Indicated Airspeed

[flyboy]

Here's a question for you physics people. I was teaching a student about performance today and we got on the subject of indicated airspeed. We discussed how headwinds and tailwinds affect IAS. I told him how a shift from tailwind to headwind will show a higher indicated airspeed and vice versa. We differed a little in when and if that airspeed goes back to its original indication. So here's the sample question:

If you are indicating 100 KIAS and your winds aloft shift to a headwind, you can expect an increase in KIAS. If the headwind stays the same and you do not change your power setting, will the IAS return to its original indication or will it remain at the higher indication.

I am afraid I might have told him the wrong thing and any insight would be greatly appreciated.

 

[bigD]

The airspeed indicator will go back to the original value as soon as the airplane "catches back up" to its original velocity through the airmass. So if I'm doing 100 knots IAS and suddenly lose 20 knots from a headwind that shears to a tailwind (or just less of a headwind), I'll get that IAS back in whatever time it takes my plane to accelerate back to 100 knots again.

 

[jergar999]

Big D is right. Groundspeed will change and remain, indicated and true airspeed will return as soon as stable airflow resumes.

 

[flyn96]

In areas of wind shear your IAS will momentarily change, then stabalize until another shear is encoutered, if you were to fly through an airmass of gradual changes in direction of wind (no wind shear) your IAS would remain unchanged.

 

[172driver]

Also, your winds aloft example might not be the best one to illustrate this point. In my experience, it is much more likely to experience this type of wind shear low to the ground, at least in small airplanes. A simple change in winds aloft will not cause the airspeed indicator to change because they usually change slowly over a large geographical area, excluding fronts, jet stream, etc. Only wind shear (a rapid change in wind speed or direction) will affect the IAS. So, unless your winds aloft shear rapidly, you won't notice the difference on your ASI.

More common is final approach, sudden change in wind speed or direction, ASI jumps up or down accordingly, and returns to normal when the a/c is stabilized in the new air mass.

 

[Brother Francis]

All things being equal The IAS will stabilize at its original value. Conservation of energy applies.

The rate and magnitude of the temporary fluctuation in IAS and rate of return to equilibrium is dependent upon the mass and velocity of the aircraft, the density of the airmass and the rate of change in the wind vector.