"C:/users/Milind/ACARS/output/"+context.fltnum+TalendDate.formatDate("yyyyMMddHHmmss", new Date()) + ".dt2"
The value in the context variable is correct. But you cant use it inside the filename. The file is opened before your data is processed and at this time the variable is null.
So the question is: Do you have different values (and would like to create a file for each line / a group of lines) or do you have only one value.
Only one value: You must process your input two times. In the first step just extract the context variable. In the second process your data.