The U.S. NTSB is urging the FAA and the AOPA to engage in an educational campaign on risks that cataracts could pose to flight safety. The Safety Board made that recommendation as a result of its investigation of a Dec. 26, 2013 crash of a Cessna 172K in Fresno, Calif. That accident involved “a pilot with progressive cataracts who had demonstrated recent difficulty landing his airplane at night but was able to pass FAA medical certification vision testing,” the NTSB said.

Witnesses told NTSB investigators that, on the night of the accident, the pilot had attempted to land at Fresno Chandler Executive Airport (FCH), an unfamiliar airport to him. The runway lighting had been working correctly. On his initial attempt, the pilot landed hard midfield on Runway 30 and then took off again. In the second attempt, he approached from the opposite direction, descending to 10 to 15 feet above Runway 12. During a third try, the aircraft’s left wingtip struck a 62-foot-tall tree 1,400 feet from the approach end of Runway 30. The aircraft continued to fly over the runway, entered a left turn and crashed. The pilot and his passenger were killed in the accident.

The NTSB cited as probable cause “the pilot’s failure to maintain adequate clearance from trees while on approach, which subsequently led to a loss of airplane control. Also causal was the pilot’s continued operation of the airplane at night with a diagnosed medical condition that degraded his night vision.”

Another witness had observed the pilot struggling to maneuver around his home airport three weeks before the accident. “The pilot had been unable to find his way off the lit runway until the witness had driven his truck onto the taxiway to better illuminate it with the truck’s headlights,” the NTSB said.

The Safety Board noted that cataracts, which are areas of clouding of the lens of the eye, aren’t easily detected by standard eye exams. Symptoms include cloudy or blurry vision and glare or halos around lights. “This accident demonstrates the hazards cataracts pose to safely operating an airplane, especially at night,” the NTSB said. “Although the pilot was able to pass certification examinations and could see adequately to operate the airplane during the day, the glare around lights and diminished visual acuity created by the cataracts degraded the pilot’s ability to see and safely control the airplane at night.”

The FAA estimated that as many as 12,000 pilots, or 4 percent of the population, could have cataracts at any given time, based on overall population diagnosed with cataracts. “It is important for pilots to understand the effects cataracts can have on vision, especially night vision, and the potential effects on flight safety,” the Safety Board said.

The recommendation also comes as the NTSB has been highlighting the importance of medical fitness for duty, which was included in its 2016 "Most Wanted" list of transportation safety improvements.