There are several situations that might cause passengers concern prior to takeoff, and flames shooting out of an engine is certainly one of them.
Passengers on an Utair flight from Moscow to Makhachkala, a town on the Caspian Sea in Russia, panicked when they spotted flames coming from under one of the engines of the Boeing 737 in which they were seated. Those in the emergency exit row opened the door and three people climbed out of the plane and onto the wing.
An airline spokesperson told Russia’s RBC News, “This was an ordinary situation, the plane was in order, passengers were in no danger.” According to the spokesperson, the flames were simply caused by a disrupted air flow — a legitimate reason for the engine to suddenly emit flames. It doesn’t indicate a potential hazard, but the burning of excess fuel.
Despite the fact that the engine fire didn’t pose any actual danger to passengers, the airline switched passengers to a different aircraft. The three passengers who climbed onto the wing were detained for questioning, and one said, “When the panic started, when people started to run, squashing each other, when they began screaming and yelling, in order to pacify them and to stop the panic we opened the emergency exit.”
H/T: Yahoo Finance