Autopilot NAV or APPR mode?

[Rocky 128]

Please excuse me if the question has been asked before.

When autopilot is engaged in NAV mode while the navigation is the GPS, it works perfect. The aircraft flies the GPS route, this is good. Now if you are in NAV mode while the CDI is set to VOR1, which means the navigation source is the VOR1 and not the GPS, I noticed the aircraft doesn't fly in the expected direction.

Let's say I am flying heading 240 with the VOR1 needle centered on 240 and the TO indicator, it means I am flying to the VOR station on radial 240. Engaging the NAV mode at this time should maintain the aircraft on the 240° radial, with a heading of roughly 240°, correcting potential cross-wind deviation. Instead of this, the autopilot commands a hard turn as soon as the NAV mode is engaged.

I noticed that if I engage the APPR mode instead, it looks like working fine. The autopilot maintains the VOR radial as expected.

So my question is: to keep the aircraft stable on a VOR radial, should I use NAV or APPR mode?
In other words, am I doing wrong or is it a bug of the sim?

Before MSFS, with FSX, I used to use the NAV mode for this and it was working fine. The APPR was used to manage the lateral and vertical guidance during approach, especially with localizer and glideslope guidance.

[leprechaunlive 1,852]

2 minutes ago, Rocky said:

Please excuse me if the question has been asked before.

When autopilot is engaged in NAV mode while the navigation is the GPS, it works perfect. The aircraft flies the GPS route, this is good. Now if you are in NAV mode while the CDI is set to VOR1, which means the navigation source is the VOR1 and not the GPS, I noticed the aircraft doesn't fly in the expected direction.

Let's say I am flying heading 240 with the VOR1 needle centered on 240 and the TO indicator, it means I am flying to the VOR station on radial 240. Engaging the NAV mode at this time should maintain the aircraft on the 240° radial, with a heading of roughly 240°, correcting potential cross-wind deviation. Instead of this, the autopilot commands a hard turn as soon as the NAV mode is engaged.

I noticed that if I engage the APPR mode instead, it looks like working fine. The autopilot maintains the VOR radial as expected.

So my question is: to keep the aircraft stable on a VOR radial, should I use NAV or APPR mode?
In other words, am I doing wrong or is it a bug of the sim?

Before MSFS, with FSX, I used to use the NAV mode for this and it was working fine. The APPR was used to manage the lateral and vertical guidance during approach, especially with localizer and glideslope guidance.

Hello. In what airplane is that? I actually had a hard time on the CJ4 Citation for the exact same reasons. Was on course ia NAV, route provided by the FMS (the GPS  essentially). Then didnt want to mess with the FMS for the approach so switch to VOR to follow the procedure on the charts. Even when reseting the nav source of the PFD and MFD to VOR1 and VOR2, the AP would NOT follow the new source. But indeed, APPR mode would. Very strange. 

[LarryD 132]

34 minutes ago, Rocky said:

Let's say I am flying heading 240 with the VOR1 needle centered on 240 and the TO indicator, it means I am flying to the VOR station on radial 240. Engaging the NAV mode at this time should maintain the aircraft on the 240° radial, with a heading of roughly 240°, correcting potential cross-wind deviation. Instead of this, the autopilot commands a hard turn as soon as the NAV mode is engaged.

But after the "hard turn", does it settle out and end up having you go direct as expected?

I tried your scenario, and sure enough, a hard turn began, but after several back and forths, it finally settled in on the radial.  So this is what seems to be happening.

Radials extend out from a VOR.  The further you are from the VOR, the farther you can be from the EXACT centerline.  So your needle looks centered on the radial on your instrument, but in reality, the plane may not be EXACTLY on the centerline.  Again, the farther you are from the VOR, the farther from the centerline you can be.  Now when you engage NAV, it tries to find the centerline, it sees its "over there", and commences a turn to fly toward it and interecept it at some preprogrammed angle and then gently turn into it, just like a plane slides into the localizer.  That initial turn is the "hard turn" you see.

