Accident Overview
A biplane, occupied by a commercial pilot and a pilot-rated passenger, was en route to a fly-in gathering in another state. Witnesses observed the aircraft flying over a frozen lake at low altitude and low airspeed. One witness reported the airplane "listing" left and right before it entered a left turn and disappeared from view. Other witnesses saw the airplane turn left and nose-dive into the ground. A postimpact fire consumed most of the airplane.
Wreckage Examination
Damage to the wreckage indicated that the airplane impacted the ground in a nose-down attitude. Examination revealed no evidence of any preimpact anomalies with the airframe, engine, or control system.
Weather Conditions
A witness reported wind from the south at about 30 mph at the time of the accident. However, a burnt area extending east from the impact point indicated wind from the west. Nearby weather stations reported varying conditions: one station about 14 miles west-northwest of the accident site reported calm wind; another about 11 miles south recorded wind from the west at 11 knots with gusts to 27 knots near the accident time, and wind from the west at 33 knots with gusts to 48 knots about an hour later. The forecast for the area called for wind gusts to 40 knots from the west-northwest. Therefore, it is likely that strong gusty west winds prevailed in the accident area at the time of the accident.
Pilot Maneuvering
Some witnesses speculated that the pilot may have been attempting to land on the frozen lake, but the airplane was not equipped to land on ice. The reason the pilot was maneuvering at low altitude in strong gusty winds could not be determined. Based on witness observations and damage, it is likely that the pilot allowed the airspeed to decrease to a point where the critical angle of attack was exceeded, and the airplane entered an aerodynamic stall/spin.
Medical and Toxicological Findings
Although the pilot was known to have heart disease, it is unlikely that this condition contributed to the accident. Witness observations indicate the pilot was actively flying before the loss of control. Toxicology testing showed the presence of chlorpheniramine in the pilot's blood at a level likely within the therapeutic range. Chlorpheniramine is a sedating antihistamine available over the counter, with a warning that it may impair mental and/or physical ability required for hazardous tasks. Its sedating effects can slow psychomotor functioning, cause drowsiness, and suppress visual-spatial cognition and visual-motor coordination, which would have been necessary for the pilot to maintain control while maneuvering close to the ground in strong gusty winds. Therefore, it is likely that the pilot's ability to safely operate the airplane was impaired by the effects of chlorpheniramine.
Probable Cause
The pilot's failure to maintain sufficient airspeed while maneuvering at low altitude in strong gusting winds, which resulted in exceedance of the airplane's critical angle of attack and an aerodynamic stall/spin. Contributing to the accident was the pilot's impairment due to the effects of a sedating antihistamine.