When you get on a plane, enabling Airplane Mode should turn off all the radios on an iPhone or iPad. You say flight mode doesn't work, so is this no longer the case?
And with Apple now being inconsistent about what a on/off toggle actually does... It's hard to tell what toggling flight-mode actually results in.
And if I were in the airline business, I would take an issue with that.
EDIT: This is what Airplane Mode looks like https://i.imgur.com/mEMYdL6.jpg https://i.imgur.com/4L3uZmm.jpg