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Airspeed Errors

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Flightinfo's sexiest user
Dec 21, 2001
Hey guys,

Another question here. I just want to make sure I am interpreting the density error correctly.

I know that one factor leading to erroneous KIAS indications is the "density error." I also understand that air density is directly related to barometric pressure and inversely related to air temperature.

Now, this is how I intrepret the effects of density error... the airspeed indicator (like the altimeter) must be calibrated during the manufacturing stage, and the standard atmosphere is used. So, on very cold days (or lets say... higher than standard pressure days), the air tends to be more dense than it is on a standard day. This increase in air density causes the pitot tube to misinterpret this dense air as an increase in ram air pressure (ie. more "air molecules" entering pito tube, makes it seem as though more air is entering, and thus, a higher ram air pressure. This all leads to a higher KIAS.... ie. on cold /very high pressure days, your actual airspeed is likely to be lower than indicated.

So, am I correct in my interpretation?

Thanks a lot for the responses. I know sometimes I ask very detailed questions and I apologize for that. I'm just the type of person that has to be able to understand every little detail, without confusion- it drives me crazy sometimes, as it is impossible to understand everything!
KIAS Variation w/altitude

I'm a new CFI, and have been over this a few times in preparation for the initial checkride. Here's my understanding of what you're asking, and if anybody else has another way of teaching it then I'd love to hear it. Indicated airspeed is not going to vary with altitude, it's always going to stay the same. The indicated airspeed at 2,000ft is going to be the same as at 10,000ft. The reason being that indicated airspeed is a measure of dynamic pressure, and a comparison between the static pressure (from the static ports) and ram air pressure (from the pitot tube) at any given altitude. Therefore, when altitude increases, the comparison between ram air and static pressure is going to vary proportionally and that ratio is going to be expressed on the airspeed indicator as indicated airspeed. What is, however, going to vary with altitude is true airspeed. True airspeed is what you are actually referring to when you talk about airspeed being corrected for "density error." It is indicated (or calibrated, if you will) airspeed corrected for nonstandard temperature and pressure. Guess I should probably explain that calibrated is just indicated airspeed corrected for instrument and position error. It is usually pretty close to indicated, but you can consult your POH for numerical specifics. In summary, indicated airspeed (KIAS) does not vary with altitude, but true airspeed (KTAS) does. A good rule of thumb is that true airspeed is going to increase 2% for every 1000ft of altitude. Hope this helps.
"So, am I correct in my interpretation?"

Bigs024 did a very nice job. You are basically correct, but 024 is precise. Since an airspeed indicator measures the difference between Ram air and Static air, the air density is relatively the same and the differences are then basically the same.

Temperature has more of an effect on the altimeter since the altimeter has a preset aneroid wafer (or bellows). "High to low and hot to cold, look out below!"
I think I get it... the ASI' reading is dependant on the RELATIVE difference between the pitot and static ports. Even though there is technically less ram air pressure in the tubes at higher altitudes due to the decrease in density, the relative difference stays the same (ie. less ram air, but also less air pressure- thus the ratio does not change).

Is this correct? I sure hope so because this time it makes sense :)

Maybe this mnmonic will help:


I = Indicated; what you are reading off the airspeed indicator.
C = Calibrated; indicated airspeed corrected for installation errors.
E = Equivalent; calibrated corrected for compressability, i.e. ram air packing into the pitot. More of a factor for supersonic aircraft.
T = True airspeed; corrected airspeed corrected for pressure and temperature.
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