Safety Issues Related to Wake Vortex Encounters during Visual Approach to Landing

Since December 1992, there have been five accidents and incidents in which airplane on approach to landing encountered the wake vortex of a preceding Boeing 757 (B-757). Thirteen occu[pants died in two of the accidents. The encounters, which occurred during visual conditions, were severe enough to create an unrecoverable loss of control for a Cessna Citation, a Cessna 182, and an Isreal Aircraft Industries Westwind. Additionally, there were significant but recoverable losses of control for a McDonnell Douglas MD-88 and a B-737 (both required immediate and aggressive flight control deflections by their flightcrews).

Safety Board data show that between 1983 and 1993, there were at least 51 accidents and incidents in the United States, including the 5 mentioned above, that resulted from probable encounters with wake vortices. In these 51 encounters, 27 occupants were killed, 8 were seriously injured, and 40 airplanes were substantially damaged or destryed.

The Safety Board conducted a special investigation to examine in detail the to examine in detail the circumstances surrounding the five recent accidents and incidents to determine what improvements may be needed in existing procedures to reduce the likelihood of wake vortex encounters.

The Safety Board's investigation initially focused on why the B-757 appeared to be involved in a disproportionate number of wake vortex encounters. Several reports indicated that the B-757 generated wake vortices that were more severe than would be expected for an airplane of its weight. However, as a result of a thorough study and analysis of the issue, the Safety Board found little technical evidence to support the notion that the wake vortex of a B-757 is significantly stronger than indicated by its weight.

The Safety Board's investigation, therefore, raised concerns about the following safety issues:

• the adequacy of the current aircraft weight classification scheme to establish separation criteria to avoid vortex encounters;
• the adequacy of air traffic control procedures related to visual approaches and visual flight rules operations behind heavier airplanes;
• pilot knowledge related to the avoidance of wake vortices; and
• the lack of available data to analyze the history of wake vortex encounters in the United States.

As a result of this special investigation, 19 recommendations were issued to the Federal Aviation Administration, U.S. Department of Transportation.