Over the past few years, the integration of drones into cellular networks has been of particular concern as airborne users experience different propagation conditions than terrestrial users. This in turn necessitates telecommunications service providers such as cellular networks to assess their capacity in providing highly-reliable command-and-control links for drones through various key indicators as specified by Third Generation Partnership Project (3GPP) standard. To do so, as a mobile network operator, in this paper we concentrate on examining the network conditions that drones experience in flights through comprehensive field trial measurements conducted in a sub-urban area where the drone flies at different altitudes in our operational LTE network. In particular, we evaluate a target performance for the command-and-control connection with a reliability of 99.9% and a maximum one-way air interface delay of 50 ms, as required by 3GPP standard. The results of this study certainly reveal the practical potential of the current deployed cellular networks for drone operations and help operators to see how far they are from the stringent requirements of the 3GPP standard. Further, we evaluate other performance criteria that can help to unleash this emerging potential.