Inside the pitot-static system
Where the air comes from, where it goes, and what each instrument does with it.
Pitot (ram) pressure
Static (outside air) pressure
Moving dots show air flowing in or out of a line as pressure changes.
Fly the airplane
100 kt
Power stays the same, so climbing slows the airplane and descending speeds it up.
What the airplane is really doing
Altitude3000 ft
Airspeed100 kt
Vertical speed0 fpm
Static26.82
Pitot (total)27.30
Dynamic0.48
Pressures in inches of mercury (inHg). Standard atmosphere.
Airspeed indicator
Altimeter
Vertical speed indicator
Key points +
- Pitot tube: faces the relative wind and collects ram (total) pressure. Only the airspeed indicator uses it. The drain hole lets water out.
- Static port: flush on the side of the fuselage, out of the airflow, so it senses the still outside air pressure. It feeds all three instruments.
- Airspeed indicator: pitot pressure inside the diaphragm, static pressure in the case. The difference is dynamic pressure, which is what the needle shows.
- Altimeter: a stack of sealed aneroid wafers, calibrated to standard pressure (29.92 inHg), sits in a case filled with static air. As static pressure drops, the wafers expand and the needles turn. The Kollsman window sets the reference pressure.
- VSI: static air reaches the diaphragm right away but reaches the case only through a calibrated leak. That difference shows the rate of climb or descent. The trend shows immediately; the exact rate takes about 6 to 9 seconds to settle.