Autopilot Troubles

I’ve noticed that on heavy aircraft, particularly the 747 and A380, that when I set the autopilot to, say, Mach 0.85 it will only go to Mach 0.83 or Mach 0.84. I’m currently flying the A380 and I’ve set the autopilot to Mach 0.85 yet it will only go to Mach 0.83 on the HUD. Is this to do with lack of engine power? Weight? Winds?

I’ve done many flights on the 787, 777, etc. and they cruise at the set autopilot speed - even with extensive loads.

I believe it’s the old flight models of the planes. I could be wrong but I noticed that issue as well, it’s not a big deal as it only adds maybe minutes on to your flight. Or you could set your throttle higher, until your true air speed indicator I think it’s called shows .85

The 744 has a known issue. You need to set it a few notches higher (I.e. .86 for cruising at .84).

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Yeah, it’s not particularly a huge deal because 0.01 Mach is only 10 knots-ish. Just kind of annoying.

Will try that, thanks.

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It can be annoying at times. Luckily there are topics available for these aircraft to get reworks where this issue will most likely get fixed if a rework were to occur. 😉

Tall* and it depends I guess I’ve flown it many times and it’s AP speed and the true speed never match so it’s a guess if you’re going to get the right speed or not

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