On February 20, 1986, a Piper PA-15 (registration N4521H) was involved in an aviation accident near Mount St.helens, WA. Investigators recorded the probable cause as: The aircraft was unable to outclimb downdrafts created by high winds flowing across a lava dome inside a volcanic crater, due to its limited engine power of 75 horsepower. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
Two occupants were rescued after their low-powered aircraft became trapped inside a volcanic crater by strong downdrafts exceeding its climb performance.
What happened
The pilot was attempting to navigate an aircraft within the confines of a volcanic crater, specifically flying around a lava dome situated at an elevation above 6000 feet mean sea level. The aircraft in question was equipped with a relatively low-output engine rated at only 75 horsepower. As the aircraft entered the crater from the northeast, it encountered high winds that flowed across the lava dome. These wind conditions generated significant downdrafts that exceeded the climb capability of the small aircraft. Unable to maintain altitude or escape the sinking air mass, the aircraft became trapped within the crater environment.
The investigation
The primary factor identified was the aerodynamic interaction between the high winds and the topography of the lava dome. The specific geometry of the crater created a downdraft that the 75-horsepower engine could not overcome. This mechanical limitation of the aircraft's power-to-weight ratio, combined with the extreme weather conditions inside the crater, rendered escape impossible under the prevailing circumstances.
Findings
The incident highlights the dangers of flying low-powered aircraft in complex terrain with unpredictable weather patterns. The downdrafts created by winds flowing over the lava dome were the critical environmental factor. The pilot's decision to fly into this specific topographic feature resulted in the aircraft being unable to outclimb the descending air. Both occupants survived the ordeal and were rescued the following day.
Probable cause
The aircraft was unable to outclimb downdrafts created by high winds flowing across a lava dome inside a volcanic crater, due to its limited engine power of 75 horsepower.