Cessna T337G N37E accident near Plano, Illinois

Probable cause

The pilot’s failure to recognize the airplane’s low altitude and arrest the airplane’s descent on approach to the airport in dark night conditions, which resulted in controlled flight into terrain. Contributing to the accident was the pilot’s degraded night vision due to glaucoma and a cataract and his impairment from the use of diphenhydramine.

NTSB analysis

The private pilot was approaching the destination airport at the conclusion of a cross-country flight in dark night, visual meteorological conditions. According to radar track data, the pilot had maintained a gradual descent profile during the final 7 minutes of the flight. About a minute before the accident, during a routine conversation, the tower controller at the destination airport told the pilot that he saw the airplane on radar at 1,800 ft mean sea level (msl). At that time, the airplane was about 1,200 ft above the ground and descending. The airplane subsequently collided with trees located along a ridge at an elevation of about 645 ft msl. The destination airport was located at 712 ft msl. According to radar track and engine monitoring data, the pilot did not attempt to slow the airplane's descent before the accident. A postaccident examination of the airplane and flight instruments did not reveal any anomalies that would have precluded normal operation during the accident flight. Additionally, the pilot had selected a Kollsman window setting that would have minimized any altimeter indication errors.

The 80-year-old pilot had moderate-to-severe coronary artery disease, which can result in a sudden loss of consciousness; however, based on available radar track data and his communications with the airport tower controller, it is unlikely that his coronary artery disease contributed to the accident. The pilot also had longstanding, open-angle glaucoma, which had required a series of medical and surgical treatments over the years, and cataracts in both eyes. The cataract in the left eye had been removed in 2008, but the cataract in the right eye had recently increased in size. An ophthalmology evaluation in the weeks before the accident noted a significant increase in the size of a central scotoma (blind spot) related to longstanding glaucoma in the left eye, and declining corrected distant visual acuity bilaterally. Although the pilot met the Federal Aviation Administration medical certification standard of 20/40 vision in daylight conditions, the glaucoma and cataract likely impaired his night vision and, as such, impeded his ability to judge altitude using available visual cues in dark nighttime conditions. Additionally, toxicology testing identified diphenhydramine, a sedating antihistamine, in the pilot's blood at therapeutic levels. Research has shown that the use of diphenhydramine can impair cognitive and psychomotor performance. The pilot's failure to identify the airplane's low altitude during cruise descent further supports that he was likely impaired by the use of diphenhydramine.

Text from the NTSB. Open the NTSB docket for CEN14FA522 for the full record.