GPSS & Waypoints

[rbridges]

I'm just following this thread to see where it goes.  It's an interesting read.  

[KLRDMD]

7 hours ago, kortopates said:

The control information (or GPS stearing on what to fly, when to turn) is provided by your Avidyne GPS. The Aspen GPSS just translates that to heading commands for an AP that doesn't know how to interpret GPS stearing commands. Thus the issue is with the Avidyne GPS.

That doesn't make sense as the airplane had an Avidyne 540 in it when I bought it and I upgraded it to a 550 after a few months. The GPSS issue has been present throughout my entire ownership period and since the Aspen has been adjusted fully, it seems the only thing left is the S-Tec autopilot. I seriously doubt two independent Avidyne boxes were both incorrect in exactly the same way.

[KLRDMD]

6 hours ago, Awful_Charlie said:

Have you tried resetting the Aspen GPSS gain to standard? I have a feeling the values in that are "inverted" (eg GPSS=2 means half rate, and GPSS=0.5 is double), but in any case setting standard can remove that unknown. I don't know the S-TEC, but some more modern autopilots have the GPSS built into them, and if this is the case then the Aspen GPSS is out of the loop.

The Aspen was originally set at 1.0 and did half standard rate or maybe a bit less. I set it to 1.5 and it was better, then set it to 2.0 and it was better yet. 2.0 is the highest possible setting. Since the turn improved with higher values, I don't believe setting it to a value below 1.0 would help, but I can certainly do that and see. I don't believe the S-Tec 60-2 has GPSS built in, I believe that all comes from the Aspen.

[KLRDMD]

2 hours ago, Marcopolo said:

I am almost 99% sure that this is standard operating procedure for the IFD units.  The pilots guide references this for any turns greater than 135*.  I have seen this behavior many times in my local area and on long cross countries (IFD 540, KFC200, GPSS).  When I see a turn on the map that doesn't depict the smooth radius I simply fly toward the intended waypoint and then activate the barber poled leg when I get within a mile or so of that waypoint where the high angled turn is depicted.  You can try this on the simulator.

From the pilot's guide "On any course change greater than 135 degrees, the FMS will treat the waypoint as a flyover"

This makes perfect sense and explains everything. If there isn't a lead-in curve depicted, it will not fly it and I'll have to manually activate the next leg as you suggest. Thank you !

[Marcopolo]

2 hours ago, Marcopolo said:

Ken,

 

  I am almost 99% sure that this is standard operating procedure for the IFD units.  The pilots guide references this for any turns greater than 135*.  I have seen this behavior many times in my local area and on long cross countries (IFD 540, KFC200, GPSS).  When I see a turn on the map that doesn't depict the smooth radius I simply fly toward the intended waypoint and then activate the barber poled leg when I get within a mile or so of that waypoint where the high angled turn is depicted.  You can try this on the simulator.

 

Ron

 

From the pilot's guide "On any course change greater than 135 degrees, the FMS will treat the waypoint as a flyover"

@KLRDMD,

 

  I edited my previous post(attached above) to change the degrees used by the IFD for changing a waypoint to a fly-over to 135*.  If the turn is less than this the IFD will give a 10 count down to the turn, if the turn is 135* or greater it will give a 30 second count down.

 

Hope this helps

 

Ron

[Marcopolo]

Just now, KLRDMD said:

This makes perfect sense and explains everything. If there isn't a lead-in curve depicted, it will not fly it and I'll have to manually activate the next leg as you suggest. Thank you !

Ken,

 

  Knowing that the IFD will instruct the autopilot to run the 270* opposite direction turn past the waypoint I have also asked ATC for "maneuvering space" beyond the waypoint and that has been granted in all three instances I've requested, but I have mostly just activated the leg beyond the waypoint within a mile or two depending on speed being flown.

 

Ron

[kortopates]

2 hours ago, Marcopolo said:

Ken,

 

  I am almost 99% sure that this is standard operating procedure for the IFD units.  The pilots guide references this for any turns greater than 135*.  I have seen this behavior many times in my local area and on long cross countries (IFD 540, KFC200, GPSS).  When I see a turn on the map that doesn't depict the smooth radius I simply fly toward the intended waypoint and then activate the barber poled leg when I get within a mile or so of that waypoint where the high angled turn is depicted.  You can try this on the simulator.

 

Ron

 

From the pilot's guide "On any course change greater than 135 degrees, the FMS will treat the waypoint as a flyover"

That definitely explains it, but is worse than I thought hoping it was an isolated waypoint issue.

6 minutes ago, Marcopolo said:

Ken,

 

  Knowing that the IFD will instruct the autopilot to run the 270* opposite direction turn past the waypoint I have also asked ATC for "maneuvering space" beyond the waypoint and that has been granted in all three instances I've requested, but I have mostly just activated the leg beyond the waypoint within a mile or two depending on speed being flown.

