CCFL (countercurrent flow limitation) is an important phenomenon for numerous engineering applications and safety of light water reactors. In particular, the possible occurrence of CCFL in the hot-leg of a PWR during SBLOCA or LOCA accidents is of special interest for nuclear safety research. A theoretical review showed that despite numerous experimental works, many scaling and geometrical effects are still not fully understood (channel diameter, inclined riser length, and inclination angle). Since most experimental work has been done in down-scaled hot-leg simulators, it becomes interesting to increase the data base in order to safely extrapolate results to a full-scale hot-leg. Another goal is to provide high quality images of the phase interface for validating CFD codes. There is an increasing interest in performing 3D CFD simulation for CCFL in hot-leg geometries, and thus good experimental data and the development of more representative closure laws for fundamental processes (momentum transfer) are an essential part of the validation and development process. A two-phase flow test facility, COLLIDER, was constructed at the Nuclear Engineering Department at the Technical University Munich in order to investigate air/water CCFL phenomena in PWR hot-leg geometry under atmospheric pressure conditions. The facility concentrates on investigations in large diameter pipe (inner diameter 190 mm) rather than quadratic cross section that although it facilitates optical measurements but does not represent the real geometry. Experimental measurements related to CCFL phenomena are limited in large diameters and hot-leg geometry. COLLIDER represent an approximate 1/3 downscaled model of standard PWR hot-leg geometry with respect to channel diameter, horizontal length to diameter ratio, inclined length to diameter ratio, and 50° inclination angle. First tests were conducted in order to determine the onset of CCFL at different water inlet superficial velocities and for a detailed tracking of the events leading to CCFL occurrence while the gas velocity was gradually increased. Additionally, the deflooding point was determined by gradual decreasing of the gas velocity after CCFL onset in each test run. Consequently, a detailed phenomenological description of flooding/deflooding was obtained besides the important critical gas velocity at CCFL onset and at deflooding in Wallis parameters (JL*0.5, JG*0.5). The results cover low and medium water inlet velocities (JL,in*0.5 = 0.085 → 0.3). Critical gas velocities at CCFL onset show usual trend behavior (decreasing with increased water inlet velocities at low water inlet velocities and increasing with increased water inlet velocities at medium water inlet velocities, see Figure 6). The deflooding line follows a linear tendency quite well. A correlation for the deflooding line based on current results was proposed. Further investigations will include visual observations of the air/water interface for CFD validation.