Mach speed discrepancy in A/P

Hi all,

On some aircraft like the A350 and A380, when I set the Mach speed in autopilot (e.g. Mach 0.85), the actual speed the aircraft is being commanded at is M0.83/0.84. Is this normal behaviour?

If I wish to cruise at Mach 0.85, do I refer to the airspeed indicator, or the autopilot SPD button?

Thanks!

On the A380, this is mainly due to the model and physics being extremely outdated.

Haven’t had this issue on the A350 so idk…

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To be honest with you, it has happened to me too on several aircraft. I think it’s either because of headwinds or crosswinds or anything else like turbulence which may keep the aircraft lower than set. I could be wrong, so don’t take this as the possible issue.

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If you fly the ERJ you’ll notice the Mach has three decimal places like 0.849 for example. I think the autopilot targets 0.850 for example but sometimes slips down to 0.849 and so you see 0.84 displayed on other aircraft. But on the ERJ you can see how close it is to 0.85

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I believe winds should only affect the difference when the wind speed is changing and it takes some time for the AP to accelerate or decelerate to match the change.

If the winds are steady over a long time, there would be no effect because Mach speed is a TAS: it measures how fast you move within the moving air. So it makes no difference because you are moving with the wind.

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For the A350, what I have encountered is the actual mach speed being at 0.84 when the selected mach speed is 0.85. For the A380 the actual mach speed was at 0.83.

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