AP not behaving

[FridayChild]

I am stabilized on the LOC, waiting for the GS to kick in, inbound Palermo. Aircraft=737-800 by KittyHawk.
Suddenly, without an apparent reason, the altitude hold (i was at 3800 feet) disconnects by itself and the plane starts pitching up. I reconnect the alt hold and start descending to 3800 feet again. Then all goes fine for a couple of minutes. At this point the GS marker starts to move down, and I lower the speed to 170, flaps 15, as I usually do. APP is of course armed and I am sound on the LOC. Then... nothing happens. The AP does not capture the glideslope. At this point I decide to disconnect the AP and fly the approach manually, which I succeed (only slightly off the center of the runway, I'm afraid).
The two anomalies, particularly the first, puzzle me. The GS may not be captured if the speed is too low or too high or if the a/c is not stable on the LOC, but what about the first weirdness? And what about the fact that both things never happened to me before, even in my first unortodox attemps?

[RollerBall]

You have to capture the glideslope from BELOW. If you don't you will get weird things happening.

Almost 4000 is a bit high to be intercepting GS anyway. Should be at only 2000-2500

[GA_Pilot]

Yeah try intercepting the localizer and glideslope between 2,000-3,000 ft agl...

[Brett_Henderson]

There's a big, cryptic section in the aircraft.cfg file that defines the autopilot. I've never taken the time to learn and interpret it, but it looks like there are limits, that if exceeded, will disengage part or all of the autopilot functions.

I've encountered this problem too and have been meaning to get some understanding of this paragraph.

Now I'm motivated to  ;)

[FridayChild]

RollerBall, FlyIn:
I was very well below the GS! 3800 feet was the altitude I was assigned to by the AI ATC. The GS marker just went past the middle without firing up the auto-descent and at this point I disconnected the AP and fled manually.

Brett:
If it has to be considered more realistic this way, I'll get used to it; the default 737 never played tricks like this to me though. This KittyHawk 737 has had an engine failure and an AP misfunction in two flights, so far.... looks like it's overhauling time at KLM!

[Ivan]

really sure you had it on NAV mode and not on GPS mode

[eno]

How much fuel do you have remaining ....... ? If you're too heavy it can have the effect your describing.

[FridayChild]

It was in NAV mode, and yes, the fuel tanks were almost full... it was a medium short flight (Milano to Palermo) and I didn't bother altering the fuel quantity. Yet, my first FS2004 flights were with the default 737, tanks full, short distance, ILS, no problem.

[ashaman]

I have an answer for your problem, and a sort-of solution too.

I've flown a lot in almost all the simulated world and I've been able to understand that in FS, when you're in an approach sometimes the LOC signal goes a long way more far than the GLIDE.

When this happens and you prepare yourself to land by using the semi-automated landing of FS, never, NEVER activate ILS hold unless both signals are received.

If your LOC needle has abandoned its central repose position but the GLIDE is still dead center inactive and you press ILS hold, the ALT hold will disconnect immediately and the plane will begin to descent to an arbitrary angle... straight in the middle of nowhere.

When this happen and you force the ALT hold to engage again, the autopilot will not recognise as valid the GLIDE when it will appear and you'll have to descend by hand.

What I've written I've verified in quite a lot of airports around the world and I'm pretty sure it's a FS bug (unless real planes behave like this too). So be sure you have both LOC and GLIDE before telling Otto to prepare to follow the ILS. Remember that in presence of a glideless LOC signal you can always use the NAV option to begin lining up (GPS/NAV switch on NAV) to the runway and when the GLIDE appears press ILS hold.

That's all folks. Fair winds and nice weather to you.

[FridayChild]

Thanks. Maybe someone with real world liners flying experience will confirm if it's a feature or a bug. I believe in real life often the LOC and the GS have different ranges, the LOC range normally being greater. I don't know if this affects the ability of arming APP at any time. I will try to fly the Palermo approach again, this time correcting my heading manually and arming the APP only when I'm in the GS range.

PS: weird thing, two italians talking to each other in english  :D

[ashaman]

PS: weird thing, two italians talking to each other in english  :D

I reckoned as much and the irony was not lost to me, but chose not to remark on it. Forum rules being what they are, we have to communicate like this, here.

[Icelandair Pilot]

I had the same altitude problem. out of nowhere it would have about 20% nose up and stall

I reset the avionics and all was well