Description: From Reeder (1989): "At 1900LT 28 April 1986, James Dickson, a resident of Unalaska, while on a Peninsula Airways Inc. flight from Dutch Harbor to Atka, observed a 500 m vertical steam plume over Makushin Volcano. The top of this plume trailed to the S for at least 40 km. James detected no obvious ash in the white steam plume. Pilot Tom Madsen, President of Aleutian Air Ltd. at Dutch Harbor, also flew to Atka from Dutch Harbor earlier the same day (left Dutch Harbor about 1120LT), but the volcano at that time was not visible due to cloud cover."
This activity probably does not constitute a volcanic eruption, and Simkin and Siebert (1994) classify this event as not an eruption. Additional reports are quoted below.
From Smithsonian Institution (1986, v. 11, n. 6): "On 28 April at 0700, James Dickson observed a 450 m vertical steam plume over Makushin that trailed SE for at least 30 km. He detected no ash in the plume."
From Smithsonian Institution (1986, v. 11, n. 4): "On 20 December, pilot T. Madsen (Aleutian Air) noticed anomalous amounts of steam rising from six large and closely spaced steam vents just E of the summit. The largest plume was 500-600 m high. No ash was observed in the white plumes. Air temperature at 2,400 m was -6.7°C, warm for that altitude. Steaming remained anomalously vigorous for the next two days before returning to a more normal level. Based on John Reeder's observations . . . since 1979, the summit steam activity is continuous and normally reaches heights of 100 m or slightly less."