Autopilots switch off before landing

[martinlest2 58]

I'd really like to know what would cause both autopilots to turn off during approach? I am on a normal ILS approach - AP1 and AP2 set, all looking good for touchdown less than a minute away, localizer and glidescope on track. Suddenly a siren and both APs switch off. A/C very wobbly, but a manual landing is fine. Had the approach configured ok in the FMC with VRef speed selected against flaps.Anyone any ideas? Or is this just a glitch, independent of anything I might have done/not done? Thanks.Martin

[martinlest2 58]

737-700 BTW!

[Mats_J 2]

Martin,My first thought is wake turbulence. Are you using Active Sky?Hope it helps,

[martinlest2 58]

Hi. Yes, I do have active sky, but I don't have wake turbulence activated.

[martinlest2 58]

This is driving me crazy! Half a minute or so from touchdown and then - siren, AP disengages, elevators (and hence a/c) start jerking up and down: inevitable crash.Probably have to reinstall the a/c - doesn't happen with the default 737-400. Unless anyone has other dieas?M.

[Mats_J 2]

M. (Martin?)Still sounds like wake turbulence to me. Try without any weather what-so-ever (No Active Sky and Clear FS weather) and see if the same thing happens. My bet is it will not.No need to reinstall the plane untill you've checked the above.Oh. And please don't compare this aircraft with the default 737. The default aircrafts are not nearly as sophisticated as these ones. ;-)Hope it helps,

[martinlest2 58]

Yes, "M." = Martin :-)I know that the default 737 is a mile behind the PMDG one; only mentioned it to show that it is not happening with every a/c. I have had the PMDG 737 pretty much since it came out and so feel I am fairly used to most of its intricacies: this problem is fairly new, which is why I wondered if the setup is corrupt in some detail.It's not due to wake turbulence as there are no a/c ahead of me: with traffic at 0% I still get this. Weather is clear, no wind.I wonder though if this could happen if my approach speed is just a little too slow? Normally, I'd expect just a stall if so (and I do follow the FMC for VRef and so forth), but I just re-ran my latest disastrous approach (AutoSave) with my airspeed a bit higher and all was OK. Not that this proves things 100% as I have often found that re-running part of a flight using autosave rarely produces the same fault as the first time round. The sim seems to wait until I have completed 99.9% of a long-haul flight before it messes things up, but that's life of course!Martin

[martinlest2 58]

BTW, despite what I say above, I will try with 'no weather' set up at all (so to speak). But, as I also said, it's often difficult to reproduce the fault by re-running the landing, so troubleshooting is doubly hard.

[Guest ColdBear]

hi Martin,Can you tell us a little bit about the airport and Nav setup? If you want both A/P's on you need to set the ILS into both Nav recivers. Is the ident code yellow with a line thrue it?have you done all the basic, like right procedures, intercepting G/S from below etc?rgds,Martin

[martinlest2 58]

Yes, I am almost 100% certain that the setup is fine (both NAV radios to same frequency, hit G/S from below etc. etc - all the 'usual suspects'!). I think that if things had been amiss in regard to those kinds of factors, the a/c wouldn't come so close to the runway - mostly only 15-30 seconds from touchdown - before the elevators go haywire and the APs suddenly switch themselves off. The aircraft then pretty much wants to drop out of the sky. Up to that point the approach goes just fine - I capture both localizer and glidescope with no problem and the a/c reacts accordingly.All very odd (and HUGELY annoying to end a long flight like that on so many occasions! Puts me in a bad mood for hours!!!). I will try keeping my airspeed about 10KIAS above that indicated in the FMC next time, see if that makes any difference.Thanks for the encouragement!Martin :-)

[rocketfs 2]

Hello,Are you sure you have a dual coupled approach engaged: is "FLARE" armed in the Flight Mode Anunciator?Best Regards,Bert.

[martinlest2 58]

Yes, I am sure. "Flare" shows up fine..M. :-)

[downscc 1,327]

What AP mode are you using for speed?My preference is to use MCP SPD.. in fact, I only use VNAV derived speed targets for the A/T during climb and cruise and change to MCP controls (SPD V/S or ALT) for descent and approach.Is the A/P disconnect only happening at one approach (location), some approaches or all approaches? Airport location has already been asked, it seems like it might be worthwhile to rule out the approach set-up (some funny things can happen if backcourse is enabled on a runway with ILS on both ends on same freq).

[martinlest2 58]

Hi. When the G/S engages I set flaps as recommended and the speed through the A/T+IAS buttons. I usually set the speed to what appears in the FMC + 5. I have set VRef speed/flap settings in the FMC too. The A/P disconnect has happened at several airports: and I know that in these instances an APR descent should be OK (as opposed to backcourse).Hmmmm. !

[downscc 1,327]

So it only happens at some airports, that rules out any one approach and now what about the speeds your are using?If you are using FMC-provided speeds then those speeds may be inappropriate for the approach. Use MCP SPD set for an indicated airspeed appropriate for the flap schedule that appears on the speed tape. I recommend using direct control via the MCP instead of letting the automation fly the aircraft, at least until you get this problem sorted out.The aircraft should be stablized on final approach with speed and configuration ready for landing before descending to 1000 agl. In practice, I try to cross the FAF at a speed that allows me to drop the final notch of flaps (eg from 25 to 30) passing the FAF.