Flight Instruments: The Vertical Speed Indicator (VSI)
The vertical speed indicator is one of the six basic flight instruments in an airplane. The VSI tells the pilot whether the airplane is climbing, descending or in level flight. The vertical speed indicator also gives rate information in feet per minute (fpm) for the climb or descent. For example, the desired climb or descent might be accomplished at 500 feet per minute, and the VSI indicator makes this task easy.
Think of the vertical speed indicator as a convenient instrument for accuracy and stability, especially for instrument pilots. Coupled with the other five basic instruments (airspeed, attitude indicator, altimeter, turn coordinator and heading indicator) the VSI gives the pilot a good indication of the airplane's status.
How the VSI Works
The vertical speed indicator is made up of a diaphragm inside of an airtight instrument casing. The diaphragm is connected by linkage and gears to the needle on the face of the instrument. Static pressure lines are connected to both the inside of the diaphragm and the instrument casing. The casing surrounding the diaphragm has a metered leak, which helps reflect the rate of climb of descent.
Pressure changes are measured instantaneously within the diaphragm as it expands and contracts from the pressure. The metered leak in the surrounding instrument casing also measures the pressure change, but the leak provides an intentional lag, allowing the instrument to measure the pressure change more gradually than inside the diaphragm.
This lag comes from the consistent pressure leak and the corresponding rate of climb or descent as it's measured on the instrument needle in feet per minute. After a few seconds of level flight, the two pressures equalize and the vertical speed indicator shows '0' feet per minute (fpm).
The result of a climb or descent is shown on the vertical speed indicator first as trend information (meaning, a sudden climb or descent) and then shown as rate information (for example, 400 fpm).
Errors and Limitations
The vertical speed indicator is inaccurate during turbulence and when maneuvering abruptly. The lag involved with the calibrated leak is about six to eight seconds, rendering the vertical speed indicator almost useless when turbulence is encountered. If turbulence is encountered, the pilot should try to maintain an appropriate pitch attitude using the attitude indicator or outside visual references, instead of "chasing the needle" or trying to maintain a steady rate.
Static Port Blockage
If a static port becomes blocked, similar to an altimeter, the vertical speed indicator will indicate '0' and no change will be witnessed with either a climb or a descent. However, some airplanes are equipped with an alternate static source which provides an alternate source of static air to the flight instruments in the event of a blockage of the main static line.