B200/C90T Autopilot Vertical Guidance - RNAV LPV approach

[tttocs]

As mentioned before, I've run into occasional problems in two ways with the 430/530 with June 2011 data. Procedures that have simply changed (or didn't even exist) since 2011, AND what I suspect (but have no way to verify, since I don't have charts from that cycle) are simply errors in the trainer's data. For example, one of the SIDs for KLAS as depicted on the Garmin just doesn't make sense - not just because it doesn't match the current cycle's data, but because it wouldn't have EVER made sense, even assuming the procedure had changed since June '11. Here I don't think the data is just out of date, I think it's wrong.

 

By and large, the data is good and works well. But there are anomalies.

 

For the procedure you're trying to fly, if the stepdowns you're using from the chart don't match where you should be on the approach as defined by the Garmin data I can well imagine that vertical guidance might be a bit dicey to make work out.

 

Scott

[Bert Pieke]

As a rule of thumb, I aim for 2000 feet AGL for the glideslope intercept.

 

That seems to work for most airports.

 

But if you have the actual approach chart, that would take precedence..

[fppilot]

Bert,

Yes. Here is the plate for RNAV 22 at Easton. Note the red and blue lines that I added. My colored lines depict what occurs during the RXP/FSX/Carenado coupled approach. Once engaged, the APR function and GPS maintain a level approach altitude until well past when the glidepath begins on the chart. The slope on the chart starts at FILRO. In this case you get almost over HILNU before the coupled RNAV/VNAV approach extinguishes the ALT hold and starts you down. That's what was causing me earlier to abandon the coupled approaches and take over vertical control myself. It appeared the vertical guidenance wasn't working. This same thing happens at KBMG RNAV 17 and may be most obvious at KAVL RNAV 16.

 

 

BTW, if you try this approach yourself with RXP you will see some different fixes than are published on this more current plate. AGARD, HILNU and JOPAG are evidently more current than the fixes in the June 2011 database, though the database fixes are in approx the same postions.

[tttocs]

OK, maybe a dumb question, but why would you not have started a descent to 1600 at the IF? I don't typically see vertical guidance until sometime (where sometime is a bit variable) after the IF on most LPV approaches. If there's an intermediate fix between the IF and FAF it usually seem to kick in about there, though obviously there isn't one on this approach. The last thing I want to do is be at the FAF well above the glideslope (virtual or otherwise) which we know on this approach will be at 1600.

 

Keep in mind that my RL experience with precision approaches is limited to ILSs, but with that disclaimer, that's how I'd fly this approach.

 

Scott

 

Edit: Re-reading, I think you're mistaking the chart depiction of the vertical profile from the IF for a virtual glideslope. The profile tells you that you can step down to 1600 at the IF, not that you can expect to grab the (again virtual) GS there. Do that stepdown, and I think you'll be fine.

 

S.

[fppilot]

Scott,

I'll be flying those three mentioned approaches (KESN, KBMG, and KAVL) again so will play around with descents a bit. What I will say is once the vertical guidence takes over and I turn my attention to the panel the dot is centered on the vertical scale on the instruments. As a matter of fact, at KAVL RNAV 16 I watched the dot gradually lower to center just as the VNAV guidence took over.

[tttocs]

I think you're mistaking the chart depiction of the vertical profile from the IF for a virtual glideslope.

 

Grrr. Let me put that another way - what's depicted is a virtual glideslope, but not necessarily an indicator of when you'll get virtual guidance. That is, it's not the same as seeing the glideslope path depicted on an ILS plate, which actually shows you the GS intercept.

 

What I will say is once the vertical guidence takes over and I turn my attention to the panel the dot is centered on the vertical scale on the instruments.

 

Yep, seems to work very accurately when you get thinks working well.

 

Scott

[Bert Pieke]

I just tried the approach and I believe your drawing is off by quite a bit..

 

The glideslope became active at about 10 nm out..

 

I measured the distance to the runway at about 6 nm at glideslope intercept, when coming in at 2000 ft ASL.

[fppilot]

Bert,

I haven't been instrument rated since 1984 and all my real-life training was on ILS and LOC approaches. Perhaps I need a better primer on RNAV GPS approaches. I expected the vertical guidence to follow the slope on the chart like with an ILS.

 

Note that on the approach you flew the slope begins 10.8 out at 2,000, and reaches the FAF 6.1 nm later, so it is unarguably certain that the slope depicted on the chart is not linear. I agree with you that the intercept is about a mile before CEPLU, but that point is approx 4.7 miles inside the point at which the plate's slope begins. That's what I found confusing. My ILS training I guess is getting in the way. Anyway, I do now have it figured out in practice.

 

BTW I flew an approach in the C210 tonight to RNAV 33 (LPV) at KPDG (Punta Gorda, FL). The slope begins at 2500 and the slope reaches 1600 6.1 nm later at the FAF, which is 4.8 out. I began descending from 2500 at a shallow rate to reach 1600 at the FAF and the whole way I was low on the HSI until I got to my intercept. I selected APR as I neared that point and let the AP took over. For similar reference look at your dot in your first photo above. Seems like you'll be shown low until you reach the intercept.

[tttocs]

For similar reference look at your dot in your first photo above. Seems like you'll be shown low until you reach the intercept.

 

This is what I was trying to point out previously, but I'm probably not articulating it well. On most ILS approaches you generally do intercept the physical glideslope from below. Same here.

 

If you look at the profile view of a typical ILS approach you'll see two things. A vertical profile to fly, and the physical radio glideslope to be intersected - usually from below. On an LPV approach that physical glideslope doesn't actually exist except as interpreted and created by the WAAS GPS unit, so the intercept point of a physical ground signal is not shown - because it doesn't exist. When you look at the profile, you'll see the vertical profile to be flown, but the actual intercept of the virtual glideslope is not depicted. Don't worry about that. Fly the profile, descending when allowed and "capture" the glideslope when it appears and all will be well and will actually work out much as ILS approaches do. You'll generally descend to a point below the glideslope as allowed by the charted approach, and should usually intercept it from below.

 

Hold off on your descent, staying above the altitude the approach allows you to descend to, (as it appears you've been doing), and you'll be too high.

 

Scott

[Bert Pieke]

Yes, from a procedural point of view, I activate ALT hold at roughly 2000 ft AGL and

when the glideslope needle/dot first shows up, verify that I am indeed below the glideslope,

and then activate APR.

 

The autopilot will hold altitude until the intercept, and then start the descent... at that point, I typically

throttle back, activate the gear and one notch of flaps...

 

If flying by hand, I would do the same thing, hold altitude until intercept and then descend,

following the glideslope down.