KAP 150 on NAV with no GPSS turns on its own

[Oldguy]

Had some time to kill Saturday morning, so my CFI and I went to shoot the RNAV 3 approach at the home drome. Filed a round robin from KPLR to KYLEE to KPLR and launched. Beautiful day on top with about a 1,500' smooth layer starting at 3,000'. Out to KYLEE it is a 187 heading and then a 14 heading back to the IAF at HANUR. As I am approaching KYLEE, the 430 starts its countdown, which I am watching, to turn right to 14. I have been talking with Birmingham  approach and am going to pass KYLEE and turn back so it will not be such a tight turn, when all of a sudden the KAP 150 starts a standard rate turn to the right. Nothing has moved on the HSI. The 430 has sequenced to the KYLEE to HANUR segment, but I don't have GPSS and was not expecting to start my turn for another 30-45 seconds. Was mentally set up to start the turn and everything went fine on the approach, but it was surprising. The KAP 150 was in NAV mode and not in Approach mode, but the approach was loaded into the 430.

Is there something I am not understanding in my autopilot? I had always been under the impression if I set it on NAV, I have to set the course pointer to the desired heading and it will hold it until changed. The pilot guide does state it will "bank as necessary to maintain course", but I was under the impression it was only to maintain the course set by the course pointer. Can anyone point me in the right direction or explain what I am missing? I would appreciate it.

John

[N201MKTurbo]

As soon as the nav outputs an error signal the autopilot will start to correct the error. There will be a heading error and a nav error. If you let it go, it would settle somewhere between the selected heading and the new course your nav was trying to get you to. If you want it to track the new course accurately you will have to change the heading pointer.

Edited September 28, 2015 by N201MKTurbo

[Oldguy]

As soon as the nav outputs an error signal the autopilot will start to correct the error. There will be a heading error and a nav error. If you let it go, it would settle somewhere between the selected heading and the new course your nav was trying to get you to. If you want it to track the new course accurately you will have to change the heading pointer.

Thanks. Knew it was probably something I didn't know. Is that knowledge gained from experience or a manual somewhere?

[N201MKTurbo]

I'm an engineer, and I like to study how things work.

[carusoam]

I'm an engineer, and my KLN90B will keep flying an extended course beyond my waypoint.  A small flashing light on the IP will tell you to begin the turn prior to reaching the waypoint.

I would need to get a GPSS or an Aspen that includes the GPSS skills.

My favorite MS used radio supplier has a GPSS unit for sale, $800(?), but it is not update specific to my BK HSI.

The ability to change headings and altitudes with automatic precision would be really helpful.

Check with Alan to see if that GPSS would be helpful to your situation.

Best regards,

-a-

Edited September 29, 2015 by carusoam

[Cruiser]

In NAV mode I believe the autopilot follows the signal from the GPS computer 

with the approach loaded, the 430W would signal the autopilot to turn inbound when you crossed the waypoint. It would then try to fly to the next waypoint in the GPS. Your HSI only indicates the lateral deviation from the "magenta line" regardless where it is pointing on the display unlike the HDG indicator that will make the plane point in whatever direction you turn the knob (indicator)

[gsengle]

In NAV mode I believe the autopilot follows the signal from the GPS computer 

with the approach loaded, the 430W would signal the autopilot to turn inbound when you crossed the waypoint. It would then try to fly to the next waypoint in the GPS. Your HSI only indicates the lateral deviation from the "magenta line" regardless where it is pointing on the display unlike the HDG indicator that will make the plane point in whatever direction you turn the knob (indicator)

Without a separate roll steering unit, this is not true

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[N201MKTurbo]

A standard attitude based autopilot will sum the roll error from the attitude indicator, the heading error from the heading bug and the nav error from the radio or GPS. The roll error will try to keep the wings level. Without it a heading or nav error could roll the plane inverted. The heading error will try to keep the plane on the heading of the bug. Without it a nav error could make the plane fly in circles. 

GPSS bypasses the heading error input to the summing amplifier. The GPS computer is smart enough to calculate the intercept course back to the flight planned course. It can constantly calculate the heading of the plane so it doesn't need the heading error input to keep it from flying in circles. The GPSS will just give a roll error proportional to the angular difference between  the desired heading and the current heading.

[Marauder]

Just to show an example of what happens to an STEC autopilot that is not receiving a valid GPSS roll signal. The autopilot in this video was setup to receive a GPSS signal to navigate to another airport.

My Aspen installation has a throw over switch which allows me to switch Nav signals from either Aspen to the autopilot. This is used in the reversion mode (if the primary PFD failed, I can make the MFD function as a PFD). The switch was stuck and the GPSS roll steering commands were not getting to the autopilot. When this happens, the fault mode would for the autopilot is for it to fly wings level at the fault point. Even though the autopilot was engaged sequence wise correctly, it was not receiving a valid Nav signal leading to the wings level mode.

It is important to understand the failure modes of your autopilot, otherwise it could fly you into the side of a hill if you aren't monitoring it. In my installation, the cue was the missing flight director cue bars.

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[Oscar Avalle]

Just to show an example of what happens to an STEC autopilot that is not receiving a valid GPSS roll signal. The autopilot in this video was setup to receive a GPSS signal to navigate to another airport.

