G1000 KAP 140 NAV to APR transition question

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Marc_CYBW
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G1000 KAP 140 NAV to APR transition question

Post by Marc_CYBW »

Over the past while, while flying on a GPS-NAV course, at the iaf when arming the approach sometimes the plane follows the approach and sometimes drifts off. I then have to use CWS to re-intercept the course. Then again, sometimes it does ARM (engage) and follow the approach.

Any ideas of what's happening here?
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carym
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Re: G1000 KAP 140 NAV to APR transition question

Post by carym »

Marc,
I have no idea but am eager to hear the responses. The last time I flew the GPS18 into my home field, after engaging APR on the KAP140 when at the IAF it flew the segment from the IAF to FAF just fine, turned me toward the final approach course but then kept turning me away from the final approach course. I just disengaged the autopilot and hand flew the approach since the ceiling was 800-900 feet. Very disconcerting.
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Re: G1000 KAP 140 NAV to APR transition question

Post by hoyaj »

carym wrote: when at the IAF it flew the segment from the IAF to FAF just fine, turned me toward the final approach course
Did you mean to say that the AP flew the IAF to IF just fine? There's no turn at the FAF on the GPS 18.

The KAP 140 does a poor job of correcting for wind, especially if the course to fly on the HSI comes in late. It has to rebracket the delta for xwind.

On the GPS 18 referenced sequencing the CRS to 182 sooner (tailwind) or later (headwind) wil impact the KAP140 tracking. If the automation is doing that, I suspect the results will vary as indicated.
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Re: G1000 KAP 140 NAV to APR transition question

Post by Marc_CYBW »

News from Bendix King and Diamond. The transition from NAV mode to APR mode causes a momentary drop in signal from the G1000 to the KAP140, and the AP switches to ROL mode. The G1000 announces the need to re-set the nav source on the KAP140 and may overshoot the track away from an intercept angle while you re-arm APR. Hence the need to use CWS to re-capture the track. BK has prepared a commentary on this and I should receive it in a day or two. I'll post it then.
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Re: G1000 KAP 140 NAV to APR transition question

Post by Eric N »

There's a lot to be said about managing the 140 in the transition to the final approach course. Since we (here in the Northeast of the US) almost always get vectors to final, we make it a point to not go to approach mode until shortly before we're about to intercept, i.e on the final vector heading. Even with an RNAV approach with a "T" configuration, if you engage approach mode before you intercept the FAC the AP will only use a half-standard-rate turn and always overshoots, so we wait until we've mostly finished the turn onto the FAC before we hit the APR button. I agree with the comment about difficulty managing significant crosswinds, and have a low threshold for disengaging the AP and hand-flying the last bit of the approach. Having said all that, by-and-large it works pretty well in most situations; last weekend we had to fly 2 ILS to near-minimums, completely coupled, and had no trouble at all.
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Re: G1000 KAP 140 NAV to APR transition question

Post by carym »

hoyaj wrote:
carym wrote: when at the IAF it flew the segment from the IAF to FAF just fine, turned me toward the final approach course
Did you mean to say that the AP flew the IAF to IF just fine? There's no turn at the FAF on the GPS 18.

The KAP 140 does a poor job of correcting for wind, especially if the course to fly on the HSI comes in late. It has to rebracket the delta for xwind.

On the GPS 18 referenced sequencing the CRS to 182 sooner (tailwind) or later (headwind) wil impact the KAP140 tracking. If the automation is doing that, I suspect the results will vary as indicated.
I stand corrected. I flew to the IAF (CUCAD) and starting flying the segment from CUCAD to SICLE (IF/IAF) and then engaged the APR. As I neared SICLE the autopilot starting turning me torad the FAF (SUBFO) but continued to turn after going through the final approach segment. My mistake was engaging the APR after reaching CUCAD so the autopilot tried to take me back to CUCAD :( This wouldn't have happened if I engaged the APR mode before reaching CUCAD. I believe my mistake came from getting vectors to the final approach coarse all the time and not engaging the APR mode until on the final approach course.

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Re: G1000 KAP 140 NAV to APR transition question

Post by hoyaj »

carym wrote: My mistake was engaging the APR after reaching CUCAD so the autopilot tried to take me back to CUCAD.
So, the only way I think that could happen is to select "Activate the approach" in the G1000 at SICLE? If so, I think there is confusion on the meaning of Approach modes, which is very common.

The approach is active in the G1000 as soon as CUCAD is the active waypoint, whether selected by activate, activate the approach, or Direct-to (i.e., all assuming the approach was loaded from the database and not manually input as a fix into the flight plan). Even at 100 miles away, using the APR mode on the autopilot would be fine.

I found the notation in the DA42 WAAS supplement that there is a signal change reaching the FAC that causes the KAP 140 to suspend nav and revert to ROL (really a flashing NAV or APR annunciation). That's the same issue with the GNS530W and the KAP140. It's an interoperability issue stemming from the GPS Select mode in the Garmin software and the safety "feature" in the KAP 140 that detects the navigation signal switch (same as changing CDI inputs, the autopilot wants confirmation that the pilot knows that the nav signal to trk has changed.)

So in addition to the 5 T's, add Toggle (the autopilot mode asneeded). My recollection is that there is no twisting required for the DTK on RNAV approaches using the G1000 HSI.

I'm intrigued by the assertion that the KAP 140 implements half standard rate turns in the APR mode. I don't see that discussed in the BK pilot manual, but if there is a reference would like to have it.
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Re: G1000 KAP 140 NAV to APR transition question

Post by Eric N »

I'm the one who posted the "assertion" about half-standard-rate turns in APR mode with the KAP140, but am a little embarrassed to say that I can't cite a reference convincingly. I agree that it does not seem to be in the AP manual, so I think I may have learned it at the new owners 3 day course we took at the factory when we picked up the plane there in 2005. Since then I've just assumed it to be true, but it's time to go out and try it out "the other way". I'll get back with the results.
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Re: G1000 KAP 140 NAV to APR transition question

Post by Marc_CYBW »

As mentioned, here is an email from a BK tech:

Usually the “autopilot not following the approach” with people flying LNAV approaches is a result of the autopilot being flown in NAV mode on the part of the approach when the transition from terminal to approach (in the GPS) occurs, and there is a momentary drop in the nav signal coming from the GPS. The autopilot correctly ceases to fly NAV, and reverts to wings level with NAV flashing.

It’s an easy “mistake” to make, and I did it myself last Friday on an approach in another airplane with a KAP 140. I’ve always found it best to fly with the HDG bug during that phase of the approach, as it will always transition from HDG to APR correctly without sensing a nav source change.

The tell tale sign that this is what happened is when you see you’re not maintaining course, look at the autopilot and see if it’s mode indication is flashing.
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Re: G1000 KAP 140 NAV to APR transition question

Post by RC7 »

We had a nickname for that autopilot - the KrAP-140. On a serious note, you really have to watch it like a hawk if you are flying a coupled approach. I taught my students to keep their hand on the disconnect button during approach as a reminder that YOU are the one flying the airplane... not George.
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