LESSONS · AIRCRAFT SYSTEMS · 12 MINUTES · AUSTRALIA AND NZ
The pitot-static system.
Three instruments run on two pressures. The pitot tube faces into the airflow. Small static ports on the side of the aircraft sample the undisturbed air. Block one of them and the instruments start telling you things that are not true.
Two pressures, three instruments
Static pressure is the pressure of the air around you. It is there whether you are moving or not, and it falls as you climb. Dynamic pressure comes from moving through the air. The pitot tube collects both together, through an opening at its front. The static ports collect static pressure alone.
The airspeed indicator (ASI) compares the two lines and shows the difference. The altimeter and the vertical speed indicator (VSI) use the static line alone.
The ASI is a sensitive, differential pressure gauge that measures and promptly indicates the difference between pitot (impact/dynamic pressure) and static pressure.
The pitot tube also has a small drain hole at the back, so moisture can run out.
In addition to the larger hole in the front of the pitot tube, there is a small hole in the back of the chamber that allows moisture to drain from the system should the aircraft enter precipitation.
In flight, ice blocks the front opening of the pitot tube. The drain hole behind it stays clear. What does the airspeed indicator do?
TRY IT
Fly it, then block it
Climb, descend and change speed. Then block a port and see which instruments go wrong. Dots show air moving in a line while its pressure changes. The model uses the standard atmosphere and an illustrative cabin pressure, so real aircraft differ in the detail.
AirspeedAltimeterVertical speed
Where the aircraft is, and what the instruments say
Quantity
Actual
Instrument
Airspeed
100 kt
100 kt
Altitude
3,000 ft
3,000 ft
Vertical speed
0 ft/min
0 ft/min
All clear. The ASI compares pitot pressure with static pressure. The altimeter and VSI use static pressure only.
The VSI lags, even when nothing is blocked
Set a climb with everything clear and watch the table. The VSI takes a few seconds to catch the new rate. It measures how fast static pressure is changing by letting air leak slowly into its case, and that leak takes time.
If the pitch attitude is held constant, the needle stabilizes after a short period (6–9 seconds) and indicates the rate of climb in hundreds of fpm.
Moisture, ice, dirt and insects can all block the pitot tube or the static ports. With the front of the pitot tube blocked and the drain hole clear, the ASI falls to zero. If the drain hole blocks too, the air in the pitot line is trapped and the ASI stops responding to speed.
Blockage may be caused by moisture (including ice), dirt, or even insects.
If both the pitot tube opening and the drain hole should become clogged simultaneously, then the pressure in the pitot tube is trapped. No change is noted on the airspeed indication should the airspeed increase or decrease.
Both the pitot opening and the drain hole are blocked, so the pitot pressure is trapped. The static port is clear. You hold your airspeed and climb 1,000 ft. What does the ASI show?
When the static port blocks
A blocked static port traps the pressure in the static line. All three instruments are affected, because all three use it.
A blocked pitot tube affects the accuracy of the ASI, but a blockage of the static port not only affects the ASI, but also causes errors in the altimeter and VSI.
The static port ices over while you cruise at 3,000 ft. You then descend to 2,000 ft. What does the altimeter show?
The ASI is still working, but it compares the pitot line with the wrong static pressure. Below the height where the port blocked, it reads faster than you are flying. Above it, it reads slower. Try both in the simulation and watch the table.
The airspeed indicates lower than the actual airspeed when the aircraft is operated above the altitude where the static ports became blocked because the trapped static pressure is higher than normal for that altitude. When operating at a lower altitude, a faster than actual airspeed is displayed due to the relatively low static pressure trapped in the system.
Some aircraft have an alternate static source inside the cabin, for when the outside ports block. Air flowing around the fuselage leaves the cabin at a lower pressure than outside, so the instruments now read wrong in a known direction:
the altimeter reads slightly higher than your real altitude
the ASI reads faster than your real airspeed
the VSI shows a brief climb when you switch over, then settles if you hold your altitude.
Due to the venturi effect of the air flowing around the fuselage, the air pressure inside the flight deck is lower than the exterior pressure.
The size of the error depends on the aircraft. The flight manual for your aircraft gives it.
Each pilot is responsible for consulting the Aircraft Flight Manual (AFM) or the Pilot’s Operating Handbook (POH) to determine the amount of error that is introduced into the system when utilizing the alternate static source.