In high mobility scenarios, cell selection in preparation for handover is a non-trivial task. Particularly a user equipment (UE) installed on board a passenger aircraft faces a high handover frequency and strong interference from a terrestrial radio access network (RAN) due to the high velocities and altitude of the aircraft. Conventional reference signal received power (RSRP) measurements do not reflect the potential for interference mitigation in the aeronautical regime. Therefore, we incorporate beamforming into conditional handovers such that signal to interference power ratio (SIR) measurements are used for the cell selection in order to find a serving base station which provides the best SIR after beamforming. Link budget simulations to evaluate the proposed method are performed using real passenger aircraft flight trajectories and real world base station deployment data. The proposed method improves the link availability by 32 percentage points compared to the conventional handover procedure while decreasing the average handover interval by up to 0.6 s at the high UE velocities. We also show that a handover candidate filtering rule based on the direction of flight offers a trade-off by increasing the average handover interval by up to 2.3 s at the cost of availability.