ICAO at global level and the EU at regional level require that instrument flight (IFR) trajectories remain within volumes of controlled airspace. To this end, a volume of specific dimensions is defined around the aerodrome, typically named Control Zone (CTR). Once outside this volume, the aircraft must integrate into other control areas, which are defined beyond the boundaries of the CTR. In many countries, in the absence of clear criteria, very restrictive aircraft climb/descent gradients are used to define the lateral boundaries of CTRs, which results in airspace volumes that are excessively large for the intended purpose. Defining volumes larger than necessary may constrain the operations of other types of aircraft, such as General Aviation conducting visual flights (VFR) in adjacent airspace or new users like Unmanned Aircraft System (UAS). This article proposes criteria for defining the lateral boundaries of CTRs based on ADS-B trajectories data from actual operations at several airports. The new criteria are based on the actual climb and descent gradients experienced by aircraft, and in particular, on the distance from the aerodrome reference point (ARP) at which the aircraft reaches a specific altitude (target height). The 99.9th percentile of flight paths were selected to define these new criteria. The results of applying the proposed criteria are compared with currently published dimensions for these zones and show that the volumes defined for the CTRs could be considerably reduced. The value of the reduction will depend on the particular aerodrome considered.