# Bernoulli-induced errors in altimeters

When an aircraft flies through things like updrafts, downdrafts, horizontal gusts and so on, does the velocity change produce Bernoulli effects that push the displayed altitude up or down wrt the correct altitude? If so, how much of a practical problem is it?

(By velocity change I mean the air velocity, of course, and not the plane's)

Homework Helper
Normally the static port(s) on an aircraft are located so that the speed of flow across the port does not significantly affect the static pressure sensed by the port(s), located inside the boundary layer(s). A gusting wind with a component of wind perpendicular to a static port would affect the reading. If the gust is momentary, the reading will only be affected momentarily. If there is a change in wind direction, eventually the aircraft will react to a crosswind by changing direction, so that there isn't a crosswind, assuming the aircraft is not being kept in a yawed state (a constant crosswind component).

Yes, there would be artifacts from how the air flow interacts with the instrument, as you describe. Those would be errors in the pressure measurement which would indirectly translate to altitude errors.

But my question was more about the relation between pressure and altitude, assuming that we can somehow measure the pressure very accurately. Would the dynamics of the gusting wind alter the relation between pressure and altitude to a degree that might matter, considering that fast moving air should have a lower pressure than stationary air at the same altitude and temperature?

For example, consider two balloons that are at the same altitude. One balloon is floating in still air and the other is being carried along in a stream of air. Would their altimeters show different pressures? If so, the pressure on the moving one would be wrongly interpreted as a lower altitude. Or... is this a naive and wrong application of Bernoulli?

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Homework Helper
Although altimeters may self-calibrate based on GPS, I doubt typical altimeters take weather into account. Rather than compare a still versus moving stream of air, consider still air in the middle of a high pressure zone versus still air in the middle of a low pressure zone. I don't know how altimeters compensate for weather.

Gold Member
I don't know how altimeters compensate for weather.
They don't. You have to zero the altimeter just before you take off and then believe what the meteorologists tell you about any changes in atmospheric pressure with time. Any other information like radar and observations of your plane from the ground could help you when you are up there.

Mentor
...considering that fast moving air should have a lower pressure than stationary air...

For example, consider two balloons that are at the same altitude. One balloon is floating in still air and the other is being carried along in a stream of air. Would their altimeters show different pressures? If so, the pressure on the moving one would be wrongly interpreted as a lower altitude. Or... is this a naive and wrong application of Bernoulli?
It's a wrong interpretation of Bernoulli (or rather, this has nothing to do with Bernoulli's principle); the two static pressures are equal.

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Mentor
Doing some quick math on this, a 20mph wind has a velocity pressure of 0.4 mm HG, or about 17' of altitude. So the effect of a sudden gust would be enough to notice if the altimeter wasn't damped, but still pretty small unless you're getting up into the jet stream.

Swamp Thing