F100 FLC CLRED TO LAND WHILE DEP ACFT CLRED TO TAKE OFF ON RWY INTERSECTING APCH COURSE AT DFW.

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ON FINAL RWY 17L; CLRED TO LAND BY TWR. DEPARTING ACFT RWY 13L. HEARD TWR GIVE RWY 13L ACFT CLRED FOR TKOF AFTER WE WERE INSIDE COPPL AT APPROX 400 FT. RWY 13L ACFT HAD LIFTED OFF; AND IT BECAME APPARENT HE WOULD BE AT OUR ALT ON NORMAL GS. NO MISSED APCH FOR THIS PASSING BELOW RWY 13L DEP. THIS COULD NOT BE REQUIRED SEPARATION AND IF IT IS; IT'S UNSAFE. TWR'S RESPONSE WAS 'YOU WERE TOLD OF DEP AND TO CALL TWR ON GND IF PROB.' THERE IS NO PLACE TO GO IN THIS SIT. IF WE MISS AND CLB WE ARE AT THEIR ALT -- CAN'T GO L OR R. AT ANY OTHER PLACE THIS HAD TO BE A NEAR MISS. WE HAD TO DUCK UNDER GS TO AVOID PROB. AND IF THIS TKOF PROC IS LEGAL AT NIGHT; IT'S AN ACCIDENT LOOKING FOR A PLACE TO HAPPEN. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR INDICATES THAT WHILE CONDITIONS WERE VFR; THE PROX OF THE ACFT DEPARTING ON RWY 13L WAS A SURPRISE TO HIS FLC. HE MENTIONED THAT SINCE THE INCIDENT IN QUESTION; HE HAS LEARNED THAT THE DFW TWR FREQUENTLY USES RWY 13L TO DEPART SMALLER ACFT BTWN ACFT ARRIVING ON RWY 17L. HE INDICATES THAT THE DEPARTING ACFT IN THIS INSTANCE; APPARENTLY BECAME AIRBORNE SOONER THAN NORMAL; CAUSING THE POTENTIAL CONFLICT. HIS CONCERN IS THAT WHILE THIS PROC IS ADEQUATE UNDER NORMAL CONDITIONS; AN IRREGULARITY COULD PUT BOTH ACFT IN DANGER.