Why did the seaplane crash?

On September 15, 2015, near Iliamna, Alaska, the N928RK tail-marked de Havilland DHC-3 Otter seaplane crashed into the hill, losing control just after takeoff, as a result of both somatographic illusion and overload and unstable loading. Three people on the plane lost their lives and 5 people were seriously injured.

The plane was carrying tourists and their guides engaged in sportive salmon fishing. The pilot of the plane that survived the crash said that he did not give the mandatory emergency briefing before takeoff, as passengers are used to air travel.

It was not yet dawn when the plane took off at 06:05 am local time. The pilot said he looked outward, not at the gauges, during takeoff, and that when the plane took off, there were no visual signs left.

A witness who saw the accident said that after takeoff, the plane first rose and then descended. The descending plane crashed its pontoons under its fuselage into the water, rose again and headed to the right, but could not rise more than 50 meters because it was too heavy, and crashed into the hill.

The company operating the aircraft did not have any procedure or equipment to calculate the weight and center of gravity of the aircraft.

When the pilot was asked how he calculated the weight and balance of the plane before take-off, he replied that I was guessing from my experience. The pilot said that the plane has a capacity of six passengers and can carry enough suitcases!...

The weight and center of gravity of the aircraft were calculated after the accident and it was understood that the maximum take-off weight of the aircraft was exceeded by 230 kilograms, and the center of gravity of the aircraft was more than 10 centimeters behind.

The NTSB (1) determined that the pilot lost his orientation as a result of looking outwards instead of the flight instruments at take-off, and when there was no visual signal around, he thought that the nose of the aircraft was too upright, and caused the aircraft to crash into the water by putting the nose of the aircraft down.

The aircraft, whose weight was beyond the limit, could not naturally rise after hitting the water, and entered the stall and crashed into the hill.

NTSB reminded that pilots who see the horizon line and the terrain can sense the nose down, up, wing down, up and left, right movements of the aircraft with the help of their inner ears, whereas the pilots who do not have a visual reference cannot correctly identify these movements and especially in night conditions, in the dark. He emphasized that during takeoffs in the environment, pilots should fly 'instrument flight' by looking primarily at the flight instruments, not outside.

(1) The National Transportation Safety Board (NTSB) is an independent U.S. government investigative agency responsible for civil transportation accident investigation. In this role, the NTSB investigates and reports on aviation accidents and incidents, certain types of highway crashes, ship and marine accidents, pipeline incidents, bridge failures, and railroad accidents. The NTSB is also in charge of investigating cases of hazardous materials releases that occur during transportation. The agency is based in Washington, D.C. It has four regional offices, located in Anchorage, Alaska; Denver, Colorado; Ashburn, Virginia; and Seattle, Washington. The agency also operates a national training center at its Ashburn facility.