G5 HSI / Century 41 / gtn750 RNAV Approach

[haymak3r]

Hey everybody!

Yesterday, I tried my first RNAV approach with the new setup. My instructor and I were a bit concerned with how this would work for vertical guidance, so wanted to test in vfr conditions of course..

We flew in GPSS from the airport out to a VOR and then to an IAF. Activated the approach with the transition from that IAF still in GPSS mode. AP flew everything as expected, up to when the GS came alive. 

 

From my research here, I think I may know the mistake I made, but wanted to see if there was anybody else with this setup and can confirm. 

But, we left ap in hdg mode, with the g5 in gpss. As we turned inbound to the FAF, I don't believe we switched the AP to APR mode. We were a bit high prior to the FAF, and so GS came in a bit early, but as we slightly expected, AP did not follow the Glidepath. After passing the FAF, we did try tapping the APR button, which the AP DID go to Apr, with GS activated. But AP did not start descending. We let it go until I was one dot above the path. At that point I hand flew it the rest of the way.

 

So did I mess up and not go to APR mode prior to the FAF? Or does the g5 hsi not have the capability to provide vertical guidance to the AP. Of course ils/loc works just fine when the gtn is in vloc mode. 

 

Lastly, I have been searching for a while, but couldn't find a thread here or elsewhere that asks this specific question, with my setup. 

 

Thanks in advance!!

Chris

[stevesm20b]

I had the same problem when doing a LPV approach. I found you have to go from HDG to approach before you get to FAF in order for the autopilot to capture the glide slope.

[Jake@BevanAviation]

The G5 via the GAD29B can only provide heading or course errors to legacy autopilot systems. The vertical and lateral deflection (analog) must come from the primary navigator to the flight computer. When trying to couple the ILS or GPS approach the A/P system must be in approach mode with full up deflection on the vertical. The system has an anticipation circuit for the G/S capture that will monitor the closure rate of the vertical deviation signal and anticipate the capture. You can use GPSS to navigate to the fix, but you must turn it off and activate approach mode if you want to capture the vertical.

 

[haymak3r]

Ok, We'll test this again next time we go up. Hopefully Friday, if weather permits...

We had a feeling, that this may be what is happening. But glad to know that it *should* be possible.

 

Thanks all! I'll report back once I test this further. 

[haymak3r]

I wanted to follow up on my test approach. I can confirm that switching from gpss to approach mode prior to the FAF is the correct method with my setup. Once  in APR mode, I saw GS on the AP annunciator, and once on the GP, the plane started descending. I stayed under the hood all the way to mins in a nice stable approach even with a slight cross wind. 

I have taken note of this new procedure. 

Cheers!

[N231BN]

FWIW, a Century 41 will capture from above via pitch mode.