AirTalk.org Aviation discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  King autopilot and GPS approaches Goto page 1, 2  Next          AirTalk.org Forum Index -> Instrument Flight Rules View previous topic :: View next topic   Author Message Viperdoc Guest Posted: Sun Apr 01, 2007 5:31 pm    Post subject: King autopilot and GPS approaches I went and did some LNAV/VNAV approaches yesterday with a King KFC 200 autopilot. Looking for some tips with these autopilots on GPS approaches. 1. In NAV or APP mode the commanded rate of turn appears slower than in HEADING mode. Spinning the HSI to the desired heading (often 90 degree turns) caused the plane to try to go in the wrong direction, or the rate of turn (slightly less than standard) caused an overshoot and didn't capture the new course. 2. If flying in APP mode, and then making a step down in altitude, the AP would not capture the glideslope (I've noticed this with ILS approaches as well, even if intercepting the GS from below) 3. Of course, roll steering would be ideal, but barring this, is there a better way to utilized the autopilot to fly these approaches? 4. Would it be better to fly in heading mode, and then engage approach mode just prior to the FAF? Back to top Matt Barrow Guest Posted: Mon Apr 02, 2007 1:19 am    Post subject: Re: King autopilot and GPS approaches "Viperdoc" <jninomi (AT) NOattglobalSPAMMS (DOT) net> wrote in message news:460fa601$0$17156$4c368faf (AT) roadrunner (DOT) com... Quote: I went and did some LNAV/VNAV approaches yesterday with a King KFC 200 autopilot. Looking for some tips with these autopilots on GPS approaches. 1. In NAV or APP mode the commanded rate of turn appears slower than in HEADING mode. Spinning the HSI to the desired heading (often 90 degree turns) caused the plane to try to go in the wrong direction, or the rate of turn (slightly less than standard) caused an overshoot and didn't capture the new course. One problem that's endemic to an analog autopilot (IMOHO) Quote: 2. If flying in APP mode, and then making a step down in altitude, the AP would not capture the glideslope (I've noticed this with ILS approaches as well, even if intercepting the GS from below) Intercepting from above or below? Quote: 3. Of course, roll steering would be ideal, but barring this, is there a better way to utilized the autopilot to fly these approaches? 4. Would it be better to fly in heading mode, and then engage approach mode just prior to the FAF? If you want a significantly increased workload, but no, it's not better. Have you run all the self tests? Sounds like the AP controller is not working at 100%. Back to top Peter Guest Posted: Mon Apr 02, 2007 12:02 pm    Post subject: Re: King autopilot and GPS approaches "Viperdoc" <jninomi (AT) NOattglobalSPAMMS (DOT) net> wrote Quote: 2. If flying in APP mode, and then making a step down in altitude, the AP would not capture the glideslope (I've noticed this with ILS approaches as well, even if intercepting the GS from below) I fly a KFC225 and have not found a problem intercepting a GS (from below, of course) while still on a descending altitude. What the KFC225 won't do is a) intercept a GS from above - I was given a dodgy final vector yesterday which led to that b) intercept the LOC at 90 degrees - you need a better angle. At 90deg it just carries straight on, not knowing which way to turn. The KFC2xx is a digital autopilot; it has a micro in it and does the algorithms digitally. A few issues to watch out for, however: http://www.peter2000.co.uk/aviation/kfc225/kfc225-1.html Back to top Jose Guest Posted: Mon Apr 02, 2007 6:50 pm    Post subject: Re: King autopilot and GPS approaches Quote: What the KFC225 won't do is a) intercept a GS from above - I was given a dodgy final vector yesterday which led to that Isn't that generally a bad idea anyway? I was taught that false lobes could lead you to grief and a GS should always be intercepted from below. Jose -- Get high on gasoline: fly an airplane. for Email, make the obvious change in the address. Back to top Tauno Voipio Guest Posted: Mon Apr 02, 2007 7:47 pm    Post subject: Re: King autopilot and GPS approaches Jose wrote: Quote: What the KFC225 won't do is a) intercept a GS from above - I was given a dodgy final vector yesterday which led to that Isn't that generally a bad idea anyway? I was taught that false lobes could lead you to grief and a GS should always be intercepted from below. Jose That's right - due to inevitable ground reflections, there are always false glideslopes above the correct one. The ICAO planning instructions require the procedure to be planned so thet the GS is captured from below. In the same way, the localizer capture planning rules require an intercept angle below 45 degrees, preferably 30 degrees. -- Tauno Voipio (avionics engineer, CPL(A)) tauno voipio (at) iki fi Back to top Matt Barrow Guest Posted: Tue Apr 03, 2007 6:30 pm    Post subject: Re: King autopilot and GPS approaches "Tauno Voipio" <tauno.voipio (AT) INVALIDiki (DOT) fi> wrote in message news:ZH8Qh.3040$i61.159 (AT) read3 (DOT) inet.fi... Quote: Jose wrote: That's right - due to inevitable ground reflections, there are always false glideslopes above the correct one. Would you be so kind as to explain that one? False glidescopes? Back to top Tauno Voipio Guest Posted: Tue Apr 03, 2007 7:18 pm    Post subject: Re: King autopilot and GPS approaches Matt Barrow wrote: Quote: "Tauno Voipio" <tauno.voipio (AT) INVALIDiki (DOT) fi> wrote in message news:ZH8Qh.3040$i61.159 (AT) read3 (DOT) inet.fi... Jose wrote: That's right - due to inevitable ground reflections, there are always false glideslopes above the correct one. Would you be so kind as to explain that one? False glidescopes? An ILS signal is finally created at the receiver antenna as a combination of three signals which are sent separately. (For the perfectionists: This is a simplified description). A localizer antenna group is built of three sections: left, center and right. There are several antennas per section, often 4 to 8. The center section radiates a signal called CSB (carrier and sidebands) which alone will create a centerline display at the receiver. The side sections radiate a signal called SBO (sidebands only) which contains a kind of difference signal between the centerline signal and a side indication. The left and right side signals are radiated in opposite phase, so the right signal is positive when the left signal is negative. When the side antennas are at equal distances from the aircraft antenna, the side signals cancel at the receiver. When there is a distance difference (as it is when the aircraft is off-centerline) the side signals combine producing the corresponding side signal. The glideslope is produced in the same way, but there is a problem: We cannot install a proper bottom antenna, as it should be some tens of feet below ground. Here, we have luck: If an antenna is put above a conductive flat surface, the field pattern is similar as the pattern with an opposite-phase antenna at the same distance at the other side of the surface, and that is just what is needed here. We'll use ground surface as the reflector to create the illusion of a bottom antenna. The pattern must then be tilted upward by the glideslope angle. This creates an asymmetry in the pattern, and we get false glideslopes above the correct one. For a 3 degree glideslope, the first false glideslope is usually around 10 degrees. HTH (hope this helps) -- Tauno Voipio tauno voipio (at) iki fi Back to top Jose Guest Posted: Tue Apr 03, 2007 7:18 pm    Post subject: Re: King autopilot and GPS approaches Quote: Would you be so kind as to explain that one? False glidescopes? The glide slope signal gets reflected off the ground, and your receiver hears the echo. Under some circumstances, it may interpret the echo as a glide slope, and falsely display a fly up or fly down indication. If you follow that, you will probably descend too steeply, and short of the runway. Jose -- Get high on gasoline: fly an airplane. for Email, make the obvious change in the address. Back to top Bill Guest Posted: Fri Apr 06, 2007 4:33 am    Post subject: Re: King autopilot and GPS approaches On Apr 1, 6:31 am, "Viperdoc" <jnin...@NOattglobalSPAMMS.net> wrote: Quote: I went and did some LNAV/VNAV approaches yesterday with a King KFC 200 autopilot. Looking for some tips with these autopilots on GPS approaches. 1. In NAV or APP mode the commanded rate of turn appears slower than in HEADING mode. Spinning the HSI to the desired heading (often 90 degree turns) caused the plane to try to go in the wrong direction, or the rate of turn (slightly less than standard) caused an overshoot and didn't capture the new course. It's true. The rate of turn is somewhat reduced. Shouldn't turn the wrong direction unless turning 180 degrees. If you start the turn as commanded by the GPS, it will make it. If it goes the wrong way on a 90, there is something wrong with it. Quote: 2. If flying in APP mode, and then making a step down in altitude, the AP would not capture the glideslope (I've noticed this with ILS approaches as well, even if intercepting the GS from below) If it's working right, it should intercept the gs from either attitude mode or altitude mode. The gs must pass thru center in either case and it may be slightly misadjusted. Descending onto it from above requires a serious rate of descent and it could be argued as unsafe. Quote: 3. Of course, roll steering would be ideal, but barring this, is there a better way to utilized the autopilot to fly these approaches? Most of the dozens of KFC200s I've run into at BPPP clinics track just fine by turning the course arrow as commanded by the GPS. If you have a sandel, this is done automatically for you. Note that in strong winds, after a 90 degree turn your ap will have to re-psych the wind; if you use APR mode this will be fairly rapid. Quote: 4. Would it be better to fly in heading mode, and then engage approach mode just prior to the FAF? That would work. There are a lot of features in the coupling modes of the 200. In some cases if something isn't working right it would be really hard to detect. It's all analog switches driven by combinational logic. I'm sure that some I've seen aren't working correctly because the vast majority of them do the problem correctly. Bill Hale BPPP instructor Back to top Bill Guest Posted: Fri Apr 06, 2007 4:37 am    Post subject: Re: King autopilot and GPS approaches On Apr 3, 8:18 am, Jose <teache...