Which is an inertial reference frame (or at least a very good approximation of one)? Which is an inertial reference frame (or at least a very good approximation of one)? A jet plane flying straight and level at constant speed. A jet plane turning takeoff, a jet plane flying straight and level at constant speed, AND a jet plane turning at constant speed. A jet plane during takeoff. A jet plane flying straight and level at constant speed AND a jet plane turning at constant speed. A jet plane turning at constant speed.