GP capture failure

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gcampbe2
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GP capture failure

Post by gcampbe2 »

I flew an RNAV LPV approach into my home airport today, and the GP failed to be captured. I *think* I know what happened, but I can't find any confirmation in the G1000 pilot's guide, so am hoping someone here can confirm my thinking.

I was being vectored to intercept the final approach course, and had received my final intercept heading and approach clearance. The AP was engaged and in HDG mode, vectors-to-final had been selected, and I had selected the APR key (the GPS and GP white armed annunciators were visible. *Before* I intercepted the FAC (so the AP was still in HDG mode) I flew through the glide path, but the GP failed to capture. Here is a photo from my GoPro recording that shows this at the moment I realize there is a problem and the GP will not capture. As you can see, I am just about to intercept the FAC, but haven't yet, and I've just blown through the GP, which is now below me.
gp_capture_failure.png
Moments later, the GPS captures the FAC, and I manually force a 700 fpm descent, to re-intercept the GP from the top. This time, the intercept is successful, and GP is captured.

My belief is that the when flying an RNAV approach, the GP will not capture UNLESS the final approach course has already been captured (i.e. AP is in GPS mode not HDG). Can anyone confirm this?
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Re: GP capture failure

Post by Rich »

gcampbe2 wrote: Sun Jun 04, 2023 9:37 pm My belief is that the when flying an RNAV approach, the GP will not capture UNLESS the final approach course has already been captured (i.e. AP is in GPS mode not HDG). Can anyone confirm this?
I believe this is correct and is correct behavior. Think about it, you don't want the plane to start descending outside the protected corridor of the FAC.
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gcampbe2
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Re: GP capture failure

Post by gcampbe2 »

Thanks Rich. That makes sense, and was my thinking as well. The navigation database does not know the safe altitude when on vectors to final, so it plays it safe, and will not capture the GP.
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Re: GP capture failure

Post by mfdutra »

That's by design on (almost?) all avionics to keep you alive. Same behavior for an ILS approach as well. It will only capture GS if LOC is active.

Once you're locked in the final approach course, gently capture it from above. Don't just increase your VS. Slow down to increase your flight path ANGLE, not RATE.
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Re: GP capture failure

Post by Boatguy »

mfdutra wrote: Mon Jun 05, 2023 3:19 am That's by design on (almost?) all avionics to keep you alive. Same behavior for an ILS approach as well. It will only capture GS if LOC is active.

Once you're locked in the final approach course, gently capture it from above. Don't just increase your VS. Slow down to increase your flight path ANGLE, not RATE.
This is probably semantics, but you need to capture the GP/GS from below. So you've captured the LOC or lateral guidance and you are below the the GP/GS with GS/GP armed. As you move down the course flying level, the AP will intercept the GP/GS as the GP/GS "descends" to meet the plane. The AP will not descend the plane to capture the GP/GS.

At my home airport ATC always vectors me to cross about 1,300' above the IF so I let the AP capture the lateral guidance, then use CWS and push the nose down, while holding lateral course, to intercept the GS/GP. As soon as the GP/GS flips to green, I let go of the CWS and I'm coupled to the approach. So you can only capture from above, if you manually push the plane down to intercept.
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Re: GP capture failure

Post by Rich »

Capturing from above the GS simply means descending at a steeper angle than the GS - which is typically ~3 degrees, though it can be steeper in unusual situations. At 90 knots groundspeed, this is roughly 477 FPM - call it 500. Moreover, you want to capture it far enough out that it happens well before the MAP. So you want to be reasonably aggressive. There are two obvious ways to approach this, but both have one thing in common: You don't want to let your airspeed increase while chasing it, so reduce power below what you would normally carry on glide slope. My baseline for 90-ish KIAS is 10" when clean, depending on weight, conditions, aloft, etc. So I can't use this setting when capturing from above.

What I do is set VS to -800 FPM and put T/O flaps in. Pull power as necessary to keep speed under control. I've also used the CWS method, but recognize that this disengages ALL servos, so you're hand-flying all dimensions during that period. And be hair-trigger on releasing it upon GS capture.
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Re: GP capture failure

Post by waynemcc999 »

Boatguy wrote: Mon Jun 05, 2023 4:18 am
mfdutra wrote: Mon Jun 05, 2023 3:19 am That's by design on (almost?) all avionics to keep you alive. Same behavior for an ILS approach as well. It will only capture GS if LOC is active.

