AP doesn’t recapture GS after temporary disconnect

[Eugr]

I was playing with the autopilot today and did a practice ILS approach. I armed NAV & VNAV modes when cleared for the approach, and the autopilot did a great job capturing the localizer and glideslope. However, somewhere near FAF I realized that I spent too much time playing with Dynon and got faster than gear extension speed, so I decided to disconnect the autopilot, throttle down, pitch up to quickly bleed the speed, deploy the landing gear and then re-engage the autopilot.

When I re-engaged it, I noticed that it was still showing both LOC and GS modes in green, so I assumed it will keep tracking the glideslope even though I got above it. Nope! I blew right through it and I had to disconnect the AP and hand fly the rest of the approach.

The question: is it normal? If it is, is there any way to know that it is actually tracking the glideslope?

I’ll try again in a few days with a safety pilot. I’m glad it happened in VFR conditions...

 

[airguy]

I was playing with the autopilot today and did a practice ILS approach. I armed NAV & VNAV modes when cleared for the approach, and the autopilot did a great job capturing the localizer and glideslope. However, somewhere near FAF I realized that I spent too much time playing with Dynon and got faster than gear extension speed, so I decided to disconnect the autopilot, throttle down, pitch up to quickly bleed the speed, deploy the landing gear and then re-engage the autopilot.

When I re-engaged it, I noticed that it was still showing both LOC and GS modes in green, so I assumed it will keep tracking the glideslope even though I got above it. Nope! I blew right through it and I had to disconnect the AP and hand fly the rest of the approach.

The question: is it normal? If it is, is there any way to know that it is actually tracking the glideslope?

I’ll try again in a few days with a safety pilot. I’m glad it happened in VFR conditions...

I believe the autopilot will only capture the glideslope from below, not while above or descending through it.

 

[Eugr]

I believe the autopilot will only capture the glideslope from below, not while above or descending through it.

Yes, it’s in the documentation. However, I wish there was more clear indication on whether it actually captured the glideslope or not. It was showing GS in green active mode, so I assumed that it was still tracking...

 

[airguy]

Yes, it’s in the documentation. However, I wish there was more clear indication on whether it actually captured the glideslope or not. It was showing GS in green active mode, so I assumed that it was still tracking...

So.... it's IN the documentation, but you want it BETTER documented? Is that what you are asking for? I'm confused...

 

[Eugr]

So.... it's IN the documentation, but you want it BETTER documented? Is that what you are asking for? I'm confused...

Sorry, I can see how it can be confusing. Let me try again.

What I’m saying is that there is no way to tell if it has actually captured and is actively tracking the glideslope after you re-engage autopilot.

So normally when you arm VNAV it will arm GS first, and activate it when about to intercept and then will track it. Now what happened is that I disconnected the AP while in active VNAV mode, but FD and modes stayed active even after disconnect (just like it should), and FD seemed to be tracking the glideslope which was already below me (just one dot).

Yes, yes, I know that the autopilot only captures glideslope from below, but in this case it WAS already captured and the modes remained active.

So when I re-engaged the autopilot, GS mode stayed active (NOT armed!), but the autopilot was not tracking it anymore, but it LOOKED like it was tracking until it flew right through it.

Does it make sense now?

 

[airguy]

Sorry, I can see how it can be confusing. Let me try again.

What I’m saying is that there is no way to tell if it has actually captured and is actively tracking the glideslope after you re-engage autopilot.

So normally when you arm VNAV it will arm GS first, and activate it when about to intercept and then will track it. Now what happened is that I disconnected the AP while in active VNAV mode, but FD and modes stayed active even after disconnect (just like it should), and FD seemed to be tracking the glideslope which was already below me (just one dot).

Yes, yes, I know that the autopilot only captures glideslope from below, but in this case it WAS already captured and the modes remained active.

So when I re-engaged the autopilot, GS mode stayed active (NOT armed!), but the autopilot was not tracking it anymore, but it LOOKED like it was tracking until it flew right through it.

Does it make sense now?

This was on an ILS, correct? I will have to go fly some scenarios now and play with this.

 

[Eugr]

This was on an ILS, correct? I will have to go fly some scenarios now and play with this.

Yep, good ole ILS... Haven’t tried with GPS approach yet, but I suspect it will be the same...

 

[kellym]

Yep, good ole ILS... Haven’t tried with GPS approach yet, but I suspect it will be the same...

You could just pull power enough to make the A/P slow to gear speed while holding GS, then after gear is down adjust power for speed you want.
Yes, probably takes a little longer, but not confusing to A/P.

 

[Eugr]

You could just pull power enough to make the A/P slow to gear speed while holding GS, then after gear is down adjust power for speed you want.
Yes, probably takes a little longer, but not confusing to A/P.

True, and if I was in IMC I would likely do just that. However, it does take much longer for Bonanza to slow down in descent with the gear retracted... I wonder if using CWS instead would allow the autopilot to continue tracking the glideslope? Based on the manual, that seems to be the case. What’s interesting, CWS is disabled in the STC installation manual for Bonanza, I wonder why.

 

[Dynon]

So just to be clear, are you saying that when you re-engage, VNAV or GS is cyan in the top bar? IE, it shows as actively engaged?

 

[Eugr]

So just to be clear, are you saying that when you re-engage, VNAV or GS is cyan in the top bar? IE, it shows as actively engaged?

Yep, that was the case. Here is what I did:

1. I armed NAV & VNAV when cleared to the approach and while in ALT HOLD mode being vectored to the final approach course
2. Autopilot activated both LOC and GS modes and started tracking the localizer and glideslope
3. I clicked AP DISC button on my yoke (the flight director and modes remained active), pulled up a little, dropped the gear. Ended up no more than one dot above the glideslope.
4. The AP modes and FD were still active. I re-engaged the autopilot by pressing AP button on the control panel
5. Both LOC and GS modes were highlighted in green (not cyan)
6. The autopilot flew through the glideslope and ended up way below it without any attempt to correct, so at that point I disconnected AP and continued hand-flying the approach.