Aspen ST-50 only turns half standard rate turns

[sailon]

STUMPED!  In my F, I have the Aspen PRO 1000 glass, STEC 50 autopilot, and the GTN 650.  All turns associated with holding patterns, and RNAV approaches are only made at half of the standard rate, NOT full rate.  I have found discussions about this in some other forums, but I have yet to find the magic setting that determines the rate of turn.  Anyone else have this issue, and what solved it?  Gain setting on the Aspen? Defecting turn coordinator?  Setting on the STEC?  

 

Thanks

[Cris]

6 hours ago, sailon said:

STUMPED!  In my F, I have the Aspen PRO 1000 glass, STEC 50 autopilot, and the GTN 650.  All turns associated with holding patterns, and RNAV approaches are only made at half of the standard rate, NOT full rate.  I have found discussions about this in some other forums, but I have yet to find the magic setting that determines the rate of turn.  Anyone else have this issue, and what solved it?  Gain setting on the Aspen? Defecting turn coordinator?  Setting on the STEC?  

 

Thanks

This autopilot is limited to 90% of standard rate. There is no pilot adjustment so it will need to go to the dealer if you are not satisfied with the turn rate. However before you do so try to see how it works in different scenarios. As an example use the left/right roll command to determine if it is giving the same rate indication as approaching a course for GPS/VOR/LOC. Use the high setting for GPS/LOC. I think Heading on the St50  and GPSS on the Aspen might also work. Remember it will use a more shallow bank angle depending on what it needs to intercept. Basically it will not intercept unless you have it within 10% of the course before you select the mode except when I GPSS mode. Finally standard rate depends on your airspeed. Check the accuracy of your Turn Coordinator by:

Take the first two digits of your airspeed (or just the first digit if you are under 100kts) and add 5 to it. That is your bank angle to get a std rate turn.

120 kts, add 12+5=17 degrees of bank gives you a standard rate turn. Stec will only go to 90% or 15 degrees in this example. Don’t confuse your attitude Indicator at 15 degrees in this example with the assumption it is the standard rate shown on the Turn Coordinator. 

90 kts, add 9+5=14 degrees of bank gives you a standard rate turn. 

Compare that to the Turn Coordinator. Last but not least you can stay on the magenta line if you set your GPS to one mile instead of the standard 3 mile enroute so it wont get to far off course before adjusting. 

Edited September 19, 2018 by Cris

[sailon]

Thanks Cris.  I supplied some incomplete data.  The autopilot / Aspen is in GNSS mode, navigation mode is RNAV.  Where is the "high setting" that you are referring to?  Lastly, the turn coordinator is lining up with the Aspen and confirming that all of the turns were made at half rate.  When I disconnect the autopilot and hand fly the turns, the bank angle doubles.  Totally understand what it needs to intercept.  Fuller disclosure, the hold and approach were at totally different points.  The approach was an RNAV with "T" waypoints.  The entry to the IAF, and the 90 degree turn to line with the FAF were both at half rate.  I had to disconnect autopilot, turn airplane by manually, then reengage GNSS after turn to follow approach.  My Century III in my old Arrow did a better job of flying RNAV approaches!  Like they say, somethin' ain't right.

[Marauder]

Thanks Cris.  I supplied some incomplete data.  The autopilot / Aspen is in GNSS mode, navigation mode is RNAV.  Where is the "high setting" that you are referring to?  Lastly, the turn coordinator is lining up with the Aspen and confirming that all of the turns were made at half rate.  When I disconnect the autopilot and hand fly the turns, the bank angle doubles.  Totally understand what it needs to intercept.  Fuller disclosure, the hold and approach were at totally different points.  The approach was an RNAV with "T" waypoints.  The entry to the IAF, and the 90 degree turn to line with the FAF were both at half rate.  I had to disconnect autopilot, turn airplane by manually, then reengage GNSS after turn to follow approach.  My Century III in my old Arrow did a better job of flying RNAV approaches!  Like they say, somethin' ain't right.

The high setting he is referring to is the approach mode. The APR button.

Below is an LPV I flew with my STEC 60-2. You will see that in GPSS mode it will fly with less than standard rate. The STEC in combination with the Aspen GPSS will fly different turn rates based on your ground speed and your intercept angle. It will also limit itself if you are approaching at an intercept angle greater than 45°. The manual doesn’t do a great job of explaining this.

If you are flying in heading mode, do a turn to either direction and note the turn rate. Using Cris’s math above, you can confirm that it is turning at the proper rate.

Is the GPSS new to the plane? It took me a while to figure out how to use it correctly with the autopilot.

If you look at this video, at the 7 minute mark it shows me attempting to do an 80° or so intercept. He doesn’t show the full turn but you will see that it will fly through the final approach course a bit. This is the 45° limit.

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[sailon]

Marauder, Thanks so much for the videos, they will really help.  I am looking forward to reviewing them and comparing with what my airplane does.  The GNSS came with the airplane.  I have over 75 hrs in it and I am still trying to figure out how to use the autopilot with the airplane.  Agree that the documentation is terrible.  Nice setup.  Same as mine except for the STEC 50 and I do not have the MFD.  Which GPS are you using?  I have the GTN 650.

