Penetration Descents seem to be the bane of many a Herk Pilot. There's lots of gouge out there--some good and some just plain wrong. That said, even with perfect numbers, you may find yourself down early or late. What I wanted to do was give people an even playing field to start with. If you’re not going to run the chart from the 1-1 each time, that means good tab data.
Those dual qualified probably realize the E-Model and H-Model use different charts. Ideally, you'll use your FE to calculate the descent distance from the appropriate 1-1. That's not always practical. Those flying "The Pain Train" know what I'm talking about. Many often go to rules of thumb or the various gouge that's out there.
The problem is that rules of thumb don't always apply universally and some of the gouge is only a wag at best. The problem I see with a lot of gouge is that it is based upon "altitude to lose" instead of pressure altitudes.
What's the big deal? I'll give you an example; Think of flying into Bagram. You ingress from FL250 and land somewhere around 5000' MSL. That's 20,000' altitude to lose. If we run just 20,000 “altitude to lose” in the chart, we get 26.3 NM. On the other hand, if we use double entry and run both pressure altitudes, we get 36.6 - 6.3 = 30.3 NM. That's a 4 NM difference from the single entry. Four nautical miles can easily turn a straight-in to an overhead or go-around.
Most of the airfields in Iraq are closer to sea level, so Pen-D's attempted there (using single entry numbers) often don't reveal the problems associated with "altitude to lose" gouge. That said…double entry is still the way to go.
Note: The new 3-3 is out and it has a great section on Pen-D's, but the associated tables in the Combat Mission Guide are misleading. The numbers are more or less straight from the 1-1, but the table is listed as "Altitude to Lose.” If you run the numbers, you’ll see that they’ve calculated it (single entry) using pressure altitude down to sea level. To get an accurate distance, you'll have to subtract the pressure altitude you are leveling off at (usually 1000’ above field elevation) from the number on that chart. Since the 3-3 chart doesn't go down to 1000' MSL or 2000' MSL, that's difficult to do using only that chart. Hooray for the 3-3 writers though--at least they included charts for both the E and H.
How I constructed the Tab Data (E-Model Version) (H-Model Version)
I scanned the Time & Distance Penetration Descent charts from the E & H Model 1-1s. Then, using digital measuring equipment, I calculated the descent distance from a given altitude for a given gross weight. For gross weights between the black lines on chart, I averaged the distances from the surrounding weights (e.g. for 130K gross weight, I averaged the distances I calculated from the 120K and 140K lines). I did this for both the E and H Model.
The second table (for a given gross weight) represents how many feet per nautical mile the aircraft will descend from one pressure altitude to another. It is calculated using the difference in pressure altitudes divided by the calculated descent distance. This represents an average rate over the entire descent. Obviously, those familiar with Pen-D's know the initial descent gradient will be greater than the rates achieved at the bottom of the descent.
Discussion: The 3-3 Pen-D tab data gives an "Average FT/NM" for 120K, 130K, and 140K. Our missions these days will often have us landing with less than 110K, so I've included more weights in these tables. But even within those weights, the 3-3’s average ft/nm is misleading. I'm not sure what it really means. If you take a C-130H at 120K and descend from 30'K to 10'K, that's 20'K altitude to lose. It will take 34.6NM overall or 578 ft/nm. From 20'K to Sea Level it takes 26.3NM or 760 ft/nm to lose the altitude. I'm not sure if using their average of 770 ft/nm can be universally applied. The 3-3 says it's a close approximation, but at lighter weights the difference between extreme ends of the spectrum can exceed 300 ft/nm. I agree with the 3-3 that crews should use altitude gates to back up the Pen-D, but I encourage crews to calculate one using the differences in pressure altitude divided by the calculated distance to descend. Again, this requires double entry.
Note: Have you ever considered the effect of the difference in altitudes when switching from QNF to QFE? Dial up the airfield altimeter prior to descent to see the difference. I've seen as much as 500' difference. If the field altimeter setting is lower than 29.92, you'll get down early. If it is higher than 29.92, you might find yourself behind in the descent. It's normally not a big deal, but knowing about this little discussed phenomenon may explain some of your 'screwed up' descents and help you adjust for it in the future.
Final Thoughts
One thing I often see missing from Pen-D gouge is L/D max information. The Dash-1 tells us to pull the throttles to flight idle and descend at L/D Max until 20,000 feet. Passing 20K, you push the nose down and accelerate to and maintain 250 KIAS. It’s hard to know what L/D Max is without the FE getting out the 1-1, unless it’s included in your gouge. I’ve included it in my tab data for your convenience.
This data should be constant. Unless the 1-1 changes, I do not foresee any changes to it. You can find checklist size tab data as well as the full size on http://www.herk-gouge.com/. Just look for the Pen-D files on the Kneeboard Gouge Page. Note that there is a 1 knot difference between -15 engines with and without the Rosemount system. If you're flying an H3 your L/D Max will be 1 knot faster than those Herks without the Rosemount (exception, at 140K you're still 170 KIAS).
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