Truck Scheduling
for Ground to Air Connectivity
Randolph W. Hall
Dept. of Industrial and Systems Engineering
University of Southern California
In recent years the fastest growing
segments of the goods movement industry in the United States have been small
shipments and air shipments. Federal Express, United Parcel Service, DHL, Emery,
Airborne Express and the US Postal Service have prospered in this environment
by creating integrated groundair networks. Air cargo terminals have also developed
a capability to rapidly unload trucks, sort shipments, load these shipments
into air containers, and load the air containers onto aircraft. These steps
can sometimes be completed within a time span of 1 hour or less.
At a destination airport the steps are reversed, allowing aircraft to be unloaded, and trucks to be loaded, within a short time span. High efficiency in sorting and loading has made it economical to send shipments across the country with next morning delivery (Analla and Helms, 1996; Chan and Ponder, 1979; Hansen and Kiesling, 1993; Jannah and Wilder, 1999; Larson, 1998; and Oster et al, 1995, examine the economic structure of the industry.) Taken as a whole, express transportation can often be divided into the 11 steps listed in Table 1. To meet time commitments, it is desirable to make all of these steps as fast as possible. And it is also desirable to expedite some of the steps in order to provide more flexibility in others (e.g., to allow later pick-up times for shipments). The sorting process at the origin airport is a particularly critical step, as it is susceptible to random delays in the arrival of work, and because it demands relatively large investments in facilities and labor. The facilities and labor are only needed within concentrated time periods, which sometimes makes it uneconomical to provide sufficient capacity to process shipments as quickly as they arrive.
Unfortunately, late truck arrivals can also delay sorting, with significant repercussions. The issue is especially critical in Southern California for three reasons: (1) west coast shipments have a 3 hour time lag relative to east coast, due to the difference in time zones, (2) Southern California is the dominant population center on the west coast, and (3) congestion in Southern California has both elongated travel times and made them less predictable. The importance of these shipments is magnified by the fact that major hub terminals (such as Federal ExpressÕ Memphis hub) cannot release their outbound aircraft until all inbound flights have arrived and been processed. Thus a single delay to a Southern California flight (due to a few late trucks) can translate into systemwide delays. From a customer service perspective, systemwide delays can force an airline to alter its delivery commitment and pickup cutoff times. Thus, airlines that are better at managing their ground operations can offer more competitive service to their customers, and capture a larger share of the express shipment market.
Analysis Approach
The fundamental unit of analysis in this paper is a "sort" at an air
cargo terminal. Some air cargo terminals schedule multiple sorts at different
times, and some terminals have multiple lines that simultaneously complete sorts.
Each sort ends when all the packages have been processed for an individual aircraft,
or for a group of aircraft that share a sort. Because different sorts process
different inbound trucks, and because different sorts feed different aircraft,
they can be analyzed independently of each other. This paper is based on a project
that was completed in cooperation with the two largest private express package
companies in the United States: Federal Express and United Parcel Service. One
objective of the project was to create a real-time/web-based tool for scheduling
trucks and managing the sort. To do this, models were developed for predicting
the performance of a sort. These models are being specified and tested with
empirical data, including travel time measurements obtained from the airlines
and real- time information on highway congestion.
Although many aspects of air freight operations have been examined, prior research has not addressed the interaction between the arrival of incoming trucks and the processing of shipments, as is the focus of this paper. Hall and Chong (1 993) did investigate queueing interactions for banked arrivals of aircraft at a hub terminal, with focus on aircraft-to-aircraft transfers, rather than ground-to-air transfers. In this case queueing appeared as a consequence of runway capacity, rather than sortation capacity. Paper Organization The paper is divided as follows. First, models are developed for the arrival of shipments (measured as "work") at a groundair terminal. Then the scheduling of the sorting process is examined, and methods for determining the start time for the sort are created. Simulations are provided next, showing the effects of different sorting schedules on the completion time of the sort. The paper ends with conclusions and a description of future work.