3 4 I 0 2 2 I 3 UP DOWN .5 .5 Diaphragm Static connection Restricted passage Bypass restriction Dashpot acceleration pump Dashpot piston 100 200 50 150 Diaphragm Handstaff Static connection Rocking shaft Hairspring Sector Long lever Pitot connection Restraining spring 0 A B Hairspring Vane Pivot Pointer and scale to fixed pressure reservoir to static source 10-24 Figure 10-42.  An airspeed indicator is a differential pressure gauge  that compares ram air pressure with static pressure.  Figure 10-41.  A variometer uses differential pressure to indicate  vertical speed. A rotating vane separating two chambers (one with  static pressure, the other with a fixed pressure reservoir), moves  the pointer as static pressure changes. Figure 10-40.  The small dashpot in this IVSI reacts abruptly to a  climb or descent pumping air into or out of the diaphragm causing  an instantaneously vertical speed indication.  While the relationship between ram air pressure and static  air pressure is the basis for most airspeed indications, it can  be more accurate. Calibrated airspeed takes into account  errors due to position error of the pitot static pickups. It also  corrects for the nonlinear nature of the pitot static  pressure  The rate-of-climb indication in a digitally displayed  instrument system is computed from static air input to the  ADC. An aneroid, or solid-state pressure sensor, continuously  reacts to changes in static pressure. The digital clock within  the computer replaces the calibrated orifice found on an  analog instrument. As the static pressure changes, the  computer’s clock can be used to develop a rate for the change.  Using the known lapse rate conversion for air pressure as  altitude increases or decreases, a figure for climb or descent  in fpm can be calculated and sent to the cockpit. The vertical is greater than the static pressure; the vane is pushed in the  opposite direction, causing the pointer to rotate and indicate  a climb.  [Figure 10-41] The relationship between the ram air pressure and static air  pressure produces the indication known as indicated airspeed.  As with the altimeter, there are other factors that must be  considered in measuring airspeed throughout all phases of  flight. These can cause inaccurate readings or indications  that are not useful to the pilot in a particular  situation. In  analog airspeed indicators, the factors are often  compensated for with ingenious mechanisms inside the case and on the  instrument dial face. Digital flight instruments can have  calculations performed in the ADC so the desired accurate  indication is displayed. The airspeed indicator is another primary flight instrument  that is also a differential pressure gauge. Ram air pressure  from the aircraft’s pitot tube is directed into a diaphragm in  an analog airspeed instrument case. Static air pressure from  the aircraft static vent(s) is directed into the case surrounding  the diaphragm. As the speed of the aircraft varies, the ram  air pressure varies, expanding or contracting the diaphragm.  Linkage attached to the diaphragm causes a pointer to move  over the instrument face, which is calibrated in knots or miles  per hour (mph).  [Figure 10-42] speed is often displayed near the altimeter information on  the primary flight display.  [Figure 10-34] Airspeed Indicators