It dropped off the radar just 21 minutes later, shortly after the crew asked air traffic control for permission to drop from 10,000 to 6,000 feet, said Daryatmo, chief of the national search-and-rescue agency.
They did not explain the change of course, near Salak mountain, a long-dormant volcano. Though drizzling, it was not stormy, and there was no obvious sign of distress.
"I saw a big plane passing just over my house," Juanda, a villager who lives near the 7,200-foot mountain, told local station TVOne.
"It was veering a bit to one side, the engine roaring," he said. "It seemed to be heading toward Salak, but I didn't hear an explosion or anything."
Cellphones of those on board were either turned off or not active.
Sergei Dolya, a photographer with the tour, posted images from earlier in the day of flight attendants pouring Champagne and himself dressed as Neptune, with beard and trident, greeting passengers.
"We continue to call our people on board," wrote Dolya via Twitter. "Sometimes the phones ring, sometimes the calls break off. But they do not pick up."
Dozens of family members gathered at the airport, awaiting news. "My husband called this morning, saying he was going to be on the test flight," said Windy Prisilla. "He wanted me to meet him at the airport before they took off so we could have lunch together, but I told him I couldn't. I had to get the kids to school."
The Superjet has been widely considered Russia's chance to regain a foothold in the passenger-plane market. The country's aerospace industry was badly undermined in the economic turmoil after the 1991 collapse of the Soviet Union.
Developed by the civil aircraft division of Sukhoi, with cooperation from Western partners, the 75- to 95-seat plane made its maiden voyage in 2008 and its inaugural commercial passenger flight in 2011.
This article contains information from the New York Times News Sevice.