Visual Descent Point

[SkyWestCRJPilot]

Here is another reason for a VDP. Let's say you come down to your MDA and you are there early. If you see the runway you shouldn't start a descent from the MDA till you are at your VDP. That is the point at which a continous descent to a landing using normal manuevers can be made. Otherwise you may end up coming down from you MDA too early and your last 1-2 miles of the approach may be at a 1-2 degree glideslope. You could end up taking out a few power lines or trees in that case. Ideally when you reach your VDP while at your MDA the VASI shows red over white. Incidentally I was reading in Aviation Week that a country in Europe has decommissioned all of thier nonprecision approaches. They believed that unstablized nonprecision approaches are a leading cause of CFIT.

 

[spanky2]

My previous airline started training (2001 or so) for CANPA...constant angle non-precision approach. No more "dive and drive" with logic being that diving to the MDA at low altitude was exceeding 1000 fpm which is not allowed. SOOO..result is a quasi-precision approach based on computing VDP (300' per mile) and backing that up to FAF altitude and then using the requried vertical descent rate ... not to exceed 1000 fpm...to arrive at VDP altitude. If rwy not insight at that point...TOGA, power, pitch, pos rate..."gear up"...etc.

Even with a depicted VDP...we still had to come up with the descent rate...ball park is always going to be 700 - 800 fpm for normal 737 speed. Not sure if this has changed.

 

[JJJ]

you must land in the touchdown zone, that is, the first 1,500 feet of the runway
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I though the Touchdown Zone was 3000 feet

or the first half of the runway which ever is shorter, right? For runways less than 6000 ft.


You can't start down before the VDP, but I think saying that leaving the MDA past the VDP is unsat or unsafe is not true. The regs say using normal manuvers, normal rate of decent, and land within the touchdown zone or first half of the runway. Starting down at the VDP normally allows for about a 3 degree decent at around 700-800 fpm at jet speeds. Usually a "stabilized approach" is 1000 fpm or less. This would allow you to decend from the MDA past the VDP and still make a stabilized approach to the runway.

If this were not true, you could only decend at exactly the VDP, not before or even .1 after.

 

[TonyC]

or the first half of the runway which ever is shorter, right? For runways less than 6000 ft.


You can't start down before the VDP, but I think saying that leaving the MDA past the VDP is unsat or unsafe is not true. The regs say using normal manuvers, normal rate of decent, and land within the touchdown zone or first half of the runway. Starting down at the VDP normally allows for about a 3 degree decent at around 700-800 fpm at jet speeds. Usually a "stabilized approach" is 1000 fpm or less. This would allow you to decend from the MDA past the VDP and still make a stabilized approach to the runway.

If this were not true, you could only decend at exactly the VDP, not before or even .1 after.

I'll defer the "first half of the runway" issue to someone who knows more about it than I. I have never heard of that being in the regs, but there's a lot that I don't know.

I will, however, address your last statement. If you compute a VDP based on a given rate of descent to a certain point on the runway, you must begin the descent from that point at that given rate of descent in order to arrive at that certain point on the runway. If you delay the descent, you must either increase the descent rate, or land beyond the planned point of landing, or both. How much deviation you allow in terms of descent rate and touchdown point are matters of judgment. Is 3.1 degrees acceptable? Is 500 feet long acceptable? You make the call, but it IS a judgment call. On an airline interview, I don't think you can go wrong being conservative.

 

[SkyWestCRJPilot]

I don't think that descending slightly past the VDP is a problem. What about headwinds or tailwinds. A 40 knot headwind vs. a 10 knot tailwind will affect your descent rate more than a few tenths of a nm. Most MDAs I've seen are around 350-600 feet above the TDZE. That creates a VDP of around 1.1 to 2 nm before the runway. At the higher MDA and a farther VDP (2 nm) you have a little more room to manuever and adjust your descent rate than with a lower MDA and closer VDP. It requires good judgement taking into account your familiarity with the airport, aicraft characteristics, runway length, and contamination. It would differ how strict I would be were I flying a DC-10 vs. an E-120. Nonprecision approaches are inherintly unstabilized approaches so you have to have good judgement when it comes to descending "slightly" past the VDP. That would be my longwinded explaination during an interview.