The pitot-static system is a very reliable set of instruments that measure air pressure and display airspeed, altitude and vertical speed to the pilot using that information.
What is a Pitot Tube?
The pitot tube points directly into the oncoming air to measure the dynamic pressure as air is forced into the tube by the forward motion of the plane. It is critical to reading airspeed and it is not used by the altimeter or vertical speed indicator. Pitot tubes will also have a drain hole to allow water to exit.
What is a Static Port?
The static port is simply a hole on the side of the plane that allows static pressure from outside to enter the pitot static system and supply the airspeed indicator, altimeter, and vertical speed indicator with static pressure. This hole is mounted sideways (or sometimes pointing down or backwards) to avoid dynamic pressure from the oncoming air.
Some planes have a pitot tube and static port combined like this Piper Archer.
Pitot-Static Ice
The pitot tube and static port are vulnerable to icing. In both cases, ice can collect and block the hole that allows air pressure to enter. When the pitot tube or static port are iced over the pressure does NOT drop to zero. Instead, it stays constant, as the air is trapped inside.
This means that if the pitot tube is iced over the airspeed needle will stay constant even if you change speed.
If you climb or descend with an iced pitot tube this will also affect your airspeed. As you climb the air becomes thinner in the static port but the air in the frozen pitot tube remains constant so the airspeed will indicate an increase. Conversely when descending the airspeed will show a decrease.
If the pitot tube is frozen most airplanes are equipped with pitot heat. You can turn this on and the tube will warm up so the ice melts.
The static port, which often does not have any heating is also susceptible to ice. If the static port is frozen then changes in airspeed will show correctly as long as you are at the altitude where it froze.
If you climb the oncoming air forced into the pitot tube will get thinner but the trapped air in the static system will remain the same so the airspeed will indicate lower than it should. Conversely in a descent with a frozen static port the airspeed will indicate higher.
If the pitot tube hole freezes but the drain hole doesn’t then the airspeed will indicate zero, because the pressure of oncoming air will be blocked and the pitot tube will measure the static pressure, like it does when the aircraft is not moving.
A static port usually cannot be heated, but planes have an alternate static source which can be turned on. This is an extra static port in the cockpit, usually under the panel (except for pressurized aircraft, in which it will sit outside of the pressure vessel). When this is turned on it will let the static pressure from the cabin into the static system and everything will indicate correctly, with a small inaccuracy.
If the static port is frozen and alternate static is not used then the vertical speed will read zero and the altitude will remain where it was when the port froze. This is because the static pressure is trapped inside the system until the static port thaws.
