(this is when aircraft skin temp. is lower that outside air temps. due to the loss of heat
The temperature of the skin cannot drop below the temperature of the outside air. Heat transfer between the skin/air only takes place when there is a temperature difference between the two. Unless you add/remove heat from the skin with a device, the skin will always reach the outside air temp eventually, point at which heat transfer between the two ceases (yes stagnation points will raise the air temp locally depending on your airspeed, but let's keep it simple
). If the skin temperature were colder than the outside air, then heat would flow from the air to the skin until both reached an equal temperature.
The problem with all these temperature/wind chill definitions is that human senses are simply terrible for determining temperature. If you stand outside in -10ºC weather and dip your hand in a stream of water, your brain will tell you that the water is much colder. Yet the water is probably near 0ºC, warmer than the outside air. But your body loses heat much faster to the flowing water and your brain interprets that as a lower temperature.
One experiment that was always fun to try on physics students was to get a hold of samples of several different materials (piece of styrofoam, wood, marble, and metal) and ask them to estimate the temperature of each by touching them. Then they were provided a thermocouple to take actual readings. All 4 were at room temperature, but by touch styrofoam and wood feel "warm" since they do not conduct heat away from your hand as well as the marble and metal pieces. The styrofoam and wood samples would always end up with marks where the students tried to drive the thermocouple in an attempt to get a "warmer" reading than the metal/marble samples as their senses had told them to expect