The Role of Computer Simulation in Reducing Airplane Turn Time


Airplane turn time -- the time required to unload an airplane after its arrival at the gate and to prepare it for departure again -- has increased since the mid-1970s. Boeing has created a computer simulation that can help airlines reduce one of the key elements of turn time: passenger boarding (enplaning and deplaning). Decreasing passenger boarding time may significantly lower the amount of time between revenue flights, and so increase profitability to airlines.

As airlines face increasing pressure to improve profitability, they are striving to carry the greatest number of passengers feasible while keeping their fleets in revenue service as much as possible -- all without compromising passenger convenience.

One way airlines can move toward this goal is to reduce airplane turn time. Turn time is the time required to unload an airplane following arrival at a gate and to ensure that the airplane is ready and loaded for its next departure. The most significant elements of turn time include passenger enplaning and deplaning, cargo loading and unloading, airplane fueling, cabin cleaning, and galley servicing. (figure 1)

For many airlines the largest factor in turn time is the passenger boarding process. Boeing has conducted studies to help understand the airplane's contribution to turn time. The company is continually working with customer airlines to develop the data and tools necessary to help reduce turn time without significantly affecting passenger convenience.

During its studies, Boeing considered the following:
1 Historical trends.
2 Existing turn time documentation.
3 Computer simulation tools.
4 Discrete event simulation.
5 Simulation validation and testing.
6 Applications of computer simulation.

1 HISTORICAL TRENDS
The majority of the standard body fleet has been experiencing a gradual increase in airplane turn time since 1975. A useful indicator is the airlines' reported increase in through-stop times, the ground time required for flights continuing on to other destinations. (figure 2) Increased turn time is further substantiated by Boeing boarding rate studies. Since 1970, the actual speed at which passengers boarded an airplane (enplane rate) has slowed by more than 50 percent, down to as low as 9 passengers per minute. (figure 3) Similar through-stop time increases and boarding rates have been observed for wide-body airplanes. The trends are generally attributed to increased passenger carry-on luggage, more emphasis on passenger convenience, passenger demographics, airline service strategies, and airplane flight distance (stage length). Boeing believes that these trends will continue unless the root causes are understood and new tools and processes are developed to reverse the trend.

2 EXISTING TURN TIME DOCUMENTATION
Boeing has documented airplane turn time, including the passenger loading process, for many years. This documentation describes the "generic" flow time expected for each airplane. (figure 1) The information helps an airline schedule ground times and establish airplane ground handling procedures. However, the general nature of this information does not help airlines evaluate alternative procedures that could shorten flow time or help the airlines predict the impact of these procedures on passenger convenience.

3 COMPUTER SIMULATION
Boeing developed a computer simulation model to help airlines reduce turn time. This model analyzes the impact of interior configuration changes and variations in passenger boarding procedures on the passenger boarding process. The simulation can quantify potential changes that an airline would normally identify only through costly and potentially disruptive in-service experiments. Though using the model does not completely eliminate the need to conduct passenger loading trials, it can effectively quantify the expected outcome. This allows the airline to limit in-service trials.

Called the Boeing Passenger Enplane/Deplane Simulation (PEDS), this simulation helps the user evaluate different passenger boarding scenarios and airplane interior configurations. The simulation:

4 DISCRETE EVENT SIMULATION
PEDS analyzes the passenger boarding process as a set of interrelated elements through a technique called discrete event simulation. This modeling technique uses computer software to combine the effects of mathematical queuing theory with an analysis of random behavior.

Random behavior associated with passenger loading can occur either in activity timing (when events happen) or in action decisions (how people act). The more complex the overall activities are, the more difficult it is to accurately predict the impacts of the random behavior.

Boeing began using discrete event simulation to understand interactions in the factory environment. In 1994, Boeing started applying the discrete event model in passenger boarding studies. PEDS assigns each passenger certain attributes, such as walking speed, type of carry-on luggage, luggage put-away time, and relationship with other passengers (traveling alone or with a group). The simulation accounts for random behavior by applying probability distributions to passenger attributes.

In discrete event simulations, each activity happens at specific intervals. The activity starts, continues for a finite period of time, then stops. Similar to a car passing along a city street, each airplane passenger enters the cabin and "travels" to his assigned seat. Various other activities, such as passengers standing in the aisle, assisting family members, or waiting to store luggage in the overhead bins, act as traffic lights that prevent passengers from speeding through the cabin to their seats. PEDS breaks down the passenger loading process into a series of finite elements of starting, moving, stopping, waiting, and restarting, beginning with the moment the first passenger enters the cabin and ending when the last passenger is seated.

In the computer model, engineers use the software to create a mathematical scene of the interior of the airplane, complete with seats, overhead bins, aisles, and doors. Each passenger is individually modeled and assigned attributes that describe some segment of the traveling population. The simulation then governs the decisions of each passenger based on these attributes and the timing of each event (waiting or moving) as he or she passes through the cabin.

In addition to varying passenger "speed," the simulation allows variation in other attributes. Each run of the simulation calculates the total elapsed time and provides visibility into the various "choke" points identified by the scenario. To eliminate statistical bias, multiple simulations are run on the identical interior configuration using different random number starting points, and the results are then averaged over the multiple runs.

The process begins by establishing a general airplane simulation. This simulation is then "tailored" using the airline's specific interior configuration, boarding procedures, and passenger demographics. A series of baseline runs is conducted to validate PEDS predictions with the airline's in-service experience. This turn time baseline allows the airline to evaluate potential changes to interior layout or boarding procedures against existing time lines.