@aol.nospam.com> wrote: Quote: Would you be so kind as to explain that one? False glidescopes? The glide slope signal gets reflected off the ground, and your receiver hears the echo. Under some circumstances, it may interpret the echo as a glide slope, and falsely display a fly up or fly down indication. If you follow that, you will probably descend too steeply, and short of the runway. Jose -- Get high on gasoline: fly an airplane. for Email, make the obvious change in the address. Worth noting: Even without grounds, the lobes on the antennas would cause false paths. To wit: The LOC has false nulls. Note how it looks as you fly around the place--swithing back and forth in a seemingly random way. A place where it really matters: Jackson Hole WY. There you need to position yourself on the loc using other navaids. Otherwise you might run into something descending on a false LOC path. Bill Hale BPPP instructor Back to top Viperdoc Guest Posted: Fri Apr 06, 2007 6:11 am    Post subject: Re: King autopilot and GPS approaches My understanding is that at least the KAP 200 and 225 will not track a glideslope (either from an LNAV+V, an LPV, or an ILS) approach when intercepting from above, which is also my experience. I also observed that adjusting the altitude with the autopilot up/down switch will negate the glideslope intercept even from below while in approach mode. Have you seen this behavior as well? Back to top Jose Guest Posted: Fri Apr 06, 2007 6:11 am    Post subject: Re: King autopilot and GPS approaches Quote: To wit: The LOC has false nulls. Note how it looks as you fly around the place--swithing back and forth in a seemingly random way. A place where it really matters: Jackson Hole WY. There you need to position yourself on the loc using other navaids. Thanks. Good point. Jose -- Get high on gasoline: fly an airplane. for Email, make the obvious change in the address. Back to top Bill Guest Posted: Sat Apr 07, 2007 12:55 am    Post subject: Re: King autopilot and GPS approaches On Apr 5, 8:51 pm, "Viperdoc" <jnin...@NOattglobalSPAMMS.net> wrote: Quote: My understanding is that at least the KAP 200 and 225 will not track a glideslope (either from an LNAV+V, an LPV, or an ILS) approach when intercepting from above, which is also my experience. I also observed that adjusting the altitude with the autopilot up/down switch will negate the glideslope intercept even from below while in approach mode. Have you seen this behavior as well? The 200 should intercept from above if in attitude mode. Does not know what glideslope is in use. You can demonstrate this: but it takes a whale of a descent to set it up. You need to pass thru the gs center to make it couple. It's a bad idea in any event. The 225 will do the same thing on ILS but not sure about GPS approaches. Bill Hale Back to top Peter Guest Posted: Sun Apr 08, 2007 11:14 am    Post subject: Re: King autopilot and GPS approaches "Bill" <33bonanza (AT) gmail (DOT) com> wrote Quote: The 200 should intercept from above if in attitude mode. Does not know what glideslope is in use. You can demonstrate this: but it takes a whale of a descent to set it up. You need to pass thru the gs center to make it couple. It's a bad idea in any event. The above makes sense to me - I don't think the 225 has software specifically looking for the *bottom* of the GS, to be seen first. It would take maybe a 10 degree descent though Back to top Viperdoc Guest Posted: Sun Apr 08, 2007 5:19 pm    Post subject: Re: King autopilot and GPS approaches My experience is that the GS has to be intercepted from below in order to couple (also true for the 225). Also, it seems like use of the pitch toggle while in approach mode will prevent the AP from capturing the GS. This has happened on multiple occasions when given a late vector to intercept, along with an altitude change, while already in NAV or HEADING mode, with the APP mode armed. In other words, using the toggle switch for pitch on the AP master panel while in APP mode seems to prevent the AP from sensing and capturing the GS. This has been true for ILS as well as GPS (LNAV/VNAV or LPV) approaches. My only thought was to not engage the APP mode until the final altitude is attained for GS intercept. So, the main question is: with a King 200 AP, does the use of the pitch toggle switch on the master panel while in APP mode negate GS tracking? How about the CWS? Does a switch from APP to NAV and back to APP then allow GS tracking? 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