- the earth is not flat
- aircraft is actually fly through air even if the air itself is moving
- Newton’s Laws of Motion apply
- the rotational speed of the earth at the equator,
- the speed of light
- the universe is expanding
I believe in the above, but only two apply. A tank full of air in a truck turning a corner with a drone inside or throwing a ball in the air in a speeding train and a few other things previously claimed do not apply.
In the early days of windshear understanding, we used a technique of increasing approach speed with the expectation of loss of IAS due to known loss of headwind closer to touchdown. Loss of headwind and then increasing tailwind is an approach that many experienced pilots would tend to avoid. My modern airliner might even warn of the the danger with a wind display or a warning. Classic horizontal windshear on approach is flight between areas with changing winds with a particularly bad case being loss of the entire headwind and increasing tailwind, with urgent action required to survive.
If the above can be reasonably accepted, we are almost there.
So what has windshear got to do with a simple downwind turn? In both cases the aircraft is flying into a reducing headwind and in the case of the downwind turn, we have set the circumstances to be eventually an equal tailwind.
In the downwind turn we are deliberately flying from an area with a headwind to an area with a tailwind.
We are effectively creating our own windshear.
On approach, windshear might develop over maybe 5 - 10 seconds with urgent action required, but our downwind turn might take a minute. Depending on the actual wind for our downwind turn, little or no pilot response may be required. If the wind was 50K, and the aircraft was already low and slow, the loss of IAS would be more noticeable.
Fortunately for aviation, turns would normally be around rate 1. The minute taken to more gently progress from a 50K headwind to a 50K tailwind gives the aircraft auto flight systems the opportunity to recover some of the IAS loss. If our 200K IAS aircraft could make that same 180 degree downwind turn in 30 seconds, recovery of lost IAS would take more effort.
My downwind turn is simply a 180 degree turn from a headwind to a tailwind. It could be at any altitude and does not necessarily refer to a light aircraft turning from take-off heading to downwind in a circuit pattern.