landing gear. In fiscal year 1975 the airplane will be modified to examine operational problems encountered in crosswinds beyond the current limits.

Wind Shear and Visibility Restrictions.-Several fatal accidents have occurred recently in which severe weather and limited visibility were identified as critical factors.

The low level (0-1000 ft) wind environment at airports contains a variety of flown-field effects influencing approach and landing. Low-level wind shear can cause long or short touchdowns, resulting in loss of control on the runway. Severe shears resulting from cold air outflow from thunderstorms can result in loss of control and structural failure. We are employing wind tunnels, field measurements and theoretical analyses to improve our understanding of these phenomena. In fiscal year 1975, emphasis will be placed on relating low level meteorological data more closely to aircraft design and operations.

Just as other elements of our research program are seeking means of safe low visibility approach and landing, we are also continuing our study of warm fog dissipation technology to improve visibility at fog-shrouded airports.

WAKE VORTEX RESEARCH

The reduction of upset hazards presented by wake vortices will permit safe traffic capacity increases at high demand airports and a reduction of inadvertent encounters throughout the system.

NASA has responded to a 1973 FAA request for NASA assistance by intensifying research in four areas: aerodynamic alleviation, ground based vortex detection and tracking, vortex marking, and anti-vortex active controls.

Aerodynamic Alleviation.-Aerodynamic alleviation concentrates on identifying practicable concepts for modifying the vortex so that large rolling moments will not develop to upset following aircraft. Several concepts have been investigated in ground facilities and flight tests (Figure 6). The result of this phase of the work were reported to the FAA on February 15. NASA finds that the vortex intensity can be reduced by the use of various add-on devices, such as trailing splines (Figure 7) located at wing tip or flap juncture points, and by altering spanwise lift distribution through differential adjustment of segmented flaps (Figure 8). Continued ground facility work will be done in fiscal year 1975, along with increased levels of flight test activity in order to validate the ground facility results in actual operations.

Vortex Detection and Tracking.-Jointly with the FAA, NASA is developing a ground based vortex detection and tracking system (Figure 9). This capability is based on space program laser-Doppler technology developed at MSFC and used in OAST's in-flight Clear Air Turbulence detection program. During the latter part of fiscal year 1974 and extending into early fiscal year 1975, we will install and test for FAA, a prototype detection system at Kennedy International Airport. This system will demonstrate the feasibility of vortex encounter avoidance by providing information to the controller and pilot as to vortex location.

Vorter Marking. During fiscal year 1975, we will continue research begun in mid fiscal year 1974 at JPL which will investigate the feasibility of visibly marking trailing vortices. The marking will be accomplished through the use of ecologically acceptable substances injected into the vortex system of large aircraft while operating in the terminal area. If successfull, this would enable other aircraft to see and avoid vortices. The sought-for properties would cause the marking additive to break down within a few minutes to an ecologically neutral and invisible compound. In addition, the marking system will be useful for research.

Control System Modification.-During fiscal year 1974, analytical studies were performed at Flight Research Center to determine the feasibility of adapting a light twin PA-30 equipped with an experimental control system to the special problem of maintaining control of an aircraft encountering the highly-structured and powerful flow field of a vortex. The analysis showed the feasibility of counteracting vortex-induced aircraft moments. During fiscal year 1975, flight studies will be conducted to confirm the analytical conclusions.

AIRCRAFT FIRE TECHNOLOGY

In recent years, about 68% of all U.S. certificated air carrier fatal accidents involved fire (Figure 10). In accidents that occurred during either landing or aborted takeoff, there is evidence that in some cases the occupants may have survived the impact, only to succumb to the ensuing fire. Smoke, noxious gas inhalation, or intolerable heat exposure can incapacitate passengers, making escape impossible. It is difficult to precisely quantify the air transport fire death toll, but NTSB attributes at least 300 deaths to fire and its effects in the past decade.

FIGURE 10