5 SIMULATION VALIDATIONAND TESTING
A simulation is valid only if it accurately predicts what happens during actual events. In order to validate PEDS, Boeing conducted two types of activities:

In-service observation.
Boeing engineers observed revenue passenger loading activities with different airlines at several different airports around the world. The loading process was timed and correlated to simulation predictions. These exercises were conducted on a random basis so that timed data would be representative of actual experience. However, this method of validation had two limitations:

Actual passenger loading tests.
In order to provide additional information, Boeing conducted a series of passenger loading tests.

The tests were conducted on a fully configured 757-200 airplane located in a Boeing factory. A loading platform was modified to simulate an airport passenger loading bridge, and a staging area was used to simulate an airport gate waiting area. Trained airline flight attendants used procedures inside the airplane and at the loading bridge that reflected actual airport and airline operations.

A total of 600 non-Boeing people, representative of the typical traveling population, participated in the enplane/deplane tests. Each passenger was requested to bring carry-on luggage typical for a three- to four-hour flight. The passenger population was varied for each test, with each passenger participating in only two sets of enplane/deplane tests to prevent "learning"that could alter the test results. The following four enplane/ deplane scenarios were then run:

The outside-in scenario involves loading passengers at the window seats first, middle seats next, and aisle seats last. The outside-in test was designed to validate the simulation's ability to handle nontraditional procedures.

Each test was timed and videotaped for comparison with the simulation's predictions. Video cameras located on the loading bridge and throughout the passenger cabin recorded each test for later analysis.

The tests showed that PEDS performed well in predicting both enplane and deplane times for each scenario. However, the videotapes documented some unexpected interaction among passengers during the loading process. For example, several passengers stowed their carry-on luggage in overhead bins located away from their seats. This happened while they were waiting behind people who were blocking the aisle, rather than waiting for the aisle to clear and then stowing their luggage when they reached their seats. This additional information allowed Boeing to refine PEDS to account for this type of behavior.

6 APPLICATIONS OF THE COMPUTER SIMULATION
PEDS offers several applications both for airlines and for Boeing:

Airplane interior configuration. Both Boeing and the customer airline can use PEDS initially to help configure the airplane interior. PEDS improves their understanding of how a proposed configuration affects passenger loading. Passenger boarding times become part of interior configuration decisions, similar to decisions made about aesthetic appearance and passenger comfort.

Evaluation of potential changes to passenger loading procedures. The simulation also helps airlines evaluate potential changes to passenger loading procedures. Boarding alternatives such as outside-in, multiple preboarding, or even unassigned seating can be evaluated using an airline's target passenger population. Airlines can quantify potential savings without incurring the expense of boarding trials or risking passenger inconvenience. Airlines can use PEDS to evaluate even small procedural changes, such as increased involvement by flight attendants or emphasis on reduced passenger carry-on luggage.

Alternatives that show significant promise can then be tested in the field. PEDS helps airlines develop detailed implementation plans for new procedures and logistics needed to decrease turn time, including flight attendant training and airport logistics.

Boeing is currently working with a number of airlines to evaluate PEDS for their specific applications.

SUMMARY
Decreased airplane turn time can positively affect airline profitability. A new computer simulation developed by Boeing offers airlines an analytical method for evaluating changes to airplane configuration or passenger loading techniques and their impact on airplane turn times. The simulation is a cost-effective tool to help airlines rapidly predict results with a high degree of success.

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GETTING HELP WITH TURN TIME REDUCTION
The Boeing Passenger Enplane/ Deplane Simulation (PEDS) offers airlines an additional tool to help reduce turn time. Depending on an individual airline's operation, other elements of turn time, such as cargo handling, cabin cleaning, or galley servicing, may also be improved.

Boeing has a team of turn time experts that can work with airlines to analyze specific areas of concern. Airlines interested in evaluating solutions to their turn time problems should contact their local Field Service or Customer Requirements representative for assistance.

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EVALUATING TURN TIME ON THE NEW 757-300
The initial application of the Boeing Passenger Enplane/Deplane Simulation (PEDS) was on the new 757-300 airplane. PEDS helps evaluate various passenger boarding scenarios to determine how to reduce airplane turn time.

The 757-300 is a stretched derivative of the 757-200. It is 23 feet, 4 inches longer than its predecessor and holds approximately 40 more seats.

Several potential customers for the 757-300 had told Boeing that, based on their experience with other standard-body airplanes, they were concerned about the new airplane's potentially greater turn time.

Boeing used the 757-200 as a baseline for evaluating passenger boarding on the 757-300. PEDS predicted that passenger loading for the 757-200 would take approximately 22 minutes and that deplaning would take about 10 minutes. The predicted overall turn time for the 757-200 was 52.5 minutes, including cargo handling, fueling, galley servicing, and cabin cleaning. The estimated time was based on actual airline in-service turn times for the 757.

Applying PEDS to a 757-300 dual-class configuration with 240 passengers showed that passenger loading would require 26 minutes, and that unloading would take about 12.5 minutes. The predicted overall turn time was 59 minutes, an increase of only 6.5 minutes over the 757-200. (figure 5a)

Boeing then used PEDS to evaluate a number of alternative boarding scenarios. Simulation predictions were compared to the 757-200 passenger boarding test to validate results. Based on these validated predictions, it was possible to identify significant potential reductions to overall turn times for the 757-300. For example:

PEDS showed that the new 757-300 could be operated within the normal 757 turn time window of 60 minutes without making notable changes to existing procedures. It also showed that turn time could be reduced significantly if airlines used alternative passenger boarding methods.

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Scott Marelli, P.E.
Senior Math/Modeling Analyst
Applied Research and Technology
Boeing Shared Services Group

Gregory Mattocks
Lead Engineer
757 Configuration and Engineering Analysis
Boeing Commercial Airplane Group

Remick Merry
Senior Specialist Engineer
New Airplane Customer Service
Boeing Commercial Airplane Group

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