The Evolving Airline Industry: Impacts on Airports
2.3 Airline Scheduling and Fleet Assignment Optimization
glob-al airline glob-alliances has encouraged these new internationglob-al services, with one or both end points being major hubs for one of the partners in the alliance. For example, Salt Lake City is a Delta hub and Paris de Gaulle is an Air France hub, and both carriers are partners in the SkyTeam alliance (seeChap. 1). In addition, the increased range capabilities of smaller international aircraft like the Boeing 767 and 787, and the Airbus A330 and A350 mean that airlines can offer these nonstop flights with a lower risk of not filling seats.
For airports, these trends in network evolution and airline route planning suggest that a proactive approach to attracting new airlines and new routes could be beneficial. Airports must, however, understand the changing business models and network characteristics of airlines with differing values and objectives, in order to offer them attractive proposals. An LCC with primarily point-to-point operations will be most interested in finding new air-port destinations with large traffic catchment areas, which can offer lower user fees and improved operational reliability than competing airports (e.g., in terms of short turnaround times and lack of congestion). A large network carrier considering a new service from an airport to its hub will be more interested in the potential for its flight to capture an adequate amount of connecting traffic from the new spoke city via its hub, above and beyond the local market potential.
The choice of what frequency to operate on a specific route depends on both competitive and economic considerations. Greater frequency of departures on a route improves the
“schedule coverage” of an airline, that is, the proportion of desired passenger departure times that can be accommodated by the airline’s flight departure times. Greater schedule coverage is particularly important for time-sensitive business travelers. More frequent flights improve the convenience of air travel for passengers and reward the airline with higher traffic, revenues, and increased market share at the expense of its competitors.
In competitive markets, airline frequency share is the most important factor that determ-ines each airline’s market share of total demand, assuming that both prices and on-board service quality tend to be similar among competing carriers. The shorter the distance in-volved, the more important frequency share is, given that actual flight times represent a rel-atively small proportion of the passengers’ total travel time. In these markets, it is common for competing airlines to operate smaller capacity aircraft with higher operating costs per seat and per seat-kilometer, trading off higher costs against the revenue benefits of higher market share.
The objective of “load consolidation” also affects airline frequency decisions on a route.
Consolidating passenger traffic from multiple O-D markets onto one aircraft can allow that airline to operate higher frequency on the route (increasing its market share) and/or larger aircraft (reducing its unit operating costs). This ability to consolidate loads is a fundament-al reason for the economic success of airline hubs.
Given a chosen frequency of departures on a route, the process of timetable development determines the specific departure and arrival times of each flight. All else equal, peak de-parture times (early morning and late afternoon) are most attractive both to business trav-elers willing to pay higher fares and to many leisure travtrav-elers as well. However, develop-ing a timetable of flight departures requires airline schedulers to make tradeoffs between aircraft utilization (block hours per day) and schedule convenience for the passengers. See Example 2.1.
Example 2.1 A peak-hour departure at 17.00 from airport A and arrival at airport B at 19.30 is more likely to be attractive to business passengers and therefore profitable for the airline. Once that aircraft arrives at airport B, a minimum “turnaround” time is required to prepare it for the next flight. Turnaround times vary by aircraft type and the characteristics of the flights involved—a narrow-body aircraft in domestic service might require as little as 30 minutes, whereas a wide-body international aircraft takes 2 hours or more to prepare it for the next departure.
Assuming a 45-minute “turnaround” time, the aircraft arriving at airport B at 19.30 could be scheduled to depart again as soon as 20.15. However, a late evening departure might not be attractive to many passengers, so the air-line scheduler must decide whether to operate the return flight at 20.15 with fewer passengers and less revenue or hold the aircraft (and potentially its crew) on the ground overnight until the next peak departure time, say at 07.30 the following day. In the latter case, the aircraft and its crew will be idle for 12 hours, reducing aircraft and crew utilization and increasing costs.
Typically, most airlines develop timetables to maximize aircraft utilization. They keep
“turnaround” times to a minimum and the aircraft and crews flying as much as possible
to reduce unit costs. This approach can lead to off-peak flights with relatively low load factors, which might be necessary to maintain competitive frequency share and to position aircraft for peak flights at other cities. It can also leave little buffer time for maintenance and weather delays, if adequate slack is not built into the timetable.
A variety of factors can constrain timetable development. Airline hubs with fixed con-necting banks require that flights arrive from spoke cities and depart from the hub at pre-determined times. At large hubs, the use of connecting banks creates surges of aircraft and passenger activity that require relatively high airport capacities, in terms of both airside ele-ments (runways, taxiways, and gates) and terminal facilities. The most successful connect-ing hub airports have been able to expand their capacities (e.g., Atlanta) to facilitate con-nections, in contrast to more constrained airports that are close to saturation (e.g., Tokyo/
Narita and London/Heathrow).
Time zone differences also limit feasible departure and arrival times, especially on long-haul routes. For example, flights from eastern cities in North America to Europe typically do not depart before 16.00, as passengers do not want to arrive at their European destination much before 06.00 local time. Regulatory constraints, such as airport arrival and departure slot times, and noise curfews can further limit the scheduling flexibility for an airline. Fin-ally, crew scheduling and aircraft maintenance requirements can also impose significant constraints on timetable development.
Fleet assignment is the problem of allocating the specific aircraft type to be flown on each flight leg, given a network of routes, a set of flight departure times and available air-craft types from the airline’s existing fleet. The objective of fleet assignment is to minim-ize the combined costs of “spill” (rejected demand and lost revenue) and aircraft operating costs. Spill occurs when the aircraft assigned to a flight departure is too small and poten-tial demand and revenues are lost to the airline. Airlines can reduce (or eliminate) spill by assigning a large enough aircraft to accommodate all possible peak-day demands for the flight in question. However, larger aircraft have higher operating costs and will fly with many empty seats on most nonpeak days.
Many airlines use fleet assignment software tools based on large-scale mathematical net-work optimization methods. These assign aircraft to maximize expected profitability, sub-ject to constraints such as minimum ground times, maintenance requirements, and number of aircraft by type available in the airline’s fleet. Aircraft routing models are used to as-sign specific aircraft “tail numbers” to each flight, creating rotations that satisfy aircraft maintenance requirements and maintain a balance of inbound and outbound flights at each airport. The use of these optimization tools has allowed airlines to achieve higher aircraft utilization rates and reduce total aircraft and crew costs.
The final product of the airline schedule development process is a detailed plan of how the airline will operate on a given date in the future. It includes aircraft schedules, crew assignments, and a large number of other operational details. This plan will have been
op-timized to reduce costs, increase revenues, and maximize profit under what are typically assumed to be favorable operating conditions. Unfortunately, almost every day presents a variety of unexpected and unplanned events that force any airline to deviate from its op-timized schedule.
Dealing with “irregular operations” requires airlines to revise their planned schedule right up until the flight departs or is cancelled. A cancelled flight can seriously disrupt aircraft rotations, crew schedules, and maintenance plans, not to mention passenger trips.
Under conditions of disruptions and/or flight cancellations, the primary objective for the airline is to return to normal operations as quickly as possible. In this effort to get the airline
“back on plan” with respect to the planned timetable, flight cancellations or aircraft rerout-ing sometimes take precedence over passenger convenience. The next section describes the characteristics of airline operations occurring at the airport that can contribute to these de-viations from the planned schedule.