Learning to fly, or thinking of learning? Post your questions, comments and experiences here

Moderator: AndyR

User avatar
By Stevefarn
#1630073
I have question on the above subject.

Flying at 80kts ( ground speed 60kts) - headwind of 20kt

All good so far.

Windshear

Ground speed still around 60 kts and now a tailwind of 10kts.

My question is - why is the airspeed now 50 and not 70kts.
By johnm
#1630080
because 50 plus 10 equals 60 and 80 - 20 also equals 60
By TopCat
#1630083
flybymike wrote:By what means are you establishing the wind speed and direction?

It doesn't matter. Imagine two anemometers, if you like, measuring the wind at the two levels in question - one where the wind is coming from in front at 20 kt, and the other where it's coming from behind at 10 kt.

The point is, at the instant the wind changes, the aircraft has a speed over the ground of 60 kt forwards. If there was suddenly no wind at all, its airspeed would be 60 kt, right? (@OP, if you don't get this, stop reading here and say so, because what follows will not help).

But actually, the wind is 10 kt from behind. So the aircraft at that instant is going only 50 kt faster than the air it's moving through. That is what the airspeed is, by definition.

Now, as the aircraft was flying at an airspeed of 80 kt just before, it must have been developing enough thrust to overcome the drag at that airspeed. At risk of a very slight oversimplification that you can ignore for these purposes, now that the airspeed is only 50 kt, the drag is less, so that thrust will be enough to accelerate the aircraft until it's doing 80 kt through the air again.

The point is, at the instant after the wind changed, the airspeed is 50. This might be below the stall speed - the question is intended to illustrate the potential dangers of abrupt wind fluctuations especially close to the ground.

@OP - if you still think it should be 70, or if you don't get that it would be 60 if the wind changed to being no wind at all, post an explanation of how you reached your answer.
User avatar
By flybymike
#1630084
If it were possible to know windspeed and direction solely from ground speed variation (and without air data) then every portable GPS could give instant wind information, instead of , for example, SD having to use forecast winds.
The only way I can think of to do it without air data, is to orbit around a given point and see which direction the wind blows you in, and at what rate, over a given period of time.
By TopCat
#1630085
flybymike wrote:If it were possible to know windspeed and direction solely from ground speed variation (and without air data) then every portable GPS could give instant wind information, instead of , for example, SD having to use forecast winds.
The only way I can think of to do it without air data, is to orbit around a given point and see which direction the wind blows you in, and at what rate, over a given period of time.

Sure, but that's not the point of the question!
By TopCat
#1630097
flybymike wrote:Sorry, I thought the question was based on an actual event in the air, not a theoretical one!

It's a theoretical one here, but it represents a very real actual event in the air.

This is quite a good article on microbursts and their very real dangers.

A classic headwind followed by tailwind situation that you really don't want, close to the ground.
By TLRippon
#1630241
There is a difference between microbursts and wind shear. I remember at one of our ATPL ground schools the debate between those with light aircraft experience and those with airline experience. The light people reckoned that if you experience wind shear in approach you would get away from the ground, go around and try again. The airline people (mostly doing durisdiction conversions) sat shaking their heads and to a person repeated, “Windshear, don’t change anything”. The rule they use is accept the wind shear, don’t change configuration or power and make minor control inputs to account for turbulence. The rationale is that any change in configuration could destabilise the approach so for example putting the gear up to go around would deploy gear doors which would provide a surface for the changing wind to affect. Adjusting power could lead to a stall if the wind shears again and anyway the cycle time for a large jet to apply the commanded power is so delayed that precise control wasn’t going to be achievable.