Leaving aside all the theoretical arguements about this subject - everyone seems to agree that rapidly changing wind/shear does affect IAS. This, of course, is because the rate of change of the wind/gust exceeds the rate at which the inherent momentum of the aircraft ie. it's inertia can change.
Basic laws of physics. Known since Newton. A moving body will keep moving in a straight line unless a force acts on it. (Newton's first law) If there is a force it will accelerate at rate proportional to the force, and inversely proportional to its mass (F=ma - Newton's second law).
So if an aircraft moves (say whilst in a descent) instantaneously from a head wind of say 30 knots to one of 20 kts, the airspeed will decrease by 10 kts. If the pilot is trying to fly at a given speed he will now need to accelerate by 10 kts. That takes time - and that's what we mean by inertia.
So,now, why is it that the "one parcel of air" concept can totally ignore inertia? Is it because the rate of change is usually so minor at usual speeds?
Perhaps we are straying into the old Bernoulli versus Newtonian argument where neither theory is totally and or wholely at work?
No it really isn't either of those. It's because if you're in a moving layer of air, it really doesn't matter how fast it's moving. There aren't two sides to this - there's a correct view and an incorrect view. And the idea that somehow turning downwind makes a difference due to inertia doesn't come into it.
I would also like someone to explain to me the cause of the large change in VSI indications at CONSTANT IAS when turning into or out of a very strong wind with respect to the TAS that I cited earlier. The only cause I can come up with is the large inertial change in speed of the aircraft wrt to it's own TAS.
Because generally when you do this you're in turbulent air. The high wind creates updrafts and downdrafts particularly in the boundary layer. Talk to any glider pilot. Fly in a high wind at a higher altitude and you won't see this.
I remember an AAIB report touched on this matter. If I recall correctly, c. 1990 a C150 flown by a father with his son as a passenger, crashed when turning 180 degrees from a headwind to a tailwind. The flight was part of a low-level navigation competition in Hampshire and the aircraft was flying slowly so as be precisely on time. Tragically both were killed in the crash. The AAIB report, if I recall correctly, gave credence to the theory of the proposing side of this argument. Perhaps somebody will find the AAIB report? It generated a huge debate.
Slow turns at low level are not a good idea, and lots of people have killed themselves doing them (see also spinning in from the final turn) but the reason for that is nothing to do with 'inertia'. See SSD's post #47 for the main reasons, but I'll add one. One effect of doing a low level turn is that your lower wing can be in air moving at a different speed to the air in which your upper wing is moving - due to wind shear. That can cause one wing to stall, followed by a spin and an impact with the ground.
Paul