The start sequence began. Both engines spooled up, and the sound filled the hangar, a deep rising roar that vibrated through the floor and into everyone's chest.
Faith closed her eyes.
She listened.
The engines idled roughly with the same unstable rhythm the mechanics had heard for fifty days. To most of the room, it sounded like a troubled turbine system behaving as expected. To Faith, it was a pattern.
She raised one finger toward Neil. "Wait."
Eight seconds passed. Then she raised two fingers. Another eight seconds passed, and she repeated the gesture.
Faith opened her eyes. "Kill the right engine. Keep the left running."
Neil hesitated because that was not part of the standard diagnostic sequence. Dr. Sutter gave him a confirming nod, and Neil shut down the right engine.
The left engine kept running.
The change was immediate. The idle was not perfect, but the rhythmic stutter that had been cycling every eight seconds nearly disappeared.
"Now swap," Faith said. "Right engine only."
Neil restarted the right engine and shut down the left. Again, the repeating stutter faded.
Faith turned toward Dr. Sutter. "It's not the engines. It's what happens when both engines run together."
She asked Neil to open the raw fault logs rather than the summary screens the teams had been relying on. She wanted millisecond-level timestamps.
"Overlay them. Left and right. Same time axis."
Neil did it.
Two data lines appeared on the screen. Once viewed together, the pattern became visible: a tiny dip in fuel pressure on the left engine lasting about three-tenths of a second appeared every eight seconds. Each dip lined up with a corresponding increase in fuel flow on the right side.
Nobody had seen it because nobody had looked at the engines as a coupled system.
Faith pointed at the traces. "Your crossfeed valve is cycling. Every eight seconds it opens when it shouldn't. It pulls fuel from the left engine feed line for a fraction of a second. That's enough to disturb combustion without triggering the alarms. Keep doing that for ninety seconds and the instability compounds until both engine-control systems interpret the condition as a flameout and shut down."
The room went silent.
Neil stared at the screen. Todd spoke from the back wall.