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'A clock on a plane goes slower than a clock on the ground'.

It's the other way around actually. I always mix them up too. See e.g. https://en.wikipedia.org/wiki/Gravitational_time_dilation

Imagine a mechanical clock battling gravity to get its hands to move. Voila, instant mnemonic.

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I think the author may have been referring to time dilation due to relative motion of the clocks as opposed to gravity. In that case, to an observer on the ground, the clock on the plane goes slower than a clock on the ground.

Yes, you're probably right. Now I'll need to figure out which effect has the upper hand. Maybe later.

EDIT: Flying clocks around: https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experim... If compared to a clock remaining on earth, then it depends on the direction the plane is flying which effect wins.

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