Originally posted 4/5/08 (this is an update)
I would venture to say that most pilots prefer normal glide path assaults these days. What normal means is relative to each pilot, but somewhere between 2.5 and 3.5 degrees. Some times terrain or other factors bring you in at a higher altitude, closer to the field.
At those times a 2-step approach is required. I have created an Excel worksheet to calculate a two step approach's initial required glidepath and VVI to arrive at a 3° glideslope at 100'AGL & 2000' feet from the threshold.
In the worksheet you can actually choose your 'normal' glideslope and the desired altitude you want to be at when you intercept it.
I'd like a little feedback on this worksheet. The math should be good, but I'd appreciate on any feedback. If you find this useful, I will post a cleaned up final version and a checklist sized tab data for common speeds and approaches.
Click on the title above to download the worksheet.
UPDATE: I got to thinking today... This works pretty good for straight line 2-step assaults, but what about for a turning approach? Say off the perch or desent to final. What is my required VVI to intercept my desired final? I added some of that math at the bottom of the worksheet to add that feature. Let me know what you think.
-RC
24 September 2009
03 September 2009
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
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
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