Jet transport response to a horizontal wind vortex

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The dynamic response of a twin-jet transport aircraft encountering a horizontal vortex (or rotor) on final approach to landing is investigated. Computer simulations determine the effects of vortex strength, vortex length, lateral entry position, vertical entry position, and encounter incidence angle on the aircraft's roll response. Maximum roll rate and roll angle increase proportionally with vortex strength and length until a saturation length is reached. Roll response is highly dependent on entry location: changes in lateral entry position largely affect maximum roll angle while changes in vertical entry position affect maximum roll rate. Peak roll rate and roll angle obtain their largest values at near-zero incidence angles. The response is highly dependent on the precise initial conditions of the encounter—even small variations in initial condition cause significant changes in aircraft roll response. IRCRAFT that encounter strong vortical winds during flight can experience violent angular and translational motions. Rotating fields are produced by the tip vortices of other aircraft or by naturally occurring sheared flows. As a consequence of structural similarities in the flows, studies of aircraft encounters with wake and vortices are com- plementary. Greater knowledge of aircraft-vortex interactions is needed for the investigation of unsolved accidents and for preventing future accidents through the development of new vortex detection and warning systems, pilot training pro- grams, and departure-preventing control system designs. The importance of understanding aircraft/wind-vortex in- teractions was highlighted by a recent jet transport accident that may have been caused by a mountain-wave vortex (also called a wind rotor).1 On March 3, 1991, a twin-jet transport aircraft on final approach to landing rolled upside down and pitched nose down before contacting the ground at high speed. The entire event, from steady, level flight 1000 ft above the ground to impact, took less than 10 s. All pas- sengers and crew members perished. The National Transportation Safety Board (NTSB) consid- ered a mountain-wave vortex as a possible cause of the accident. Strong winds flowing over mountains prevailed on the day of the accident; such winds can produce downslope windstorms characterized by wave features extending for many miles beyond the mountains.24 If encountered at the proper orientation, a vortex may well have the strength to roll the aircraft to an unrecoverable roll angle. Neverthe- less, limited knowledge of how a vortex affects a trans- port aircraft along with lack of flight evidence prevented pos- itive determination of this accident's cause.1 While the research reported here is motivated by the ref- erenced accident, it is not a study of that specific accident. Instead, it is an exploration of the rotor and aircraft flight conditions that could lead to severe flight path upset for small jet transports, with emphasis on open-loop aircraft