How it works

You probably know that the vertical speed indicator uses the airplane’s static system to function, but maybe you don’t know how. The centuries-old concept is actually so simple it’s surprising. At its core, the VSI is a basic instrument that simply measures changes in pressure.

HOW IT WORKS

• Each airplane has a small hole in its side called a static port. This port senses ambient air pressure, which changes as we climb or descend.
• That ambient air is plumbed up to the panel and into the back of the VSI’s (mostly) sealed case (as well as the airspeed indicator and altimeter), and the inside of a sealed disc called an aneroid or bellows. This aneroid is connected through a mechanical linkage with the needle on the face of the instrument.
• As the airplane climbs, pressure inside the case and aneroid decreases, the aneroid expands, and the indicator needle moves up. When the airplane descends, the opposite happens.
• If it ended there, the instrument would be similar to an altimeter. But the VSI is different because it needs to go back to zero when the climb or descent stops. It does this very simply through a small metered hole in the static line called a release valve. That way the pressure changes more slowly than the aneroid inside the case, allowing the aneroid to expand or contract.The pressure equalizes in the case after a second or two—and no more pressure is exerted on the aneroid.

When it doesn't work

The most common VSI failure is caused by a blocked static port. If this happens, the VSI will show zero, regardless if the airplane is climbing or descending. If the airplane is equipped with an alternate static source, apply that. Otherwise, the VSI is considered to be somewhat of a supporting instrument, and you can fly just fine without it.