Sinking air will give you a negative vertical speed but it won't kill your forward speed.
10540
It most certainly will kill your forward speed purely through the laws of triangulation. That is presuming that you do not try to maintain altitude and go for maintaing IAS in sinking air.
Trying to maintain altitude will kill your forward speed as well as quickly reducing to stall speed.
Trying to maintain your IAS will also result in an increase in VS as well as a decrease in forward speed as stated due to triangulation.
You can only maintain forward speed by maintaining altitude.Take a stupid example. You are flying level at 100 kts you hit a pocket of air descending at 200 kts are you going to travel the same distance forward as if you were still maintaining 100 kts in the horizontal plane?
The angle you would have moved forward would probably be without working it out about 30 degrees from the vertical rather than 90 degrees in level flight. Your vertical speed would also have increased dramatically over the 200 kts aircurrent trying to maintain your IAS.
Regardless an aircraft in severly descending air is going one way and thats down. It would be natural for the pilot to see ground fast approaching to minimise the impact by pulling back to near the stall.
His vertical speed would reduce as would his forward speed at impact point.
My concern here is the belief that you can fly mountains when winds are low and be immune to severe down draughts unless CBs are also present.
This is far from the truth and a dangerous conception in mountain flying.
Pace