Mitigating Wake Turbulence Risk During Final Approach via Plate Lines

To mitigate the risk of wake vortex encounters during final approach, so-called plate lines have been developed. Data collected during a six-month measurement campaign at Vienna International Airport are used to assess the potential for reducing minimum aircraft separations facilitated by plate lines during approach and landing following the re-categorisation (RECAT-EU) methodology for revised wake turbulence categorisation. To ensure that no other parameters controlling wake vortex decay bias the analysis, it is verified that wind speed, atmospheric turbulence, thermal stratification and flight altitude reside in similar ranges with and without the plates. The analysis follows the steps of the RECAT-EU method to generate non-dimensional so-called reasonable worst-case circulation decay curves; one as a reference for nominal operations without plates and one representing the accelerated wake vortex decay brought about by the plate lines. The difference between the two circulation decay curves corresponds to the non-dimensional time-based aircraft separation reduction potential that can be translated into distance-based separation gains. Depending on the particular RECAT-EU category combination, the attained aircraft separation reduction potential ranges between 12% and 15%. Constricting the analysis to wake vortices generated by one aircraft type representing the Upper Heavy RECAT-EU category, the separation reduction potential amounts up to 24%.

Enhancement of wake vortex decay by air blowing from the ground

Assessment of dynamic pairwise wake vortex separations for approach and landing at Vienna airport

PurposeIn this study, 12 potential wake vortex encounters that were reported at a major European airport have been investigated. Because almost all encounters occurred in ground proximity, most pilots conducted a go-around. The primary purpose of this study is to discriminate between incidents caused by wake vortices or rather by effects like wind shear or turbulence. Detailed knowledge of real-world encounter scenarios and identification of worst-case conditions during the final approach constitute highly relevant background information to assess the standard scenario used for the definition of revised wake turbulence separations.Design/methodology/approachWake vortex predictions using the probabilistic two-phase wake vortex model (P2P) are used to investigate the incidents in detail by using data from the flight data recorder, meteorological instrumentation at the airport and numerical weather prediction.FindingsIn the best documented cases, the flight tracks through the vortices could be reconstructed in good agreement with wake vortex predictions and recorded aircraft reactions. Out of the eight plausible wake vortex encounters, five were characterized by weak crosswinds below 1.5 m/s combined with tailwinds. This meteorological situation appears favourable for encounters because, on the one hand, weak crosswinds may compensate the self-induced lateral propagation of the upwind vortex, such that it may hover over the runway directly in the flight path of the following aircraft. On the other hand, tailwinds limit the propagation of the so-called end effects caused by the breakdown of lift during touchdown.Practical implicationsThe installation of plate lines beyond the runway tails may improve safety by reducing the number of wake vortex encounters.Originality/valueThe conducted investigations provide high originality and value for both science and operational application.

A new parametric wake vortex transport and decay model is proposed that predicts probabilistic wake vortex behavior as a function of aircraft and environmental parameters in real time. The probabilistic two-phase wake vortex decay model (P2P) accounts for the effects of wind, turbulence, stable stratie cation, and ground proximity. The model equations are derived from the analytical solution of the spatiotemporal circulation evolution of the decaying potential vortex and are adapted to wake vortex behavior as observed in large-eddy simulations. Vortex decay progresses in two phases, a diffusion phase followed by rapid decay. Vortex descent is a nonlinear function of vortex strength. Probabilistic components account for deviations from deterministic vortex behavior inherently caused by the stochastic nature of turbulence, vortex instabilities, and deformations, as well as uncertainties and e uctuations that arise from environmental and aircraft parameters. The output of P2P consists of cone dence intervals for vortex position and strength. To assign a dee ned degree of probability to the predictions reliably, the model design allows for the continuous adjustment of decay parameters and uncertainty allowances, based on a growing amount of data. The application of a deterministic version of P2P to the Memphis wake vortex database yields favorable agreement with measurements.

In order to study the wake vortices generated by landing aircrafts, we conducted an experiment on the airfield of Tolmachevo Airport in 2018, which involved the Stream Line lidar, AMK-03 sonic anemometer and MTP-5 temperature profiler. The experiment determined the limits of applicability of the radial velocity method used to obtain estimates of the parameters of wake vortices from lidar measurements, depending on the type of aircraft and the intensity of wind turbulence. An analysis of the experimental results made it possible to identify a number of features of the spatial dynamics and evolution of aircraft wake vortices during different states of the surface layer of the atmosphere. In particular, it was found that in the case of low average lateral wind speed and moderate wind turbulence, the lifetime of the vortex formed behind the landing aircraft MD-11F can reach almost 4 minutes.

