GPS approach not sequencing

[Tokirbymd]

Practicing GPS approach into KMDQ Rwy36 from KUCPE (GTN650). Flew from north and did hold/procedure turn inbound back to KUCPE. Large crosswind required fairly large angle for wind correction and on inbound back to KUCPE got GPS error that angle was too great to give VNAV info and stayed in TERM mode. Reloaded the approach with KUCPE as the IAF and deleted the hold waypoint to show KUCPE to ZUPIM (faf) as correct sequence. It gave me a direct to KUCPE heading rather than re-intercepting the correct inbound course of 005 so I put in an OBS of 005 for the KUCPE waypoint to get back on course and intercept the correct inbound course. After passing KUCPE GPS didn't sequence to ZUPIM. I had planned on flying it as LNAV only without the LPV vertical guidance. I selected the flight plan and activated the correct segment KUCPE--> ZUPIM and then received the LPV notification on the GTN650 and was able to fly the rest of the approach as published.

Question for real IMC- if this happens in real life should I do as I did, or would I be better to declare a missed approach, stay at altitude and fly to KMDQ and then the published missed and re-enter the fill approach (or of course request vectors back to the IAF). ?

 

[midwestpa24]

Just my opinion, but shooting an approach in actual is the last place I want to be fumbling around trying to reload or re sequence the approach. You are already task saturated during the final phases of an actual approach, a navigation unit not doing what you want it to do just adds to it. Guys have died that way. Unless you were confident in the buttonology to correct the issue quickly, I'd go missed and take the time to get it right.

 

[RussR]

Practicing GPS approach into KMDQ Rwy36 from KUCPE (GTN650). Flew from north and did hold/procedure turn inbound back to KUCPE. Large crosswind required fairly large angle for wind correction and on inbound back to KUCPE got GPS error that angle was too great to give VNAV info and stayed in TERM mode.

Do you remember the exact message it gave you?

Reloaded the approach with KUCPE as the IAF and deleted the hold waypoint to show KUCPE to ZUPIM (faf) as correct sequence. It gave me a direct to KUCPE heading rather than re-intercepting the correct inbound course of 005

That would be the normal, expected behavior.

so I put in an OBS of 005 for the KUCPE waypoint to get back on course and intercept the correct inbound course.

I would have just left it as direct to KUCPE at that point - you were likely very close to the inbound course anyway, correct?

After passing KUCPE GPS didn't sequence to ZUPIM.

If you were still in OBS mode, then no, it wouldn't automatically sequence to the next waypoint. You would need to come out of OBS mode (though it would keep the new desired inbound course you selected of 005).

I had planned on flying it as LNAV only without the LPV vertical guidance. I selected the flight plan and activated the correct segment KUCPE--> ZUPIM and then received the LPV notification on the GTN650 and was able to fly the rest of the approach as published.

Yes, I believe activating the KUCPE to ZUPIM leg would override the OBS mode so that would return things to "normal".

Question for real IMC- if this happens in real life should I do as I did, or would I be better to declare a missed approach, stay at altitude and fly to KMDQ and then the published missed and re-enter the fill approach (or of course request vectors back to the IAF). ?

I think you hosed yourself by the whole OBS mode step. But I am interested in the error message that got you to that point in the first place.

I do agree with @midwestpa24 that any time you don't know what's going on with the GPS (or autopilot, etc.), you should go missed, figure it out, and try again. I presume you would be in radio contact with Huntsville Approach, so just let them know and get vectors back around.

 

[benyflyguy]

What kind of degree course correction are we talking about???

 

[Hang 4]

If there's a way to recreate the situation on a VFR day at a sleepy airport with an approach, why not just fly direct to the IAF and see if it corrects. I think reacting to the message by touching the instrument was the beginning of making it worse vs just flying direct to the fix.
In actual IMC, I agree with the go missed crowd.

 

[Tokirbymd]

30kt wind- was flying teardrop entry and got the message when I turned back to re-intercept the approach course before KUCPE (faf). I was within 4mi but at a 45deg angle from the faf course so I assume that is what triggered the message. Yeah probably could have just flown the original approach and navigated to KUCPE rather than reloading the approach would have been better. I agree, setting the OBS is likely what prevented sequencing to ZUPIM (I didn't think about that originally). Was thinking that I needed to get back on the course prior to reaching KUCPE however. Once I realized it wasn't sequencing correctly- (only took a second or two)my choices were to manually seuqnce it and see what it gave me, or try to manually go direct to the MAP/airport at altitude and then fly the missed. I'll have to see if I can recreate it again on the simulator to get the exact message.

