According to the requirements for stall warnings on a large aeroplane, when must the warning begin during a speed reduction?

The correct answer states that the stall warning should begin at a speed exceeding the stall speed by not less than 5 knots or 5% of calibrated airspeed (CAS). This requirement is grounded in safety considerations and operational practicality.

Stall warnings are crucial for alerting pilots to imminent aerodynamic stalls—where the aircraft can lose lift due to exceeding critical angles of attack. Initiating the stall warning at just the stall speed could leave little room for pilot reaction, especially during critical phases of flight like approach or climb. By establishing a threshold that exceeds the stall speed, the warning system provides an additional margin of safety, allowing pilots more time to react to a potential stall situation.

The thresholds set by this requirement aim to ensure that pilots receive an effective warning that enables them to take appropriate corrective action before reaching the critical stall condition, thus enhancing flight safety.

The other options suggest different margins where the warning might begin, but they either do not provide a sufficient buffer for pilot reaction or incorrectly define the threshold. This emphasizes the critical nature of effective stall warning systems in maintaining safe flight operations.