static line that prevents the pressure inside the case from changing immediately when air flows into the case.
- Capsule: The capsule is connected directly to the static line. The capsule is connected to the VSI pointer via linkage. Because the capsule must compress or expand against an opposing pressure that lags behind, the calibrated leak creates the pressure difference necessary to drive the VSI.
- Case: The case is the housing that contains the other components of the VSI. The case is connected to the static line via a connection with a calibrated leak.
- Diaphragm: The diaphragm is a flexible metal container located inside the case and attached directly to the static line. Rods and gears connect the diaphragm to the needle on the face of the Vertical Speed Indicator’s cockpit display gauge.
How is a Vertical Speed Indicator constructed?
A Vertical Speed Indicator is constructed using a number of different parts. It has a capsule placed in a sealed case and the capsule is fed with static pressure from the pitot-static system, while the case is connected to that system through a calibrated nozzle. Because the calibrated orifice causes pressure inside the case to change more slowly than pressure inside the capsule, any climb or descent produces an immediate pressure difference. As the airplane climbs, pressure inside the case and capsule decreases, the capsule expands, and the needle - connected to the capsule by gears and rods - moves up. Conversely, as the airplane descends, pressure inside the case and capsule increases, the capsule compresses, and the needle moves down.
An instantaneous vertical speed indicator contains additional accelerometer-actuated air pumps to reduce the lag time inherent in simple VSIs. Inside the IVSI, a dashpot or vane-type accelerometer and a weighted piston held in place by springs inside a cylinder detect vertical acceleration the moment it begins. These accelerometer units, mounted in the vertical axis, drive tiny pumps that instantly adjust the pressure inside the instrument case, so the capsule responds without waiting for the calibrated leak to create the pressure difference.
A sealed alloy container is fastened to the aircraft's static-pressure line through a calibrated hole that acts as a restrictor. Inside the casing, a thin membrane divides the container: one side faces the static line while the other side faces the same air trapped within the instrument. As the aircraft climbs or descends, the change in force per unit area between the two sides causes the membrane to extend or shorten. This motion is transferred by a gear device to the pointer on the face, showing the rate of ascent or descent. During assembly, a skilled worker performs a precise compression experiment, simulating several speeds of climb and descent, and adjusts the restrict or until the pointer's motion exactly matches the artificial vertical speed.
What are the types of Vertical Speed Indicators?
Two kinds of vertical speed indicators appear in modern cockpits. The uncompensated variometer, a simple aneroid capsule linked to the static line, gives the conventional indication of rate of climb or descent. A needle moves round an arced dial or a vertical speed tape on the EADI shows the trend information. Lag, however, inhibits accurate readings during turbulence or abrupt control movements. To solve that problem, the second design, termed Instantaneous Vertical Speed Indicator, supplements the pressure capsule with accelerometers: a dashpot or a vane-type accelerometer actuates miniature air pumps that anticipate pressure changes. Whether styled as an arced indicator or as vertical speed tape on the EADI, both presentations are driven from the same two information sources: trend information, shown by the direction of needle movement, and rate information, the stabilized value once the movement stops. Thus, the pilot judges at a glance both instantaneous motion and final vertical speed.
What are possible errors of a Vertical Speed Indicator?
The possible errors of a Vertical Speed Indicator are outlined below.
- Position Error: Position error is due to the incorrect location of static vents and is most noticeable during takeoff acceleration when a sudden speed change produces a false climb or descent.
- Hysteresis Error: Hysteresis error occurs when the aircraft has been at a flight level for a considerable period, making the instrument unwilling to respond promptly to changes in static pressure values. This is the main source of the six-to-nine-second lag that pilots observe.
- Instrument Error: Instrument error is on account of manufacturing imperfections.
- Elasticity Error: Elasticity error is caused by metal fatigue of the aneroid wafers.
- Maneuver Induced Error: Maneuver induced error, caused by different changes in attitude and configurations of the aircraft, leads to false indications of rate of climb or descent and leaves the needle off zero after a maneuver.
- Turbulence Sensitivity Error: In turbulence sensitivity, turbulence, abrupt control movements, and acceleration or deceleration errors cause the instrument to fluctuate or show false climbs and descents while the aircraft is in level flight. In turbulent conditions, the lag inhibits accurate readings entirely.
- Static Port Blockage Error: Static port blockage is a special case of instrument error: a blocked static line causes the VSI to show a continuous zero indication or to momentarily show a climb and then freeze, while alternate static source restores function.
What causes a Vertical Speed Indicator failure?