I guess my bigger question was when you get unexpected results on a RNAV only approach what is the best way to handle it? Staying high and going missed and setting up again would be the obvious answer but in this case it seemed like setting up a new navigation path to the MAP would have taken more effort and button pushing than simply reselecting the appropriate segment to activate.

 

[drjcustis]

I’ve done something similar (a few times). The last was an approach into KBHM and after I was given the intercept to final, I realized I had forgotten to activate the approach. On activating the approach, the GPS wanted to take me back to the IAF which was the opposite direction. A few quick button pushes and I was on course, missed the intercept by about a half mile and got queried by ATC.

My thought is if the GPS is doing something different than you want it to do and you don’t know why, then you’re in trouble and need help right away. Ask for a vector. I don’t think you’ll need to fly the entire missed approach, the controller can just vector you back.

Keep messing with the GPS until you know what it does when you hit the right buttons and you know what it does when you hit the wrong buttons. And, the autopilot is your friend, until it’s not.


Sent from my iPad using Tapatalk

 

[RussR]

I guess my bigger question was when you get unexpected results on a RNAV only approach what is the best way to handle it? Staying high and going missed and setting up again would be the obvious answer but in this case it seemed like setting up a new navigation path to the MAP would have taken more effort and button pushing than simply reselecting the appropriate segment to activate.

If all else fails, this situation is almost exactly perfect for using the MSA, or in this case the TAA in lieu of the MSA (as one is not published when there is a TAA). Since you had KUCPE shown as your "to" waypoint (before you solved the sequencing error), and you know you were north of KUCPE because you had passed it, you are in the 4700 MSL sector of the TAA. Climb to 4700, stay within 30 nm of KUCPE and you at least won't hit anything. That should require no immediate button pushing.

 

[Fracpilot]

Just hit “activate vectors to final” and it would’ve fixed it. You could’ve still went direct to a point in the approach. Anytime I see something I don’t like with the Garmin, hitting “activate vectors to final” usually fixes it.

Not sure why you selected OBS mode. But yes you pretty much screwed yourself by going into that mode. I believe it puts the approach into “suspend”.

Even after they missed approach you can just hit “activate approach” or “activate vectors to final” and it would’ve fixed it.


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[Tokirbymd]

I was able to recreate it in the simulator. The exact wording is “ VNAV unavailable. Excessive track angle error” I researched it and this is given when you are more than 70def off the correct course which I was while flying the teardrop. When I repeated it I left the approach unchanged and just flew to the iaf and the LPV and correct sequencing worked fine. Was a very good lesson however in managing things when the GPS goes awry during an approach. Glad I wasn’t in the soup when it did. Will keep practicing.

 

[midwestpa24]

I guess my bigger question was when you get unexpected results on a RNAV only approach what is the best way to handle it? Staying high and going missed and setting up again would be the obvious answer but in this case it seemed like setting up a new navigation path to the MAP would have taken more effort and button pushing than simply reselecting the appropriate segment to activate.

You are correct. Trying to push the buttons and reprogram, again at a time you are now task saturated going missed while IMC, could cause more problems. Remember to Aviate first. Most of your missed approach instructions are for a straight or climbing turn. It would be best to start the climb, report the missed to ATC, and ask for vectors until you can get sorted out.

 

[chemgeek]

I assume you were significantly off the final approach course tracking inbound to KUCPE in the hold to generate the tracking error? Otherwise you may be able to simply delete the hold and navigate direct to the IAF if it can be done safely, e.g. by flying the hold manually.

Sometimes, the GPS/autopilot is not a help. In a recent thread, I described a situation with an approach to my home field, newly redesigned with a HILPT, that my GNS430 insists that I fly the hold, regardless of direction of flight to the IAF. The only solution I could come up for the NoPT approach headings is to manually delete the hold prior to activating the approach. In IMC, that could be a handful if it happened the first time when you are figuring out how to make it work properly.