Vertical speed indicator (VSI)
The vertical speed indicator (VSI) shows rate of climb or descent in hundreds of feet per minute (fpm). Uses static pressure only, yet is a differential pressure instrument (PHAK ch.8).
| Part | Connection | Pressure response |
|---|---|---|
| Diaphragm | Direct to static line | Feels change at once |
| Case (airtight, around diaphragm) | Static line via restricted orifice (calibrated (metered) leak) | Follows late |
- Ground or level flight: both pressures equal, pointer reads zero.
- Climb or descent: case pressure stays higher or lower briefly, diaphragm contracts or expands, needle shows the difference. Difference stabilizes at a definite ratio: rate.
Two kinds of information (PHAK ch.8):
| Type | Meaning |
|---|---|
| Trend | Direction needle moves: immediate sign of more or less climb or descent |
| Rate | Stabilized rate of altitude change |
Lag = time from initial change in climb rate until VSI shows accurate new rate (PHAK ch.8):
- Pitch held constant: needle settles in about 6 to 9 seconds.
- Rough control and turbulence extend the lag, give erratic readings.
- Instantaneous VSI (IVSI): accelerometers compensate for the lag.
Pitch change sets the lag (TP 1102):
| Pitch change | Lag |
|---|---|
| Any | Directly proportional to speed and magnitude of change |
| Slow, smooth | Minimum lag |
| Large, abrupt | Erratic needle movement, longer delay before it stabilizes |
Preflight check (PHAK ch.8):
- Reads near zero before leaving the ramp and again just before takeoff.
- Not zero: use indication as the zero mark. Small offset is normal.
- After takeoff: trends upward (positive rate of climb), rate shows once climb stabilizes.
Trap: needle moving at once is only trend. True rate shows after 6 to 9 second lag; don't chase first reading.
References
- PHAK ch.8 (FAA-H-8083-25), Vertical Speed Indicator
- TP 1102 Instrument Flying: Level Flight