Using Winds to Adjust Slowdown Point

Min slowdown approaches seem to be the bain of the Tactical Airlifters existence. It's not that difficult to understand. When we do penetration descents, we adjust our descent point for a ballastic wind. When we CARP an airdrop we adjust the release point based upon a ballistic wind. In both cases, the time that the wind affects that object determines how much we offset for that wind.

Similarly we look at the time that a wind affects us during slowdown to adjust that point of initiation. How do we do this?

Let's assume we're doing a level slowdown from 250 KTAS to 150 KTAS. Experience (and nothing else) has shown us that the C-130 will slow down in 0.25 NM for each 10 knots of airspeed lost. Using that rule of thumb, it will take 2.5 NM to slow to my desired speed. Based on an average speed of 200 KTAS, it will take me 45 seconds to complete the slowdown.

The wind is either pushing me faster or slower during that time. Wind doesn't change my true airspeed, so the time and distance to complete the slowdown don't change. Only my resultant position over the earth changes with the wind. The stronger the wind the more it will affect me. Wind probably won't affect me too much until at least 10 knots, so I want to know how much the wind will affect me for each 10 knots of a direct headwind or tailwind.

If I convert 10 knots into 10 NM / 3600 seconds, I can multiply that by 45 seconds (the time required to complete the slowdown). The result is 0.13 NM. If I had slowdown to 120 KTAS, the wind would have had more time to affect me and would result in 0.18 NM change for each 10 knots of wind.

The average is around 0.15 NM. Funny enough, this is the number that the Combat Planning guide tells us to use for adjusting the slowdown on a Straight-In approach.

Since our FMS only shows us distances to the first decimal place (i.e. 5.2), I recommend the following technique: Round up for tailwinds and round down for headwinds. This a more conservative way to apply. For a 30 knot tailwind I would execute the SD .5 NM earlier.

Refer to the excel spreadsheet for the calculations. Change the SD factor in the file to switch from J to E/H Model C-130.

Excel Sheet

-RC