2020 Alaska mid-air collision

NTSB narrative

Two airplanes, a float-equipped DHC-2 and a PA-12, collided in midair near a local airport shortly after departing from different locations. The DHC-2 was traveling northwest about 1,175 ft mean sea level (msl) and gradually climbing when it crossed a local highway, and the PA-12 was traveling northeast at about 1,175 ft msl north of and parallel to the highway.

A witness located near the accident site observed the DHC-2 traveling in a westerly direction and the PA-12 traveling in a northerly direction. He stated that the PA-12 impacted the DHC-2 on the left side of the fuselage toward the back of the airplane. After the collision, he observed what he believed to be the DHC-2's left wing separate, and the airplane entered an uncontrolled, descending counterclockwise spiral before disappearing from view. Both airplanes sustained substantial damage during the collision and impact with terrain.

An NTSB cockpit visibility study revealed that for the 53-second period before the collision, the PA-12 would have been unobscured and visible to the DHC-2 pilot through the DHC-2 left windshield. The DHC-2 would have been unobscured and visible to the PA-12 pilot through the PA-12 windshield for 28.3 seconds before the collision. Federal Aviation Administration guidance indicated that 12.5 seconds is the minimum time for a pilot to visually acquire another aircraft, judge a collision course, and take evasive action. Therefore, there was adequate time for both pilots to see and avoid the other airplane.

The airplanes flew on converging flightpaths for about 1 minute before the collision. The NTSB determined that, if a common traffic advisory frequency area (CTAF) had been established for the Soldotna area and an FAA requirement had been in place for pilots to communicate their positions when entering the CTAF, the accident might have been avoided. As a result, the NTSB issued recommendations to the FAA to require that pilots report their positions on the designated CTAF when operating in CTAF areas (A-22-4) and create additional dedicated CTAF areas in locations with a high risk for midair collisions (A-22-5).

Weather conditions about the time and location of the collision were characterized by a thin ceiling and high broken-scattered clouds resulting in a mix of direct sun and shaded conditions in the general vicinity both airplanes were operating in, which resulted in a complex background for both pilots. Perception of form is determined by a form’s interaction with background features, and the presence of a complex background can cause melding of a form with that background, making it harder to see. The sun was about 84° azimuth and its elevation was about 18° above the horizon, within 20° of the PA-12’s track. Consequently, to spot the DHC-2, the PA-12 pilot would have to be looking toward the sun.

The pilot of the PA-12 was denied medical certification in June 2012 due to open-angle glaucoma and had severely impaired peripheral vision. Additionally, even in his central vision where acuity was preserved, he very likely had significantly diminished contrast sensitivity, making areas of light and dark appear less distinct, and colors less vibrant. This likely impaired his ability to resolve lower-contrast features of a visual scene. His severe glaucoma also likely increased his susceptibility to adverse effects of glare. Given the environmental and geometric conditions of this accident, it is likely that the PA-12 pilot’s severe visual impairment reduced his opportunity to identify the Beaver and avert the collision.

The DHC2 was being operated as a Part 135 ondemand charter flight, and the PA-12 was operating as a Part 91 personal flight. The DHC-2 had no traffic awareness equipment installed, nor was it required by the Federal Aviation Administration (FAA), but ADS-B Out and In were installed on the PA-12 with avionics capable of providing traffic alerts similar to an airborne traffic advisory (ATAS)-capable device conforming to FAA DO-317B standards. However, it could not be determined if an alert was generated.

However, the NTSB believes the lack of a requirement for ADS-B In-based traffic awareness displays for all aircraft conducting Part 135 operations fails to take advantage of the demonstrated benefit of this technology in mitigating the midair collision hazard. In addition, aircraft without ADS-B do not demonstrate the “appropriate level of safety” for passenger-carrying operations conducted under Part 135 regulations. Therefore, the NTSB issued Safety Recommendation A-21-17 to the FAA to require the installation of ADS-B Out- and In-supported airborne traffic advisory systems that include aural and visual alerting functions in all aircraft conducting operations under 14 CFR Part 135.

An NTSB performance study concluded that if both aircraft had been equipped with airborne traffic advisory (ATAS)-capable devices conforming to FAA DO-371B standards, the PA-12 pilot would have received an alert 26 seconds before the collision and another alert 9 seconds before the collision. The DHC-2 pilot would have received an alert 26 seconds before the collision and another alert 19 seconds before the collision. It is likely that the pilots of both aircraft could have maneuvered to avoid the collision if their aircraft were equipped with ATAS-capable or similar devices, and these devices were operational.

Verbatim NTSB analysis and probable cause from the NTSB dataset