Narrative:

As the first officer; I was the pilot flying. Around 15-20 miles at 6;000 feet; we were at approximately 200 knots; flaps 5 and had just intercepted the localizer to xxr (cleared for the approach) and with the approach tile armed to intercept the glideslope. We were then told to reduce to minimum approach and to not descend below 5;000 feet. We configured to flaps 20; dropped the landing gear; and bugged flaps 20 minimum maneuver speed. On the TCAS; about 3 miles in front we could see another aircraft 2;000 feet below (the aircraft was so low; we weren't sure if he was crossing or actually on the approach). We leveled at 5;000 and continued slowing; but the controller wasn't comfortable with the distance and asked us either to 'turn right 30' or to 'turn right to 330.' we never got a chance to clarify; as things started happening rapidly.we discussed briefly the best way to discontinue the approach; and I hit the toga switch; as we needed the aircraft out of the approach mode. The aircraft appeared to begin climbing even though 5;000 feet was set in the altitude window. Rather than wait and see if the trend continued; I disconnected the auto throttles and autopilot and manually started the turn. As I asked the captain for clarification on the heading; and while attempting to avoid overshooting it; I noticed the airspeed dropping below flaps 20 minimum maneuver. We had been attempting to slow down this whole time; and the throttles were at idle with the gear down. Although I initiated the correct throttle input immediately; it took several seconds to spool up and the speed went below flaps 20 minimum maneuver before climbing back up; almost prompting me to begin descending to avoid losing any additional speed.I asked for gear up; and right at that moment the controller told us to turn to a heading of 240; descend to 3;000 and resume the approach. We had both assumed that we'd been vectored off the approach in order to be sent around for a new one; and so we were caught by surprise when he wanted us back on the ILS. In our haste; we never selected flch after inputting 3;000 which added to some confusion as I was trying to look 'through' the flight director. We got configured yet again and tried to re-automate. Once on the localizer and glideslope; we noticed there was still only a 2.5-3 mile distance to the aircraft in front of us. We weren't sure if this was a different aircraft or the same one; but we discussed going around and were prepared for the possibility.we were fully stable by 1;500 feet and weren't gaining on the aircraft in front; but the distance was still uncomfortable. At this point; the tower controller asked us if we could accept the visual to xxl. With the short separation; and not wanting to deal with the same controller during a go around again; I told the captain that I would prefer the visual to xxl rather than continuing to xxr. He agreed; and we made an uneventful visual approach to xxl and did a thorough debrief in the chocks.the big takeaway for us was remembering how to handle a discontinued approach as well as how best to handle being put back on that same approach. We were really put in a difficult position by the controller; but the fact that you cannot easily go out of approach mode in the 757 makes it very challenging at times. Also; how often do the controllers vector 757s off of the approach due to minimum separation with slower aircraft? It could be a reoccurring theme.

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Original NASA ASRS Text

Title: B757-200 First Officer reported difficulties dealing with an airborne conflict on approach to landing.

Narrative: As the First Officer; I was the pilot flying. Around 15-20 miles at 6;000 feet; we were at approximately 200 knots; Flaps 5 and had just intercepted the localizer to XXR (cleared for the approach) and with the approach tile armed to intercept the glideslope. We were then told to reduce to minimum approach and to NOT descend below 5;000 feet. We configured to Flaps 20; dropped the landing gear; and bugged Flaps 20 minimum maneuver speed. On the TCAS; about 3 miles in front we could see another aircraft 2;000 feet below (the aircraft was so low; we weren't sure if he was crossing or actually on the approach). We leveled at 5;000 and continued slowing; but the controller wasn't comfortable with the distance and asked us either to 'turn right 30' or to 'turn right to 330.' We never got a chance to clarify; as things started happening rapidly.We discussed briefly the best way to discontinue the approach; and I hit the TOGA switch; as we needed the aircraft out of the approach mode. The aircraft appeared to begin climbing even though 5;000 feet was set in the altitude window. Rather than wait and see if the trend continued; I disconnected the Auto Throttles and Autopilot and manually started the turn. As I asked the Captain for clarification on the heading; and while attempting to avoid overshooting it; I noticed the airspeed dropping below Flaps 20 minimum maneuver. We had been attempting to slow down this whole time; and the throttles were at idle with the gear down. Although I initiated the correct throttle input immediately; it took several seconds to spool up and the speed went below Flaps 20 minimum maneuver before climbing back up; almost prompting me to begin descending to avoid losing any additional speed.I asked for gear up; and right at that moment the controller told us to turn to a heading of 240; descend to 3;000 and resume the approach. We had both assumed that we'd been vectored off the approach in order to be sent around for a new one; and so we were caught by surprise when he wanted us back on the ILS. In our haste; we never selected FLCH after inputting 3;000 which added to some confusion as I was trying to look 'through' the flight director. We got configured yet again and tried to re-automate. Once on the Localizer and Glideslope; we noticed there was still only a 2.5-3 mile distance to the aircraft in front of us. We weren't sure if this was a different aircraft or the same one; but we discussed going around and were prepared for the possibility.We were fully stable by 1;500 feet and weren't gaining on the aircraft in front; but the distance was still uncomfortable. At this point; the Tower controller asked us if we could accept the visual to XXL. With the short separation; and not wanting to deal with the same controller during a Go Around again; I told the Captain that I would prefer the visual to XXL rather than continuing to XXR. He agreed; and we made an uneventful visual approach to XXL and did a thorough debrief in the chocks.The big takeaway for us was remembering how to handle a discontinued approach AS WELL AS how best to handle being put BACK on that same approach. We were really put in a difficult position by the controller; but the fact that you cannot easily go out of approach mode in the 757 makes it very challenging at times. Also; how often do the controllers vector 757s off of the approach due to minimum separation with slower aircraft? It could be a reoccurring theme.

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.