# Speed discrepancy

Hello guys
I’m currently cruising at FL390 with the beautiful B777-200ER… My cruise speed is supposed to be M .84 as selected but it shows that the speed is M .83… What is the reason for this discrepancy?!

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hello! it all depends on winds as i’m aware :) but some else’s may be able to give you a better answer

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Yes, it is winds. Ground speed also is a factor when it calculates, as you do have a tailwind. Remember, ground and airspeed are very different.

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So do you mean that based on the winds; the selected speed corresponds to the calculated one?!

Yes. The newer models have been known to have some extra discrepancies. This is also more realistic to what happens irl. The speed of the plane will always be slightly deviated than what is calculated by environmental factors such as winds, altitude, weight, etc.

This is also prominently found on the A359 as well

hello!
also referring to what @Thunderbolt said! about airspeed and Ground speed being very different here is a video explaining in great detail what it is i hope you enjoy ur flight :) https://youtu.be/zCxMh_1n0uc

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I tried to increase it manually but after selecting SPD; it gets back to M .83

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Thanks a lot… I know this by heart… Indicated Air Speed is completely different from ground speed…

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Yes, a little bit of deviation is expected. If you want to be precise with the speeds, I suggest simply setting the aircraft to M 0.85, that way, the aircraft will actually fly at M 0.84

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While climbing; i increased the speed up to 315 KIAS which corresponds to M .84 but the aircraft was struggling to maintain 315 knots and instead it maintained 314 sometimes fluctuated to 313

AP sometimes does end up slightly slower than the requested speed when A/C is heavy. Nothing to do with winds.

Also, you could have set a speed extremely close to M.835

The aircraft wasn’t heavy… It was 22% loaded… I used to climb at 315 knots in all B777 variants which corresponds to M .84; the aircraft was struggling to maintain 315 knots which eventually resulted in M .83

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