Once you're locked in the final approach course, gently capture it from above. Don't just increase your VS. Slow down to increase your flight path ANGLE, not RATE.
This is probably semantics, but you need to capture the GP/GS from below. So you've captured the LOC or lateral guidance and you are below the the GP/GS with GS/GP armed. As you move down the course flying level, the AP will intercept the GP/GS as the GP/GS "descends" to meet the plane. The AP will not descend the plane to capture the GP/GS.
Russ, with all due respect, I beg to differ. Although your CWS technique is clever, it's unnecessary. I have MANY many times (as recently as last week) had the Autopilot (GFC700) intercept the GS/GP from above (e.g. as Rich and Marion previously noted, once the FAC is captured -and- either by slowing down and/or increasing VS descent).
Wayne
Last edited by waynemcc999 on Mon Jun 05, 2023 3:30 pm, edited 1 time in total.
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Rich
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Re: GP capture failure

Post by Rich »

There is the step of starting the descent. The starting state is that my AP is in ALT HOLD mode and already at ~90 KIAS, 15" MAP. So the first step in the KAP140 is to toggle to VS and set it to descend, picking a rate significantly higher that 500 FPM and pull power/add flaps. If the GS bar/diamond is still in range I can just monitor that it starts to move upward, adjusting until it does so. If it's off range I have to treat this as a non-precision for the time being, watching waypoint crossing altitudes. If I never capture its non-precision MDA, which usually is OK, anyway.

NOTE: If this happens in an IPC I push the big red button and hand-fly. CFII's normally like to see this kind of response :D
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Re: GP capture failure

Post by Boatguy »

waynemcc999 wrote: Mon Jun 05, 2023 3:14 pm
Boatguy wrote: Mon Jun 05, 2023 4:18 am
mfdutra wrote: Mon Jun 05, 2023 3:19 am That's by design on (almost?) all avionics to keep you alive. Same behavior for an ILS approach as well. It will only capture GS if LOC is active.

Once you're locked in the final approach course, gently capture it from above. Don't just increase your VS. Slow down to increase your flight path ANGLE, not RATE.
This is probably semantics, but you need to capture the GP/GS from below. So you've captured the LOC or lateral guidance and you are below the the GP/GS with GS/GP armed. As you move down the course flying level, the AP will intercept the GP/GS as the GP/GS "descends" to meet the plane. The AP will not descend the plane to capture the GP/GS.
Russ, with all due respect, I beg to differ. Although your CWS technique is clever, it's unnecessary. I have MANY many times (as recently as last week) had the Autopilot (GFC700) intercept the GS/GP from above (e.g. as Rich and Marion previously noted, once the FAC is captured -and- either by slowing down and/or increasing VS descent).
Wayne - I think we're in agreement. The point of agreement is that the AP will not descend the plane until it has captured the GP/GS. The AP will not begin a descent in order to capture the GP/GS. The capture can come from above, but only if the pilot manipulates the plane to descend to intercept.

In my example, crossing the IF 1,300' high, well above the GP/GS, the AP will never intercept the GP/GS, it will hold altitude at 4,300'.

By contrast, if I crossed the IF at 3,000', below the GP/GS, with the AP in ALT mode and GP/GS armed, the AP will fly 3,000' until it captures the GP/GS, then begin the descent under AP control.

There are a lot of variations with capturing lateral guidance at a waypoint, or in the middle of a leg with an intercept vector from ATC, and the AP will capture the lateral guidance and turn to course. But I think we're in agreement that if the plane is above the GP/GS, the pilot will need to do something to descend the plane and then the AP will intercept when it vertically crosses the GP/GS.

Are we confused yet?
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Re: GP capture failure

Post by waynemcc999 »

Russ, indeed we are in agreement... the Autopilot above the GS/GP in ALT mode will, well, hold that altitude. The pilot needs to get a descent underway (manually or by VS on the Autopilot) that's steeper than the GS/GP and then the Autopilot will indeed capture the GS/GP from above. Now I feel better :) .
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