Pseudo RNAV approach with vertical guidance

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40flyer
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Pseudo RNAV approach with vertical guidance

Post by 40flyer »

I have an ‘05 DA-40 with the KAP 140 A/P. Because the G1000 does not provide vertical guidance for an RNAV approach I found I can create the same lateral path by entering the approach waypoints and putting a VNAV alt target (or the flight path descent angle listed on the approach plate) on the last waypoint prior the airport. If there isn’t a convenient one near airport I create one using the along track offset. Then I can get a psuedo glideslope to that last waypoint. I think there are two problems with this - 1) without an approach selected, the last waypoint would be the airport reference point, not the appropriate runway threshold. They could be significantly different at large airports, and 2) I’m not sure sure you can get the KAP 140 into approach mode meaning if I only have nav mode it will mot track the final approach course with the same tolerance it does in approach mode. Has anyone tried this? I wish I had WAAS but it’s no longer available
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Re: Pseudo RNAV approach with vertical guidance

Post by linzhiming »

Are you looking to create a vertical guidance approach for an airport without instrument approach (akin to the GTNs) or get vertical guidance for an LNAV approach?

From your description, I understood it to be latter - in that case why do you need to create new waypoints? Wouldn't you be able to use the existing RNAV waypoints which should have a RWxx MAP for the start of the runway?

I have a 2007 non-WAAS DA40 and generally found it sufficient to set a vertical descent rate on the A/P (-500 fpm for a 3°-descent at 100 kts GS) and then to monitor altitude based on distance from the MAP and cross-checking against the approach plate. If there is a decision, increase/decrease descent rate by 100 fpm until back on desired descent path.
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40flyer
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Re: Pseudo RNAV approach with vertical guidance

Post by 40flyer »

Yes, it's the latter. If I don't select the approach I don't get the MAP. If I select the approach you cannot put a VNAV target on the MAP so that's why I was trying to fake one, usually by putting in the required FPA of 3 degrees (usually) on a user created waypoint just shy of the runway. Just trying to avoid dive and drive with the stepdowns. I can just monitor to ensure I clear any intermediate altitudes. 500 fpm can be too shallow or too aggressive in many cases, especially where I live where it's often windy with gusts. Dive and drive I'm usually 700 fpm. The vertical cue would allow me to manage the descent path with the throttle, just like a 737 works.
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Re: Pseudo RNAV approach with vertical guidance

Post by vontresc »

Or you could use a couple basic rules of thumb my old CFII taught me to fly stabilized non precision approaches.

at a 3 degree glideslope it take about 3 nautical miles to lose 1000' of altitude
to achieve that 3 degree glideslope multiply your ground speed by 5 and descend at that rate in fpm.

Those two calcs are relatively simple and no G1000 gymnastics are required. This method is even compatible with the KAP140 in approach mode.

Works surprisingly well.
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