Nobody else laughed.
Faith stood there for 40 seconds, then she came back. "There's a chemical in the left engine exhaust that shouldn't be there. It's faint, almost nothing, but it's there. It smells like burning plastic mixed with vinegar."
She turned to Neil. "Your crossfeed valve has a fluoroelastomer O-ring that seals the internal chamber. That O-ring is degrading. The smell is the rubber breaking down under heat stress. It's been cooking every time the valve cycles, which is every 8 seconds over and over for 50 days."
She explained it simply: fluoroelastomer—FKM rubber—is standard in aerospace fuel systems. Tough material, built to handle extreme temperatures, but it has a limit. If the valve is cycling abnormally, opening and closing thousands of times more than it should, the O-ring heats past its rated threshold. It starts to decompose slowly, microscopically. The decomposition releases trace volatile compounds into the fuel stream. Not enough to trigger the chemical sensors in the diagnostic system—those sensors are calibrated for major contamination. This was minor, a whisper of contamination, but it was enough to affect combustion stability in the left engine's feed line: a tiny impurity in the fuel mixture that made the already starved engine just a little more unstable during each cycle.
"Pull a fuel sample from the left engine's direct feed line," Faith said. "Not from the tank. From the line after the crossfeed valve."
Neil drew the sample into a clear container. He held it under the hangar's fluorescent lights. There it was: a faint amber tint, so slight you'd miss it if you weren't looking for it. But jet fuel is supposed to be water-clear; any discoloration means something is in it that shouldn't be.
Dr. Sutter stood up 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 40 years. He never trusted a sensor he couldn't double-check with his own hands."
Preston Caldwell uncrossed his arms. For the first time in 50 days, he wasn't performing authority; he was watching, actually watching. Todd Whitmore hadn't spoken since his last comment; his face had lost some color.
Revelation three: the touch.