From a conversation I had with Airbus a while ago:
The 5 minutes period mentioned in the smoke/fumes procedure comes from the original design of the AVIONICS SMOKE ECAM alert, which included a 5-minute timer on older Flight Warning Computers (before and until FWC F4). This older procedure referred to a 5-minute countdown during which the flight crew had to perform troubleshooting to identify the source of the smoke. If the smoke was no longer detected in the avionics bay, before the countdown reached 5 minutes, then the AVIONICS SMOKE ECAM caution disappeared. And if the countdown reached 5 minutes and smoke was still detected, the AVIONICS SMOKE ECAM was latched, remaining displayed on the ECAM (even if after that, the source of the smoke was discovered and stopped).
Since then, Airbus has modified this ECAM logic so that the countdown is removed from the AVIONICS SMOKE ECAM procedure. Feedback has shown that, depending on the smoke scenarios, this 5-minute threshold was not always applicable: in some cases, it was more appropriate to set the electrical emergency configuration well before the 5-minute period, as the situation was deteriorating rapidly. In other cases, even though the smoke source had been isolated, more than 5 minutes were necessary to clear the smoke from the detectors.
The 5-minute requirement to restore normal ventilation was introduced in the Smoke/Fumes procedure to ensure consistency between the various smoke procedures.
However, taking into account the diversity and the complexity of the smoke scenarios, only the flight crew is in a position to assess the situation and to take the decision to restore the ventilation, if they have determined that the source of the smoke has been isolated.
The condition: "If smoke disappears within 5 minutes: NORMAL VENTILATION . . . . RESTORE" requests the flight crew to restore the ventilation system if the smoke disappears
within a 5-minute timeframe after the smoke is detected.
The items to restore are
the pushbutton switches on the VENTILATION overhead panel: BLOWER, EXTRACT, CAB FANS.
