IAS with tail wind or head wind

[aslan1982]

can someone please explain to me what happens to your IAS when u encounter a tail wind or head wind and why.
thanks in advance

[Rainboe]

Because of inertia, the plane will tend to maintain current groundspeed rather than maintain constant IAS if you run into an increasing headwind. So if you run do run into increasing headwind, there will be an immediate increase in IAS. If you are flying along in still air and you are hit suddenly by a tailwind the same value as your IAS, what will happen to your IAS, and what will happen to the plane?

[Chris Scott]

Hi aslan1982,

I shall assume that you are thinking of sudden changes in wind?

Let's say a light aircraft is cruising straight and level at sea-level in calm conditions (no wind), nose heading north (HDG 360) at a steady IAS of 100kts. Because the wind is calm, the aircraft's GS (speed over the ground) is also 100kts, and its track over the ground (TRK) is also north.

(1) Now, say a wind of 20kts from the north (360/20) springs up instantly [we call this wind sheer]. The IAS will rise instantly to 120, increasing the lift over the wing by about 40%. The GS will still be 100. However, unless the power is increased, the extra drag created will slow the aircraft down to 100kts IAS again, at which speed the GS will have fallen to 80kts. The track of the aircraft (TRK) will not have changed.

2) Instead of (1), let's say the wind springs up finstantly from calm to 20kts from the south (180/20). In this case, the IAS will drop instantly to 80kts, resulting in a loss of lift of about 35%. The GS will still be 100. The reduction in drag will allow the aircraft to accelerate gradually to IAS 100 again, at which point the GS will be 120.

If the above is of any use, let me know and I can have a go (later) at the crosswind case if necessary.

Chris

PS (by EDIT): Sorry Rainboe, it's all yours!

[aslan1982]

thanks alot for both your replys. great help :ok:

[lion-g]

HI there,

Good post there, just wanna know if this applies to jet aircrafts. And can you give an example with crosswinds, and turning into crosswind, with wing low? Just out of curiosity.

Cheers,
lion-g

[wlee]

Jeez, I wish it were that simple. Simplified version:

Assumptions: Ship is trimmed to level flight at 100 knots TAS, and thrust is unchanged.

An instantaneous headwind component increase of +20 knots will produce an immediate increase in the ship's lift vector. Given constant thrust and no pilot inputs, the ship will pitch up, climb and deccelerate until it's TAS again equals 100 knots, at which point it will (after a series of Phugoid occillations) attain stable flight (TAS = to about 100 knots and GS = to about 80 knots), assuming no pitch control inputs from the pilot flying.

If the pilot flying desires to maintain altitude during the headwind event, he will apply nose down pitch to counter the increase in lift. This input will be translated into an increase in TAS and GS. The additional drag created by this scenario of increased airspeed will gradually slow the ship to its original TAS, and the resulting GS will be TAS - HW or about 80 knots, again, assuming thrust is unchanged.

In the tail-wind scenario, from stable cruise, an instantaneous tail-wind encounter will reduce the lift vector, causing the ship to pitch over and descend. TAS will drop to 80 knots and GS will gradually fall off. As the ship descends, it will accelerate, until reaching its trimmed speed, at which point, it will go through a series of Phugoid occillations until it again achieves stabilized, level flight (assumming no pilot inputs and a constant thrust.) At this point, TAS will be about 100 knots and GS will be about 120 knots.

Fly in any turb. (wind shear) and this is what you will experience.

Regards,
wlee

[bArt2]

just wanna know if this applies to jet aircrafts

In the region of normal command (speed above minimum drag speed) the answer is yes.

In the region of reversed command (speed below minimum drag speed) the answer is no.

The region of reversed command is sometimes called "behind the power curve" (but it should be named "behind the thrust required curve").
In this region the slower the aircraft flies, the greater the thrust required to maintain this speed.

Anyway, when you are flying in this speed region with a constant thrust output, a decrease in TAS due to gust will lead to continuous decrease in TAS if thrust is not changed (or the nose lowered). This is a dangerous situation

An increase in TAS due to gust or windshear will lead to a continues increase in TAS untill you reach the TAS that equals the intersection of the thrust required curve with the thrust set on the side of the region of normal command.

[ExSp33db1rd]

Interesting when trying to maintain the ILS G/s. !!

As the Boy Scouts say - Be Prepared ( clue - comparison of present IAS and G/s with Twr. reported wind on the ground will help )

[ferudun63]

Chris Scott, I Agree With You Exactly What You Explained Above. But I Just Need A Legal Documents About This Information. Because Some Of My Friend Want Me To Show Them Legal Documents About Your Explanation. Do You Have Any Legal Documents About Wind Effect Of Ias. Thank You For Your Help

[con-pilot]

An excellent example of the affects of changing upper winds is how a 'mountain wave' condition effects the IAS of jet aircraft. I have seen changes in excess of 20 knots or more, plus and minus in these conditions. Although it is termed 'mountain wave', it can be encountered when not flying over mountainous terrain.

The worse I was ever in we went from the Mach warning going off to the stick shaker in a matter of what seemed just a few seconds, this was in a 727 over the Rocky Mountains one night. Needless to say we descended as soon as we could. By the way, there was no turbulence involved. Sometimes you can actually feel the wave before it affects the aircraft. It will sometimes feel as if the back of the aircraft will start rising, as if going down a ocean wave on a surfboard.

For some reason this seems to happen more at night, why I've not a clue. While encountering these wave conditions can be very irritating, I've never heard of an accident happening because of a mountain wave. I'm not saying there never has been, I just cannot recall any.

As for a legal document, all I can suggest is to Google 'Mountain Wave' and see what comes up. What ever comes, I don't think it could be called a 'Legal Document'.

[p51guy]

I was flying an MD80 over the Sierras at FL350 one night and encountered a mountain wave. I got the rising wave so the autothrottle went to idle but was still going to exceed mach limit so I used full speed brakes to maintain altitude. Next came the descending wave and I retracted the speed brake and the autothrottle went to max cruise thrust. We were still losing airspeed so went to max continuous thrust. We were just able to maintain our speed above low speed buffet so didn't have to descend with no clearance. The changes were smooth but rapid, not like windsheer where the airspeed change is abrupt. There was no turbulence during this encounter.

[Rainboe]

Chaps, the question was ''IAS with tail wind or head wind''. With limited English he is probably going to get a bit confused. Mountain waves are a different subject.

Mountain waves? I can tell you about mountain waves!

[Pugilistic Animus]

http://www.pprune.org/professional-pilot-training-includes-ground-studies/388963-principles-flight.html

PA

[Microburst2002]

Ferudun

Physics don't need legal texts. Whatever happens to IAS after a gust, happened and will happen the same regardless of ICAO, JARs, FARs or whatever.

[framer]

We get mountain waves in the sth Island of NZ quite often. They can be pretty full on, once in a 737-300 I was PM and the PF had to disconnect the auto-throttle in order to prevent overspeeding, didn't need speed brake like in P51's encounter though.
What sort of "legal Document" do you require? Newton wrote a few, maybe google him?