LAUSR

North Atlantic air traffic is broadly organised into a track system; daily sets of tracks are defined by air traffic control which are vertically stacked, such that the same set of tracks is used for all flight levels, regardless of any vertical variations in wind. This work uses minimum-time routes, previously shown to be a good proxy for the location of the North Atlantic track system, to understand whether vertical variations in wind speed and direction significantly affect minimum-time routes optimised at different altitudes; this is to examine whether (all other factors assumed equal) there is potential for improvements in fuel efficiency. The optimum cruise altitude over the North Atlantic is determined, focusing on the New York – London route. It is found that eastbound routes, which take advantage of the jet stream, are on average faster at 250 hPa (flight level (FL) 340) than at 300 hPa (FL300) or 200 hPa (FL390) by approximately 2 minutes (compared to the annual-mean route time of about 330 minutes, assuming a true air speed of 250 m s-1). For westbound routes, the route time increases with height: aircraft flying at 300 hPa are on average 3 minutes faster than at higher levels (the annual-mean optimum time being about 400 minutes). These estimates are compared with the time penalty which arises from flying a route optimized at 250 hPa at the other two altitudes. The time penalty is generally less than a minute, compared to the minimum-time routes calculated at those altitudes.