The Vertical Speed Indicator (VSI) shows vertical speed (climb or descent rate) in feet per minute.
Operating principle
Static pressure flows into a capsule in the VSI. From the same static port, a calibrated leak (capillary) with defined flow restriction leads to a chamber. The pressure difference between capsule and chamber is proportional to climb/descent rate.
The VSI therefore compares the current static outside pressure with the previous static pressure in the reservoir — the delay in the reservoir (through the calibrated leak) produces a pressure difference proportional to the climb rate:
- Climb → current pressure < reservoir pressure → positive indication.
- Descent → current pressure > reservoir pressure → negative indication.
- Level flight → pressures equalise → zero indication.
| State | Pressure behaviour |
|---|---|
| Level flight | both equal → no difference → zero indication |
| Climb | capsule pressure falls faster than chamber → difference → positive |
| Descent | capsule pressure rises faster than chamber → difference negative → negative |
Quick response to altitude changes
The VSI responds very quickly to changes in altitude (climb or descent rate) — within a few seconds of a pitch change, the pressure difference builds and the needle moves.
QNE — altitude above 1013.25 hPa reference
QNE is the altitude above the reference pressure level of 1013.25 hPa (standard pressure altitude). With the subscale set to 1013.25 hPa the altimeter shows pressure altitude = QNE = corresponding flight level.
(Note: this affects primarily the altimeter; in the VSI the subscale is irrelevant — the VSI only measures the differential drift.)
Construction types — capsule vs baffle plate
| Type | Mechanism | Response speed |
|---|---|---|
| Classic VSI with aneroid capsule + capillary leak | Capsule + reservoir, calibrated hole as leak | Slower — 6-9 sec lag |
| VSI with baffle plate | Instead of a calibrated capillary, a baffle plate with a hole creates the pressure difference via flow restriction | Faster — shorter lag, better response to altitude changes |
| IVSI (Instantaneous VSI) | Additional accelerometer compensates the lag | Practically lag-free — indication almost instantaneous |
→ Baffle-plate-type VSIs provide faster readings than the classic capsule-type VSIs with capillary.
Size of equalising reservoir
The size of the equalising tank (reservoir) affects the indication scale:
- If the reservoir is too big: the reservoir pressure changes more slowly, so the pressure difference to the capsule is larger than calibrated → VSI reads too high (indications are too high).
- If the reservoir is too small: opposite effect — VSI reads too low.
Structural lag
Due to the calibration chamber the classic VSI has a lag of several seconds. Negligible in cruise but can cause brief misreadings at transitions (start/end of climb/descent).
Behaviour with static blockage
With blocked static port pressures in both chambers cannot change → VSI shows zero, regardless of actual vertical motion. Alternatives: alternate static source, or ignore the VSI and work with altimeter + stopwatch.