 

Ron

This is flat out wrong to fly the turn that way and could get anyone that doesn't correct it into trouble if not violated. Avidyne should really correct it - its a safety hazard. That said, real life IFR flight plans that would really have a greater than 135 turn required would be really rare. At least I can't imagine a valid enroute situation that would require that and wasn't a course reversal in an approach ( a course reversal in an approach would be coded differently anyway). But @Marcopolo comment above suggest it still happens though.

[Marcopolo]

1 minute ago, kortopates said:

At least I can't imagine a valid enroute situation that would require that and wasn't a course reversal in an approach ( a course reversal in an approach would be coded differently anyway). But @Marcopolo comment above suggest it still happens though.

Paul,

 

  Most of my high angle course changes were created for me during my IFR training by my instructor due to how the IFD handle these compared to the G1000 he was used to.  He wanted to make sure we had the bottonolgy or the phraseolgy (Atlanta Center, Mooney 1079B requests maneuvering space to the northeast of VOR ABC due to course change angle) down pat, most were in actual as he preferred real life training towards the end of the syllabus.  I have however run into these situations a couple of times in real IFR life due to some funky flight plans or course reroutes due to airspace or MOAs.

 

Ron

[Marauder]

1 hour ago, KLRDMD said:

This makes perfect sense and explains everything. If there isn't a lead-in curve depicted, it will not fly it and I'll have to manually activate the next leg as you suggest. Thank you !

I guess this leads to the question, is this okay? I think it was Anthony who mentioned that flying that loop could put you into conflict with traffic if ATC is expecting you to make the turn towards the new waypoint (airport in your case) rather than fly the loop. It also makes me wonder why the Garmin GTN series does depict this differently and shows the curved route.

I fly later this afternoon and see if I can duplicate this on my GTN and STEC.

[Marauder]

And I echo Paul's comments that seeing this in real life would be an exception rather than the rule.  

[kortopates]

1 hour ago, Marauder said:

And I echo Paul's comments that seeing this in real life would be an exception rather than the rule.  

Although it got to be rare, should this happen in the busy terminal environment its a set up for triggering an RA on a larger commercial plane with it taking evasive action like climbing or descending and then the controller having his/her hands full dealing with the aftermath, or causing a loss of separation. Either way the pilot is going to be in trouble.

And Maro's follow up comment 

2 hours ago, Marcopolo said:

I have however run into these situations a couple of times in real IFR life due to some funky flight plans or course reroutes due to airspace or MOAs.

suggest its just a matter of time before others unexpectedly run into this. Although Marco is prepared for this, I would think most won't be.

1 hour ago, Marauder said:

I guess this leads to the question, is this okay?

So based on my FAASTeam role with my local FSDO seeing pilots get pilot deviation for unexpectedly turning the wrong way over a fix in busy terminal airspace I think its definitely not okay. Ken is really in a bad position with this IMO given his STEC can only do 3/4 standard rate turns leading to extra large 270 pattern (which of course is a separate issue entirely).

1 hour ago, Marauder said:

I fly later this afternoon and see if I can duplicate this on my GTN and STEC.

I am very certain you will merely get an annunciation of "Steep Turn ahead" followed by Garmin's turn anticipation s/w starting the turn inside of the turn to avoid over shooting. But please do report back. 

So the question remains why would Avidyne treat steep this way and turn off turn anticipation to fly over the waypoint? Has anyone called for an explanation? You'd think the FAA would have required them to annunciate a warning to the pilot akin to garmin's "Steep Turn ahead" to warn you?

[flight2000]

Ken,

My thoughts are that the 60-2 control head may need to be sent to STEC to get re-calibrated if it's that far out of whack.  I can't think of any reasons why it shouldn't do standard rate turns other than calibration.  Mine will do standard rate turns all day, but it's also driven by an ST-901 GPSS with data coming from a GTN-750. 

You're correct on the 60-2 not having built in GPSS.   

Cheers,

Brian

[KLRDMD]

Just now, flight2000 said:

My thoughts are that the 60-2 control head may need to be sent to STEC to get re-calibrated if it's that far out of whack.  I can't think of any reasons why it shouldn't do standard rate turns other than calibration.  Mine will do standard rate turns all day, but it's also driven by an ST-901 GPSS with data coming from a GTN-750. 

I'm looking at upgrading to the 3100 so have no plans to put $$$ into he 60-2 at this time.

[flight2000]

Just now, KLRDMD said:

I'm looking at upgrading to the 3100 so have no plans to put $$$ into he 60-2 at this time.

Don't blame you one bit.  I'm going to wait and see on the 3100.  Maybe when I retire or the 60-2 pukes itself (whichever comes first) and I add the Aspen to the panel. 

