Updated: Dec 8, 2020
Pitot-Static System Lesson by wifiCFI
How it Works
The pitot-static system comprises of a pitot tube and one or multiple static ports.
These components are responsible for giving data to the aircraft’s: airspeed indicator, altimeter, and vertical speed indicator.
For this lesson, we will discuss the inner workings of the pitot-static system and associated instruments.
We will also discuss the errors that can occur with this system and instruments when flying in the IFR Environment.
To start, we will diagram how the pitot tube and static ports are connected to their respective cockpit instruments.
As can be seen, the pitot tube is only connected to the airspeed indicator.
While the static port is connected to all three instruments.
The Airspeed Indicator
The airspeed indicator is connected to both the pitot tube and static ports.
The line from the pitot tube connects to a diaphragm type balloon in the instrument while the static port provides air pressure to the rest of the empty space.
The altimeter works in much the same way as the airspeed indicator.
However, the diaphragm in the altimeter is pre-sealed and is referred to as an Aneroid Wafer.
The Vertical Speed Indicator
The vertical speed indicator compares static air pressure to a change in static air pressure.
Two lines of static pressure are connected to the VSI and the change is static pressure is measured through the use of a “Calibrated Leak” as shown below.
Now we will discuss how each instrument will be affected with a pitot tube or static port clog.
Clogs happen most often in flying through ice accumulation.
A clogged pitot tube will only affect the airspeed indicator.
While a clogged static port will affect all of the pitot-static instruments.
Clogged Pitot Tube
If the pitot tube becomes clogged in flight, only the airspeed indicator will be affected since the altimeter and VSI are not connected to the pitot tube.
This would mean no ram air pressure is being sent to the diaphragm.
The diaphragm will cease to expand and contract with increases and decreases in airspeed.
Essentially making the airspeed indicator act like an altimeter.
Clogged Static Port
A clogged static port will affect the airspeed indicator by displaying slightly erroneous information.
Let’s assume the static port gets clogged while flying at 5,000’ MSL.
This would trap 5,000’ air in the static area of the airspeed indicator and cause the diaphragm to expand and contract incorrectly at higher and lower altitudes.
The altimeter will freeze at the altitude where the clog occurred.
This is because there will be no changing static pressure inside the instrument casing and therefore no expanding and contracting of the pre-sealed aneroid wafer.
A clogged static port will cause no change in static pressure in the Vertical Speed Indicator’s Diaphragm or in the pressure flowing out of the calibrated leak.
Therefore, the VSI will drop to a 0 indication and freeze there.
Fixing the Clogs
If the pitot tube becomes clogged the pilot can utilize pitot heat to try and eliminate the blockage.
Pitot heat is activated by a cockpit switch and it will electrically heat the pitot tube to try and melt any blockage that has occurred.
Alternate Static Source
If a static port becomes clogged, the pilot can open up the Alternate Static Source with a switch in the cockpit.
When this switch is activated the aircraft instruments will start receiving their Static Air Data from a source inside the aircraft cabin.
This will cause slight errors to all three of our pitot-static instruments that will be discussed in the following slides.
Aircraft manufacturers place the external Static Ports in such a way that they can evaluate Static (non-moving) Air Pressure.
However, the Alternate Static Port is typically located in the aircraft cabin.
Because aircraft cabins are not sealed air-tight, this will cause movement of air inside the cabin.
This means that the air, inside the cabin (at the Alternate Static Port), is not completely Static (non-moving).
The movement of this air is caused due to a lower pressure outside of the aircraft than the relative pressure inside the cabin.
This means air will flow from high to low (from inside the aircraft to outside the aircraft) as depicted below.
Alternate Static Source
Because the Alternate Static Source is sensing lower air pressure than the air pressure at the Normal (external) Static Port, there will be a slight increase seen on all three of the pitot-static flight instruments.
For this reason, many airplanes will be accompanied with conditions to be met when operating the Alternate Static source such as: closing all windows, closing all vents, turning off windshield defrost, etc.
This is done in attempt to make the cabin air as Static (non-moving) as possible.
Below is depicted the inner workings of why each pitot-static instrument will show a slight increase with use of the Alternate Static Source.
FAA Sources Used for This Lesson
Instrument Flying Handbook