Which soot pattern indicates the engine was in flight?

• A. In flight: Dark soot from origin, gradient aft in direction of the airflow (Correct)
• B. Window pane: On the ground. Black soot all the way around; No aerodynamic flow pattern
• C. Uniform soot around engine
• D. No soot deposition

Answer: (A)

Soot tells you about the interaction between the exhaust and the surrounding air. When an aircraft is in flight, the engine plume vents into a fast-moving airstream, so the soot appears darkest at the engine origin and forms a clear gradient trailing downstream in the direction of the airflow. That distinct aft gradient together with soot starting at the engine is the signature of in-flight exhaust.

If the engine were on the ground or stationary, there’s no strong forward airspeed to carry the plume in a single direction. The soot tends to coat things more uniformly around the engine or follow a local wind pattern, rather than showing a clean gradient trailing in the air’s direction. Patterns described as covering a windowpane or having uniform soot around the engine without a directional trail point to ground conditions rather than flight, and no soot deposition would imply either no exhaust residue is present or it’s been removed.

So the flight-indicating pattern is the one with dark soot beginning at the origin and fading back along the direction the air is moving.