Trailing Vortex Wake Encounters at Altitude - A Potential Flight Safety Issue?

The potential hazard of one aircraft encountering the trailing vortex wake of another aircraft during either take-off or landing is well known. The wake vortex hazard in the terminal area has been virtually eliminated using the operational procedures and aircraft separation criteria mandated by the air traffic authorities. This paper presents arguments that wake encounters at cruise altitude are a potential safety issue that needs to be examined. Wake vortex encounters at cruise altitudes are likely to increase due to three factors. These factors include; the reduction of the minimum vertical separation distances between aircraft, the increased air traffic allowed by the reduced vertical separations, and the large difference in the size of aircraft operating at the cruise altitudes. The arguments are supported by simple analysis and a review of simulation and flight test results obtained from earlier studies of wake encounters at low altitudes. Simple analysis techniques show that the rolling moment induced on a following aircraft is comparable in magnitude to that which could occur during take-off or landing. In addition reported wake encounters at cruise altitude are discussed.