Pattern 14 · During
Alert Triage
Alerts are ranked by how urgently a human must act, and the ranking is visible in colour, position and sound, so the critical signal is not buried in a flood of equals.
The mechanism
condition detected → link to the response
- 01Condition detectedSomething the operator may need to act on
- 02 · Key momentRank it by urgencyAct now, act soon, be aware
- 03Encode the rankColour, position and sound carry it
- 04Suppress the restConsequences of a fault, not new faults
- 05Link to the responseThe alert carries its procedure
At 02: CS 25.1322 uses three levels: Warning for immediate awareness and immediate response, Caution for immediate awareness and subsequent response, Advisory for awareness that may need a response.
At 03: Colour is reserved, not decorative: red for Warnings, amber or yellow for Cautions, and Advisories any colour except red or green. Warnings and Cautions have to reach the crew through at least two senses.
Without / with the pattern
Anti-pattern: Alarm Flood
Fault cascades, then Everything alarms, then Nothing is legible
Alarm Flood: at Three Mile Island a hundred alarms arrived within minutes of the first, and the alarm printer fell more than two and a half hours behind events. An operator told the President's Commission he would have liked to throw the alarm panel away, because it gave no useful information.
With the pattern
Fault cascades, then Ranked and encoded, then The critical one stands out
Rank is carried by the display itself, so the one alert that needs action now is distinguishable from the ninety-nine that are its consequences.
Related patterns
- Complements · 10Explainable FaultTriage decides which alert reaches the operator first; the other decides whether it names anything useful when it gets there. A ranked flood of error codes is still unreadable.
- In tension with · 03Graceful DegradationStepping down produces alerts by design, and each rung has consequences that look like faults. The ladder is only legible if the step-down is one event rather than fifty.
Sources
The detail that makes Three Mile Island more than a volume problem is the printer falling two and a half hours behind.3 The alarms were not just too many to read; the record of them stopped being a record of the present. Whatever the operators were looking at had come loose from what the plant was doing, which is the state this pattern exists to prevent — and the operator's remark about throwing the panel away is what a display looks like once it has lost the ability to distinguish anything.2
The aviation answer is worth borrowing precisely because it is so unglamorous. Three levels, defined by what the crew must do and when.1 Colours that mean one thing and are not available for anything else. Two sensory channels for anything above Advisory. None of that is clever; it is a budget, spent in advance, on the assumption that attention is finite and the worst moment is when everything happens at once. The part most often skipped is the fourth step, suppressing the consequences of a fault so they do not arrive as peers of their own cause — which is the difference between a hundred alarms and one.