A helicopter carrying ammonite - used to break ice - crashed in Irkutsk region after 'exploding in mid air'.
Crew of Siberian UTair plane tell of 'confusion and silence' from air traffic controllers at key moment.
Aircraft had been flying as part of a test flight following an engine change when it came down.
Modern-day zeppelins costing $30 million each would revolutionise transport links and economic exploitation of Siberia in $240 billion plan.
The Mi-8 with 25 passengers and a crew of three on board crash landed some 45 km into a flight, said early reports.
Pilot error, technical malfunction and bad weather are all possible causes checked by investigators.
The helicopter carrying 17 people lurched on its side on takeoff, with 5 described as being in a 'grave' condition.
Mystery over circumstances of crash when chopper was on 'unsactioned' flight.
Now the Far East of Russia linked to beaches and cities in the sun with direct routes from Vladivostok on low-cost airline Vietjet.
A desperate rescue mission saved four people - but up to 24 are feared dead in the air tragedy in the Sakha Republic.