A Puzzling Problem with the Autopilot

[JerryH 1]

I've finally given up trying to solve an FS2004 problem and am in need of an autopilot wizard. The problem was first noticed with an add-on B732 when I was approaching KSEA and selected APP. Since then I've tried the same flight many times with the default B734, but the same thing happens. Let me explain the sequence of events.1. Everything normal during the descent to KSEA; NAV1 radio at 116.80; autopilot set to NAV and tracking the 340 radial; ALT is set to level out at 3400 feet.2. Levels out at 3400 feet; DME about 30; the ILS comes alive a short time later; set NAV1 to 110.30; click on APP and it lights up; at the same time, ALT is disabled (Problem #1).3. Next, click ALT back on (pretending everything is okay); continuing to track the ILS localizer; nearing the glideslope; when the glideslope centers, nothing happens - the B734 maintains level flight (Problem #2).4. After several failures, I've learned that if I quickly click APP OFF and ON, I can salvage the approach, because ALT then turns off and the aircraft makes a nearly normal descent to the runway.I've deleted (renamed) FS9.cfg, let FS create a new one, then tried the above approach with the same aircraft and got identical results. Searched the forums and Googled the subject with nothing useful so far. With FS2002, I've performed that autopilot approach several hundred times. Only purchased FS2004 a few months ago and can't believe the AP operation should be any different. Many thanks for any suggestions.JerryH

[Egbert Drenth 250]

>2. Levels out at 3400 feet; DME about 30; the ILS comes alive>a short time later; set NAV1 to 110.30; click on APP and it>lights up; at the same time, ALT is disabled (Problem #1).>to the runway.Hi,My guess is you tune in the ILS freq too early. At distances of 20+nm the signal is weak and might get interupted causing APP to disangage.Set/Engage ILS at about 15nm and try again. BTW: How do you know the ILS comes alive shortly after 30nm while you are still on the VOR frequency?

[JerryH 1]

Hi,You make a good point about early ILS engagement, but I'm very familiar with the KSEA approach and have never had a problem with the signal. However, I will try ~15 nm and see what happens.I didn't mention it, but I always tune the ILS frequency into NAV2 so I'll know when I'm in range.Regards,JerryH

[XWRed5 0]

Jerry,Without knowing the add on you are talking about (which I doubt I have anyway) it sounds like the autopilot is just lying to you.I have two suggestions :1) When you arm the APP and ALT disappears (you describe as problem #1) that could be normal. It's odd I agree but it could be the autopilots way of telling that G/S capture has been armed (so you're not strictly in ALT any more). So don't click on ALT again (you'll disarm G/S capture and it'll fly right through the glideslope - your problem #2).If, on selecting APP, the aircraft doesn't maintain altitude try suggestion number 2.2) Instead of selecting APP miles out, is there not a LOC button? Select this first and wait until the glideslope comes alive (maybe 1 dot above you) before selecting APP.In real life, the SOP for the airline I fly for is that we are not allowed to arm G/S capture until we are fully established on the LOC so we always select LOC first then APP later.Hope this helps,Ian

[pschlute 37]

It is normal on almost all planes I have flown that APP coming live will turn off the ALT function.Think about it: you are moving from a altitude hold mode to a ILS mode. The lateral (VOR) element of the ILS will capture first but the glideslope will not have captured yet. You will continue to fly at the current altitude until you hit the glideslope...at which point that will be the vertical mode.So what mode are you in between the 2 stages ? Certainly not ALT hold....whose function is to maintain you at a specified altitude until switched off. You are in APP altitude hold mode : it means "the current altitude will be maintained until we hit the glideslope"

[HoratioWondersocks 4]

Yep selecting APP Hold should turn of ALT hold at some point,otherwise i would imagine you would just continue at whatever ALT setting you have selected and not descend for your landing.Somebody more knowledgeable can probably explain the process in more detail.Cheers Andy

[old_doggie 0]

Here are some pics of the app to KSEA 16L - ILSAircraft B737400(default)Panel is home grown from the boneyard5000 intercept as per IAP at KENMOPic 1 - 21 dme, 5000ft - ALT HLD and APP engaged ( you can use LOC HLD until the GS is active or alive) Pic 2 - 16.1 dme 5000ft. the GS has just been captured, alt hld is off on capture and trend of IVSI is descendPic 3 - 8.3 dme ,2500ft, in the slot, app hld still functioning as it should, no problems so far, config for landingpic 4 - at the outer marker, all stablePic 5 - 100 above, viz is 1/4 mile fog, slightly high on the slope, app hld all ok.was unable to duplicate the problems, tried with various combos of VORLOC(or NAV HLD) and the result was the same, normal capture and tracking all the time, so I guess there is something amiss in the aircraft config. Maybe let us know what aircraft/panel you are using.Douglashttp://forums.avsim.net/user_files/178979.jpghttp://forums.avsim.net/user_files/178980.jpghttp://forums.avsim.net/user_files/178981.jpghttp://forums.avsim.net/user_files/178982.jpghttp://forums.avsim.net/user_files/178983.jpg

