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Contrail and Cirrus Cloud Avoidance

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Contrail and Cirrus Cloud Avoidance 25th INTERNATIONAL CONGRESS OF THE AERONAUTICAL SCIENCES CONTRAIL AND CIRRUS CLOUD AVOIDANCE Frank Noppel, Riti Singh Cranfield University, Bedfordshire, United Kingdom Mark Taylor Rolls-Royce plc, Derby, United Kingdom Keywords: contrails, cirrus clouds, mitigation, environment, sustainability Abstract evolve from spreading persistent contrails known as primary cirrus or contrail cirrus (2). Secondary Recent climate assessments have stressed the im- cirrus occur due to locally increased soot and portance of perturbations in the Earth’s radiation aerosol concentration, which might lead to the budget caused by air traffic. They are caused by formation of cirrus clouds that would not form the emission of greenhouse gases, aerosols, con- in the absence of air traffic (3; 4; 5). An indirect trails and aviation induced cirrus clouds. The climate forcing of aircraft emissions is possible contribution from contrails and cirrus clouds by changing particle size and ice particle number might be larger than that of all other aircraft emis- density of natural cirrus clouds (6). sions combined. Facing an increasing demand in Aviation induced pollutants have been identi- air travel, the challenge is to find technological fied and assessed in terms of radiative forcing by and operational enablers for a more sustainable the Intergovernmental Panel on Climate Change. air traffic. A preliminary analysis of the proposed Radiative forcing is a metric where the expected techniques is necessary for an adequate compar- steady state equilibrium change in terms of global ison. In this paper, an analysis tool is presented mean surface temperature is related linearly to to compare different technologies regarding their the observed radiative forcing of a certain per- environmental compability. A sample study is turbation. Linear persistent contrails and avia- presented where the effect on block fuel con- tion induced cirrus clouds were identified as main sumption is examined if contrails were avoided contributors to the overall aviation induced radia- by avoiding regions facilitating the formation of tive forcing. It is estimated that linear persistent persistent contrails. For this purpose meteorolog- contrails contribute approximately 20% to the to- ical data is evaluated regarding persistent contrail tal aviation induced radiative forcing (7). This formation. estimate considers a year 2000 scenario where cirrus clouds are excluded. Aviation induced cir- 1 Introduction rus clouds have the potential to cause a radiative forcing which exceeds the radiative forcing of all Aviation has been identified as contributor to an- other emissions due to air traffic combined. An- thropogenic changes in the Earth’s radiation bud- nually and globally averaged total contrail cover get. In particular this is due to the emission of and the associated radiative forcing is expected greenhouse gases, soot, aerosols, and the for- to quadruple during the next decades due to the mation of contrails and aviation induced cirrus increase in air traffic (8). clouds. Linear persistent contrails occur in an ice Depending on the allocated importance of supersaturated atmosphere if the Schmidt Appel- the radiative forcing due to persistent contrails man criterion is satisfied (1). Cirrus clouds can 1 Contrail and Cirrus Cloud Avoidance and cirrus clouds relative to that of other air- craft emissions, it might occur that the avoid- engine aircraft aircraft ance of persistent contrails and cirrus clouds be- parameters performance parameters comes the most important and pressing issue to be addressed in the future. The challenge is to accommodate the forecast growth in air traf- emissions/ fic of 5% p.a. during the next decades (9; 10) environmen- global data tal impact whilst reducing the emission of greenhouse gases matrix flight path and aerosols, and mitigating the radiative forc- ing associated with persistent contrails and cirrus clouds. This can be achieved by introducing rev- optimiser olutionary technologies. In order to assess technologies regarding their environmental impact, a tool has been developed Fig. 1 model layout combining engine performance, aircraft perfor- mance and meteorological data. It enables op- specified departure and destination points for a timisation of the aircraft, engine and flight path given engine/aircraft combination is carried out for least fuel burn or other desired metrics. in order to calculate the fuel burn penalty associ- In principle, persistent contrails could be ated with contrail avoidance. Fuel consumption avoided by avoiding regions in the atmosphere along the flight route is calculated considering a that support their formation (11). Avoiding the changing aircraft weight and the performance of formation of persistent contrails would also avoid the aircraft depending on altitude. Two optimi- the formation of contrail cirrus clouds. This sation algorithms are taken into account: genetic avoidance technique will in all likelihood cause algorithms and the simplex search method. A de- an increase in block fuel