Is it normal for jet engines to flash flame sometimes?

A jet engine burns kerosene continuously in an annular combustor and expels gases at very high temperature. In certain situations, some of that fuel finishes burning outside the combustor, in the nozzle or just behind it, producing an orange flash lasting a fraction of a second. This happens mainly during startup, when the mixture hasn't stabilized yet; during rapid accelerations from idle, when the fuel control unit injects more fuel than the airflow can consume at that instant; and in some engines during shutdown, while the fuel lines are purged.

At night the effect is exaggerated: the eye catches a flare that would go unnoticed in daylight. The same happens with heavy rain or snow, which momentarily disrupt combustion. None of these situations damages the engine or shows up as a fault in the cockpit.

The spectacular phenomenon people usually film is different: compressor stall, also known as surge. The compressor blades are small airfoils, and like any airfoil, they stall if the air reaches them at the wrong angle. When this happens to many blades at once, the compressor stops sustaining pressure upstream and the flow reverses violently: hot gases shoot forward out of the intake and backward out of the nozzle, with a sharp bang audible from the ground and a much longer, louder flame.

Common causes include bird ingestion, foreign object damage, ice, an extreme angle of attack or a strong crosswind that distorts the inlet flow, and worn or misaligned variable guide vanes. The engine may recover on its own or require the crew to reduce the throttle and follow the corresponding procedure; if it recurs, the engine is shut down and the aircraft lands at a suitable airport. Commercial aircraft are certified to fly and land with one engine inoperative, and crews train for this scenario periodically.

There's a third case where the flame is the point of the design. Military fighters carry afterburners: they inject additional fuel into the exhaust jet, where unburned oxygen remains, and burn it to boost thrust by up to 50% at the cost of enormous fuel consumption. There, the bluish flame with visible shock rings — the so-called Mach diamonds — is the sign that the system is working correctly. No current commercial aircraft carries an afterburner; the last one was the Concorde, which used it for takeoff and transonic acceleration.

The rule of thumb is simple: a brief, silent flash, usually during startup or a power change, is routine. A series of audible bangs, perceptible cabin vibration and repeated flames mean something is wrong with that engine. In that second case the crew detects it on their instruments long before any announcement, and the aircraft keeps flying normally: the design is exactly meant for that.