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VNAV capture

Postby 1 Victor Fox » Mon Aug 07, 2017 2:28 pm

My RV7 has an HXr, GTN650,and a TruTrak Vizion A/P. All software is current. When I load, activate, then execute a Lnav/Vnav approach, I capture the Lnav course fine but the aircraft flies through the Vnav path, maintaining the last altitude hold and does not descend on path. All of the Autopilot presets on the PFD page are correct to my knowledge. Ideas?
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Re: VNAV capture

Postby Bobturner » Mon Aug 07, 2017 11:56 pm

Are you sure you meant LNAV/VNAV? Not LNAV+V? or LPV? I didn't think the 650 offered LNAV/VNAV as an option.
I'll take a wild guess, since I recently encountered this with someone else. Did you get a glide slope indication on the PFD? If "no", you may have mistaken a step down on the chart as a GS. What approach was this?
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Re: VNAV capture

Postby 1 Victor Fox » Tue Aug 08, 2017 2:10 am

Hi Bob, I loaded the RNAV (GPS) RWY 3 at IMS (Madison, IN) using the 1321 LNAV/VNAV mins. Shouldn't I have vertical path guidance for this approach? I saw the yellow cdi needles on both axis' but no capture.
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Re: VNAV capture

Postby Bobturner » Tue Aug 08, 2017 7:54 am

First of all, did the 650 ever say "LNAV/VNAV"? I didn't think a 650 could authorize that, but I don't have one so I may be wrong. Do not confuse "LNAV+V" with LNAV/VNAV.
Approaching KOYGE what did the 650 say? Probably "TERM" for terminal mode. On the FAA chart it looks like a glide slope starts at KOYGE but it does not. See the little arrow on the diagonal line? This is a step down fix. Passing KOYGE you need to reset the altitude to 2400'. The autopilot should descend and level at 2400'. If you have pushed execute, then about 3 miles outside of the FAF (BULLG) the GS needle should come alive, the 650 should change from TERM to LPV. At GS intercept (BULLG) the autopilot should start the plane down. Use whatever minimums the 650 authorizes, most likely LPV.
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Re: VNAV capture

Postby 1 Victor Fox » Tue Aug 08, 2017 4:04 pm

Ahhh. Good stuff Bob. Thanks. In the GTN650 manual it does list LNAV/VNAV approaches and uses the annunciation: L/VNAV. It uses LNAV+V for those approaches (where vertical "guidance is only provided". I will fly this again using your tips and report back. I had flown it just drilling along at 2700 at KOYGE waiting for the decent with the approach executed going wtf? Ill go try it again treating it as a step down fix, expecting the descent to intercept at BULIG..

One more question maybe you could answer: In the Garmin 650 manual at the beginning of chapt. 6, the section on Procedures, there is a note that states: "to take full advantage of the GTN unit's capabilities, an optional baro-corrected altitude source is recommended for auto sequencing of altitude leg types. If no baro-corrected altitude data is provided to the GTN unit, altitude leg types must be manually sequenced." I'm not sure what that means......

Cheers!
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Re: VNAV capture

Postby Bobturner » Tue Aug 08, 2017 4:42 pm

It means that it is best if you have an RS-232 serial connection from your GRT to the 650, feeding it barometric altitude data. If you followed the GRT setup and installation manuals then you have this. Otherwise the 650 might stop sequencing until you confirm your altitude by pushing an acknowledgement. Pretty rare.
You want to get into the habit of watching those GPS annunciations (term, lpv, etc). Every once in a while it will detect bad satellite geometry or other issues, and either authorize a lesser approach (e.g., LNAV instead of LPV) or maybe no approach at all.
I personally dislike the government charts for this approach. Notice that from the IAF (KOYGE) to the FAF you only lose 300' over many miles, so the diagonal descent line should be very shallow. But the chart is not to scale, so it is easy to mistake it for the GS. Jepp charts depict this differently, much harder to mis-interpret.
You can also 'cheat the box'. Although not the way intended to fly the approach, you can, once established on a straight in course, change the 650 to 'vectors to final'. Some of the intermediate fixes may disappear, but you will get GS enunciation earlier. You can then stay at 2700' (note 2400' is a minimum but not mandatory altitude) until GS intercept (in this case, about 1nm outside the FAF) and then follow the GS down.
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Re: VNAV capture

Postby 1 Victor Fox » Wed Aug 09, 2017 2:08 pm

Thanks Bob. Good explanation of the RS-232 connection.
I flew the approach again yesterday and watched all of the GRT and GTN annunciations throughout the approach. All were correct and sequenced correctly. I did two approaches one with the flight director and one with the needles and both times the Vizion would not capture the Vpath. I have been having this discussion with GRTJeff as well. I basically have confirmed that the GTN is behaving and communicating correctly as well as the HXr. Maybe there is a Vizion setting I have wrong. The fact that it climbs and descends and captures selected altitudes though has me confused as this is in effect the same type of signal according to Jeff. I am going to talk to some TruTrak people...
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Re: VNAV capture

Postby 1 Victor Fox » Fri Aug 11, 2017 12:08 am

TruTrak say I have everything setup correctly. I am in GPSS and VS. HXr Annunciator fields are all correct.
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Re: VNAV capture

Postby Bobturner » Fri Aug 11, 2017 3:35 am

I'm not so sure about VS mode. Tracking the GS the autopilot should accept whatever the VS and airspeed are, but track the GS. If you reduce the throttle VS and IAS should reduce, while the autopilot still stays on the GS. Since I have a Trio I'm not much help here. You want GPSS for sure.
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Re: VNAV capture

Postby 1 Victor Fox » Fri Aug 11, 2017 1:51 pm

VS is misleading... (not vertical speed) The annunciation stands for GPS Vertical Steering. Across the bottom of the Vizion you will see: GPSS VS
Its indicates that the "Vertical Navigation signal is present" TruTrak uses SVS for a pilot selected vertical speed, so you would see something like GPSS SVS400^
for example.
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