An airspeed indicator on an aircraft functions as a precise pressure gauge. The indicator measures the difference in air pressure between the pitot, or ram air pressure, and the static or undisturbed air pressure. Accurate airspeed measurement is essential for the safe operation of the aircraft. Airspeed indicators are so sensitive to pressure that blowing into the instrument to test it damages the indicator. Damaged indicators or indicators that are out of calibration need fixing.
Obtain a manometer. A manometer precisely measures small pressures necessary for testing an airspeed indicator. You may be able to borrow one from your local aircraft mechanic. You can also build a manometer yourself using simple tools and materials from the hardware store.
Remove the airspeed indicator from the aircraft's instrument panel. Use a flat-bladed screwdriver to loosen the hose clamps and disconnect the two hoses on the back of the unit. Be sure to label and cap the hoses to prevent contamination from entering the lines. Remove the front mounting screws with a Phillips screwdriver. Remove the airspeed indicator from behind the panel.
Connect the manometer to the line marked "P" for "pitot" on the airspeed indicator. Leave the line marked "S" for "static" open.
Apply air pressure slowly to the manometer. Confirm that there is a corresponding rise in airspeed on the indicator. Do not apply so much pressure that the needle pegs.
Calibrate the airspeed indicator. Check the indicator's accuracy by comparing a table of pressures vs. airspeed with your indicator. For example, a water column height of 6.5397 inches on the manometer equals .2362 psi and 100 knots of airspeed. Send the indicator off for an overhaul if the device is not accurate.
Re-install the airspeed indicator. Make sure you reinstall the hoses in the same positions they were in previously and from which they were removed. Confirm that the connections are tight.
Blowing directly into the airspeed indicator will cause damage requiring an expensive overhaul.