Non-Precision Approaches; MAP vs. VDP

[Futterman]

Hey everyone,

I'm in the midst of working on my IR here at Purdue, and while looking at some local non-precision approaches, I can't help but wonder - to put it bluntly - why the Missed Approach Point is at the beginning of the runway. I'd think that one would have to be quite hopped up to consider starting a descent through the MDA over the numbers...

Instead, consider the fact that Visual Descent Points ensure a normal 3-degree glide path from a specific point along the MDA. Wouldn't this be a more prudent choice of location for executing a missed approach? Clearly, any point beyond the VDP would require an increasingly steep descent to the point of absurdity at the threshold.

Any and all comments, clarifications, and corrections are greatly appreciated.

Thanks,

Brian

[tgrayson]

Originally Posted by Futterman

I'd think that one would have to be quite hopped up to consider starting a descent through the MDA over the numbers...

Mostly yes, although with many pistons you could do it with a 6,000 ft runway. Remember, too, that you have the option to circle if you get the airport environment in sight even very late.

Instead, consider the fact that Visual Descent Points ensure a normal 3-degree glide path from a specific point along the MDA. Wouldn't this be a more prudent choice of location for executing a missed approach?

For jets, probably so, but pistons can easily achieve a descent angle twice that, so it would be unduly restrictive for us. Plus, not all NPA's have VDP's.

You certainly can go missed at the VDP, as long as you don't turn until you reach the MAP.

[fish314]

There's also a human factors component to locating the MAP after the visual descent point. At the visual descent point you are focused on finding the runway and beginning a descent. Beginning a descent is a relatively complex task, as is beginning a missed approach. Beginning a descent involves a 3 degree nose low pitch change, and a corresponding power reduction, whereas a missed requires raising the nose a certain amount from the level flight pitch picture and a power addition. Tying both of those actions to the same point as well as adding an additional task (making the descision which course of action is appropriate), seems like it could lead to greater task saturation at an already critical point in time.

By having them separate, you are focused on beginning a descent as you approach the VDP, and as you pass it and head to the MAP it gives you some time to realize and prepare yourself for the eventuality that you are going to go missed even if you see the runway. It allows you to "shift gears", and gives you some extra time to make that decision.

[PanJet]

Another consideration is how you identify the MAP. For example, many MAP's are a VOR crossing, and I think you'll find in your training that many MAP's don't correlate with being lined up on the runway at all. We have a VOR approach at our airport where the MAP is actually at the departure end of the runway it is set up for. Very little chance of you landing off that one if you actually have to go all the way to the MAP. Just my $.02