KiloAlpha

I saw a different way to calculate the VDP distance on flightinfo.com and I was wondering why the discrepancy.

The rule of thumb said: multiply HAT by 3, then divide by 1000

loudgarrettdriver

Tony,

Thanks for clearing that up.

Sincerely,
LGD

loudgarrettdriver

I didn't manage to make it through your 12 million word post.

TonyC

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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TonyC

I'm sorry to offend your short attention span, but that's OK. It was redwave that asked the question, anyway.







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HSLD

That's a good point, although I think the "correct" definition only matters during a type ride

TonyC's technique works for a charted or uncharted decent. It's so rare [for me] to actually fly a non-precision approach in the wx that I'd be doing the mental math charted or not!

Call it what you will, the important point is to have the aircraft in position that you can land (safely?) at the completion of the approach. Or so I'm told

loudgarrettdriver

Yup- That Adult ADD is flaring up again. Must be runing low on meds.

Groundhog

Gotta disagree with the part about going missed. You've just calculated a PDP, not a new MAP.
I know where you're coming from. You're saying that if you do get the approach lights or the runway environment in sight after your PDP, you're not going to be able to fulfill the other requirement of 91.175 of an approach to landing using normal maneuvers. That's not necessarily true.
Also, the way you wrote your response, you implied that you would go missed at your PDP. You would still want to continue to the MAP before executing the missed approach procedure.

Hog

TonyC

If you've computed the PDP or VDP correctly, it means you'll begin your descent above the normal glidepath. IF I'm flying a turbine aircraft (and I do) I am required to land in the touchdown zone. If I use a normal descent rate and normal maneuvers (also required by 14 CFR) I will likely land beyond the TDZ. If I use maneuvers or descent rates that are not normal, I have violated 14 CFR as well. So, either way, I violate the regs.

The way I look at it, reaching the VDP, or PDP, and not being able to begin a descent, for whatever reason, to the runway, means I won't be landing there this time.


Necessarily.



You can begin the Missed Approach procedure at any point. You can make no turns proscribed by that procedure until reaching the Missed Approach Point. In other words, if one reaches the VDP and does not have the runway environment in sight, he may immediately commence a climb according to the Missed Approach Procedure, and then at the MAP can begin whatever turns are called for.

Perhaps it's just semantics, but I wanted to clarify this in order to avoid confusing people. You can execute the Missed Approach Procedure at any time, but you want to continue to the MAP before commencing any turn.





The truth only hurts if it should.

Groundhog

Except that the original post that started this thread did not indicate a particular type of operation. For operations other than 121 or 135, as you indicated, there is no requirement to land within the touchdown zone. A small aircraft (not operating under 121 or 135) approaching a long runway could elect to proceed beyond the PDP, break out, and execute a landing safely and legally. It depends on the equipment and the type of operation.

So, no. Not necessarily.

As for starting the missed approach procedure at the PDP/VDP, I stand corrected. You can start the vertical portion, but you can't start the lateral portion until reaching the MAP. That was the original intent of my post, but my response was just as ambiguous as the post I was replying to.

Hog