The VSI displays two different types of information:  The ASI is a sensitive, differential pressure gauge that  measures and promptly indicates the difference between pitot  (impact/dynamic pressure) and static pressure. These two  pressures are equal when the aircraft is parked on the ground  in calm air. When the aircraft moves through the air, the  pressure on the pitot line becomes greater than the pressure  in the static lines. This difference in pressure is registered by  the airspeed pointer on the face of the instrument, which is  calibrated in miles per hour, knots (nautical miles per hour),  or both.  [Figure 8-7]  The ASI is the one instrument that utilizes both the pitot,  as well as the static system. The ASI introduces the static  pressure into the airspeed case while the pitot pressure  (dynamic) is introduced into the diaphragm. The dynamic  pressure expands or contracts one side of the diaphragm,  which is attached to an indicating system. The system drives  the mechanical linkage and the airspeed needle.  Just as in altitudes, there are multiple types of airspeeds.  Pilots need to be very familiar with each type.  The trend information is the direction of movement of the  VSI needle. For example, if an aircraft is maintaining level  flight and the pilot pulls back on the control yoke causing the  nose of the aircraft to pitch up, the VSI needle moves upward  to indicate a climb. If the pitch attitude is held constant,  the needle stabilizes after a short period (6–9 seconds) and  indicates the rate of climb in hundreds of fpm. The time  period from the initial change in the rate of climb, until the  VSI displays an accurate indication of the new rate, is called  the lag. Rough control technique and turbulence can extend  the lag period and cause erratic and unstable rate indications.  Some aircraft are equipped with an instantaneous vertical  speed indicator (IVSI), which incorporates accelerometers  to compensate for the lag in the typical VSI.  [Figure 8-6]  As part of a preflight check, proper operation of the VSI  must be established. Make sure the VSI indicates a near zero  reading prior to leaving the ramp area and again just before  takeoff. If the VSI indicates anything other than zero, that  indication can be referenced as the zero mark. Normally, if the  needle is not exactly zero, it is only slightly above or below  the zero line. After takeoff, the VSI should trend upward to  indicate a positive rate of climb and then, once a stabilized  climb is established, a rate of climb can be referenced.  Figure 8-6.  An IVSI incorporates accelerometers to help the  instrument immediately indicate changes in vertical speed.  Figure 8-7.  Airspeed indicator (ASI).  •  Indicated airspeed (IAS)—the direct instrument  reading obtained from the ASI, uncorrected for  variations in atmospheric density, installation error,  or instrument error. Manufacturers use this airspeed  as the basis for determining aircraft performance.  Takeoff, landing, and stall speeds listed in the AFM/  POH are IAS and do not normally vary with altitude  or temperature.  •  Trend information shows an immediate indication of  an increase or decrease in the aircraft’s rate of climb  or descent.  •  Rate information shows a stabilized rate of change in  altitude.  Airspeed Indicator (ASI)  Instrument Check  8-8  4 3  I  0  2  2 I  3  UP  DOWN  .5  .5  Accelerometer  Inlet from static port  Calibrated leak  100  200  50  150  Diaphragm  Handstaff pinion Static air line  Pitot tube  Ram air  Sector Long lever  Pitot connection 