consumption. In this pa- scription of the methodology applied during this per, a sample study is presented examining the in- study is given in the following sections. crease in block fuel consumption by avoiding re- gions in the atmosphere which support the forma- 2.1 Aircraft and engine model tion of contrails. The departure and destination Engine off-design performance is calculated for points considered are London and New York. different throttle settings and altitudes. The con- 2 Model sidered engine represents a modern three spool high bypass ratio turbofan. Fuel flow and thrust Engine performance, aircraft performance and at- for the different altitudes and throttle settings is mospheric data are combined in an integrated stored in an engine deck. An aircraft perfor- model. Therefore, multiple existing algorithms mance table comprising inverse specific air range are linked with each other. They include Turbo- depending on altitude and the aircraft weight match, a Cranfield University gas turbine perfor- (percent fuel burned) is computed using the en- mance code (12), FLOPS, the NASA flight op- gine deck. The considered aircraft technology timisation code (13), an ESDU aircraft perfor- represents an advanced modern long-range mid- mance code (14), and APPEM, a Cranfield Uni- size subsonic transport aircraft for about 350 pas- versity engine emission prediction code based on sengers. The performance table covers an alti- (15). Figure 1 shows the model with its modules. tude range from 6000 m to 12000 m with a res- Optimisation can be carried out by varying the olution of 500 m, weight varies in 10 steps rep- parameters of the different modules using differ- resenting 100% fuel and 5% fuel. The calculated ent optimisation algorithms. aircraft performance is given in figure 2, which In this study, flight path optimisation between shows inverse specific air range depending on al- 2 Contrail and Cirrus Cloud Avoidance titude and aircraft weight for a fixed Mach num- 12000 ber of 0.85. Knowing the inverse specific air range for dif- 8000 ferent altitudes and aircraft weights, it is possible 4000 to obtain the fuel burned during an entire flight [m] altitude path. During optimisation, linear interpolation is 0 applied to calculate the inverse specific air range 40 in between the sampled data points. latitude [deg] 50 −3 x 10 0 12 -40 -20 60 -80 -60 longitude [deg] 6.5 11 Fig. 3 Flight path parameterisation with base co- 10 6 ordinates (dashed), the deviation vectors in hori- 9 zontal and vertical direction (thin solid) and the 5.5 altitude [km] flight path (thick solid). 8 5 7 flight path optimisation are the deviation vectors 6 full 80 60 40 20 reserve which determine the major waypoints. In order fuel [%] to increase the accuracy of the calculation, ad- ditional points are considered. They are equally Fig. 2 Inverse specific air range [kg/m] calcu- spaced between the major waypoints, but only lated for a modern subsonic commercial transport the major waypoints are accessed during optimi- aircraft at Mach number of 0.85. sation. major waypoint 2.2 Flight path parameterisation additional waypoint destination In order to optimise the flight path for contrail formation and fuel burn, an adequate parameter- isation of the flight path is necessary. The short- departure est connection between two points on the Earth’s surface is defined by the great circle. Equally Fig. 4 Major waypoints and additional waypoints spaced points along the great circle between de- (solid and hollow circles) along the flight path parture and destination on ground level define the (dashed). The flight path as considered for the base coordinates. The waypoints along the flight calculation for fuel burn is shown by the thick path are defined by the horizontal and vertical solid lines. deviation from the base coordinates. The ver- tical deviation is the flight altitude whereas the For each waypoint, inverse specific air range horizontal deviation is a vector perpendicular to and the probability of contrail formation is read the great circle on ground level. Figure 3 shows from the aircraft performance table and the global the flight path (solid), the base coordinates on data matrix. The fuel burned along the entire the great circle (dashed) and the deviation vec- flight path is calculated starting at the destina- tors (thin solid) in horizontal and vertical direc- tion point, which makes iteration redundant to tion. Horizontal deviation for departure and des- match a given destination aircraft weight. Air- tination is set to zero for all calculations. craft weight at destination includes 5% fuel re- Figure 4 shows the flight path projected on serves in this study. two dimensions. The variable parameters for 3 Contrail and Cirrus Cloud Avoidance 60 2.3 Global data matrix saturation pressure w.r.t. ice saturation pressure w.r.t. liquid water In order to calculate the fuel burn penalty asso- 50 theoretical mixing line critical mixing line ciated with contrail avoidance, climate data for ambient conditions the year 2005 was evaluated regarding contrail 40 formation. Analysis data from an unified field model has been made available by the MET Of- 30 fice.
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