swift, 163 bytes

Result

AC

Duration

1072ms

Code [DL]

var b=false,c=0,d=1
while let l=readLine(){
print(String(
l.map{(c:Character) -> Character in
if"T"==c{d=0}
b="K"==c||b&&d>0
d=(d+1)%50
return d==1||b ?"a":c
}))
}

stdin

                 T                                
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
              K                                   

stdout

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              aaaa                                

stderr

strace