What type of advisory is issued for aircraft departing behind strong turbulence-producing aircraft?

A wake turbulence advisory is specifically issued to inform pilots of the presence of wake turbulence generated by preceding aircraft, particularly those that are heavy or large. This type of turbulence can pose a significant hazard to smaller aircraft taking off or landing behind them, as it can lead to loss of control or unexpected maneuvers.

By issuing a wake turbulence advisory, air traffic control provides essential safety information, allowing pilots to take appropriate measures, such as increasing their separation distance or adjusting their flight path. This advisory is crucial for maintaining safety during critical phases of flight like takeoff and landing, where the risk of encountering turbulence is heightened.

In the context of the other options, a flight advisory refers more generally to any advisory information provided to pilots, a separation directive is focused on ensuring safe distance between different aircraft during flight, and quality assurance advisory does not pertain to flight operations. Thus, the wake turbulence advisory is the most fitting term for the situation described in the question.