A/P will not switch to HDG mode when internal GPS is active

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smbohn
Posts: 7
Joined: Fri Aug 04, 2017 9:35 pm

A/P will not switch to HDG mode when internal GPS is active

Post by smbohn »

Testing out my newly installed GRT Autopilot with Horizon HX EFIS (V9 software.) GNAV works flawlessly (both on the ground and in the air.)

The problem I have is this: I was ground testing yesterday and could not get the HDG mode engaged. When I activate the A/P in HDG mode, it appears to briefly go in to HDG mode and then immediately switch to GNAV or ENAV. Every time.

Any ideas?

Steve
N44VF
Velocity XLFG
smbohn
Posts: 7
Joined: Fri Aug 04, 2017 9:35 pm

Re: A/P will not switch to HDG mode when internal GPS is act

Post by smbohn »

After additional testing, I was able to determine that I can't select HDG mode when GPS2 (the internal GPS) is active.

After powering up the EFIS, the A/P will be in HDG mode until the internal GPS activates. At that point it will switch to ENAV mode and stay there.

Here is a link to a video of the situation:
https://www.dropbox.com/s/da21rrifz1x5w ... n.mp4?dl=0

A little more background on the EFIS set up: The panel has three EFIS. From left to right, they are labelled DU2, DU1, DU3. The A/P servos are connected to Serial 8 of DU2 (the left most unit) and only to that unit.

Some operational aspects of the system seem strange:

1. DU1 and DU3 can display the A/P menu, however they can't engage the A/P. Engaging the A/P can only be done from DU2.
2. When invoking the A/P from the soft keys, the "LAT A/P" softkey is only displayed on DU1. DU2 and DU3 only show the "VERT A/P" softkey. See this video: https://www.dropbox.com/s/u798e1nr9os4n ... y.mp4?dl=0
GRT_Jeff
Posts: 802
Joined: Tue Dec 11, 2012 12:11 am

Re: A/P will not switch to HDG mode when internal GPS is act

Post by GRT_Jeff »

Check the Analog # Function settings in SET MENU, General Setup, after the serial ports. The screen that's missing the LAT A/P button probably has an analog function set to "Ext A/P HDG Mode Select (direct)". Change it to Off.
smbohn
Posts: 7
Joined: Fri Aug 04, 2017 9:35 pm

Re: A/P will not switch to HDG mode when internal GPS is act

Post by smbohn »

Hi Jeff... I finally got to the hangar this past weekend and you were spot on.

According to the wiring diagrams (which may be a bit dated) the OBS Annunciation (P1001-14) from the GTN 650 is wired in to Analog 4 (B-18.) When I changed the function to 'Off', I was able to engage in HDG mode.
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