The most common VSI failure is caused by a blocked static port. When the static port is blocked, the vertical speed indicator moves to 0 FPM and no longer changes and will remain at zero even if the aircraft is climbing or descending. Static port blockage traps static pressure, so without varying static pressure the calibrated leak inside the instrument cannot equalize, and the capsule connected directly to the static line cannot move. Altimeter freeze and VSI zero are a clear sign of a blocked static port.
Static port blockage is caused by ice, dirt, moisture, or nest material like insect nests. Aircraft equipped with an alternate static source apply the alternate static source when the static port is blocked. Opening the alternate static source causes the VSI to indicate a temporary climb and then returns to normal operation. Aircraft not equipped with an alternate static source break the glass face of the VSI to introduce static pressure, although breaking the glass renders the VSI inoperative.
Turbulence and abrupt control movements cause the VSI to be inaccurate, but the indication returns once the motion stops. A disconnected static line inside the cabin of a non-pressurized aircraft causes higher readings on the altimeter and airspeed indicator, but the VSI shows only a momentary direction change when the line is first disconnected.
Vertical Speed Indicator failure is most often caused by an obstruction in the device's static system. The obstruction is probably a choked static port or a twist in the related pipe and either defect prevents case pressure from equalizing with varying ambient pressure. When this happens the pointer stays below the zero sign and resists motion. A second source of failure is internal: the inner workings become jammed through endured pollution over time, producing a mechanical breakdown unconnected to in-flight compression alterations. During pre-flight check the device may seem to be running normally, yet once airborne the pointer continues to deflect and will not fall to 0 even after the aircraft is leveled at the designated elevation. Cross-checking with the orientation dial and the measuring instrument confirms the aircraft is keeping height, proving the deviation is an error inside the device itself.
How to adjust a Vertical Speed Indicator?
A Vertical Speed Indicator can be adjusted by turning a screw on the lower left corner of the instrument case. If the indication is off by more than 75 feet (22.86 meters) from the surveyed field elevation, the instrument must be referred to a certificated instrument repair station for recalibration.
What is the difference between an altimeter and a vertical speed indicator?
An altimeter and a Vertical Speed Indicator serve two separate but adjacent tasks. The altimeter displays the airplane's exact height above sea level at every instant, while the Vertical Speed Indicator shows only the rate at which that height is varying. Because the altimeter is coupled directly to the static line, it will indicate climb or descent sooner than the Vertical Speed Indicator, giving the pilot the earliest cue that the flight path has altered. Once the airplane stabilizes at a new altitude, the altimeter freezes at that value, whereas the VSI returns to zero. Thus, the altimeter is useful mainly for determining when to level out, and the VSI is used to verify that level flight has actually been captured and is being maintained.
What is the difference between a vertical speed indicator and an airspeed indicator?
A vertical speed indicator is a diaphragm-and-calibrated-leak instrument that tells the pilot how fast the airplane is climbing or descending in feet per minute. It senses the rate of change of static pressure: as the aircraft climbs the casing pressure drops faster than the trapped pressure inside the diaphragm, and the needle deflects upward; the opposite occurs in descent. No pitot input is used, no temperature or density correction is required, and the reading is independent of forward speed. While the ASI supplies speed relative to the airflow, the VSI supplies speed relative to the ground plane, giving the pilot an instantaneous trend of vertical motion that complements the instantaneous horizontal motion shown by the airspeed indicator.
An airspeed indicator (ASI) is a differential pressure gauge that measures the difference between pitot impact pressure and static pressure. The resulting reading on the dial is the indicated airspeed (IAS), the raw speed the airplane shows relative to the surrounding air mass. The instrument displays airspeed in knots, and on many panels it carries an additional colored pointer to mark critical speeds. From that IAS the pilot can derive calibrated airspeed, then equivalent airspeed (EAS) corrected for compressibility, and finally true airspeed (TAS) by accounting for outside-air temperature and altitude. When plotted against the local speed of sound, the TAS gives the Mach number displayed on the Machmeter. Ground speed, by contrast, comes from navigation equipment and adds wind to TAS. Thus, the ASI answers only one question: how fast the aircraft is moving through the air at any moment.
Is a Vertical Speed Indicator required?
FAR 91.205 does not require a Vertical Speed Indicator, therefore, the instrument is not required for VFR day flight, nor is it demanded for IFR certification. Because no federal aviation regulation mandates the instrument, a Vertical Speed Indicator does not have to be functional for the flight to be legal, and it is not obliged to meet any calibration standard. Although US FAA has no specific requirement for aircraft to carry a Vertical Speed Indicator, the instrument is traditionally included in the six pack of cockpit instruments, so when the static pressure system is inspected within the preceding 24 months per FAR 91.411, the VSI is examined along with the altimeter and airspeed indicator.
