Piper PA 25 N4369Y accident near Llano, California

Probable cause

The pilot's loss of airplane control during the landing approach due to an incapacitating medical event.

NTSB analysis

The tow plane pilot had performed six uneventful glider launches on the morning of the accident with turnaround times of about 15 minutes. The seventh launch was uneventful, and, after releasing the glider, the pilot made an appropriate radio call and was observed normally entering the downwind traffic pattern. Video data and witness accounts indicated that, shortly after, the airplane descended well below pattern altitude and overshot the runway centerline by about 1/4 mile during the turn from the base leg to final approach. The airplane then began to maneuver erratically as it passed back across the centerline, and subsequently collided in a dirt field 900 ft southwest of the threshold of the gravel portion of the runway.

Postaccident examination of the airframe and engine did not reveal any anomalies that would have precluded normal operation, and the engine data monitor information indicated that the engine performed similarly for all of the flights on the day of the accident, including the accident flight.

The pilot was issued a second-class medical certificate 6 days before the accident. At that time, he reported having high blood pressure, which had been controlled with medication for many years. Two years before the accident, he had taken a cardiac stress test, which had to be stopped prematurely due to shortness of breath. Further, his wife reported that he had a bout of unusual dizziness while hiking on a hot day about 1 year before the accident.

The autopsy revealed hypertrophic heart disease, a condition associated with an increased risk of developing atrial fibrillation, an arrhythmia that typically causes a fast heart rate, which may lower blood pressure, even to the point of causing fainting. Onset of atrial fibrillation is typically sudden. It was a hot day, and there may have been a component of sweating and heat effects, particularly during the periods when the airplane was on the ground, which may have increased the amount of physiologic stress and the likelihood of an arrhythmia. The sudden onset of an arrhythmia can cause symptoms ranging from palpitations to loss of consciousness but leaves no evidence after death.

The lack of mechanical problems, the pilot's successful completion of six tow flights before the accident flight, the radar track, and the pilot's failure to make a radio call regarding an emergency are consistent with sudden incapacitation, and it is likely the pilot's incapacitation caused the accident.

Text from the NTSB. Open the NTSB docket for WPR15FA250 for the full record.