Date: 201507
Local time of day: 0001-0600
Location: DFW.Airport, TX
Flight conditions: VMC
Light: Daylight
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B DFW
ATC / advisory: Tower DFW
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude
Detector: Automation Aircraft RA; Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance
Contributing factors: Airspace Structure
Primary problem: Airspace Structure

Narrative: [Inbound to] DFW. ILS/visual approach 17L. Approaching DRAAK at 5000 FT. Received a TCAS RA command to 'descend; descend.' (No preliminary 'traffic' alert). We complied with the RA and advised tower controller. The FO sighted the aircraft above and to our right on the course for 17C. The TCAS indicated they were 400-600 FT above us and on a parallel descent path. We also had visual on the ground and the runway 17L. We were below 3000 FT when we received a TCAS 'level off' alert; and another 'descend' alert shortly thereafter. Finally got a 'Clear of Conflict' at about 1500 AGL when we offset slightly left of course and the other aircraft pulled slightly ahead. We also got a 'low altitude alert' from tower at about 2000 FT AGL; even though the controller was aware of our RA maneuver. The RA maneuver resulted in our being below the altitudes at INWOD and GBUSH. We were fully stable by 1000 FT AGL. If this had taken place in IMC conditions it would have been a much trickier situation. We question why the other aircraft did not receive a corresponding RA to climb. Additionally; our situational awareness was hampered by having the other aircraft on a different tower frequency. It should be noted that we received a 'LEVEL OFF' command; but no such command is listed in the table of [our] 737 Ops Manual. Lastly; if I had it to do over; I would have offset to the left of course slightly sooner in order to be clear of the conflict.


Second narrative: We did not experience a quick resolution to the conflict as the target aircraft was following the 17C GS and basically forcing us lower and lower. The entire process took place from 5000 FT just outside of DRAAK to approximately 1500 FT AGL when we finally received the TCAS 'clear of conflict.' After receiving the 'clear of conflict'; we leveled off and intercepted the ILS GS for a stabilized approach and uneventful landing.I'm curious to know why the tower controllers did not direct the aircraft on approach to 17C to level off and discontinue their approach; knowing that we were continuing to receive an RA to descend. Had we elected to go-around; we would have been going directly against the TCAS guidance. Breaking off the approach and making a turn to the east could have put us in conflict with traffic on downwind or traffic going into DAL. The captain did an excellent job of complying with the TCAS guidance; yet minimizing the descent rate to the extent possible given our altitude; and remaining cognizant of the obstacles left of course; all while slowing; configuring; responding to normal checklists; and listening to me...great job. Needless to say; this would have been quite a handful in IMC.


Synopsis: B737-800 flight crew reported receiving TCAS 'descend' commands on a parallel approach to DFW that took them down to 1500 FT and prompted a low altitude alert from Tower.