Narrative:

We declared the field in sight and were cleared for the visual approach; with our cleared descent to 2000 feet changed to 3000 feet. We were given a 7 mile base turn restriction. Tower reported a challenger on final as the traffic we were following. We saw it on TCAS over the final approach fix but never called it 'in sight' to ATC; therefore were never advised to maintain visual separation. The challenger was on short final when tower eliminated the 7 mile restriction and gave us base turn at our discretion. I assumed this was because traffic separation was assured. I turned base. Visibility was VFR; but the vog (volcanic fog) made it difficult to discern finer visual cues. Depth perception suffered. My base turn was at 175 knots; gear down; flaps 15. Intercepting final; the PAPI showed I was above path. We quickly configured into final landing configuration. It got busy. Fixated as I was on flying; cross-checking inside and outside; it was a shock when we got a traffic TA.we then realized there was a smaller airplane between us and the runway. It was not the challenger previously reported; but rather a caravan. Where did this plane came from? If this traffic had been called out by tower; neither the pilot monitoring nor I recall hearing it. I had the awful thought how lucky we'd been to roll final behind it; and not on top of it. This airplane was going s-l-o-w. A go around was the only decision.the pm informed ATC we were going around; and I hit the toga button. Immediately; ATC advised the aircraft in front to go around instead. Apparently; ATC told the aircraft to turn right; a nuance I (and the caravan pilot) missed. I hesitated in completing the actions for a go around. I knew when I went around I would lose sight of this airplane which was not yet turning; but was climbing. I was now clearly out of profile and out of any other option but going around. I was uncertain which way the caravan would turn. The caravan pilot asked for go around instructions. ATC advised the caravan to go right. We immediately went left. I completed the go around and the pilot monitoring told ATC we would head west and climb to 3000 feet. ATC acknowledged. They had not yet issued any formal go around directions to us. I admit; I was totally rattled. I did not expect to see an aircraft so close on final. At tower controlled fields and when cleared for visual approaches; when we haven't declared the traffic in sight; I'm used to hearing traffic callouts with closure rates. We both sincerely believed our challenger traffic had landed (which it had). The go around was ugly. I was so concerned about hitting something and was so outside; it was hard to pull myself inside. As soon as I initiated; I indeed lost sight of the airplane. We practice go arounds during training but those always come with a heading (usually initially straight ahead) and an altitude and never requiring a steep turn while overrunning a smaller airplane also climbing out in front whose direction of turn is unknown. Afterwards; I conversed briefly with the challenger pilots. They saw and heard the whole thing from the vantage point of a taxiway. They said that tower's go around instructions to the caravan were stepped on; and the caravan had to ask which way to turn. They also wondered; as we did; why we were not given the go around; instead of the caravan. Having the airplane in front of us go around; when we were the faster airplane and above it; put us both in jeopardy. Stepped on radio communications is an annoyance; until your life depends on it. Then; it's tragic. Cannot technology find a way to prioritize some communications over others? Cannot ATC have an override button; so that urgent instructions are not stepped on?

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Original NASA ASRS Text

Title: B737 Captain reported initiating a go-around after observing an unexpected Caravan lined up ahead for the same runway.

Narrative: We declared the field in sight and were cleared for the visual approach; with our cleared descent to 2000 feet changed to 3000 feet. We were given a 7 mile base turn restriction. Tower reported a Challenger on final as the traffic we were following. We saw it on TCAS over the Final Approach Fix but never called it 'in sight' to ATC; therefore were never advised to maintain visual separation. The Challenger was on short final when Tower eliminated the 7 mile restriction and gave us base turn at our discretion. I assumed this was because traffic separation was assured. I turned base. Visibility was VFR; but the vog (volcanic fog) made it difficult to discern finer visual cues. Depth perception suffered. My base turn was at 175 knots; gear down; flaps 15. Intercepting final; the PAPI showed I was above path. We quickly configured into final landing configuration. It got busy. Fixated as I was on flying; cross-checking inside and outside; it was a shock when we got a traffic TA.We then realized there was a smaller airplane between us and the runway. It was not the Challenger previously reported; but rather a Caravan. Where did this plane came from? If this traffic had been called out by tower; neither the Pilot monitoring nor I recall hearing it. I had the awful thought how lucky we'd been to roll final behind it; and not on top of it. This airplane was going s-l-o-w. A go around was the only decision.The PM informed ATC we were going around; and I hit the TOGA button. Immediately; ATC advised the aircraft in front to go around instead. Apparently; ATC told the aircraft to turn right; a nuance I (and the Caravan pilot) missed. I hesitated in completing the actions for a go around. I knew when I went around I would lose sight of this airplane which was not yet turning; but was climbing. I was now clearly out of profile and out of any other option but going around. I was uncertain which way the Caravan would turn. The Caravan pilot asked for go around instructions. ATC advised the Caravan to go right. We immediately went left. I completed the go around and the Pilot monitoring told ATC we would head west and climb to 3000 feet. ATC acknowledged. They had not yet issued any formal go around directions to us. I admit; I was totally rattled. I did not expect to see an aircraft so close on final. At tower controlled fields and when cleared for Visual Approaches; when we haven't declared the traffic in sight; I'm used to hearing traffic callouts with closure rates. We both sincerely believed our Challenger traffic had landed (which it had). The go around was ugly. I was so concerned about hitting something and was so outside; it was hard to pull myself inside. As soon as I initiated; I indeed lost sight of the airplane. We practice go arounds during training but those always come with a heading (usually initially straight ahead) and an altitude and never requiring a steep turn while overrunning a smaller airplane also climbing out in front whose direction of turn is unknown. Afterwards; I conversed briefly with the Challenger pilots. They saw and heard the whole thing from the vantage point of a taxiway. They said that tower's go around instructions to the Caravan were stepped on; and the Caravan had to ask which way to turn. They also wondered; as we did; why we were not given the go around; instead of the Caravan. Having the airplane in front of us go around; when we were the faster airplane and above it; put us both in jeopardy. Stepped on radio communications is an annoyance; until your life depends on it. Then; it's tragic. Cannot technology find a way to prioritize some communications over others? Cannot ATC have an override button; so that urgent instructions are not stepped on?

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.