STEC Autopilot, NAV mode and that pesky Heading Bug

[Mr.T]

So when your STEC autopilot is engaged in NAV or APR mode, what does the Heading Bug on the DG do?

I find that moving the heading bug while in NAV mode definitely causes the plane to change heading toward the bug, but I don't understand why or what the "correct" position for the heading bug is supposed to be.

Logically, it seems to me that the heading bug should be ignored by the autopilot when tracking an OBS (or in GPSS mode). But that doesn't seem to be the case.

What am I missing?

 

[Grum.Man]

Something isn't right with the autopilot then. The heading bug on my Stec 50 does nothing while in approach mode.

 

[Mr.T]

Something isn't right with the autopilot then.

That's what I thought until I flew two different planes -- one with an STEC-60 and one with an STEC-55x and they both act similarly.

The STEC operation manual says to set the Heading Bug and OBS to the and then press the NAV button:

 

[Grum.Man]

That is probably good practice, not a functional requirement. So you are saying if you are in NAV mode heading 360 and set the obs to 090 it will start a turn to the right until reaching 090?

 

[Mr.T]

That is probably good practice, not a functional requirement. So you are saying if you are in NAV mode heading 360 and set the obs to 090 it will start a turn to the right until reaching 090?

No it doesn't turn to 090 but it turns "a little bit" toward the bug - not enough to move the OBS more than one or two dots from center as far as I can tell and maybe it's not doing anything more than just a short-term deviation; I haven't flown for great distances with the AP engaged and the bug intentionally offset to see what it does.

 

[Grum.Man]

No it doesn't turn to 090 but it turns "a little bit" toward the bug - not enough to move the OBS more than one or two dots from center as far as I can tell and maybe it's not doing anything more than just a short-term deviation; I haven't flown for great distances with the AP engaged and the bug intentionally offset to see what it does.

It could be the drag of the bug on the HSI. I know on mine if I move the heading bug it will tug on the cdi card enough to cause an ever so slight deviation.

 

[Let'sgoflying!]

I have experienced the same thing and had the same thoughts; that aint raight.
It was not my a/p so I did not pursue it.
Seems like if in Nav mode it should strictly follow the nav inputs and ignore the bug (unless it is also commanding an intercept on a heading -but usually both hdg and nav are pressed in that case)

 

[John Collins]

On autopilots without an HSI, the heading bug is used to communicate the course information to the autopilot. With an HSI, the course pointer performs this function. So, close your eyes and pretend you are the autopilot and are walking along the intercept course. Assume that the CDI is based on audio and there is a voice saying left or right every few seconds and the sound gets lighter as the needle centers. As you walk along in this simulation with your eyes closed, what direction do you turn to when the sound is very low and indicating you are passing thru center where it switches from left to right.

 

[Mr.T]

On autopilots without an HSI, the heading bug is used to communicate the course information to the autopilot. With an HSI, the course pointer performs this function. So, close your eyes and pretend you are the autopilot and are walking along the intercept course. Assume that the CDI is based on audio and there is a voice saying left or right every few seconds and the sound gets lighter as the needle centers. As you walk along in this simulation with your eyes closed, what direction do you turn to when the sound is very low and indicating you are passing thru center where it switches from left to right.

Ahhh... I think I get it now.

I has just assumed the AP knew (or could figure out out) that, when the needle is centered, it needs to be heading roughly in the direction of the OBS course, plus or minus wind correction and TO/FROM vagaries. But you're saying that the need deflection only tells it to go left or right without any "target" heading to aim for. Re-reading the STEC 55x manual makes a bit more sense now when it says to set the heading bug to the desired course.

I do also notice in the manual that the heading bug (and HSI course pointer) are definitely ignored in GPSS mode since the GPS is (presumably) feeding precise left/right steering commands to the autopilot and managing wind drift correction, etc.

 

[John Collins]

Ahhh... I think I get it now.

I has just assumed the AP knew (or could figure out out) that, when the needle is centered, it needs to be heading roughly in the direction of the OBS course, plus or minus wind correction and TO/FROM vagaries. But you're saying that the need deflection only tells it to go left or right without any "target" heading to aim for. Re-reading the STEC 55x manual makes a bit more sense now when it says to set the heading bug to the desired course.

I do also notice in the manual that the heading bug (and HSI course pointer) are definitely ignored in GPSS mode since the GPS is (presumably) feeding precise left/right steering commands to the autopilot and managing wind drift correction, etc.

GPSS uses the heading error signal that is normally generated by the DG or HSI and is a voltage proportional to the difference between the current heading and the where the heading bug is set to. When in heading mode on the GPSS converter, the heading error signal is passed thru to the autopilot. The autopilot commands a bank angle that corresponds to the difference in the bug and heading, so a 10 degree difference causes the autopilot to command a 10 degree bank towards the bug, a 15 degree difference commands a 15 degree bank and so on up to the maximum bank or turn rate supported by the autopilot. When the GPSS is used to select GPS for steering, the GPS generates a precise bank angle for the autopilot to fly and the converter creates a voltage that acts like the heading error signal. So it sorta works like the GPS has a little electronic leprechaun that twists the heading bug to get the autopilot to fly what the GPS wants it to. GPSS aka roll steering is not governed by CDI deviations and this is what allows the GPS to anticipate turns and fly curved paths and other paths such as DME arcs, hold entries, holds, and procedure turns which have no CDI guidance.

 

[MauleSkinner]

Ahhh... I think I get it now.

I has just assumed the AP knew (or could figure out out) that, when the needle is centered, it needs to be heading roughly in the direction of the OBS course, plus or minus wind correction and TO/FROM vagaries. But you're saying that the need deflection only tells it to go left or right without any "target" heading to aim for. Re-reading the STEC 55x manual makes a bit more sense now when it says to set the heading bug to the desired course.

Put another way, autopilot “thinks” the heading bug is a course needle when in NAV mode.

Once it got stabilized on course, it “thought” it was correcting for a huge crosswind.