B737-800 flight crew reported receiving TCAS 'descend' commands on a parallel approach to dfw that t...

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 first officer 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.

Original NASA ASRS Text

Title: 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.

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1276702
Time
Date	201507
Local Time Of Day	0001-0600
Place
Locale Reference	DFW.Airport
State Reference	TX
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Route In Use	Visual Approach
Flight Plan	IFR
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Conflict Airborne Conflict Deviation - Altitude Excursion From Assigned Altitude

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.