Virticle Speed Indicator

Question 1

What system runs the VSI and how does it work with that system?

Answer 1

The static system.

• A bellows inside the VSI is connected to the static port. There is also a calibrated leak, which allows air/pressure around the bellows to escape the VSI at a calculated rate.
• In straight and level flight, the pressure inside the VSI and the bellows inside it are equal, giving a reading of 0fpm.
• As we descend and the pressure from the static port increases and expands the bellows, it pushes air out the leak and gives us a reading of descending by x amount of fpm
• The opposite is true for when we climb

Question 2

What happens if the static port gets blocked?

Answer 2

The VSI will read zero, only way to fix this is if you have an alt static knob

Question 3

What two errors is the VSI subject to?

Answer 3

• Lag (takes a second for the pressure to leak out)
• Reversal Error (if you pitch up very suddenly, it will actually show a little descent)

Question 4

How long does it take for the lag error to stabilize?

Answer 4

6-8 seconds

Question 5

If you are flying at 6500 and you know the circuit height is 1500ft AGL, how long will it take you to descend at 500ft/min?

Answer 5

5000 ft difference so it will take you ten minutes to descend to 1500ft from 6500ft.

If you know your GS, you can calculate how for out you need to start your descent

120kts GS = 2NM/min

2x10=20NM out is when you should start your descent