How do you read the vertical speed indicator?
| Marking | What it means |
|---|---|
| 0 at 9 o’clock | Level flight: neither climbing nor descending. |
| UP (top half) | Climbing. The needle moves clockwise from 0. |
| DN (bottom half) | Descending. The needle moves anticlockwise from 0. |
| 5, 10, 15, 20 | Hundreds of feet per minute: 5 is 500, 10 is 1,000. The 20 at 3 o’clock is 2,000 up or down, where the needle stops. |
| Small marks | Every 100 feet per minute up to 1,000, either way. |
| 100 FEET PER MIN | Printed on the dial: the numbers are in hundreds of feet per minute. |
| Screw at the lower left | Sets the needle to zero. On the ground, the needle should point at 0. |
Trend and rate: why does the needle lag?
The vertical speed indicator gives two kinds of information:
- Trend: the moment the aircraft starts to climb or descend, the needle starts to move that way.
- Rate: the exact number of feet per minute. The needle needs a few seconds to settle on it.
Because of this lag, chasing the needle with the controls leads to over-correcting. Set the attitude with the attitude indicator, wait, then read the rate. Rough air also makes the needle swing.
How does it work inside?
Like the altimeter, it is connected to the static port. The trick is a small hole called a calibrated leak.
- Static air goes straight into a diaphragm inside the instrument, so the diaphragm always has the pressure outside right now.
- The case around the diaphragm gets static air too, but only slowly, through the calibrated leak.
- When you climb or descend, the pressure in the diaphragm changes at once, while the case lags behind. The difference moves the needle.
- When you level off, the case catches up, the pressures match, and the needle returns to 0.
What can go wrong?
- Chasing the needle. It lags, so steer by attitude and use the vertical speed indicator to confirm.
- A blocked static port. No pressure change reaches the instrument, and the needle stays at 0 whatever the aircraft does.
- Not checking zero. If the needle is not on 0 on the ground, the readings in flight are off by the same amount.
Questions
What does the vertical speed indicator measure?
How fast the outside air pressure is changing, shown as feet per minute of climb or descent.
Why is the vertical speed indicator slow to respond?
It compares the pressure now with the pressure in its case, which catches up slowly through a calibrated leak. The direction shows at once; the exact rate takes a few seconds.
What does 5 mean on the VSI?
500 feet per minute. The numbers are in hundreds of feet per minute.
What happens to the VSI if the static port is blocked?
It reads 0 and stays there, whatever the aircraft does.
Is SkyDeck’s vertical speed indicator laggy too?
In the app it shows the vertical speed your simulator gives its own gauge, so it behaves the way the simulator’s does.
Source
- Federal Aviation Administration, Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 8, Flight Instruments.