Flight tracking data suggests the airliner dropped 6,000 feet (2,000m) in a matter of minutes after crossing the Bay of Bengal.
"So I started bracing for what was happening, and very suddenly there was a very dramatic drop so everyone seated and not wearing seatbelt was launched immediately into the ceiling," 28-year-old student Dzafran Azmir said.
"Some people hit their heads on the baggage cabins overhead and dented it, they hit the places where lights and masks are and broke straight through it."