Narrative:

Sfo ATIS reported cig 1800 broken; few 1200. Cleared for the ILS 28L; on 10 mile final with bridge and airport in sight. Given the ATC instruction to maintain 220 or better to the 7 mile final; we were at 240. Pairing traffic to 28R visually acquired by both pilots and communicated this to ATC who then told us to maintain visual separation. Both pilots recognized excessive closure rate to E170 and pilot flying started to pair airspeed towards 220 by disengaging autothrottle and using manual thrust (idle). Around this point (@9miles) ATC instructed 'slow to 180 knots to san mateo bridge'. Approaching san mateo bridge; we entered IMC (broken layer) with E175 @ 500 above; offset to 28R facility (final approach course); and slightly in trail. Pilot monitoring advised ATC that we were IMC and lost sight of pairing traffic. ATC responded with 'maintain visual separation'; to which pilot monitoring advised we were unable due to IMC. ATC instructed 'go around; maintain runway heading' (emphasis mine). By this point; we were VMC with field in sight again (@1400ft) but E175 was in the layer above. Pilot flying disengaged autopilot and began a methodical go-around conscientious of the presence of the E175 above and slightly in trail and that winds at altitude were @220/25kts. With this in mind; pilot flying instructed a slight offset upwind of rh to pilot monitoring to enter into heading window and @275 was entered with pilot flying executing a reduced rate climb back into the IMC. ATC nearly simultaneously instructed a left turn towards a downwind and remainder of FM GA/mapp was executed.

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Original NASA ASRS Text

Title: B737 flight crew reported entering IMC conditions and losing visual separation with landing aircraft on parallel runway while on an ILS approach. Go-around and missed approach conducted.

Narrative: SFO ATIS reported cig 1800 BKN; Few 1200. Cleared for the ILS 28L; on 10 mile final with bridge and airport in sight. Given the ATC instruction to maintain 220 or better to the 7 mile final; we were at 240. Pairing traffic to 28R visually acquired by both pilots and communicated this to ATC who then told us to maintain visual separation. Both pilots recognized excessive closure rate to E170 and Pilot Flying started to pair airspeed towards 220 by disengaging Autothrottle and using manual thrust (idle). Around this point (@9miles) ATC instructed 'slow to 180 knots to San Mateo Bridge'. Approaching San Mateo Bridge; we entered IMC (BKN layer) with E175 @ 500 above; offset to 28R FAC (Final Approach Course); and slightly in trail. Pilot Monitoring advised ATC that we were IMC and lost sight of pairing traffic. ATC responded with 'maintain visual separation'; to which Pilot Monitoring advised we were unable due to IMC. ATC instructed 'Go Around; maintain RUNWAY HEADING' (emphasis mine). By this point; we were VMC with field in sight again (@1400ft) but E175 was in the layer above. Pilot Flying disengaged autopilot and began a methodical go-around conscientious of the presence of the E175 above and slightly in trail and that winds at altitude were @220/25kts. With this in mind; Pilot Flying instructed a slight offset upwind of RH to Pilot Monitoring to enter into HDG window and @275 was entered with Pilot Flying executing a reduced rate climb back into the IMC. ATC nearly simultaneously instructed a left turn towards a downwind and remainder of FM GA/MAPP was executed.

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.