Hi Milind,
now I got it. If you define a variable inside a tMap it is only visible to this tMap. If you need to store a value for later use, You should use a context variable for this. So define a context variable, add before your tMap where you defined the actual var a tJavaRow. In tJavaRow press the button "generate code" to generate the mapping from in to out for all of your metadada and add the following code as additional line:
context.fltnum= input_row.fltnum_new;
Now you could change the string for creating the file name to:
"C:/users/Milind/ACARS/output/"+context.fltnum+TalendDate.formatDate("yyyyMMddHHmmss", new Date()) + ".dt2"
Bye
Volker