Aircraft in the UA and UB series employ a bladder-type fuel tank system in the wings. Later 1900Cs use a wet wing fuel system: entire sections of the wing are sealed off for use as fuel tanks. This design change allowed more fuel to be stored, substantially increasing the 1900C's range. The wet wing 1900Cs were assigned serial numbers beginning with "UC." These aircraft are also referred to as 1900C-1s. The wet wings proved popular, and the UC is the most common version of the low-ceiling 1900, with 174 UC airframes built.