Five cabin channels (O2, CO2, pressure, temperature, humidity) stream continuously noisy readings. On reset, the first calibration window records each channel's mean μ and standard deviation σ while the system is nominal. Every tick after that, each reading is converted to a standardized score and fed into a two-sided CUSUM (cumulative sum) control chart — the same statistical process-control algorithm used to catch slow instrument drift in real industrial and aerospace monitoring, not a scripted "value > limit" alarm:
z = (x - μ) / σ
Sh = max(0, Sh + z - k) // accumulates upward drift
Sl = max(0, Sl - z - k) // accumulates downward drift
ALARM when Sh > h or Sl > h
A single noisy sample rarely trips a CUSUM alarm on its own — it takes a sustained drift away from baseline to accumulate past the decision threshold h, which is exactly what a slowly degrading component produces and what a single-sample threshold check misses until the drift is already large. The slack k discounts small deviations so pure noise never accumulates; the threshold h trades detection speed against false positives, same as in a real annunciator design.
Inject a fault and its primary channel begins a slow ramp away from nominal. Left undetected, the fault cascades: after a fixed dwell it starts pulling a second, physically-coupled channel off nominal too (a degrading scrubber raises CO2, which — if unresolved — pushes crew metabolic load and starts eating into O2; a coolant fault raises temperature, which then degrades humidity control). Catch the CUSUM alarm and click Isolate fault before that dwell elapses and the fault stays contained to one channel; miss the window and the cascade is already running by the time you isolate it.
- Fault drift rate — how fast the injected fault ramps toward its full magnitude; faster ramps are easier for CUSUM to catch quickly but harder to distinguish from a genuine fast failure.
- CUSUM slack k / threshold h — the same sensitivity knobs a real statistical process-control chart is tuned with: lower k or h catches drift sooner at the cost of more false alarms from noise alone.