YOU SAID:

They're playing the piano while flying in the plane.

INTO JAPANESE

彼らは飛行機で飛んでいる間ピアノを弾いています。

BACK INTO ENGLISH

They are playing the piano while flying on a plane.

INTO JAPANESE

彼らは飛行機に乗ってピアノを弾いています。

BACK INTO ENGLISH

They are playing the piano on an airplane.

INTO JAPANESE

彼らは飛行機でピアノを弾いています。

BACK INTO ENGLISH

They are playing the piano on an airplane.