All of the people on the aircraft – four navy officers and four rescuers – were killed when it reportedly hit a cliff on Butak Mountain in the Temanggung district of Central Java province.
It came down just minutes before it was due to arrive at Dieng Plateau, where the Sileri Crater had been spewing cold lava, mud and ash as high as 164ft (50 metres) into the air since erupting on Sunday morning.
Amateur footage of the eruption tweeted by the Indonesian National Agency for Disaster Management showed panic on the streets as people fled the area in cars, on their motorbikes and on foot – against the backdrop of a cascade of volcanic ash.
— BNPB (@BNPB_Indonesia) July 2, 2017
Sileri is the most active and dangerous among some 10 craters in the volcanic plain, situated in the Central Java district of Banjarnegara.
Emergency services and the military were deployed to the scene following the sudden eruption of the volcano while up to 17 visitors were around the crater.
Residents and tourists have been evacuated from the area after around 10 people were injured and needed hospital treatment.
Brigadier General Ivan Tito, of the National Search and Rescue Agency, told TVOne station the Indonesian-made Dauphin AS365 helicopter was airworthy.
Sileri last erupted in 2009, when it unleashed volcanic materials up to 656ft (200 metres) high and triggered the creation of three new craters.