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Autopilot disengaging on long flights

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Autopilot disengaging on long flights

Postby HFC3 » Sat Nov 03, 2018 1:48 am

Flying a 747-8. I've got a long distance flight setup from Phoenix to Chicago (OK, medium distance). Auto-speed set to 250. Everything works like a charm (autopilot wise) until about 90-120 minutes into the flight, and not always at the same point. For some reason, the AP is disengaging, doesn't appear to matter if it's in the middle of a turn or mid-leg. Once it's disengaged, I cannot get the AP to re-activate even if aircraft is at 0 climb 0 bank.

I've tried at 200nmh and 300, with no change in behavior.

How do I tell what caused the disengage and/or why it won't re-engage again?

I'm going to test with the 777 this weekend to see if it behaves the same way.
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Re: Autopilot disengaging on long flights

Postby HFC3 » Sat Nov 03, 2018 4:07 pm

777-200 appears to be doing something similar - I didn't get far enough for the AP to disengage as FG crashed about 2 hrs into flight.

However, I did notice the following;
AT set for 200
ALT set for 39,000

Once I finally closer to cruising altitude (w/in 10k ft) , the aircraft leveled off at about 32,000 and would not increase altitude AT lock. By using VS set to + value, I was able to coax it up to cruise altitude. Setting ALT lock to 39,000 would then maintain that altitude.

Everything was find for 30-45 minutes), then I started to get shaker noises and warning buzzer. Nothing else changed; level was staying at 39k, speed at 200. I reduced altitude to 35,000 thinking maybe I was pushing against 777 max altitude; no change. I tried again at 29,000 - same.

After about 30 more minutes, the aircraft started a gentle seesaw - nose dip about 5deg down and lose 1-2k alt, nose rise to about 5 deg up and regain the 1-2k ft. Movement was gentle, but constant. I reduced FL ultimately to 25,000 and this behavior continued. I tried increasing speed with no result - except airspeed increased and alt lost/regained was a bit more (2-3k). I tried reducing airspeed to 180, which only threw a stall alert requiring flaps set to 1 or 5; seesaw continued.

So, how do you determine what the alert warning is going off for? I know the shaker is supposed to activate when aircraft is approaching stall speed. But I have no idea why the warning buzzer went off earlier (at @ 30-45min).
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Re: Autopilot disengaging on long flights

Postby sidi762 » Sun Nov 18, 2018 12:55 pm


There are different versions of 777 and they have different autopilot. I've also experienced the 'seesaw' problem before on an older version, but not a single time in the current version, so please check if you are using the correct version, that might be the cause.

And I think 200kts for cruising is a little bit low. For a 777 at cruising altitude, the cruising speed should be around 0.82 Mach.

The cruising altitude depends on your weight a lot. If your aircraft is light, then I think the 777 in FG should be no problem above FL400, but if you have a heavy plane then usually the cruising altitude won't be higher then FL350. You might have pushed your plane too high, causing you to stall.

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Re: Autopilot disengaging on long flights

Postby Octal450 » Sun Nov 18, 2018 3:41 pm

Yes, please try the newest version. I rewrote the autopilot a while back to be more stable, so it should be okay now.


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Re: Autopilot disengaging on long flights

Postby HFC3 » Mon Nov 19, 2018 4:13 pm

I'll try that. I was thinking speed, but never that it was too low. Couldn't find anything general on cruising altitudes for either airframe, but remember vaguely pilots announcing high 30's and in TV low 40's, so I didn't thing I was too far off.
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Re: Autopilot disengaging on long flights

Postby Isaak » Mon Nov 19, 2018 9:16 pm

In the 777 and most other airliners you get both visual and aural indications of how fast and how high you should be flying. In the 777 there are are color codes on the speed tape on the primary flight display (PFD, the one straight in front of you with a blue and brown horizon). Stay away from the orange and red dashes/bars. When they come too close to each other, you are constantly on the verge of stalling and thus flying too high for the current weight. Normally you would cruise between Mach .80 and .85 in the 777 at altitude. You can switch from KIAS to Mach by pushing the mach/ias swith near the speed selection button. Normally you would fly somewhere between FL300 and FL400 depending on your weight and the distance you have to fly. The service ceiling in the 777 is 43100 ft, so FL430 is the absolute maximum when you are really light loaded. Have fun exploring the world of airliners. You can find a lot on the FG wiki to get started!
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