NTSB Cites ADS-B Lapses In Alaska Midair Collision

NTSB investigators view wreckage from the DHC-2 Beaver that was involved in a midair collision near Ketchikan, Alaska in May 2019.
Credit: NTSB

A Safety Management System (SMS) might have forewarned the operator of a sightseeing floatplane involved in a 2019 midair collision over Alaska that the aircraft’s avionics were not providing adequate situational awareness, the NTSB contends.

Six people—the pilot and four passengers on a de Havilland Canada DHC-2 Beaver and a passenger on a DHC-3 Turbine Otter—were killed when the aircraft collided in midair 8 mi. northeast of Ketchikan, Alaska, on May 13, 2019. The DHC-3 pilot sustained minor injuries and nine passengers were seriously injured.

Meeting virtually on April 20, the NTSB determined that “the inherent limitations of the see-and-avoid concept” along with the absence of alerts from the traffic display systems on the aircraft were the probable cause of the accident. Among recommendations, the board reiterated its call that the FAA require Part 135 operators to implement SMS systems.

The DHC-2 Beaver, operated by Mountain Air Service, and the DHC-3 Turbine Otter, operated by Taquan Air, were returning to the Ketchikan Harbor Seaplane Base after sightseeing flights to Misty Fjords National Monument. Operating as Part 135 on-demand sightseeing flights, the floatplanes were converging on a scenic waterfall over George Inlet when they collided at 3,350 ft. mean sea level altitude, at midday in clear weather.

NTSB investigators determined that the pilot of the DHC-2 would not have had the opportunity to see and avoid the DHC-3 because his view was obscured by the cockpit structure, the right wing and a passenger in the right co-pilot’s seat. The lack of apparent motion of the DHC-2 and the obscuration of the DHC-2 by the window post for 11 sec. before the collision made it difficult for the DHC-3 pilot to see the other aircraft, according to the safety board.

Both of the aircraft were broadcasting their positions by automatic dependent surveillance-broadcast (ADS-B) Out, and both were capable of displaying nearby traffic targets by ADS-B In. But the ADS-B equipment on the DHC-3—which originally consisted of a Garmin GDL 90 transceiver and a Chelton FlightLogic display installed under the FAA’s Capstone demonstration program—no longer had aural and visual alerting capability when a Capstone upgrade in 2015 replaced the GDL 90 with the FreeFlight Systems’ RANGR 978 transceiver.

The pilot of the DHC-2 was using an Apple iPad tablet running a ForeFlight ADS-B In display application. If the setting was enabled, the application would have provided visual and aural traffic alerts when a target approached within 1.8 nm horizontally or 1,200 ft. vertically of the aircraft’s own-ship position. But the DHC-3 was not broadcasting pressure altitude data, which ForeFlight would have needed to generate an alert. The reason was that a Garmin GSL 71 control panel selector knob in the DHC-3 that supplied data from an altitude encoder had been switched off, likely during maintenance, according to the NTSB.

Because the GSL 71 can be manually turned off without any indication on the Chelton display that pressure altitude is not being transmitted with ADS-B Out broadcasts, the NTSB recommended to Taquan Air that it revise its preflight checklist to ensure the GSL 71 selector knob is in the “On” position and the unit is in ALT mode before takeoff.

Among other recommendations, the board reiterated its call—for the sixth time since 2017—that the FAA require Part 135 operators to implement company-wide SMS programs, something the agency now requires for just Part 121 airlines. The FAA is expected to release a notice of proposed rulemaking for SMS in 2022.

“An SMS might have provided Taquan Air an opportunity to discover and mitigate the risk introduced by the changes to the Capstone-affiliated avionics and aircraft,” NTSB Chairman Robert Sumwalt said. “Other Capstone operators were also affected by the avionics upgrade and the chances of finding and mitigating this risk would improve with each participant that was regularly and methodically exercising safety management principles. Even if one operator missed the risk, others might have caught it.”

The NTSB also called on the FAA to identify high-traffic air tour areas and require through a special federal aviation regulation that Parts 91 and 135 air tour operators within those areas equip their aircraft with ADS-B Out and In avionics with aural and visual alerting capability.

Bill Carey

Based in Washington, D.C., Bill covers business aviation and advanced air mobility for Aviation Week Network. A former newspaper reporter, he has also covered the airline industry, military aviation, commercial space and unmanned aircraft systems. He is the author of 'Enter The Drones, The FAA and UAVs in America,' published in 2016.