Abstract:Ice on airplanes can cause serious accidents. In order to understand the characteristics and mechanism of natural freezing of aircraft under the background of cold vortex, seven flight detections were conducted using Y-12 aircraft under the upper-air cold vortex in Hulunbuir in May 2021. Among the seven flight detections, two moderate and two intense ice accumulation occurred. This paper discusses the atmospheric environments suitable for aircraft icing detection. Characteristics and mechanism of aircraft icing are speculated based on data obtained from the Droplet Measurement Technologies, Inc. (DMT) equipments carried by the aircraft for the two intense aircraft icing processes on 31 May 2021. The results show that while the aircraft was flying at altitudes of about 3.5-4.2 km, with 〖JP2〗the ambient temperature of -8-〖JP〗-4℃, intense aircraft ice accumulation occurred in the water vapor saturated area in the southeast of the cold vortex. The averaged supercooled liquid water content （LWC） during the high LWC value periods could reach as high as 0.25-1.04 g·m-3, leading to instantaneously aircraft icing, and the particle number concentration increased to 100-200 cm-3 for drops and ice particles with mean effective drop diameter in the range of 15-100 μm. Particle images and their growth patterns in the southeast of the cold vortex were brought out. Finally, the atmospheric icing conditions in the southeast part of the cold vortex are discussed.