But if you engage APPR instead, and this is just a guess here, the programming is designed to "ease" its way back to the centerline rather than "get there now", so the initial turn is much gentler.

On my test, both NAV and APPR centered me on the radial, with APPR doing in gently and NAV with the hard turn.

[vgbaron 3,620]

Several things going on here.

there is a known issue that, when you activate the approach and are beyond the actual approach fix, the a/c will turn back to the start of the approach.

It is normal for the a/c to "search' to hone in on the radial. Also regardless of heading be sure the VOR is set to the proper radial, just because you are pointing 240 doesn't mean you are on the 240 radial.

The approach mode won't follow a VOR radial, it's looking for the localizer signal which is different from the vor signal.

[LarryD 132]

16 minutes ago, vgbaron said:

The approach mode won't follow a VOR radial, it's looking for the localizer signal which is different from the vor signal.

That may very well be the case in a real airplane, but both MSFS and the XPlane simulator G1000 both turn to and track the tuned in radial when putting it in APR mode.

[leprechaunlive 1,852]

56 minutes ago, vgbaron said:

 

The approach mode won't follow a VOR radial, it's looking for the localizer signal which is different from the vor signal.

Thats IRL. But my understanding is that the MSFS APPR mode is just a glorified LOC capture (bit more gentle than NAV) attached to a vertical capture at 3degrees slope. 

I can capture a VOR in APPR mode. Just like i can capture an ILS loc in NAV (but thats normal). 

[Rocky 128]

2 hours ago, leprechaunlive said:

Hello. In what airplane is that? I actually had a hard time on the CJ4 Citation for the exact same reasons. Was on course ia NAV, route provided by the FMS (the GPS  essentially). Then didnt want to mess with the FMS for the approach so switch to VOR to follow the procedure on the charts. Even when reseting the nav source of the PFD and MFD to VOR1 and VOR2, the AP would NOT follow the new source. But indeed, APPR mode would. Very strange. 

This happens with the C172 and the TBM 930, I didn't try with other aircraft but I can bet all would behave the same.

[Rocky 128]

1 hour ago, LarryD said:

But after the "hard turn", does it settle out and end up having you go direct as expected?

I tried your scenario, and sure enough, a hard turn began, but after several back and forths, it finally settled in on the radial.  So this is what seems to be happening.

Radials extend out from a VOR.  The further you are from the VOR, the farther you can be from the EXACT centerline.  So your needle looks centered on the radial on your instrument, but in reality, the plane may not be EXACTLY on the centerline.  Again, the farther you are from the VOR, the farther from the centerline you can be.  Now when you engage NAV, it tries to find the centerline, it sees its "over there", and commences a turn to fly toward it and interecept it at some preprogrammed angle and then gently turn into it, just like a plane slides into the localizer.  That initial turn is the "hard turn" you see.

But if you engage APPR instead, and this is just a guess here, the programming is designed to "ease" its way back to the centerline rather than "get there now", so the initial turn is much gentler.

On my test, both NAV and APPR centered me on the radial, with APPR doing in gently and NAV with the hard turn.

I must admit that when I saw this "hard turn", I didn't let it go and I switched back to HDG mode to go to the VOR manually, so I can't tell if it would come back to the correct heading. I will try again for testing.

I know that the aircraft may not be perfectly on the expected raidal when the needle looks centered, but what was strange here is that the aircraft turned away from the needle. It should turn right to be perfectly positioned on the radial but it turned left... In addition, I paid attention to the distance to the VOR. I know that you can't expect any precision guidance when you are close to the station, but in that case I was approximately 20 NM away from the VOR.

[fppilot 4,371]

1 hour ago, LarryD said:

But after the "hard turn", does it settle out and end up having you go direct as expected?

I tried your scenario, and sure enough, a hard turn began, but after several back and forths, it finally settled in on the radial.  So this is what seems to be happening.

