Editing Airspeed indicator (section)

== Operation ==
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The ASI is the only flight instrument that uses both the static system and the pitot system.  Static pressure enters the ASI case, while total pressure flexes the diaphragm, which is connected to the ASI pointer via mechanical linkage. The pressures are equal when the aircraft is stationary on the ground, and hence shows a reading of zero.  When the aircraft is moving forward, air entering the pitot tube is at a greater pressure than the static line, which flexes the diaphragm, moving the pointer.  The ASI is checked before takeoff for a zero reading, and during takeoff that it is increasing appropriately.<ref name=ASI />{{rp|8–10}}<ref>{{cite magazine|url=https://books.google.com/books?id=0SkDAAAAMBAJ&q=Popular+Science+The+lads+who+fly+Britain%27s+1944&pg=PA117 |title=How Aircraft Instruments Work|magazine=Popular Science|date=March 1944|at= pp. 117, mid page}}</ref><ref>{{Cite magazine|url=https://books.google.com/books?id=0SkDAAAAMBAJ&q=Popular+Science+The+lads+who+fly+Britain%27s+1944&pg=PA116 |title=How Aircraft Instruments Work|magazine=Popular Science|date=March 1944|at=pp. 116}}</ref>

The pitot tube may become blocked, because of insects, dirt or failure to remove the pitot cover. A blockage will prevent ram air from entering the system.  If the pitot opening is blocked, but the drain hole is open, the system pressure will drop to [[ambient pressure]], and the ASI pointer will drop to a zero reading.  If both the opening and drain holes are blocked, the ASI will not indicate any change in airspeed.  However, the ASI pointer will show altitude changes, as the associated static pressure changes.  If both the pitot tube and the static system are blocked, the ASI pointer will read zero.  If the static ports are blocked but the pitot tube remains open, the ASI will operate, but
inaccurately.<ref name=ASI />{{rp|8-10, 8-11}}