The NRC has undertaken re-instrumentation of the NRC CT-133 ruggedised research aeroplane, for high altitude atmospheric research. In the process, a new air data system has been designed, and optimized for high data acquisition rates, in particular matching the capability of the aircraft’s FRL inertial reference and navigation system, to derive and acquire wind vector data at 600 Hz. The platform has been used to conduct transport aircraft wake vortex (WV) surveys, and acquire instability wake vortex flowfield data, most recently in comparative straight flight and turning flight manoeuvres, using a Falcon 20 wake generator, to 3 nm distance. Vortex core traverse analysis has highlighted a greater frequency of small-scale vortex segments with high (>150% exceedance of the primary vortex) tangential velocities in turning flight; also, a greater bandwidth in short-wave instability wavelengths, and higher amplitude in long-wave instability, also multi-wavelength, but centred at 3 to 5 generator wingspans, whether in straight or turning flight. Such observations and general considerations of possible tactical jet aircraft wake vortex flowfields suggests the need to establish from flight data, wake vortex flowfield environmental design requirements, in order that contemporary and new tactical aircraft types may be developed with robust flight control systems, able to handle the [frequency amplitude] environments experienced during wake vortex encounters.

The design of the Wake Vortex Prediction and Monitoring System WSVBS is described with all its components and their interaction. The WSVBS was developed to tactically increase airport capacity for approach and landing on closely-spaced parallel runways. The WSVBS supports dynamic adjustment of aircraft wake vortex separations dependent on weather conditions and the resulting wake vortex behavior without compromising safety. Dedicated meteorological instrumentation and short-term numerical terminal weather prediction provide the input to the prediction of wake-vortex behavior and respective safety areas. The prediction tools employ a number of conservative aircraft parameter combinations that represent the medium and heavy aircraft weight categories. Safe aircraft separations correspond to times when the predicted safety areas associated with wake vortices generated by leading heavy aircraft cannot overlap the arrival flight corridor. A LIDAR monitors the correctness of WSVBS predictions in the most critical regions at low altitude.

In order to study the wake vortices of landing aircraft, we carried out an experiment on the airfield of Tolmachevo Airport in 2018 using Stream Line lidar, an AMK-03 sonic anemometer, and an MTP-5 temperature profiler. The limits of applicability of the method of radial velocity in estimation of wake vortex parameters from lidar measurements were determined for different aircraft types and wind turbulence strength. The analysis of the experimental results makes it possible to identify features of the spatial dynamics and evolution of aircraft wake vortices under different states of the surface air layer. In particular, we found that the lifetime of a vortex generated behind a large MD-11F aircraft on landing can reach 4 min in the case of low average crosswind speed and moderate wind turbulence.

The wake vortices of an aircraft descend by self-induction that enables a pilot to identify approximate wake location based on the preceding aircraft path. A pilot taking off an aircraft can avoid a wake vortex encounter by performing early rotation and flying at or above the climb path of the preceding aircraft. Such operation results in a situation in which two wake vortices are present in the air simultaneously in close proximity. In this study, the transport and decay characteristic processes involving interactions between the wake vortex pairs formed by the preceding and the following aircraft have been analyzed. The high altitude of the following aircraft was included as initial conditions, and atmospheric turbulence conditions were applied to large eddy simulations to account for the vortex core instability and non-linear transport and decay. The obtained results were used to identify the decay processes that induce the rapid dissipation of wake vortices such as merging into a single counter-rotating pair, rapid dissipation, and formation of a vortex ring with or without deformation of the lower vortex pair. The third process could be effectively used to adjust the take-off separation time for increasing the capacity at airports.

This study investigates the impact of meteorological parameters (crosswind, headwind, wind shear, thermal stratification, turbulent kinetic energy, dissipation rate, and density) and aircraft parameters (position, heading, speed, weight, span, and spanwise load factor) on wake-vortex behavior. Typical measurement uncertainties of these parameters, on one hand, and the range in which they typically reside, on the other hand, are mapped on variations of lateral and vertical position and lifetime of the wake vortices. For this mapping process, the deterministic two-phase wake-vortex model is employed, complemented by some simple considerations, dimensional analysis, and sources from literature. Three scenarios comprising cruise conditions, flight within the atmospheric boundary layer, and ground proximity are considered. From these investigations, the following ranking of the impact parameters has been deduced: wind, thermal stratification, turbulence, position, mass, and spanwise load factor. The relevance of the remaining parameters appears small.

Large-scale distortion of aircraft wake vortices appears to play a crucial role for aircraft safety during approach and landing. Vortex distortion is investigated based on large eddy simulations of wake vortex evolution in a turbulent atmosphere. A vortex identification method is developed that can be adapted to the vortex scales of interest. Based on the identified vortex center tracks, a statistics of vortex curvature radii is established. This statistics constitutes the basis for understanding, modeling, and exploiting mitigation effects of vortex distortion on wake vortex encounters.

