To measure the airplane descent or climb velocity. I.E...if you are flying by instrument approach such as VOR LOC, you must fly the plane
from one point to another with a specific descent vertical speed until reach the point at required altitude and distance specified in instrument approach chart.
This vertical speed is indicated by the IVSI instrument on board, (Instantaneous Vertical Speed Indicator).
On gliders this instruments gives the ascending velocity on thermals.
Mohr,
It also gives the descending velocity that you really, did not want to know about, when you are out of the thermal. The instrument used in a sailplane is a variometer, even more sensitive than an IVSI.
B.E.
You are judged not by what you know, but by what you can do.
It's a direct indication of whether or not the plane is changing altitude and at what rate.
It saves you the trouble of fixating on the altimeter to see if there is a rate of change over time. Without it you would have to track elapsed time and subtract the difference in altitudes and calculate a rate.
There are no super accurate outside visual references for judging vertical speed in an airplane. It would really be hard to judge the rate at which the altimeter needle unwinds in order to level off at an altitude.
I believe ATC has certain expectations with regard to minimum climb rates when flying IFR.
My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm professionally affiliated with.
My explain was intended to be easy to understand. You are right about gliders.
The variometer on gliders is a Total Energy Variometer type in order to read the real rate of ascending on thermals.
