ORIGINAL: AH1G
Well... A turn is a turn is a turn HOWEVER, When you turn downwind you need to hold a little up elevator in your turn & increase the throttle to increase your flying speed over the ground and maintain level flight. (depends on wing cross section)
If you are flying at say, 85 kts, you have a wind speed of 20 kts, you need to increase your throttle to maintain 85 kts IAS or 105 kts over the ground. If you don't, your air speed could actually be 65 kts. That could put you under your stall speed and you prang your plane.
In other words, those that crash/stall on the downwind leg have insufficient air speed to maintain flight.
No offense intended, but this makes no sense. Why would your airspeed downwind be 20kts less at a given power setting just because you are making a turn downwind? A model at a given airspeed will have the same airspeed reguardless of direction of wind unless you are not changing the power setting(and not changing altitude). You will loose a nominal amount of speed in any turn, but that's not what we are discussing here.
What you are describing would be true of a wind shear or gust situation where the wind suddenly changes speed or direction, but wouldn't apply to a aircraft changing direction.
The downwind turn problem is related to the pilots fixed position point of view and the pilots trying to judge airspeed by how fast the plane is moving over the ground.