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My aircraft heading is 321 with constant Static pressure, wind speed and steadily increasing Ground Speed. Meanwhile, there was a sudden jump of 10kts in IAS from 140 to 150 kts withing a span of 1 sec. For this span, Wind direction is rapidly changed from 176 tp 300 degree A.

Is wind direction is cause for IAS jump ?

DeltaLima
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Vijnana Yogi
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1 Answers1

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Yes, exactly, the change of wind direction is the cause for your sudden increase of IAS.

I have marked your aircraft heading with a magenta arrow and the direction the wind comes from with a brown arrow. So, you are going 140 KIAS at this moment and you have a steady 5 knot tailwind. That means you have a GS of roughly 145 kts (we will ignore air density and that it's not a perfect tailwind in this example).

Flight

Now the wind direction changes all of a sudden.

Flight 2

You have now got a nearly direct headwind. That means there are in total 10 kts more worth of air going into your pitot tube and thus indicating 150 KIAS. That's because your aircraft lost the 5 knots tailwind and at the same time got a 5 knot headwind. Because of the fast wind direction change, the aircraft kept its "momentum" and the airspeed increases by double the wind speed.

Federico
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Noah Krasser
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  • Can you explain little more : " How tailwind and headwind both can contribute to increase in Indicated airspeed ? What I think only TAS should be affected , not IAS ! – Vijnana Yogi Aug 14 '18 at 09:54
  • @VijnanaYogi When there is wind suddenly hitting the plane from the front, like a windshear, there is more air going over the aircraft and also the pitot tube. Same with tailwind, but there is less air and as such a decrease in IAS. – Noah Krasser Aug 14 '18 at 11:05
  • Yes, But in above explanation ( That means there are in total 10 kts more worth of ....) its seems tailwind is also causing increase in IAS , that's point of confusion .... – Vijnana Yogi Aug 14 '18 at 11:35
  • @VijnanaYogi The tailwind itself does not increase IAS, only it's sudden decrease. If tailwind decreases, IAS increases by this value, because of the plane's inertia. In your case the tailwind completely disappears, hence the first 5 knot increase in IAS. At the same time the aircraft experiences a 5 knot headwind, again increasing IAS by 5 knots, to a total of 10 knots. It's only because of the planes inertia. – Noah Krasser Aug 14 '18 at 12:13