SlotsAirport slot availability is an increasing number of cases aconstraint on route entry and scheduling At the moment, theindustry bases slot allocation at congested airports on the‘Grandfather Rights’ principle. Opportunities to land and takeoff at particular times are retained by established airlines on amore-or-less permanent basis, from one season to the next.This can mean that there will be significant difficulties fora new airline wishing to begin services at a congested airportwhere all the attractively-timed slots will be in the possessionof incumbent airlines. Even if slots can be obtained to allowservices to begin, they may be at unsuitable times. It may alsobe difficult to get sufficient slots to allow the frequencies ofestablished airlines to be matched.5.3.2 Network , Frequencies and TimingIssues about slots leads t o developments of largeaircraft such as the A380 which is able to carry largernumber of passengers in a take-off / landing slotsEnvironmentEnvironmental factors are often another practicalconstraint. Many airports now impose curfew on nightflying . Whilst many airlines try to avoid ‘dead-of night’ or‘red eyes flights’ because of their unpopularity withpassengers and difficulties with airport access, night flying isstill a way for leisure orientated airlines to boost aircraftutilisation and lower their unit costs.5.3.2 Network , Frequencies and TimingNetwork PlanningIn terms of current controversies regarding network andschedules planning, airlines are having to make a number ofdifficult decisions, many of which involve the familiartradeoff between costs and product quality.On long-haul routes, a very clear passenger preferencehas emerged in recent years. Passengers prefer non-stopflying to flights involving intermediate stops. To meet thisrequirement, aircraft manufacturers have responded byproducing families of aircraft with longer and longer ranges,and the opportunities provided by such planes have beentaken up by some airline such as SQ