Learn what an aircraft vertical speed indicator (VSI) shows, how it works, how to read it, why it lags and what false readings mean.
A vertical speed indicator (VSI) is an aircraft flight instrument that shows how quickly the aircraft is climbing or descending, normally in feet per minute. It detects the rate of change in static air pressure. A zero reading normally indicates level flight, while positive and negative values indicate climb and descent respectively.
In real-world aviation and flight simulation, the VSI is also called a rate-of-climb indicator or vertical-velocity indicator. Unlike an altimeter, it shows a rate rather than an altitude; it also measures movement through pressure levels, not height above terrain.
How does a vertical speed indicator work?
A conventional VSI calculates vertical speed from a temporary pressure difference inside the instrument. Its diaphragm receives static pressure directly, while the instrument case receives the same pressure through a calibrated restriction.
During a climb, outside static pressure falls immediately in the diaphragm but changes more slowly in the case. That pressure difference moves the pointer towards climb. During a descent, the pressure relationship reverses. Once pressure stops changing and both sides equalise, the pointer returns to zero.
An instantaneous VSI adds acceleration sensing to reduce the initial delay. Electronic flight displays generally use an air-data system to calculate vertical speed and present it beside the altitude tape; this FSX panel demonstrates vertical-speed data integrated into a primary flight display.
How do you read a VSI?
Read the VSI as a rate of altitude change, using the units and scale printed on that particular instrument.
- Above zero or UP: the aircraft is climbing.
- Below zero or DOWN: the aircraft is descending.
- At zero: static pressure is no longer changing after the instrument has settled.
Most light aircraft display vertical speed in feet per minute. On many analogue dials, a figure such as 5 represents 500 ft/min, but scales vary, so the instrument markings must be checked rather than assumed. Our Cessna 172 cockpit-instrument walkthrough shows how the VSI fits into a familiar training-aircraft panel.
In a conventional six-pack, the VSI is normally the lower-right instrument. Glass cockpits instead place a pointer, scale or numeric value beside the altitude display. Simmers looking for the older layout can see which simulator aircraft retain traditional six-pack instruments.
Does zero on the VSI always mean level flight?
No: zero means the instrument senses no change in static pressure, which is not always proof of level flight. A mechanical VSI may still show zero just after a climb or descent begins because of lag, and a blocked static system can make it settle at zero even while altitude changes.
Why does a VSI lag or show the wrong rate?
A conventional VSI takes several seconds to settle because its calibrated restriction deliberately delays pressure equalisation. The needle first provides a useful trend, then approaches the actual rate.
- Needle chasing: repeated pitch changes made before the indication settles cause oscillations. Hold a stable attitude and power setting, then allow the instrument to respond.
- Turbulence or disturbed airflow: pressure fluctuations around the static port can make the needle move rapidly even when the average flight path is stable.
- Blocked static source: the VSI generally returns towards zero and becomes unreliable. An altimeter and airspeed indicator sharing that source may also give incorrect indications.
- Alternate static source: selecting cabin static pressure may cause a temporary jump because cabin and outside pressure differ.
- Leaks, calibration errors or instrument faults: persistent sluggishness, an unexplained offset or disagreement with other instruments requires inspection against the aircraft's approved limits.
In a simulator, compare the VSI with the altimeter, standby instruments and a default aircraft. If every aircraft is affected, inspect simulated failure settings and weather conditions; if only one add-on panel is affected, its instrument modelling or configuration is the more likely cause.
How should pilots use the VSI?
Pilots should use the VSI to confirm a climb or descent established with attitude and power, not as the sole instrument for controlling pitch.
- Set attitude and power: establish the expected climb, descent or level-flight configuration.
- Cross-check the trend: confirm movement with the altimeter, airspeed indicator and VSI.
- Let the indication settle: make small corrections rather than chasing every needle movement.
- Lead the level-off: use altitude and attitude to anticipate the target, then use the VSI to confirm that vertical movement has stopped.
For planning a three-degree descent, groundspeed multiplied by roughly five gives an approximate required rate in feet per minute; 120 knots is therefore about 600 ft/min. This is a cross-check, not a substitute for published procedures or vertical-path guidance.
When an autopilot is using vertical-speed mode, it changes pitch to hold the selected rate. In a climb, an excessive selection can let airspeed decay, so airspeed and power still require close monitoring.