During the field experiment EFEDA-91 in central Spain the turbulent fluxes of sensible and latent heat were measured by an aircraft and by surface stations. A comparison of surface measurements and turbulent flux values derived indirectly by simple linear extrapolation of the aircraft-measured fluxes to the surface, shows a general disagreement. Part of the difference can be explained by the aircraft ground speed variation in a convective boundary layer.A second topic is the development of a simple model for calculating aircraft speed under the given meteorological conditions. With this tool it is possible to give an estimation, whether an aircraft is suitable to flight with rigid altitude control in a boundary layer with strong convection.