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Aircraft general knowledge

8.12.7 Variometer (VSI Vertical Speed Indicator)

To indicate whether you are climbing, flying straight or descending, we have the

variometer.

An extremely thin tube with a very small opening is placed in the inlet pipe of

the capsule.

If you climb with the aircraft, the static pressure in the instrument housing

becomes smaller, and this will allow the capsule to expand.

This causes the pointer to go to + and show climb. This climb is expressed in feet

per minute for motor aircraft. When flying straight ahead, the pointer comes to
rest at zero after a delay (like the altimeter).

Here is a schematic diagram of
the variometer.

The air in the capsule will, with a

small delay, get the same

pressure as the air in the

instrument housing. This delay

causes the pointers to move.

Here is an example of the actual pointer instrument.

The variometer is used especially for instrument

approaches.

If the static port is blocked by ice, you must open the “alternate static port”. If

the aircraft does not have one, you can break the glass on your VSI and thus get
static pressure from the cabin for all the instruments that use it.

Flyveteori PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 774