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A Simplified Hazard Area Prediction (Shape) Model for Wake Vortex Encounter Avoidance

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A Simplified Hazard Area Prediction (Shape) Model for Wake Vortex Encounter Avoidance 24TH INTERNATIONAL CONGRESS OF THE AERONAUTICAL SCIENCES A SIMPLIFIED HAZARD AREA PREDICTION (SHAPE) MODEL FOR WAKE VORTEX ENCOUNTER AVOIDANCE Klaus-Uwe Hahn, Carsten Schwarz, Holger Friehmelt German Aerospace Center, Institute of Flight Systems Lilienthalplatz 7, 38108 Braunschweig, Germany Keywords: Safety, Wake Vortex, Encounter, Hazard Area, Separation Distance Abstract List of Symbols and Abbreviations This paper gives details from offline simula- a/c aircraft tions, full-flight simulator studies and the ATTAS Advanced Technologies Testing Aircraft preparation of a flight test experiment using the System b wing span In-Flight Simulation capability of DLR’s (Ger- cg center of gravity man Aerospace Center) test aircraft ATTAS DoF degrees of freedom (Advanced Technologies Testing Aircraft) to DLR German Aerospace Center validate safe boundaries around a wake vortex IFS in-flight simulation for encounter avoidance. The so developed haz- IMC instrumental meteorological conditions ard zone is an element in the Wake Vortex Pre- MTOW maximum take-off weight diction and Observation System which is under NLR National Aerospace Laboratory development in the frame of the DLR “Wake P2P probabilistic two phase model * Vortex II” project to reduce aircraft approach RCR roll control ratio (= x ) separation and to improve flight safety. t time co-ordinate In any case the separation distance of ap- SHA simplified hazard area proaching aircraft has to be determined in such SHAPe simplified hazard area prediction v lateral velocity component a way that the approach corridor is free of haz- V airspeed (no index), velocity ardous vortex flows for the trailing aircraft. The VMC visual meteorological conditions question to be answered is what is the meaning w vertical velocity component of “hazard free”? The presented approach for ZFB Zentrum für Flugsimulation Berlin solving this fundamental problem follows the G circulation idea that it is possible to define an area around h elevator deflection a wake vortex outside which the vortex flow is x aileron deflection definitely not hazardous to an aircraft. The rea- z rudder deflection son for this “inverse” definition is the fact that D difference a clear criterion of what is hazardous is difficult F bank angle to set up (especially if a pilot is in the loop). The Q pitch angle so defined boundaries of the hazard zone can y azimuth easily be determined by the developed Simpli- indices fied Hazard Area Prediction (SHAPe) Model. It g geodetic allows the computation of hazard zone dimen- K kinetic (refers to flight path) sions for any aircraft pairing (real or non exist- max maximum ing aircraft). This potential of a universal appli- nom nominal cation is based on the parameterization of the W wind relevant aircraft wake vortex characteristics WV wake vortex which are fitted by simple functions. WVL wake vortex line * denotes normalized parameters 1 K.-U. Hahn, C. Schwarz, H. Friehmelt 1 Introduction - wind (e.g. cross wind, wind shear) atmospheric turbulence The well known phenomenon of wake vortices - VMC, IMC (ceiling, visibility) behind a lift producing wing can adversely af- - fect flight safety if encountered by trailing air- · Aircraft Control craft. The strength of the vortices increases with - individual pilot behavior (skill) the weight of the vortex generating a/c. There- - performance of automatic controllers fore, weight dependent separation distances The list above claims not to be complete. But it have been established for approach and landing underlines the difficulties (especially if a pilot is to avoid dangerous wake vortex encounters. in the loop) to set up a clear criterion of what is These proven separation distances have to be hazardous (in terms of constraints leading un- investigated to discover possible margins which questionably into an unsafe situation) as many could be used to solve the current and future ca- attempts illustrate. pacity problems at airports. This demand forms Approaching the problem from a safety the need for more flexible separation procedures point of view leads to the idea that it is possible taking into account the actual weather situation to define an area around a wake vortex outside and the parameters of the individual a/c pairing. which the vortex flow is definitely not hazard- Within this scope a Wake Vortex Prediction and ous to an aircraft. This does not imply that any Observation System is under development in the encounter (slight penetration) of the so defined frame of the DLR “Wake Vortex II” project to Hazard Zone must result in a threatening situa- reduce aircraft approach separation and to im- tion. But the area outside the Hazard Zone has prove flight safety [1]. to be an absolute Safety Zone. Following this approach it is necessary to identify safe boundaries around a wake vortex 