Radials extend out from a VOR.  The further you are from the VOR, the farther you can be from the EXACT centerline.  So your needle looks centered on the radial on your instrument, but in reality, the plane may not be EXACTLY on the centerline.  Again, the farther you are from the VOR, the farther from the centerline you can be.  Now when you engage NAV, it tries to find the centerline, it sees its "over there", and commences a turn to fly toward it and interecept it at some preprogrammed angle and then gently turn into it, just like a plane slides into the localizer.  That initial turn is the "hard turn" you see.

But if you engage APPR instead, and this is just a guess here, the programming is designed to "ease" its way back to the centerline rather than "get there now", so the initial turn is much gentler.

On my test, both NAV and APPR centered me on the radial, with APPR doing in gently and NAV with the hard turn.

So after reading this my question is this.  Were you flying a flight plan?  Or were you just out flying and on impulse decided to tune a VOR and have the AP fly to it?  i.e. no flight plan in play.

[Rocky 128]

33 minutes ago, fppilot said:

So after reading this my question is this.  Were you flying a flight plan?  Or were you just out flying and on impulse decided to tune a VOR and have the AP fly to it?  i.e. no flight plan in play.

I had no flight plan.

[Rocky 128]

I have just tested again with the Bonanza and it worked fine. The turns may be too violent but it works, the aircraft flies to the VOR in NAV mode. I am now lost...

I will test again with the C172 that has no Garmin 1000, maybe it makes a difference? Maybe I did something wrong before?
Anyway I will test again and let you know and I will capture screenshots.

[fppilot 4,371]

21 minutes ago, Rocky said:

I had no flight plan.

The reason I asked is this.  When on a flight plan, when the AP is activated and switched to NAV, the AP in this sim will take you to the first point after your departure airport.  It still lacks the important feature of allowing a Direct-To [D→] a point in the flight plan.  So instead it turns the aircraft back to the direct vector from the departure airport and the first flight plan point.  That causes the type of serpentine behavior you described in your original post.  

Your description sounded like even in VOR1 CDI mode it was steering you back on line, from some previous position, to the VOR you just tuned.  That instead of taking you direct from your current position.

Edited October 17, 2020 by fppilot

[Will Fly For Cheese 1,710]

28 minutes ago, Rocky said:

I have just tested again with the Bonanza and it worked fine. The turns may be too violent but it works, the aircraft flies to the VOR in NAV mode. I am now lost...

I will test again with the C172 that has no Garmin 1000, maybe it makes a difference? Maybe I did something wrong before?
Anyway I will test again and let you know and I will capture screenshots.

Glad you tested again and saw the result.

When you hit NAV on the AP the aircraft will turn to intercept the radial immediately. So it can be violent - but after all - you've ordered the AP to intercept that radial - it's going to expedite your instruction immediately and without finesse. it's a little overdone in MSFS but it does do as you've seen.

Real world we do the same thing but in a more leisurely fashion. For example; If I'm flying south on a heading of 120° and want to intercept the 200° for SAM (Southampton, 113.35) from 60 NM away, I'll dial it up on the VOR and just set a course to intercept it in a few miles time - no drama required, not NOW, NOW ,NOW! You can do the same in the sim.

Techniques like this will start to really finesse your navigation. And you'll burn less Avgas! 😀

 

[Rocky 128]

1 hour ago, fppilot said:

The reason I asked is this.  When on a flight plan, when the AP is activated and switched to NAV, the AP in this sim will take you to the first point after your departure airport.  It still lacks the important feature of allowing a Direct-To [D→] a point in the flight plan.  So instead it turns the aircraft back to the direct vector from the departure airport and the first flight plan point.  That causes the type of serpentine behavior you described in your original post.  

Your description sounded like even in VOR1 CDI mode it was steering you back on line, from some previous position, to the VOR you just tuned.  That instead of taking you direct from your current position.

I guess you're right because I made another test flight with the C172 with no flight plan and everything worked fine. I believe it works fine when no flight plan is defined. I will now test again with a flight plan.

[Rocky 128]

Guys,

I don't know what went wrong before but all the flights tonight went perfect and I used the NAV mode based on VOR1 each time (to test it). Sorry for bothering you all for a problem that I finally couldn't reproduce... It's the first time I am happy to be unable to reproduce a problem 🙂