 

Art

[Marauder]

Marauder, Thanks so much for the videos, they will really help.  I am looking forward to reviewing them and comparing with what my airplane does.  The GNSS came with the airplane.  I have over 75 hrs in it and I am still trying to figure out how to use the autopilot with the airplane.  Agree that the documentation is terrible.  Nice setup.  Same as mine except for the STEC 50 and I do not have the MFD.  Which GPS are you using?  I have the GTN 650.
 
Art

Art - I have a 650 as well. The STEC was designed well before GPS and GPSS roll steering was available. Hence the reason for the poorly written manual. I think I have a current one and it still doesn't address the usage of GPSS.

Fly with it some more and let us know what you find out.


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[Cris]

If you Google you will find the Stec ST 901 Manual. This is the info page for the T approach you described. From your description it seems that the issue you are experiencing is that the S-Tec is not intercepting the various segments in a timely manner. I think you believe that it needs to have a greater bank angle to resolve the issue. However typically the IAF’s are not fly over points. Therefore the unit should calculate and turn prior to reaching the IAF so that it smoothly ends on the next segment. If this is not occurring and you are using the unit in accordance with the manual I’d suspect it needs adjustment. Best course of action is to take it to an Stec dealer who regularly works on this autopilot. It could be a simple cable adjustment or reracking the control head etc. The servo might also be drawing high starting voltage which would slow its startup and cause it to fly thru the course. These autopilots are relatively bullet proof so should not be that costly to diagnose and repair. 

3-6 5th Ed.  Feb 15, 08

S–TEC

Fig. 3-2.  GPS-T Approach

1. a. Set the Heading Bug under the Lubber Line.

b. Engage the GPSS Converter's HDG mode.

c. Engage the autopilot's HDG mode.

d. Program the GPS Navigator with the desired approach.

e. Engage the GPSS Converter's GPSS mode.

f. The autopilot begins tracking to the Initial Approach Fix (IAF).

2. a. The autopilot intercepts the IAF, then makes a 90° turn, and begins tracking to the Final Approach Fix (FAF).

3. a. The autopilot becomes established on the inbound course.

b. At the Missed Approach Point (MAP), disconnect the autopilot for a landing or go-around, as required.

Edited September 21, 2018 by Cris

[sailon]

Ok starting to think something is out of whack.  Here is what I noticed on tonight's flight:

1.  in heading mode, when I enter course change of 90 degrees or greater, left turns are nearly at the standard rate.  Right turns are a bit less.  

2.  GPSS mode enroute I note that the autopilot stabilizes to the left of "the magenta line."

3  90 degree  left turn to the approach course was dead on.  This was a left turn, turns described in earlier post the hold and approaches were right hand turns

 

Whadda you think?  Servo shot?  Autopilot out of adjustment?  Note that the servo is a single unit on the pilot side aileron.  

 

[Marauder]

Ok starting to think something is out of whack.  Here is what I noticed on tonight's flight:
1.  in heading mode, when I enter course change of 90 degrees or greater, left turns are nearly at the standard rate.  Right turns are a bit less.  
2.  GPSS mode enroute I note that the autopilot stabilizes to the left of "the magenta line."
3  90 degree  left turn to the approach course was dead on.  This was a left turn, turns described in earlier post the hold and approaches were right hand turns
 
Whadda you think?  Servo shot?  Autopilot out of adjustment?  Note that the servo is a single unit on the pilot side aileron.  
 

I would love to see this on video, including what is showing on the Aspen and controller.


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[Andy95W]

4 hours ago, sailon said:

Ok starting to think something is out of whack.  Here is what I noticed on tonight's flight:

1.  in heading mode, when I enter course change of 90 degrees or greater, left turns are nearly at the standard rate.  Right turns are a bit less.  

2.  GPSS mode enroute I note that the autopilot stabilizes to the left of "the magenta line."

3  90 degree  left turn to the approach course was dead on.  This was a left turn, turns described in earlier post the hold and approaches were right hand turns

 

Whadda you think?  Servo shot?  Autopilot out of adjustment?  Note that the servo is a single unit on the pilot side aileron.  

 

How does the airplane fly without the autopilot on, in the same scenario?  

Your description is very similar to my autopilot before I re-rigged it to fly straight.

My guess is your airplane has a slight left hand turning tendency, which explains why it turns left normally and to the right "a bit less".

At approach speeds, at a slightly higher angle of attack, the left turning is compounded by normal aerodynamic left hand turning tendencies.

Try nudging a bit of right rudder in the right turns.  I bet it'll fix most of the difference.  And your rigging may need adjusted if the plane doesn't fly straight on its own.

Good luck, please post when you get it working right.

[sailon]

Flies straight as an arrow hands off with perfect coordination.  BUT speaking of rigging, I am starting to wonder if something is amiss or loosened up in the rigging of the autopilot.  Next time I am up, I'll take some photos or videos.  What is also interesting is that enroute while it flies to the left of the rhumb line, the actual track and desired track values line up.  Now that it is starting to cool off in the hangar, I may also take the inspection plate and take a look at the autopilot servo rigging.

 

Will keep all posted.

 

[Cris]

Cable adjustment of the roll servo is my best guess. Go to the Stec dealer for correct adjustment. Flying to the left of the line is a simple adjustment on the panal controller. 

Edited September 26, 2018 by Cris
Added adjustment