Century III and G5 Heading Mode Offset

[jbrrapa]

I have dual G5s coupled to a Century III autopilot. When the autopilot is in heading mode (non GPSS), the heading will be a constant 4 degrees to the left of the heading bug, e.g if the heading bug on the G5 is set to 90 degrees, the G5 will show an actual heading of 86 degrees.

Is there an adjustment so that the heading bug and heading will be the same?

 

[cowman]

I don't know about the adjustment but by any chance is one of those things using true heading while the other one uses magnetic? That came up for me right away when I put in GI-275s.

 

[jbrrapa]

I don't know about the adjustment but by any chance is one of those things using true heading while the other one uses magnetic? That came up for me right away when I put in GI-275s.

They are both using magnetic.

 

[bbarrett]

There is a roll centering adjustment pot on the Century III for this case (it's adjustment B, second from the left). Find a copy of the Century II/III Service Manual; there is a calibration routine that includes aligning flight heading and requested heading.

 

[jbrrapa]

There is a roll centering adjustment pot on the Century III for this case (it's adjustment B, second from the left). Find a copy of the Century II/III Service Manual; there is a calibration routine that includes aligning flight heading and requested heading.

If I make the adjustment you suggest, will it have any affect on the omni, nav or localize modes? Those modes now work properly.

 

[bbarrett]

If I make the adjustment you suggest, will it have any affect on the omni, nav or localize modes? Those modes now work properly.

Yes, it changes the system, but in practice, I think adjusting the roll center change gets nulled out just like a wind correction angle. In other words, I think the AP will still end up flying needle centered. There’s a whole sequence of adjustments to dial in the Century II and III, outlined in the manual, which include steps to adjust the VOR/LOC so it flies needles centered. They aren’t hard, but some require you to be in the air and really require two pilots to do safely.

The AP adjustment needs to be logged, so really needs your A&P on the flight with you, which can be a pain for all the scheduling reasons. But the adjustments are worth it if the autopilot is a bit off.