Flame flickering of jet diffusion flames reflects the instability of the hot gases ascending column, This phenomenon, which is related to buoyancy, has been characterized in terms of pressure and gravitational force by experiments conducted on the ground and on board of an aircraft. The measurements show that the relationship between the frequency F and the gravity g is very close to F∼ & g2/3when the burners have a sufficiently small outlet diameter. These measurements are entirely in agreement with a theoretical model based on the hydrodynamic instability of the layer of burnt gas under the influence of buoyancy. It is also confirmed, by varying the ambient pressure, that a model of the form F ∼ P1/3closely fits the observed phenomena with gas jet diffusion flames.