Cheers,

Brian

[Marauder]

Well I did some experimenting with my Aspen, STEC 60-2 and the GTN 650. I picked several waypoints that forces the autopilot to make at least 90° turns.

Well, it did a great job of anticipating the turns and did an even better job of flying them.












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[KLRDMD]

In summary:

In the Avidyne where there is a turn greater than 135º, the flight plan will consider the waypoint a "fly over" versus a "fly by" waypoint. That means you will fly to the waypoint and only *after* flying over the waypoint will GPSS start the turn. That's illustrated in the photos of the flight plan: 57AZ-CROME-KAVQ-57AZ. 9 photos.

In the Avidyne where there is a turn less than 135º, the flight plan will be a typical GPSS routing. That's illustrated in the photos of the flight plan: 57AZ-CROME-KMZJ-57AZ. 4 photos. 

I hope all the photos are in the correct order . . .

Special thanks to Ron !

[jetdriven]

That’s a terrible operating system flaw.  

[KLRDMD]

16 minutes ago, jetdriven said:

That’s a terrible operating system flaw.  

As long as you understand it, it can be dealt with. Overall I still think the Avidyne is a superior unit compared to the current generation Garmins.

[jetdriven]

I used to think so until the GtN750’s latest software update with real VNAV. It just boggles me how an IFR navigator will fly over a flight plan leg waypoint and then make a right 270. That will get you a violation quick as it goes. And you have to watch out for it because it will do that if the turn is over 135 degrees.  I’m not a fan of aircraft systems that do things like that 

[kortopates]

I used to think so until the GtN750’s latest software update with real VNAV. It just boggles me how an IFR navigator will fly over a flight plan leg waypoint and then make a right 270. That will get you a violation quick as it goes. And you have to watch out for it because it will do that if the turn is over 135 degrees.  I’m not a fan of aircraft systems that do things like that 

Agreed, and I can't imagine how the FAA folks approved that. It's crazy.


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[Marcopolo]

34 minutes ago, kortopates said:

Agreed, and I can't imagine how the FAA folks approved that. It's crazy.

Hi Paul,

  First let me put it out there that I am not arguing with you about the "craziness" part of this, I get it.  If the GPS commands the turn 2 miles to the inside of the fix or a mile or two outside of the fix is there a difference in the violation threshold?  Maybe the 270* turn in the opposite direction is the issue, not sure.  The answer to all of our questions on this, and how Avidyne got it certified, may be in this document RTCA DO-229 MOPS, but I haven't gotten to the exact answer(s) yet as I'm a slow reader.

 

Ron

[Marcopolo]

  The difference in how Garmin and Avidyne handle these course changes may also have something to do with the two different architectures or formats that these navigators use – the Garmin format and the FMS format, which is the format used by most airlines and also used in the Avidyne IFDs.  Garmin arranges their flight plan in chapters where the FMS uses a continuous waypoint flow.  

 

  Anyone flying "Big Iron" that can comment here on how these >135* course changes get handled by their FMSs?

 

Ron

[jetdriven]

We’ll airways are 4 NM wide so flying up to and over a waypoint on a 135 degree turn puts you outside the airway width even with a proper direction turn. The wrong way turn for sure is going to get you nailed. 

When I first installed GPSS I went out and flew a very convoluted flight plan. The GTn750/GDC31 combo lead every turn and the final issue is the CIIB autopilot won’t bank more than 22 degrees. At groundspeeds higher than 135 knots it turns a little wide. But it always approximates it pretty well.  But this flight plan was 3 90 degree turns just a few miles apart. 

[jetdriven]

2 minutes ago, Marcopolo said:

  The difference in how Garmin and Avidyne handle these course changes may also have something to do with the two different architectures or formats that these navigators use – the Garmin format and the FMS format, which is the format used by most airlines and also used in the Avidyne IFDs.  Garmin arranges their flight plan in chapters where the FMS uses a continuous waypoint flow.  

 

  Anyone flying "Big Iron" that can comment here on how these >135* course changes get handled by their FMSs?

 

Ron

It always anticipates the turn based on the ARINC429 label 121, bank angle. The FMC derives a turn radius based on ground speed and it leads the turn to intercept the new leg. Regardless of turn angle. It will always do this unless the waypoint is marked specifically as a “flyover” waypoint which is rare but they are out there. 

[carusoam]

Adding in a mountain or tower...

Turning the long way and going outside the four NM flyway buffer...

Can lead to a bit of a surprise...

Looking at Ken’s display... it shows some 3D obstacles nearby...

 

Great conversation...

 

I have started re-reading how my ancient KLN90B thinks it will handle this... tight turn challenge...

Best regards,

-a-