Grumman G164 N6917K accident near Springfield, Colorado

Probable cause

The pilot’s loss of visibility in the cockpit due to foam created by the chemicals loaded in the hopper, which resulted in collision with terrain. Contributing to the accident was the combining of chemicals known to produce foam and the pilot’s failure to perform a jar test prior to flight as recommended.

NTSB analysis

The pilot had just departed the airport to perform an aerial application flight utilizing a combination of a herbicide and fertilizers. An employee of the operator reported that the pilot was having issues with foam exiting from the hopper, which was located in front of the cockpit. The pilot had returned to the airport prior to the accident to add more de-foaming agent to the hopper and had activated the circulating pump to correct the problem. The pilot took off and a witness reported observing white smoke coming from the cockpit prior to impact; however, the smoke was "granular in nature" and "looked solid." The airplane impacted a remote flat field, a postimpact fire ensued, and the airplane was destroyed.

After the accident, the pilot reported to his wife, that there was a reaction of the chemicals in the hopper, and it was foaming “really bad” while in flight. The foam was coming out of the hopper, and it covered the windshield. The pilot reported he couldn’t see while flying.

A postaccident examination of the airplane did not reveal any mechanical anomalies.

The airplane hopper had been loaded with a glyphosate herbicide, Atarrus fertilizer, and Kugler KQ-XRN fertilizer. The Kugler fertilizer technical sheet contained a caution stating that when mixing KQ-XRN with phosphate, sulfur, humic acid, and products containing glyphosate, a chemical foaming reaction may occur. Kugler also recommended performing a jar test when mixing KQ-XRN with any pesticides and fertilizers. Postaccident testing of the chemicals used by the pilot produced significant foam when they were combined and agitated. Neither the pilot nor the ground crew performed the recommended jar test.

It is likely the pilot was unable to maintain control during flight due his view from the cockpit being obscured from foam originating from the hopper, which resulted in an impact with terrain. The foam was the result of the mix of chemicals used in the hopper, which was a combination known to produce foam, and the pilot did not perform a jar test prior to flying with the chemicals as recommended. Had a jar test been performed the pilot likely would have recognized the foaming problem and adjusted the chemical mixture appropriately.

Text from the NTSB. Open the NTSB docket for CEN20LA278 for the full record.