Noorsalu, an experienced helicopter pilot and flight instructor, said at this stage no viable cause of the accident can be ruled out, be it a technical failure, human error, or a natural factor such as bird strike.

Preliminary data reveals the two helicopters, which had taken off in Estonia and were destined for an aviation day in Finland, were flying approximately 50 meters apart and at an altitude of around 300 meters when they collided, near Eura, in the southwest of the country.

For reasons as yet unknown, one of the helicopters suddenly changed course, leading to the crash.

Speaking to "Terevisioon" Monday, Noorsalu said: "50 meters is a correct distance to maintain and should provide the opportunity to avoid various danger situations, such as when one helicopter needs to maneuver suddenly. Under normal conditions, 50 meters is safe, but we don't yet know what happened there. If a helicopter is flying at 200 kilometers an hour, covering 50 meters takes only a few seconds."

In any case two helicopters flying in tandem is riskier than one flying alone, and both parties must take into account the other, Noorsalu said. The lead helicopter must take into account the follower, and vice versa.

"You can't rule out a technical failure, you can't rule out human error, and you can't rule out a natural factor either. For example, a flying bird, sunshine, or turbulence. There are various such factors. Too little time has passed since the accident to say exactly what caused it, but the investigators will reach conclusions," Noorsalu continued.

A sudden maneuver is a natural human reaction for instance when spotting a bird in flight, and wanting to avoid striking it, he said.

"But when flying as a pair, you have to bear in mind that someone is following you. When we are driving a car and a fox jumps suddenly onto the road, we also make a maneuver without thinking whether there is someone in the other lane or not. This is natural behavior."

Noorsalu said that investigators need to amass all available evidence, and as much as remains of the wreckage of both 'copters.

Investigators will need to: "Check for points of impact. Listen to witnesses who saw it happen. Plus video recordings are especially useful. Usually an investigation takes about a year before conclusions are reached. A preliminary report may come sooner," Noorsalu went on.

Finnish emergency services received notification that two helicopters had collided near Eura airfield at around 12.30 p.m. on Saturday. The wrecks were soon located, aided by smoke issuing from one of them. All five occupants of the two craft were declared dead at the scene. Four of the deceased have been named in the media. They are: Wind turbines businessman Oleg Sõnajalg, construction firm Mapri board chair Priit Jaagant, Lillit Jaagant, wife of the latter, and motorsport competitor Tiit Kuusk.

Finnish authorities have said the full investigation could take up to a year.

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