G5 and Tru Trak/Aerocruze 100 Altitude Hold Question

[SoCal 182 Driver]

On the Tru Trak/Aerocruze 100, in order for it to couple on an RNAV approach, the plane has to be at or below the glide slope, and the autopilot has to be displaying ALT HOLD or ZERO VERTICAL SPEED (SVS).

Let's say I'm being vectored by ATC for an RNAV approach. I have the approach loaded in my autopilot, and have activated the anticipated leg. Even though the approach is loaded and the leg activated, at this point in the approach I'm using the Garmin G5 for headings, and it is set for my current altitude (4,000'). The autopilot shows ALT HOLD on the display. ATC gives me the following instruction:

Skylane 89A, turn right heading 230. Maintain 4,000 until established. Clear RNAV Z.

I turn right to 230, still at 4,000, but I know I need to be at 3,000 to couple with the glide slope on the approach. As the needles begin to come alive, I set the G5 altitude for 3,000, and SVS on the autopilot to 1,000' FPM. I begin my descent as commanded. In theory, the autopilot should level off at 3,000' and indicate ALT HOLD, and when I switch the autopilot input from G5 to GPSS, the GPS should fly me into the glide slope, the autopilot should couple, and down I go to my decision altitude or 700', whichever is higher (700' is a limitation of the Tru Trak/Aerocruze on an IFR approach). Here's my question:

Sometimes, my autopilot and/or G5 gets a mind of its own (probably because of operator error) and blows through a set altitude. If I see that I'm going to descend through 3,000 in this scenario, even though I'm controlling the altitude from the G5, can I push ALT and the main knob on the autopilot, and manually have it go into ALT HOLD?

I'm sorry for the lengthy question, but I appreciate any help anyone can offer.

 

[AlphaMike]

I'm not 100% sure of exactly what you're asking but I do have about 250 hours in my 182 with the Tru Track and G5's. One thing I can definitely suggest. Stay on top of your elevator trim! The biggest issue I have with this autopilot is the elevator servo seems underpowered. It does fine if you keep it trimmed properly but if not, it will overshoot. If I'm descending (or climbing) at 1000 FPM I will start trimming up well before the target altitude just to help the elevator servo level off. As far as your question about manually activating ALT HOLD. I can't say I've ever tried that, but I do know you won't arm the glide slope until you switch to GPS mode. Hope this helps. If not let me know and I can try to recreate what you are experiencing.

 

[Hang 4]

Won't you be getting vectored to a point where you would intercept the approach course below the glide slope? Thought that was standard ATC practice from the false glide slope issue with ILS.

I fly in very flat Florida, so perhaps there are approaches that don't do that in other areas.

 

[SoCal 182 Driver]

Won't you be getting vectored to a point where you would intercept the approach course below the glide slope? Thought that was standard ATC practice from the false glide slope issue with ILS.

I fly in very flat Florida, so perhaps there are approaches that don't do that in other areas.

The problem is that, at least for the RNAV Zulu approach into KCMA, typically ATC holds you at 4,000' until established, but they turn you into the approach between OCESI at 3,300' and HOKIV at 2,800' (and sometimes they turn you in late). By the time the needles come alive, often there's not much time to descend from 4,000' to an altitude below the glide slope so the autopilot can couple. Then it "flags," and the descent has to be hand-flown while GPSS handles the approach laterally.

 

[Jesse Saint]

If you are far enough out, then 4,000’ may be below the glideslope and you can just maintain that until capture. If you need to dive below the glideslope, then you will need to trim to level off or you will overshoot. The G5 doesn’t control the altitude. If you have an altitude bug set on the G5, then hitting the Alt button on the AP will start at that altitude, but the altitude is still set on the AP. To transition between the G5 and GPS, hit the Mode button to change to Track mode (vertical should continue doing what it is doing), flip the source select switch, wait for about 5 seconds, then activate the GPSS by hitting the Mode button. When you first hit the Mode button, the AP will maintain whatever ground track you are currently flying (magnetic heading given corrected for wind), so as soon as they give you a final heading to maintain until established, you can turn to that heading then hit Mode and flip the source select switch. Make sure not to change back to GPSS mode until you are close enough to take the turn to the approach course or the AP will adjust to a 45 degree intercept.

 

[SoCal 182 Driver]

If you are far enough out, then 4,000’ may be below the glideslope and you can just maintain that until capture. If you need to dive below the glideslope, then you will need to trim to level off or you will overshoot. The G5 doesn’t control the altitude. If you have an altitude bug set on the G5, then hitting the Alt button on the AP will start at that altitude, but the altitude is still set on the AP. To transition between the G5 and GPS, hit the Mode button to change to Track mode (vertical should continue doing what it is doing), flip the source select switch, wait for about 5 seconds, then activate the GPSS by hitting the Mode button. When you first hit the Mode button, the AP will maintain whatever ground track you are currently flying (magnetic heading given corrected for wind), so as soon as they give you a final heading to maintain until established, you can turn to that heading then hit Mode and flip the source select switch. Make sure not to change back to GPSS mode until you are close enough to take the turn to the approach course or the AP will adjust to a 45 degree intercept.

VERY helpful Thank you.