 

My Aspen installation has a throw over switch which allows me to switch Nav signals from either Aspen to the autopilot. This is used in the reversion mode (if the primary PFD failed, I can make the MFD function as a PFD). The switch was stuck and the GPSS roll steering commands were not getting to the autopilot. When this happens, the fault mode would for the autopilot is for it to fly wings level at the fault point. Even though the autopilot was engaged sequence wise correctly, it was not receiving a valid Nav signal leading to the wings level mode.

 

It is important to understand the failure modes of your autopilot, otherwise it could fly you into the side of a hill if you aren't monitoring it. In my installation, the cue was the missing flight director cue bars.

 

 

 

 

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Thank you for clarifying this. I was wondering what the difference between the NAV mode and the GPSS mode on my STEC was. Normally I use the GPSS mode on my ASPEN when I was navigating based on the inputs from my GPS 750. When I switched over to LOC or VOR I used NAV mode. But once I used the NAV mode with my GPS and it worked fine. But obviously I should not rely on NAV mode when the autopilot is receiving inputs from a GPS because of what you guys were mentioning.

 

 

[Marauder]

Thank you for clarifying this. I was wondering what the difference between the NAV mode and the GPSS mode on my STEC was. Normally I use the GPSS mode on my ASPEN when I was navigating based on the inputs from my GPS 750. When I switched over to LOC or VOR I used NAV mode. But once I used the NAV mode with my GPS and it worked fine. But obviously I should not rely on NAV mode when the autopilot is receiving inputs from a GPS because of what you guys were mentioning.

 

 

When I am navigating enroute and before an approach, I have the GPSS engaged on the Aspen and the STEC is set to HDG and ALT. When I reach the approach, I activate the NAV button on the STEC. This refines the sensitivity for the approach and also engages the vertical guidance (glide slope) if the approach has one.

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[Cruiser]

snip........

Is there something I am not understanding in my autopilot? I had always been under the impression if I set it on NAV, I have to set the course pointer to the desired heading and it will hold it until changed. The pilot guide does state it will "bank as necessary to maintain course", but I was under the impression it was only to maintain the course set by the course pointer. Can anyone point me in the right direction or explain what I am missing? I would appreciate it.

John

Perhaps. According to the KCS 55A manual (I assume this is your system)

"6. For normal navigation, set the NAV receiver to the desired VOR or Vortac station and the red navigation flag (NAV) will disappear from view if a usable signal is being received.

7. Rotate the Course Select knob to position the Selected Course Pointer to the desired VOR course.

8. The VOR Deviation Bar represents the selected course and the relationship of this bar to the symbolic aircraft in the center of the instrument visually presents the actual relationship of the selected course to your aircraft position. (in other words, if the symbolic aircraft on the display indicate approaching the deviation bar at 45”, that is the angle at which your aircraft is actually approaching the selected course." 

In the example provide by the KCS 55A manual the airplane was flying on a 060 heading and the "selected Course Pointer" (the yellow arrow with the split center deviation bar) was set to 110° (a 50 degree angle to the right of the display, i.e. it was not pointing directly ahead) This arrangement allows the pilot to "pictorally" see the position of the airplane represented by the fixed airplane symbol in the center of the HSI in relation to the desired course. the course deviation bar was above the airplane indicating the plane was flying toward the desired course (060) and the desired course was indicated by the arrow head of the "course pointer" (110).

As the airplane approaches the desired course the deviation bar gets closer and closer to the center, lining up with the tail and the pointer when "on course" The airplane would then start turning to the right and maintain the desired course of 110° inbound. Notice that the 'heading bug' was never moved or set and did nothing to control the flight of the airplane. ....see the KCS 55A manual for a more detailed description......

Edited September 29, 2015 by Cruiser

[Oldguy]

 

Perhaps. According to the KCS 55A manual (I assume this is your system)

In the example provide by the KCS 55A manual the airplane was flying on a 060 heading and the "selected Course Pointer" (the yellow arrow with the split center deviation bar) was set to 110° (a 50 degree angle to the right of the display, i.e. it was not pointing directly ahead) This arrangement allows the pilot to "pictorally" see the position of the airplane represented by the fixed airplane symbol in the center of the HSI in relation to the desired course. the course deviation bar was above the airplane indicating the plane was flying toward the desired course (060) and the desired course was indicated by the arrow head of the "course pointer" (110).

As the airplane approaches the desired course the deviation bar gets closer and closer to the center, lining up with the tail and the pointer when "on course" The airplane would then start turning to the right and maintain the desired course of 110° inbound. Notice that the 'heading bug' was never moved or set and did nothing to control the flight of the airplane. ....see the KCS 55A manual for a more detailed description......

Tom,

You are correct; I do have the KCS 55A and what you describe is exactly what I was expecting. To use your example to expand on my question (not my actual headings, but for discussion's sake), I was on the 110 heading and had been for several minutes. Without moving the "Course Pointer", the AP commenced a right standard rate turn towards the heading of the new segment on the 430. What N201MKTurbo (sorry, don't have everyone's names straight yet) posted seems to be what happened in my case. Seems as if the NAV error and the heading error were attempting to resolve themselves through the AP by making the right turn. I may go up again and run the same flight path in VMC but let the AP make the turn and see where it settles without any input from me.

Thanks for the explanation. As a computer guy, I like knowing the "why" of something.

John