[BUG] Autopilot loses altitude on sharper horisontal turns (LNAV+VNAV)

[hheexx]

Version of the mod latest dev

Describe the bug While on LNAV and VNAV autopilot if there is sharper horisontal turn autopilot will not follow flight director but will start losing altitude very fast...

To Reproduce

Expected behavior stay at same altitude

Screenshots

References

Additional information Is this bug present in the default 747-8, or only in the mod?

Your Discord: hex#7106

[giridat]

Download and re-install fresh DEV version all re-worked AP system, VNAV and further tuned LNAV.

(Issue to be closed or resolved)

Thanks

[hheexx]

Issue is not resolved in latest DEV

[giridat]

Request you to please include a video of your issue to better understand the bug.

Also, ensure your flight model is set to modern.

[Carodejko]

Its true, descent till 3000ft is correct, but with latest DEV version dowloaded today autopilot at 3000ft lost rapidly altitude by himself for unknown reason.

[giridat]

Its true, descent till 3000ft is correct, but with latest DEV version dowloaded today autopilot at 3000ft lost rapidly altitude by himself for unknown reason.

Have you check flight model to be modern? because I haven't such issue at all in my recent flights.

https://github.com/saltysimulations/salty-747/releases/download/vdev/salty-747-dev.zip

just to be sure remove everything related to Salty and re-download from the above link and install.

[Carodejko]

Hello, can confirm its last DEV version and flight model to MODERN is set.

[mobbe123]

I have the same issue. On LNAV and VNAV then set "approach" on localizer and a few second after intercepting the glide slope, the aircraft starts the descent properly but then it starts to pith down to a crazy rate of descent until it crash or I take control! This is happening at every flight.

[hheexx]

Yea. One more info: Flight director shows good path but AP is pushing plane below FD line.

[giridat]

AP isn't polished properly but your issue, I have tried alot but haven't been able to reproduce it. My flights and sharp horizontal turns have been fine. I'll let the devs check the issue because I haven't been able to face any such issue.

[flthal9]

I was experiencing the same issue when I was on LNAV, HDG SEL, and LOC modes. Long story short, at least in my case, this only happens when the "Total Takeoff Weight" in the "Fuel" window exceeds 900,000LB. Also, I was able to reproduce the same issue with Asobo's default B747, so this issue is most likely not directly related to Salty, but rather something related to Asobo's flight models. By adjusting the payload and bringing the "Total Takeoff Weight" below 900,000LB, I was able to resolve the nose dive issue.

To reproduce the issue

• I've tested this in MSFS Sim Update 7. No additional mods in the community folder.
• I was able to reproduce the same results with or without Salty.
• Load Asobo's default B747 in the vicinity of RJAA (location most likely does not matter). Bring the aircraft to 10,000ft at 250KIAS with auto-thrust enabled and altitude hold mode turned on. In the "FUEL" window, bring the "Total Takeoff Weight" above 900,000LB by sliding the payload slider to the right. With 25 degrees max bank angle, make a 180 degrees turn with "HDG SEL" mode. After the AP starts banking, it starts to lose altitude rapidly in a few seconds (3,000 - 5,000fpm drop). When I did the same experiment while "Total Takeoff Weight" was below 900,000LB, the AP was able to maintain the altitude while banking left or right.

Details

I was always using the "REQUEST" button in the "PERF INIT" page in the FMC to pull the weights configuration from Simbrief during pre-flight.

When I checked the "FUEL" window, however, this appeared to overload the payload and it frequently brought the "Total Takeoff Weight" above 900,000LB. I'm still a beginner flight-simmer, so I'm not sure if Salty is pulling the accurate payload information from Simbrief or Simbrief itself is calculating the payload wrong (I always set "Passengers" and "ZFW" to "AUTO" on Simbrief). I'm not sure if the original requester's issue is due to the same root cause as mine, but I hope this information helps!

[DesertRick13]

The most accurate RL 747-8 stats come from the Boeing website (Search 747-8 airport compatibility, and you'll find a lot of information). One parameter I wish Asobo would put in the weights table is ZFW, Zero Fuel Weight. It can be found on the FMC once you're in, but would be nice to have it as you are adding payload. The RL ZFW in a 747-8 is 651,000 and a 7478F is 727,000 (I go with the Freighter guidelines, but usually for no more than 8-10 hour flights.) It makes perfect sense that in a sharp turn at a slow airspeed, the aircraft will lose altitude. At FL100 and above, the airspeed really should be 340. In the simbrief, it has "Climb Profile" for the 747-8. It is 250/340/84, the 340 referring to IAS above 10,000 ft. I like to fly as a BCF. A quick and dirty way to approximate max ZFW and maintain good Center of Gravity is to add 121,000 lbs in the rear baggage and 60,000 in the front baggage, with everything else in the payload section default Asobo. I often take off at 987, 000. But watch the airspeed, and use flaps as appropriate until above 285 KIAS.

[flthal9]

As you mentioned, it looks like speed and flap settings do seem to play a role. Even at above 900,000LB, nose dive did not occur if the aircraft was at 340KIAS at 10,000ft. Similarly, at 5,000ft and 250KIAS, the nose dive didn't happen while turning if I brought the flaps to the correct configuration indicated in the PFD. Again, I'm not sure if my issue is related to the original requester's problem, but I hope this sheds light on the issue hheexx was facing as well. Thanks a lot for the pointers!