Little Help Please with IFR Test Question

[eetrojan]

Masters of the IFR Universe -

I’m a little confused by this question on the IFR written:

(Refer to figure 152). At what point would a pilot execute a missed approach for the Baro NAV and LNAV approaches at LBF?

A - 3100 feet DA for Baro NAV, or 1.5 NM from the RW30 waypoint for LNAV
B - 3280 feet DA for Baro NAV, or the BEMXI waypoint for LNAV
C - 3100 feet DA for Baro NAV, or RW30 waypoint for LNAV

I selected A, but the answer is C.

This was my thought process. It seemed obvious to me that “Baro NAV” was referring to the DA of 3100 feet for the “LNAV/VNAV” approach since “Baro NAV” relates to vertical guidance implied by VNAV. So, that eliminated B, and the answer must be A or C.

I then noticed that the side view specifically shows a point that is “1.5 NM to RWY 30,” along with the asterisked indication that it’s “LNAV Only.” And, based on the fact that the missed approach seems to depart more or less from that point, I answered A.

As already noted, however, the correct answer is C.

It't not at all obvious to me that the LNAV approach terminates at the RW30 waypoint.

Can someone explain this a bit? How would I know for sure what the MAP is for the LNAV approach? Is this related to the fact that RW30 waypoint is shown as a circled "fly-by waypoint" in the top down view?

Also, if the RW30 wayoint is the MAP for the LNAV approach, why is the 1.5NM location identified? What's it for?

As I typed this, it dawned on me that 1.5NM from the RWY is much too far to be a MAP, so perhaps I should have logically eliminated A based on that observation, but still want to understand more.

Thanks!

 

[tsts4]

The MAP for an LNAV is always a flyover waypoint. What you are seeing at 1.5 nm is the VDP for the LNAV. Remember for the LNAV you're decending to an MDA (classic dive and drive). The VDP gives you the optimal point from which to begin your descent from the MDA to the runway assuming the runway environment is in sight. As such the VDP has nothing to do with the missed approach

 

[eetrojan]

The MAP for an LNAV is always a flyover waypoint. What you are seeing at 1.5 nm is the VDP for the LNAV. Remember for the LNAV you're decending to an MDA (classic dive and drive). The VDP gives you the optimal point from which to begin your descent from the MDA to the runway assuming the runway environment is in sight. As such the VDP has nothing to do with the missed approach

Thank you Todd!

I now remember reading about a bolded V symbol representing a “visual descent point.” In the test supplement's fuzzy chart, the bold-V has morphed into a fat arrowhead.

When I reviewed VDPs just now, I see that the VDP is where an airplane flying at MDA, after doing a “dive and drive,” can descend below MDA if the runway environment is visible.

One website I stumbled onto says that “the VDP is the last point at which a descent from the MDA to the touchdown zone can be made at a stable three degree glideslope. If the runway is not in sight by the VDP, a missed approach should be executed.”

http://www.aviationchatter.com/2009/12/how-to-calculate-a-visual-descent-point/

If that’s accurate, though, it sounds like the VDP really is the MAP. So, I’m still confused.

Maybe it’s more correct to say that you are encouraged to go missed if you can’t see the runway environment at the VDP, or very quickly thereafter, but you don't legally have to if you're willing to make steeper descent when the runway comes into view after passing the VDP, but prior to the actual MAP?

Does that seem like a correct view of the world?

 

[wlawr005]

The VDP is the earliest point you can safely begin a descent and maintain a 3° glideslope. The very extreme example given is if you had a 3000 ft mountain 2 miles from the runway. At night, you might not see that mountain and fly into it if you began your descent too early because you had the runway in sight. Not having the runway environment in sight at the VDP is no reason to execute a missed approach assuming the visibility is within limits for the approach you're flying. In this case, if the vis was at mins (3/4), then there would be no way you'd see the environment 1.5 miles away.

Additionally, you may elect to accept a steeper glideslope and begin your descent after the VDP. Think of it like this, the VDP is designed to keep you at the MDA until a certain point, even if you have the runway in sight. They were implemented when an Air Force cargo plane began a descent too early at night...they didn't see the dark mountain until the runway disappeared and they hit it.

 

[flyingron]

The VDP is the earliest point you can safely begin a descent and maintain a 3° glideslope.

It's the only point you can maintain a 3 degree glideslope to the touchdown zone.

 

[wlawr005]

Agreed. I should get better at using commas. It is the earliest point you can safely begin a descent, and the only point that gives a 3° glideslope to touchdown.

 

[Sam D]

Thank you Todd!

I now remember reading about a bolded V symbol representing a “visual descent point.” In the test supplement's fuzzy chart, the bold-V has morphed into a fat arrowhead.

When I reviewed VDPs just now, I see that the VDP is where an airplane flying at MDA, after doing a “dive and drive,” can descend below MDA if the runway environment is visible.

One website I stumbled onto says that “the VDP is the last point at which a descent from the MDA to the touchdown zone can be made at a stable three degree glideslope. If the runway is not in sight by the VDP, a missed approach should be executed.”

http://www.aviationchatter.com/2009/12/how-to-calculate-a-visual-descent-point/

If that’s accurate, though, it sounds like the VDP really is the MAP. So, I’m still confused.

Maybe it’s more correct to say that you are encouraged to go missed if you can’t see the runway environment at the VDP, or very quickly thereafter, but you don't legally have to if you're willing to make steeper descent when the runway comes into view after passing the VDP, but prior to the actual MAP?

Does that seem like a correct view of the world?

The article in the referenced website is misleading and wrong. Read AIM 5-4-5 which lays it out pretty clearly and is consistent with the first part of your comment.

 

[eetrojan]

Thanks all!