Using NAV mode for VOR navigation in Cessna 172

[737Andi 6]

Hi, 

in this video beginning from 10:20 Philipp is explaining how to use the autopilot NAV function to automatically fly to a VOR on a desired radial. The plane is turning left and then right onto the desired radial automatically :)
 

I have tried this successfully on the XPlane standard Cessna 172 but its unfortunately not working for me on the A2A cessna under P3D :(

I have attached a picture of a quite similar situation under P3D where its not working when I press NAV. I'm pressing NAV out of the roll mode. It says NAV armed but the plane is not turning left to fly in direction of the 10° radial.

https://imgur.com/a/EGV8DQ7

What am I doing wrong?

Thanks!

Best regards Andi

Edited October 28, 2019 by 737Andi

[ryanbatc 12,798]

Autopilots vary from aircraft to aircraft.  I suggest first tuning the course (IE turn the OBS knob to make the white line center up the middle)

Then turn on NAV mode and the autopilot should go direct.  You may even need to turn your aircraft's heading bug to the same course to allow the AP to intercept a direct course.

[737Andi 6]

Thx for your answer. I will try that!

I find it rather strange that when I try to reproduce exactly the same scenario as shown in the video I'm successful when using the STec autopilot and I'm unsuccessful with the Kap 140. I think it is either a question of me not correctly activating the nav mode and/or handling the Kap 140 autopilot or that the A2A aircraft is closer to reality. 

Can anyone confirm that the scenario in the video can be flown with a Kap 140 in A2As cessna?

Edited October 28, 2019 by 737Andi

[Bert Pieke 7,584]

Not all autopilots are the same..

[ark 1,288]

18 hours ago, 737Andi said:

have attached a picture of a quite similar situation under P3D where its not working when I press NAV. I'm pressing NAV out of the roll mode. It says NAV armed but the plane is not turning left to fly in direction of the 10° radial.

I rarely fly the 172 and am not particularly familiar with the KAP 140 AP, but I asked my friend Mr Google for help (downloaded the manual) and tried a few flight experiments. What I came up with is this (corrections welcome):

When you are in ROL mode and press NAV, the AP assumes you have established an appropriate (i.e., workable) radial intercept angle ( typically 45 degs or less ) with the current a/c heading. From the GTN750 display in the picture you posted, you can see that if you push NAV when in ROL mode with the current a/c heading, the a/c will not intercept the desired radial.

However, if you are in HDG mode when you press NAV, the AP will turn the a/c to set up a fixed intercept angle of 45 degs.

So the 'advantage' ( if we want to call it that ) of ROL mode is you can pick your own NAV intercept angle. For example, if the a/c is already almost on the desired radial, an intercept angle of 20 degs may be more appropriate then 45 degs. Or the situation may call for an intercept angle greater than 45 degs, etc.

Of course, this all assumes that when you press NAV and HDG blinks on the AP, you set the DG heading bug to match the OBS course you want to intercept (10 degs in your example).

As an aside, note you also need to periodically reset the DG (which drifts) to the heading indicated by the magnetic compass when the a/c is in straight and level flight.

Al

 

Edited October 29, 2019 by ark

[737Andi 6]

Its working perfectly this way! Thanks a lot! 🙂

[ark 1,288]

1 hour ago, 737Andi said:

Its working perfectly this way! Thanks a lot! 🙂

Good -- glad to hear you got the AP working.

Al

Edited October 29, 2019 by ark