As aircrafts transit thousands of miles through the sky, they settle at appropriate altitudes & maintain cruising, all palpable to those inside who feel each pitch & acceleration. Asking pilots, they will often just tell you plainly “it’s flat,” which checks out. Then viewing artificial horizons during flight more or less confirms it, as once jostling & adjusting are completed, the plane balances the line to what is straight & level, no dip.
