WASHINGTON — The pilots of two Alaskan sightseeing planes that collided in midair couldn't see the other aircraft because airplane structures or a passenger blocked their views, and they didn't get electronic alerts about close aircraft because safety systems weren't working properly.

That's what the staff of the National Transportation Safety board found in their investigation of the May 2019 crash, which killed six people.

The board is meeting Tuesday in Washington to determine a probable cause of the crash and make recommendations to prevent it from happening again. 

Mountain Air Service pilot Randy Sullivan and his four passengers, and a passenger in a plane owned by Taquan Air were killed. Ten people suffered injuries when the aircraft converged at 3,350 feet (1,021 meters).

The Ketchikan-based floatplanes carrying passengers from the same cruise ship, the Royal Princess, were returning from tours of Misty Fjords National Monument.

Mountain Air's single-engine de Havilland DHC-2 MK 1 Beaver and Taquan's larger turboprop de Havilland DHC-3 Otter collided just after noon over the west side of George Inlet.

Staff members told the board the Otter pilot recalled seeing a white and red flash, then a tremendous collision. 

Aircraft rely on "see and avoid" by pilots to prevent midair crashes, the staff said. But the crash occurred on a clear day in the afternoon. 

The Beaver pilot's view would have been obstructed by the airplane's structure and a passenger seated to his right during the critical moments before the crash. The Otter pilot's view was obscured by a window post, the staff said. 

William Bramble, the NTSB's human performance specialist, told the board that both planes were equipped with systems that track other planes, but visual and audible alerts weren't working in either plane due to malfunctions.

"The Otter pilot seemed to miss seeing the target (the other plane) on the display because he last recalled looking at the display about four minutes before the collision," Bramble said.