There are 3 main instruments connected to an airplane's pitot-static system. The Airspeed Indicator, The Altimeter, and the Vertical Speed Indicator.
Here's what you need to know.
THE PITOT TUBE
The Pitot Tube is connected to the Airplane's Airspeed Indicator and measures Ram Air Pressure. This air pressure fills a diaphragm (or balloon) inside the Airspeed Indicator. As the airplane flies faster, more ram air pressure causes the balloon to inflate and subsequently show the aircraft flying at a faster Indicated Airspeed. Conversely, as the airplane flies more slowly, the ram air pressure inside the diaphragm will not be as great. This will cause it to deflate and show the aircraft flying at a slower Indicated Airspeed.
THE STATIC PORT
The Static Port(s) are connected to all 3 of the Pitot Static Instruments (Airspeed Indicator, Altimeter, and Vertical Speed Indicator) and measure outside Static Air Pressure (the pressure of non-moving air).
THE AIRSPEED INDICATOR
The diagram below shows how Ram Air and Static Air Pressure work in combination to move the needles on the Airspeed Indicator and inform the pilot of his or her Indicated Airspeed.
Remember, the Altimeter is not connected to the Pitot Tube and therefore does not receive a Ram Air Pressure Input. Instead, the Altimeter is fitted with a Pre-Sealed Aneroid Wafer that contains an air pressure of 29.92 while the Static Air Pressure fills up the space around the wafer.
The diagram below shows what happens inside The Altimeter when an aircraft climbs or descends.
THE VERTICAL SPEED INDICATOR
This instrument compares Static Air Pressure to changes within itself through the use of a Calibrated Leak. In other words, there are 2 lines from the Static Port(s) to the Vertical Speed Indicator. One line feeds into the instrument housing while the other feeds into an inflatable diaphragm as can be seen in the diagram below.
To learn more about the Pitot-Static Instruments such as dealing with clogs and the Alternate Static Ports, be sure to watch our Pitot-Static System Lesson at wifiCFI.com.
Author - Nate Hodell
CFI/CFII/MEI/ATP - Creator of wifiCFI - Owner of Axiom Aviation Flight School.
This information is included in the Pitot-Static System Lessons on wifiCFI. Sign up today to watch videos, listen to podcasts, take lesson quizzes, join live webinars, print lesson quicktakes, and more by clicking this link >