Accident Sequence
The pilot conducted two RNAV (GPS) runway 20 instrument approaches to Burley Municipal Airport, Burley, Idaho, in instrument meteorological conditions. The accident occurred during the second approach. For the first approach, the pilot flew with flaps up at speeds above the operator's training standard of 120 KIAS. After a low pass over the runway, likely to assess landing conditions per company policy, the pilot initiated a missed approach and requested to repeat the approach.
During the second approach, the pilot did not use flaps but reduced speed on the final approach leg. The reported weather had improved with visibility about 2.5 miles. The airplane intercepted and remained on the glide path to the stepdown fix. The last ADS-B plot recorded the airplane about 0.6 nautical miles from the displaced threshold, on the glide path, at an estimated 85 KCAS—slower than the airplane's 95-knot minimum speed for flaps up in icing conditions. Shortly after, the airplane descended about 130 feet below the glide path, striking an agglomerate stack atop a potato processing plant, fatally injuring the pilot and substantially damaging the airplane.
A witness reported seeing the airplane emerge from clouds and immediately enter a steam cloud from six other stacks before hitting the accident stack. Security camera footage captured the airplane in a wings-level, flaps-up, nose-high descent just before impact. The witness's account of engine noise increase and nose lift may have been an obstacle avoidance attempt.
Investigation Findings
Postaccident examination of the airplane, conducted hours later, revealed no structural icing on the wings and empennage. No mechanical malfunctions or failures were found. The flaps were up, and the airplane was loaded within specifications and center of gravity limits.
Between 2016 and 2017, the FAA conducted two aeronautical studies regarding the stacks. The first study determined many stacks were a hazard to air navigation, requiring mitigation. As an interim measure, the runway 20 VASI was taken out of service because stacks penetrated the obstruction clearance surface. After the plant proposed increasing stack heights, a second study determined the agglomerate stack and row-of-six stacks exceeded CFR section 77 standards and mandated painting with high visibility white and aviation orange and equipping with red flashing warning lights. The VASI was permanently removed.
On the accident day, the stacks had not been painted to the required standard. Warning lights were installed on five of the row-of-six stacks; the agglomerate stack's warning light was stolen after the accident, so its operating status could not be determined. The existing paint scheme and visible moisture from the stacks provided low contrast against the background. The FAA's Aeronautical Information Manual advises pilots to avoid overflight of exhaust stacks, but the accident stack was directly under the approach course.
Probable Cause
The probable cause as determined by the official investigation: The pilot's failure to maintain altitude during an instrument approach, which resulted in a descent below the approach path and impact with a vent stack. Also causal was the failure of the processing plant to correctly paint the vent stacks, which had been determined by the FAA to be a hazard to navigation due to their proximity to the landing approach path. Contributing to the accident was the likely distraction/illusion/obscuration created by steam from the processing plant, which intermittently obscured the runway.