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organizations. Significantly, the character of turbojet transport aircraft has changed
dramatically in terms of operating weights and wing design. These two principal factors
in wake vortex signature are significant in relevance to the 587 accident. Industrial efforts
to determine the wake signatures of new-generation aircraft, such as the 747-400,
however, are absent. Efforts to assess risk in determining the adequacy of wake
turbulence separation criteria applied by Air Traffic Control (ATC) authorities are also
equally absent.
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Based on a 10- year period of data collection, the Civil Aviation Authority of Great
Britain (CAA) changed from a three- group airplane weight category to a four-group
weight category. In 1991, the CAA presented a paper to participants of an FAAsponsored
conference on aircraft wake vortices held in Washington, D.C. This paper
stated, “The four-group scheme (weight categories) introduced in 1982 was divided as a
result of incident data gathered in earlier years and was designed to provide extra
protection for some types of aircraft found to suffer particularly severe disturbance
behind heavy group aircraft” (Wake Turbulence Training Aid, Appendix 4-A, 12). The
CAA, in analysis of events reported between 1972 and 1990, found that “…the B-747
and B-757 airplanes appear to produce significantly higher incident rates than the other
airplanes considered, indicating prima facie that they produce stronger and more
persistent vortices than other aircraft in their respective weight categories…” (Wake
Turbulence Training Aid Appendix 4-A 13). The maximum take-off weight for the B-
747 at the time of the study was 317,000 kg. or 818,000 lbs (Wake Turbulence Training
Aid, Appendix A, 14). The take-off weight of JAL Flight 47 was 780,000 lbs. Clearly, the
CAA’s four-group weights classification system is relevant to AA 587’s wake encounter.
The FAA divides groups of aircraft in the following manner:
Small aircraft <41,000 lbs.
Large aircraft are between 41,000 lbs. and 255,000 lbs.
Heavy aircraft > 255,000 lbs.
Aircraft are now approaching maximum take-off weights of 1 million pounds. A different
weight group is required for 214,000 pounds of difference in weight (Small to Heavy) but
not for 615,000 pounds of difference (Heavy minimum weight to 747 maximum take-off
weight). NTSB Safety Recommendation A-94-043 clearly states:
“...the Board still believes that the most significant problem related to establishing
adequate separation standards is the great range of aircraft weights involved. Thus the
Board believes it would be prudent to create four weight categories in which the ratios of
the high and low weights within each category are similar.”
Accordingly, separation standards should reflect the revised weight categories.
The FAA Wake Turbulence Training Aid states on page 2.3:
“The strength of wake turbulence is governed by the weight, speed and wingspan of the
generating aircraft. The greatest strength occurs when the generating aircraft is heavy,
at slow speed with a clean wing configuration.”
This is precisely the situation encountered by AA 587.
The FAA has limited knowledge of the strength of wake vortices but has seen multiple
incidents and accidents due to this phenomenon. The wake from one of the heaviest
among heavy aircraft, one that the CAA found to be “stronger and more persistent,”
created forces that absolutely demanded a response from the pilot of AA 587 (Wake
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Turbulence Training Aid Appendix 4-A 13). This response to stabilize the aircraft’s flight
attitude triggered the ensuing APC.
Computational Fluid Dynamics (CFDs) and wind tunnel modeling have been used to
predict aircraft behavior, but they are unsatisfactory in determining characteristics and
handling qualities which can involve the coupling of an aircraft, pilot, and environments
such as the counter-rotating air mass of wake vortices. In December 1999, the Safety
Board stated in Safety Recommendation A-94-043 that it “…does not believe that
unvalidated [sic] theoretical evaluations should be used to justify decreased safety
margins such as exemptions to the weight classification system.” Additionally, in the
same recommendation, “The Safety Board has no evidence that the FAA/NASA wake
vortex simulation has sufficient validity to define risk in an actual wake vortex
encounter.”
The principal conclusions previously reached by the NTSB, and which are cited in the
Wake Turbulence Training Aid (Appendix 4-A 28) of 1994, remain. Currently, the
conclusions stand as:
1. Current Air Traffic Control (ATC) procedures and pilot reactions can
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