[JerryH 1]

Thanks to all who have responded.Ian: 1) I understand that disabling ALT could be normal, but in my case the aircraft does not maintain altitude. 2) In my testing, I tried to be as "plain vanilla" as possible, so I'm flying the default Boeing 737-400 with the default panel. This panel is lacking a LOC button.pschlute and Andy:As above - if normal means ALT is disabled and the APP mode maintains the current altitude, that would be great. But (for me) when ALT goes off, the aircraft begins a shallow dive that will not have a happy ending.Douglas:Thanks for all the testing and screen shots! I'm pleased that you feel something is amiss, though I'd prefer a solution. As mentioned earlier, my recent testing was with the default B737-400 and panel. I don't normally fly the default B734, but was just trying to be as close as possible to an out-of-the-box aircraft with no tweaks or modifications.I didn't mention in my original post that a few test flights have behaved normally. Tried the default C172 on an approach to KSEA and it was okay. Earlier, when I was "playing" with an add-on 737-200, the failure was inconsisent. Switching panels sometimes seemed to help, but success was erratic.Thanks again,JerryH

[pschlute 37]

JerryHWell, I tried this flight as you described in the default 734 and.....I had the same problem !I dont have these problems with the PMDG 737 that I fly, but then my procedure is slightly different from yours. Anyway back to the default 734...... if I am in ALT Hold (at 3400) and select NAV (which will capture the VOR of the ILS) first, then I press APP (about 18 nm DME)......the ALT hold light stays on until i intercept the glide slope, at which point the a/c descends correctly on the GS.I believe it is usual practice to capture the VOR first...then press APP. Certainly works with the default 734.As an aside...the runways at KSEA use the same ILS frequency whether you are landing on 160 or 340. This always seems to present a problem at KSEA ......eg. when it first comes into range I find that my instruments tell me i am too far left...when i can clearly see i am to the right of the VOR. As I get closer the diplay corrects and I can land without problem. It may be that your problem is caused by you pressing APP when the ILS it is picking up is for some reason the one for 340 not 160. Just a thought ? Incidentally my home airport EGLL also has the same two runway setup using the same frequency for 270/90, but i never seem to get false ILS readings there. Try an airport with only one ILS.Good luck

[JerryH 1]

pschlute,A very interesting response. As I said earlier, most of my experience is with FS2002 where my procedure worked without a hitch.I will try your procedure with the default B734, first capturing the ILS localizer with NAV, then pressing APP when closer to the GS.Incidently, I had noticed on some KSEA approaches that after pressing APP, the instrument had the localizer to my right, then it jumped over to my left. Maybe that was a clue that I missed.If I can duplicate your KSEA approach doing it the "usual" way, then the FS2002 and FS2004 autopilots must have some differences.I'll report back when time permits.Many thanks for your effort.JerryH

[JerryH 1]

pschlute,That did it! Just a simple procedural change that had eluded me. Now if this old dog can just remember the new trick.I was so hung up on the way it worked for me in FS2002 that I never considered doing it any differently. Capturing the localizer with NAV (or with LOC) makes perfectly good sense. Others had hinted at doing it that way, but your demo proved it.So my friend, I owe you one. My wife has plans to drag me over your way sometime next year. If I happen to see you, I'll buy you a pint or two.Best regards,JerryH

[tgibson 11]

Hi,The typical problem with these runways with the same ILS frequency at both ends is that if you turn on the APP too soon, the AP gets mixed up with the back course from the other end and it messes up the approach.BTW, if you have (or create) an AFCAD and uncheck the back course boxes for each end of this runway, it usually fixes this problem.Hope this helps,--Tom GibsonCal Classic Propliner Page: http://www.calclassic.comFreeflight Design Shop: http://www.freeflightdesign.comDrop by! ___x_x_(")_x_x___

[JerryH 1]

Thanks, Tom.I think I've got it sorted out now, but I've made a note of your AFCAD fix and will check it out.JerryH

[pschlute 37]

JerryHI am glad you got it sorted.I dont flightsim as often as I used to. I usually fly the PMDG 737; the dreamfleet 727; the Flight1 ATR 72; and the PSS Concorde.Biggest problem I find is remembering correct procedures for these craft if I havnt flown for a while......especially the concorde !Good luck