Large-eddy simulations (LES) have been employed to investigate the far-field four-vortex wake vortex evolution over 10min behind an aircraft formation. In formation flight scenarios, the wake vortex behaviour was found to be much more complex, chaotic and also diverse than in the classical single aircraft case, depending very sensitively on the formation geometry, i.e. the lateral and vertical offset of the two involved aircraft. Even though the case-by-case variability of the wake vortex behaviour across the various formation flight scenarios is large, the final plume dimensions after vortex dissolution are in general substantially different from those of single aircraft scenarios. The plumes are around 170 to 250m deep and 400m to 680m broad, whereas a single A350/B777 aircraft would produce a 480m deep and 330m broad plume. Formation flight plumes are thus not as deep, yet they are broader, as the vortices do not only propagate vertically but also in span-wise direction. Two different LES models have been employed independently and show consistent results suggesting the robustness of the findings. Notably,$CO_{2}$emissions are only one contribution to the aviation climate impact among several others like contrails and emission of water vapour and nitrogen oxides, which would be all affected by the implementation of formation flight. Thus, we also highlight the differences in ice microphysical and geometrical properties of young formation flight contrails relative to the classical single aircraft case.

Aparallelcodeisdevelopedtosimulatenumericallywakevortexdetectionusingaradioacousticsoundingsystem (RASS). The code is written in FORTRAN 90 with the message passing interface for parallel implementation. The numerical simulation solves simultaneously the linearized Euler equations for a nonuniform mean e ow and the Maxwellequationsforanonhomogeneousmedium.Theradartransmitterandreceiverantennasaremodeledusing an array of point sources and a beam-forming technique, respectively. Many features of the RASS are explored using thenumericalsimulation.First,a uniformmeane owisconsidered,and theRASSsimulation isperformedfor two different types of incident acoustic e eld: a short single-frequency acoustic pulse and a continuous broadband acoustic source. Both monostatic and bistatic cone gurations are examined, and their results are compared. Taylor and Oseen vortex velocity proe les are used as samplemodels, and their mean e owe eldsare reconstructed from the backscattered electromagnetic signal using the Abel transform. The effect of radar beam width is also considered, as are the issues of nonaxisymmetric and interacting vortices. I. Introduction T HEcapacity of airports is constrainedseverelyby the airtrafe c control system’ s consideration of aircraft wake vortices. The present Instrument Flight Rules (IFR) restrictions are based on aircraft weight. The “ 3‐ 4‐ 5‐ 6 rule” sets the separation distances of aircraft by categories from small, less than 41,000 lb (18,600 kg), to heavy, greater than 255,000 lb (116,000 kg). These separations are viewed as very conservative. However, because there remains signie cant uncertainty about wake vortex behavior under different atmospheric conditions, there are considerable technological barriers to improvements in terminal area productivity. In an attempt to enhance airport capacity, an aircraft vortex spacing system (AVOSS) has been under development at NASA Langley Research Center. The AVOSS will provide the means to allow air trafe c control to reduce spacing safely in instrument operations when the appropriate weather conditions exist. A key element in AVOSS is the development of a real-time reliable wake vortex detection system (WVDS). A radio acoustic sounding system (RASS) is a promising candidate for a WVDS. The basic concept of RASS is based on the tracking of sound waves with radar. When acoustic waves are transmitted, they produce pressure perturbations, which lead to e uctuations in the atmospheric permittivity. An incident electromagnetic e eldisthenscatteredduetothepermittivityvariationsandgenerates an echo that can be detected by a receiver antenna. The spectrum of this echo shows a Doppler shift proportional to the local speed of sound. However, the backscattered signals are so weak that no measurable signal would be received unless the Bragg condition is satise ed.The Bragg condition is a relationshipbetween the incident electromagnetic and acoustic wavelengths that ensures a constructive interference between the two waves. The Bragg condition may be written as ¸e D 2¸a sin®

This paper presents three-dimensional large eddy simulations (LES) and two-dimensional direct numerical simulation (DNS) of wake vortices of an aircraft in a realistic atmospheric boundary layer (ABL). These calculations are performed in order to parametrize an operational model aiming at managing the separations between aircraft at take-off and landing. The computation considered here is taken from twelve selected cases corresponding to trials made at Idaho Falls sponsored by the Federal Aviation Administration (FAA) in 1990. After computing the evolution of a convective atmospheric boundary layer with a non-hydrostatic meteorological model, the evolution of the wake vortices (with their characteristics measured during the experiments) is computed. The threedimensional behavior of the wake vortices as well as the effects of the atmospheric parameters are shown. The results of the simulations are compared to experimental data.