"That's a coincidence, not a diagnosis. A three-tenths-of-a-second fuel dip doesn't ground an aircraft."
Faith did not turn toward him. "One dip doesn't. Fifty of them in ninety seconds can."
Dr. Sutter wrote something in her notebook.
Faith was not finished.
She asked Neil to restart both engines. Once they stabilized into the familiar rough idle, she walked away from the diagnostic console and the nacelle and stood roughly twenty feet downwind of the left exhaust.
She breathed slowly through her nose for about forty seconds.
Todd laughed. "What's she doing now, yoga?"
Nobody joined him.
Faith returned to the group. "There's something in the left engine exhaust that shouldn't be there. Very faint. Burning plastic mixed with vinegar."
She looked toward Neil. "The crossfeed valve uses a fluoroelastomer O-ring to seal the internal chamber, right?"
Neil nodded.
"That O-ring is degrading. If the valve is cycling every eight seconds instead of operating only when commanded, it's heating and moving thousands of times more than intended. The seal is beginning to break down under the repeated heat stress."
Faith explained that fluoroelastomer, or FKM rubber, was common in aerospace fuel systems because of its durability and resistance to fuel and heat. But no material was immune to abnormal cycling. Repeated overheating could begin decomposing the seal microscopically, allowing trace volatile compounds into the fuel stream.
The quantity could remain below the threshold of chemical contamination sensors while still affecting combustion stability in a feed line that was already suffering momentary starvation. It was not a dramatic contamination event. It was a whisper of contamination layered on top of a repeating flow disturbance.
"Pull a fuel sample from the left engine's direct feed line," Faith said. "Not the tank. The line after the crossfeed valve."
Neil drew a sample into a clear container and held it beneath the hangar's fluorescent lights.
The fuel showed the faintest amber tint. Easy to miss unless someone knew to look for it. Jet fuel should have been clear.
Dr. Sutter stood from her chair. "How did you know to smell for it?"
Faith looked at her. "My grandfather used to say your nose knows before the gauge does. He worked on engines for forty years. He never trusted a sensor he couldn't double-check with his own hands."
Preston uncrossed his arms. For the first time since the crisis began, he was not performing authority. He was watching.
Todd had stopped laughing.