Narrative:

We were being vectored around cells on the arrival. We were given an altitude of 6000 ft and we were descending to that. My first officer was flying and we were accepting direct to a fix; with the clearance to deviate 15 degrees as needed. During this time the speed bug dropped uncommanded. I'm not even sure how low it went but it went extremely low (we couldn't see it) and the aircraft slowed drastically with the thrust levers. My first officer (first officer) was on top of it and we never got slow; but he had a handful with trying to overcome the thrust levers while flying in turbulence. I'm not sure if the ap kicked off; or he clicked it off; but I am trying to figure out what is going wrong and help; while guiding him around a cell. [The first officer] was trying to explain but was still battling the speed issue. Approach is calling us with instructions and I think he told us to maintain 11;000 ft but I'm not sure. I answered that we are battling a problem with the aircraft. He asked if we needed assistance and I said I didn't think so (I was still not totally sure; and was a bit shaken). He asked if it would help if he have us lower and I said yes. He assigns us 9000 ft and we descend to that. My first officer gets the speed bug spun back to match our actual speed and I get the ap on. The speed seems to maintain and the bug stayed as commanded. I told the controller we were ok; he was very nice and handed us off to approach control. We then were able to shoot the approach and land without incident. I think he had wanted us to maintain 11;000 ft and we didn't hear him. I never answered him or repeated it as I was still trying to understand what my first officer was battling. We were descending below (I'm not sure but I think we were at about 10;800 ft when I realized he was calling me). That's when I said that we were troubleshooting a problem with the plane. It was at a difficult time when we are dodging weather; troubleshooting (my first officer was quite stressed and just kept saying 'auto pilot...I need the auto pilot') in increasing turbulence. I think this is why it took me a few minutes to understand what was actually the problem. I completely understand why my first officer couldn't articulate better; because he was also trying to figure out what was wrong; how to maintain the aircraft and navigate through turbulence and weather.this was a true maintenance issue and I did write the uncommanded speed bug and uncommanded speed reduction up on that aircraft; with maintenance control. I do think that it might be helpful [training] to practice disconnecting the auto throttles when the there is an uncommanded speed reduction; in the sim. We get so use to relying on the at (autothrottles) that we just override them when we don't like what they are doing; instead of clicking them off completely; hand controlling the speed and then problem shooting the issue. For me; it was a sign that I need to click off the at more so I don't lose that comfort level of hand controlling the speed. I do think we could have trouble-shot this situation quicker if we weren't also working around weather and turbulence; but things usually go wrong when you are work saturated.

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Original NASA ASRS Text

Title: EMB-175 Captain reported a problem with the autothrottle and speed control during initial approach.

Narrative: We were being vectored around cells on the arrival. We were given an altitude of 6000 ft and we were descending to that. My first officer was flying and we were accepting direct to a fix; with the clearance to deviate 15 degrees as needed. During this time the speed bug dropped uncommanded. I'm not even sure how low it went but it went extremely low (we couldn't see it) and the aircraft slowed drastically with the thrust levers. My FO (First Officer) was on top of it and we never got slow; but he had a handful with trying to overcome the thrust levers while flying in turbulence. I'm not sure if the AP kicked off; or he clicked it off; but I am trying to figure out what is going wrong and help; while guiding him around a cell. [The First Officer] was trying to explain but was still battling the speed issue. Approach is calling us with instructions and I think he told us to maintain 11;000 ft but I'm not sure. I answered that we are battling a problem with the aircraft. He asked if we needed assistance and I said I didn't think so (I was still not totally sure; and was a bit shaken). He asked if it would help if he have us lower and I said yes. He assigns us 9000 ft and we descend to that. My FO gets the speed bug spun back to match our actual speed and I get the AP on. The speed seems to maintain and the bug stayed as commanded. I told the controller we were ok; he was very nice and handed us off to approach control. We then were able to shoot the approach and land without incident. I think he had wanted us to maintain 11;000 ft and we didn't hear him. I never answered him or repeated it as I was still trying to understand what my FO was battling. We were descending below (I'm not sure but I think we were at about 10;800 ft when I realized he was calling me). That's when I said that we were troubleshooting a problem with the plane. It was at a difficult time when we are dodging weather; troubleshooting (my FO was quite stressed and just kept saying 'Auto Pilot...I need the auto pilot') in increasing turbulence. I think this is why it took me a few minutes to understand what was actually the problem. I completely understand why my FO couldn't articulate better; because he was also trying to figure out what was wrong; how to maintain the aircraft and navigate through turbulence and weather.This was a true maintenance issue and I did write the uncommanded speed bug and uncommanded speed reduction up on that aircraft; with Maintenance Control. I do think that it might be helpful [training] to practice disconnecting the Auto Throttles when the there is an uncommanded speed reduction; in the sim. We get so use to relying on the AT (Autothrottles) that we just override them when we don't like what they are doing; instead of clicking them off completely; hand controlling the speed and then problem shooting the issue. For me; it was a sign that I need to click off the AT more so I don't lose that comfort level of hand controlling the speed. I do think we could have trouble-shot this situation quicker if we weren't also working around weather and turbulence; but things usually go wrong when you are work saturated.

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.