Three of your most important instruments, the airspeed indicator, altimeter, and vertical speed indicator, all run on air pressure from the pitot-static system. Understanding it explains how those instruments work and what happens when a port gets blocked.
Part of our How Airplanes Work guide.
Two sources of pressure
The system has two inputs. The pitot tube faces forward and captures ram (dynamic) air pressure from the airplane’s motion. The static port sits flush with the side of the fuselage and senses the ambient (static) air pressure around the airplane.
Which instrument uses what
- Airspeed indicator: compares pitot (ram) pressure to static pressure. The difference is your airspeed.
- Altimeter: uses static pressure only. As you climb, static pressure drops and the altimeter reads higher.
- Vertical speed indicator: uses static pressure only, measuring the rate of pressure change to show climb or descent rate.
What happens when ports block
Blockages are a classic exam and safety topic:
- Pitot blocked (drain open): the airspeed indicator reads low or drops toward zero.
- Pitot and drain both blocked: the airspeed indicator behaves like an altimeter, reading higher as you climb and lower as you descend.
- Static port blocked: the altimeter freezes at the blocked altitude, the VSI stops, and the airspeed indicator becomes inaccurate, reading lower than actual above the blockage altitude and higher than actual below it.
Pitot heat, an electric heating element in the pitot tube, is the main defense against pitot icing, which is a common cause of blockage. Turn it on when flying in visible moisture near or below freezing.
The alternate static source
Many airplanes have an alternate static source, often venting to the cabin, that you can select if the primary static port blocks. Because cabin pressure differs slightly from outside, your instruments will read a little differently, and your handbook lists the corrections. Knowing how to use it is part of handling an instrument failure calmly.
What you'll need
The FAA handbooks that explain how your airplane works, from PilotMall.com.
Frequently asked questions
Which instruments use the pitot-static system?
The airspeed indicator, the altimeter, and the vertical speed indicator.
What does the pitot tube measure?
Ram (dynamic) air pressure from the airplane’s forward motion, used by the airspeed indicator.
What happens if the static port is blocked?
The altimeter freezes, the VSI stops, and the airspeed indicator reads inaccurately (low above the blockage altitude, high below it). The alternate static source can restore usable readings.