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Following the magenta line
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cavu View Drop Down
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    Posted: 12 Apr 2016 at 9:43pm
I have the 540, an STEC 30 and GPSS.

With holds the airplane does not overfly the magenta line.  It seems to be independent of winds as it overshoots the upwind and downwind legs the same. Winds were 15-20kts.  The autopilot initiates a beautiful standard rate turn but doesn't initiate in time to stay on the magenta line.  It comes back in to re-intersect the inbound and outbound legs.

I'd say 1/4 - 1/2 mile deviation from the magenta line at times.

Is this normal behavior with GPSS?
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oskrypuch View Drop Down
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What airspeed are you flying in the hold?

* Orest



Edited by oskrypuch - 12 Apr 2016 at 10:19pm
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AzAv8r View Drop Down
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GPSS adds dynamic heading inputs in addition to CDI and rate inputs to the autopilot, which in principle provide a means to turn before you've past a waypoint.  Is the GPSS new?  Perhaps the heading input is not configured correctly.  With no heading input, CDI error must integrate (accumulate) to correct the course.  At least that is how it would work with my Navomatic and Aspen GPSS.  I believe STECs are at least functionally similar, although perhaps a tad more modern in implementation.
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rpostmo View Drop Down
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Make sure your STEC 30 is in "heading" mode and not one of the track modes.
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cavu View Drop Down
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Points: 152 		Post Options Post Options   Thanks (0) Thanks(0)   Quote cavu Quote  Post ReplyReply Direct Link To This Post Posted: 13 Apr 2016 at 10:27am

130 Kts but the 540 adjusts hold pattern size based on ground speed.

GPSS is new, its an STEC brand.  Don't know what you mean by "heading input not configured correctly".  The GPSS simply converts the deviation from the magenta line as + and - signals just like deviations in heading from the heading bug.  GPSS actually doesn't receive "heading (ie 270 degrees) information.  It tracks legs fine and anticipates turns at fixes.

Stec-30 is in heading mode, with this Stec GPSS unit, even if you are in another mode, it toggles heading mode when you select GPSS.  I suppose I could override it but have never tried.
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rpostmo View Drop Down
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You must have a newer version than mine.  
If I have the STEC 30 in either Hi or Lo track modes when activating
the GPSS (STEC brand), my Comanche will wallow back and forth over the
Magenta line, and will not capture for an appropriate turn.   My STEC 30
does NOT auto toggle to HDG mode when I activate GPSS, I must do it
manually.  It then tracks and turns very accurately once both are set to HDG.
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ddgates View Drop Down
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Originally posted by cavu cavu wrote:

130 Kts but the 540 adjusts hold pattern size based on ground speed.

GPSS is new, its an STEC brand.  Don't know what you mean by "heading input not configured correctly".  The GPSS simply converts the deviation from the magenta line as + and - signals just like deviations in heading from the heading bug.  GPSS actually doesn't receive "heading (ie 270 degrees) information.  It tracks legs fine and anticipates turns at fixes.

Stec-30 is in heading mode, with this Stec GPSS unit, even if you are in another mode, it toggles heading mode when you select GPSS.  I suppose I could override it but have never tried.


GPSS really isn't new.  Been available for at least 10 years, likely more.  (I installed an ICARUS GPSS converter in 2006.

The navigator instead of left/right error outputs computed bank angles; the box (Aspen, STEC 31, others) convert that bank angle command to a left right error command which the autopilot can use in HDG mode (substitutes the converted bank angle to error command for the heading error datum).

I don't know of any autopilot which automatically switches, not sure it could, basically fooling the autopilot to thinking the GPSS datum is the heading error datum).

David
David Gates
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MysticCobra View Drop Down
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Originally posted by ddgates ddgates wrote:

Originally posted by cavu cavu wrote:

GPSS is new, its an STEC brand.

GPSS really isn't new.  Been available for at least 10 years, likely more.  (I installed an ICARUS GPSS converter in 2006.
I think he simply meant the GPSS feature was newly-added to his plane, not that he thought he was introducing us all to a new technology.


Edited by MysticCobra - 16 Apr 2016 at 10:46am
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cavu View Drop Down
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Back to my question.  Should the airplane follow the magenta line of the hold tightly or is there something wrong that it doesn't.

Lets assume the heading mode is active and GPSS is selected.
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FEisert View Drop Down
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I have flown several airplanes with other navigators and GPSS, seems pretty common in the hold for them not to follow the magenta line very tightly. I do not have GPSS in my airplane with the IFD540, so I have no data points there.
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ddgates View Drop Down
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Points: 1100 		Post Options Post Options   Thanks (0) Thanks(0)   Quote ddgates Quote  Post ReplyReply Direct Link To This Post Posted: 19 Apr 2016 at 10:21pm
Originally posted by cavu cavu wrote:

Back to my question.  Should the airplane follow the magenta line of the hold tightly or is there something wrong that it doesn't.

Lets assume the heading mode is active and GPSS is selected.


Can't speak for others but my setup operating under GPSS follows the magenta line spot on ; except holds, where it wanders around the hold oval a bit, which seems to some degree airspeed dependent.
David Gates
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DavidBunin View Drop Down
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Originally posted by ddgates ddgates wrote:

my setup operating under GPSS follows the magenta line spot on ; except holds, where it wanders around the hold oval a bit,


I thought this whole conversation was about flying a hold?

David Bunin
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ddgates View Drop Down
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Points: 1100 		Post Options Post Options   Thanks (0) Thanks(0)   Quote ddgates Quote  Post ReplyReply Direct Link To This Post Posted: 22 Apr 2016 at 3:44pm

David:

My point was that my GPSS and autopilot follow is spot on; thus the hold behavior seems intrinsic to the hold.
David Gates
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