At a certain altitude, why does the speed do this?

Look at speed

Past FL 280, the autopilot selection menu for speed changes to Mach number, instead of knots. Hope that helps.

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At FL280 the speed turns into mach speed. That’s why.

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It does thank you

Over FL260 (or 280??) the A/THR selector changes to Mach, the standard cruise speed in Mach (I believe) is 0.78-83.

Two eighty.

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Just checking you don’t fly with your spoilers on do you ?

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I think he misinterpreted the “flight” selection on the spoilers

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https://www.quora.com/What-is-a-crossover-altitude

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Yea, the real question is why you have flight spoilers and flaps at FL 320

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Probably to slow down.

I wouldn’t see why he would try to slow down with throttle at 100% , anyways, back on topic