Airspeed Indicator

                                                              How is the ASI able to indicate my Airspeed?


Simple! What the ASI shows you, is the difference in pressure between the air received in the Diaphragm coming from the Pitot-Tube (Relative Wind/Ram Air, which is acting as pressure <> called Dynamic Pressure) and that received within the instrument's case coming from the Static Ports (undisturbed air). The Static-Port (piece of metal with tiny hole(s)) is attached directly to the fuselage at a completely different angle into the wind (where it's not affected by direct airflow/ram air) compared to the Pitot Tube.  Air basically streaks/passes the hole instead of going right into it.

Example of Operation

Imagine the following situation:

Your aircraft is parked on the Runway ready for Take-Off in a 'zero' wind condition.


→ NO PRESSURE IN THE SP => Instrument Case

(both are basically receiving the same outside air pressure)

At this point your airspeed indicator needle is not moving, indicating 0 kts IAS.

1. You slowly advance the throttle to Full-Power. What happens?

You aircraft starts to move forward creating a relative airflow around it.

The Pitot-Tube is pointing directly into that airflow, while the Static-Port still only receives the undisturbed airflow (outside air pressure) around the fuselage.


2. So, what happens next?

During Ground Roll pressure will not change within the Static Line, resulting in no pressure difference within the instrument's case either. Within the Pitot-Tube Line, on the other hand, there is a pressure change, created by the (relative) wind or Dynamic Pressure that goes directly into the PT => Diaphragm. As a result of the pressure difference between both lines (more pressure in the PT-Line VS. less outside air pressure in the Static-Line), the Diaphragm will expand. This expansion makes the needle of your ASI move clockwise, showing you an increase in airspeed. A decrease in pressure in the Pitot-Line would lead to a contraction of the Diaphragm and have the opposite effect on the needle (counter-clockwise movement).

Now, how can the needle start moving just because the Diaphragm is expanding?

Very simple! The Diaphragm is connected through linkages and gears to the needle, thus the needle will respond to every movement of the Diaphragm very sensitively.


So as you get faster during your Take-Off Ground Roll, relative wind increases (= equal reaction) as well, giving you a progressively Higher Pressure within the Pitot-Tube line, while you Static-port's pressure remains the same. This will show as an increase in airspeed on your ASI (needle continues to travel clockwise) all the way to Lift-Off/Point of Rotation and after wards of course when the initial climb is established  (~ 2-3 seconds delay).


Other Pitot-Tube Components

2 Heater Elements, which can be activated in the cockpit. Pitot Heat will only be able to eliminate very small amounts of ice accumulation. Thus, it should be turned on to prevent ICING in first place.

(can be tested during preflight to make sure its working properly, should not be left on for an extended period of time during the testing phase)


→ Drain Holes, should prevent moisture from getting stuck within the tube, which could lead to ICING eventually depending on temperature of course.

→ Static Holes (combined with PT), provide static (outside air) pressure just like the Static Ports attached to the fuselage. Important to know: Some Pitot-Tubes combine these 2 air pressures in one piece instead of having the ports attached to the fuselage.


What are possible Malfunctions/Errors of the Airspeed Indicator?


The ASI is subject to the following Errors:


a) Position Error

b) Pitot-Tube 'Blocked'
c) Static Ports 'Blocked'


What is Position Error?


As previously mentioned, the Relative Wind is always opposite your Flight Path. When an aircraft (airplane) is climbing the Relative Wind will hit the front inlet hole of the Pitot-Tube at an angle. That is because, even if you climb you will move forward not only gain height, so there is some other Relative Wind or air that comes in contact with the Pitot-Tube. This angle, at the the air hits the inlet, might lead to pressure changes/fluctuations within the Pitot-Tube.


The same happens during descend of course. This problem might lead to a slight pressure decrease or differences received in the Diaphragm, which in turn can lead to an erroneous IAS eventually. The error is very small though!

What happens IF the Pitot-Tube Front-Inlet becomes blocked?


If this would be the case, there would be an equal pressure in both,

the Diaphragm as well as in the inside of the case.


Well, do not forget about the drain hole, which is located at the back of the Pitot-Tube. It basically receives the same pressure, (outside air pressure) as the Static Port, since it's not pointing directly into the Relative Wind. This situation would result in a '0' KIAS, as we have no more pressure differences between the Diaphragm and the Instrument Case.


What happens if both Pitot-Tubes Inlets (front & drain hole) become blocked?

If this would be the case, the pressure within the PT-Line (the Diaphragm) is trapped.

What would be the result of that?

front & drain hole blocked.JPG

The ASI would act as an Altimeter, since the inside of the case would still receive outside air pressure. In other words, the Diaphragm is now being controlled by the pressure within the case. That it turn means, if you start a climb pressure within the case is going to decrease, which lets the Diaphragm expand, showing a higher than usual climb airspeed. If you start a descend the opposite would be the case.


And, what happens if the Static Port becomes blocked?


If this would be the case, a certain amount of pressure would remain in the Static-Line (Instrument's case). Due to the fact or pretended that the Pitot-Tube Inlets stays unaffected, the Diaphragm can still expand and contract. As a result, the ASI works contrary to an Altimeter.

So, what happens to my IAS then?


During climbs you will experience a lower than usual airspeed. During descends you will experience a higher than usual airspeed. Thus, a decrease in ALT leads to an increase in airspeed, because the trapped air in the static lines will be less than actual and vice versa.


How can these Blockages or Malfunctions occur?


There are probably many ways this could happen, whereby the following

are the (most) common ones:

Ice Accumulation around the PT or the SP.

→ Believe it or not – Insects building a nest withing the PT.

This has already caused accidents in the past.

You will find a link to such an accident at the end of this page.



What Airspeeds do we have?


Indicated (KIAS) → the airspeed that you read of the Instrument


Calibrated (CIAS) → IAS corrected for Position* and Installation Error

What does Installation Error mean?


As the name already implies, it's about the installation of the airspeed indicator.

This might affect the Pitot-Tube or something inside the Instruments case.

And No!, there is nothing wrong with it! ;-)

Your indicated airspeed is simply adjusted for that error.

*position = relates to the climb/descend phase previously mentioned, where the

airflow hits the Pitot-Tube at an angle.


Equivalent (EAS) → CAS corrected for air compressibility at high speeds


True (TAS) → CAS corrected for higher ALT and nonstandard temperature

What does that mean?


The ASI was build for Standard Atmospheric Conditions (15 deg Celsius, 29.92 in.Hg) So, your IAS and your TAS will only be the same IF you have standard conditions. Since they exist very seldomly, your TAS will vary from IAS most of the time. Air Temperature, Density and many other factors play a role in your TAS. Thus, your airspeed on the Ground will obviously be different from the one at your cruising altitude. That's the reason we must calculate it to 99% of the time before flying. Again, TAS is being used for Flight Planning Calculations!


Ground (GS) → TAS corrected for Head/Tailwind

your actual airspeed with which you are flying (crossing) over the ground


ASI - Markings

This speed
should NEVER
be exceeded

Yellow arc
'Caution range'
should only be used 
while flying in 
'Smooth air'


Start of white
= Vso

(stalling speed 
In LDG config)

Start of the
Green arc  = Vsi

(stalling speed in LDG 

End of White arc = Vfe 
(max speed for full flaps)

End of the Green arc = Vno (max structural cruising speed)