2 Safety and Hazard defining this Hazard Zone. The today policy to In any case the separation distance of approach- avoid hazardous situations is formulated by the ing aircraft has to be determined in such a way obligation that “no vortex should be encoun- that the approach corridor is free of hazardous tered by intention [2]”. But what means “no vor- vortex flows for the trailing a/c. The question to tex encounter”. A qualitative statement can eas- be answered is what is the meaning of “hazard ily be made: the more distant the encountering free”? For solving this fundamental problem a/c flies away from the vortex cores, the less are two different methodologies are applicable. One the noticeable effects of an encounter. From this approach can be to define the constraints for declaration at least the following two conditions which an encounter becomes hazardous. The for a “no vortex encounter” situation can be de- difficulty of this procedure is the great variety rived: of possible encounter situations and the numer- · no vortex core encounter ous parameters of influence: · the a/c behaviour should be not abnormal compared to a flight in normal atmospheric · Encounter Scenario disturbances (natural gusts and turbulence) - state of the a/c - orientation of flight path and wake vortex The latter statement is the more demanding one (encounter angles) which has to be regarded for the development of - relative positions of wake vortex lines safe boundaries around a Hazard Area. - encounter altitude · Aircraft Pairing - vortex generator (a/c dimension, vortex 3 Simplified Hazard Area (SHA) strength, shape of velocity distribution) For parallel-like encounters the aircraft’s roll - follower (mass, a/c dimension, airspeed, response is the dominating motion. The worst aerodynamics, cg, control power) case occurs when the wake vortex is parallel to · Meteorological Conditions the approach path and the following a/c is per- 2 A SIMPLIFIED HAZARD AREA PREDICTION (SHAPE) MODEL FOR WAKE VORTEX ENCOUNTER AVOIDANCE manently exposed to the vortex flow field in a 4.1 Numerical Offline Simulations quasi stationary flight. The effect on roll within The numerical offline simulation is the nowa- a wake vortex flow field then can be calculated days universal state-of-the-art analysis tool. The using the required control power normalized by reliability of the simulation results depends on the maximum available control power of the the quality of the applied models. The choice of encountering a/c expressed in terms of aileron models was based on the rule “as complex as deflection [3]. The wake vortex induced roll necessary but as simple as possible”. Neverthe- moment can be compensated if equation (1) ap- less, great store was set by realistic modeling. plies. * x = x x < 1 4.1.1 Modeling and Validation max (1) A complex simulation system [3] was devel- Areas of various roll control demands can be oped including the main modules: defined by this normalized aileron deflection x* · Vortex Generating Aircraft which is equal to the roll control ration (RCR) (® flow field of a wake vortex) known from other publications. Thus, x* (re- · Encountering Aircraft spectively RCR) is a suitable measure to quan- (® 6 DoF simulation and wake vortex/ tify the effect of a wake vortex flow field on an aerodynamic interaction model) aircraft. Fig. 1 illustrates the flow field behind a · Encounter Control generic vortex generating a/c (MTOW=79 to) in (® ILS model and autopilot / autothrottle) * terms of normalized aileron deflection x for a While the basic simulation system is based on light aircraft (MTOW=4.35 to). Different col- standard modules and models particular effort ored regions represent different levels of re- was dedicated to include the special effect of quired normalized aileron deflections. wake vortices. In the frame of the S-Wake pro- The complex shape of the areas for a spe- * ject [4] sponsored by the European Commission cific nominal x nom can be conservatively ap- real wake vortex encounter flight experiments proximated by a simple rectangle which covers were executed to gather a valuable high quality the respective region (Fig. 1). SHA includes the data base from real flight tests. For the flight closer regions around the vortex lines. As a re- tests DLR’s Advanced Technologies Testing * sult, for small values of x nom all other encounter Aircraft System VFW614/ATTAS was used as parameters of interest (e.g. roll acceleration, an- vortex generator. The encountering aircraft were gle of attack and load factor, lateral accelera- the NLR’s Cessna Citation and the Dornier tion) are covered and remain within acceptable Do128 test aircraft of the University of Braun- limits in terms of a/c controllability (which will schweig [Fig. 2]. The latter is ideally suited for be shown later). So if the term encounter is used in-situ wake vortex measurements since it has 4 in this paper the respective situation has to be different stations with flow probes (nose boom, understood to be rather a passing by than a hit wing tips, fin).
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