There's a difference because it's just that, a "Rule of Thumb." Most useful rules of thumb are convenient approximations that avoid complex math. For the exact distance for a VDP using a 3 degree descent gradient, one would divide by 318 ft. A corresponding "Rule of Thumb" would be to divide by 300 - - the math is much easier, even though the resulting distance would be slightly longer, that is, slightly farther away from the runway. (I would consider this longer distance to be a conservative approach - - if you start down at this point, you should not get high and require excessive descent rates.)

The Rule of Thumb you cited, multiply by 3, and then divide by 1000, would yield a result that is slightly less than the actual value, that is, slightly closer to the runway. The mathematical equivalent of the "multiply by 3 and divide by 1000" method would be to divide the HAT by 333.333333..... As you can see, it's close, but not exact. For someone who is challenged by dividing by 318, or even dividing by 300, it's a reasonable technique. The resulting answer is not on the "conservative" side, though, so I would shy away from it. Of course, I'm using it with a B-727 on shorter runways, and I can't afford to put too much runway behind me. A 172 landing on 8,000' of concrete can afford to land a few feet longer.


I like Rules of Thumb, as long as the person using one knows the theory behind it, the conditions upon which it is based, and its limitations and shortcomings. That's why I like to start with the mathematical formula, and then work from there.


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