United States

Environmental Protection

Agency

Air and Radiation EPA420-R-99-007

May 1999

 Printed on Recycled Paper

Technical Support for

Development of Airport

Ground Support Equipment

Emission Reductions
EPA420-R-99-007

May 1999

Technical Support for Development of Airport Ground Support

Equipment Emission Reductions

Prepared by:

Sierra Research, Inc. for the

Office of Mobile Sources

U.S. Environmental Protection Agency

Contract No. 68-C7-0051

NOTICE

This technical report does not necessarily represent final EPA decisions or positions.

It is intended to present technical analysis of issues using data which are currently available.

The purpose in the release of such reports is to facilitate the exchange of technical information and

to inform the public of technical developments which may form the basis for

a final EPA decision, position, or regulatory action.
Report No. SR98-12-05

Airport Ground Support

Equipment Emissions 

prepared for:

U.S. Environmental Protection Agency

December 31, 1998

prepared by:

Sierra Research, Inc.

1801 J Street

Sacramento, California 95814

(916) 444-6666
Technical Support for Development of 

Airport Ground Support Equipment Emission Reductions

prepared for

U.S. Environmental Protection Agency

Contract No. 68-C7-0051

Work Assignment No. 0-05

December 31, 1998

prepared by:

Sierra Research, Inc.

1801 J Street

Sacramento, CA 95814

(916) 444-6666
Disclaimer

Although the information in this report has been funded wholly or in part by the

United States Environmental Protection Agency under Contract No. 68-C7-0051, it has

not been subjected to the Agency’s peer and administrative review and is being released

for information purposes only. It therefore may not necessarily reflect the views ofthe

Agency, and no official endorsement should be inferred.
Technical Support for Development of 

Airport Ground Support Equipment Emission Reductions

Table of Contents

Introduction

Available Information Sources

Potential Control Strategies

Basis for GSE Population Estimates

GSE Emissions and Activity Estimates

LPG and CNG Control Strategies

Electric GSE

Emissions Aftertreatment

Fixed Gate Support

GSE Control Strategy Summary

Estimating GSE Activity

Appendix A - GSE Model Instructions
INTRODUCTION
-1-

INTRODUCTION

Background

The significance of air pollution emissions from airports is growing as airports expand to

meet increasing demand for air travel and as the relative contribution of other sources

declines under the pressure of progressively more stringent emission control programs. 

As a result, airports are being targeted for more aggressive emission control programs. 

Aircraft ground support equipment (GSE) represents one of three groups of mobile

emission sources at airports. Together with aircraft and ground access vehicles, GSE

contribute a small but significant share of the hydrocarbon (HC), carbon monoxide (CO),

oxides of nitrogen (NOx), and particulate matter (PM) emitted in metropolitan areas. 

Today, total emissions from these three source categories comprise on the order of 2-3

percent of total manmade emissions in a typical metropolitan area, but this share is

expected to increase as air travel continues to grow while emissions from other,

non-airport sources are subject to increasingly stringent controls. 

Since airports represent a growing source of emissions, EPA issued a work assignment to

gather information and develop methodologies that can be used to develop a protocol on

how to obtain State Implementation Plan (SIP) credit for emission reductions associated

with the retrofit/replacement of airport ground support equipment by third parties, or

governmental entities. This project is in support of the U.S. EPA Office of Mobile

Sources Transportation and Market Incentives Group’s (TMIG) efforts to develop

protocols for quantifying voluntary mobile source emissions programs (VMEPs). 

Voluntary Measure Protocols are stand-alone guidance documents that describe the steps

and methodology necessary for states and/or third parties to quantify emission reductions

for SIP credit under EPA’s Voluntary Measures Policy. Steps include requirements for

quantifying emission benefits and estimating uncertainties in emission reductions and

compliance, as well as administrative requirements for record keeping and/or SIP

submission.

Project Scope

The purpose of this work assignment was to examine the control measures available to

reduce emissions from one component of airport emissions, ground support equipment,

and to recommend methodologies to quantify the reductions associated with

implementing those control measures. This effort included the following key tasks:

Literature/Data Search

List of Equipment and Control Strategies

Description of Individual Control Strategies

Summary of Available Control Strategies

Methods for Estimating Ground Support Equipment Activity

Preparation of a Spreadsheet to Quantify Ground Support Emissions

This report presents a compendium of the work products produced under each task. To
-2-

aid the reader in identifying products of interest, a brief summary of the contents of each

section of this report is presented below.

Literature/Data Search - An extensive review of the literature was conducted to identify

additional reports and bodies of information that provide insight into GSE emission rates,

activity levels and available technologies to reduce emissions. The results are

summarized in a series of tables addressing the following:

•General reports;

•Journal articles;

•Airport reports; and 

•GSE manufacturer product information.

List of Equipment Control Strategies - Based upon a review of the available literature,

a series of tables were constructed to list the control strategies available for each category

of ground support equipment. Separate tables were prepared for each pollutant (i.e., HC,

CO, NOx, PM, and carbon dioxide [CO2]). Each table is organized by GSE type (e.g.,

airport pushback tractor, forklift, etc.), engine type, estimated U.S. population, fraction of

total GSE population, fraction of emissions, and percent emission reduction potential for

each control strategy.

Basis for GSE Population Estimates - Since there are no registration requirements for

GSE or any other national organization charged with tracking GSE activity, there is no

reliable database from which accurate GSE populations can be determined. To provide a

reliable estimate of national GSE population, a detailed statistical regression analysis of

the available data was conducted. This section presents the results of that review by

airline classification and by equipment and engine type.

GSE Emissions and Activity Estimates - Accurately characterizing the emissions

performance of a particular GSE requires detailed knowledge in two specific areas: 

(1) the rate of equipment emissions per unit of activity, and (2) the amount of activity

performed during the period of interest. Generally, the unit activity emission rate can be

either measured directly or estimated from previous measurements taken for similar

equipment or engines. Because emission rates typically vary with engine speed and load

(a measure of how “hard” the engine is being worked), emission rates for GSE-type

equipment are commonly measured over a broad series of constant speed and load

operating modes that, when weighted in accordance with the amount of time the engine

spends in each mode, can describe the average emission rate of the equipment. This

section presents a methodology for estimating equipment specific activity and related

emission levels.

LPG and CNG Control Strategies - The majority of conventionally powered GSE can

be converted to either liquefied petroleum gas (LPG) or compressed natural gas (CNG)

fueling or replaced with a specially manufactured LPG- or CNG-powered counterpart. 

The basic issues surrounding the use of LPG or CNG as a GSE fuel are quite similar and,

therefore, both fueling strategies can be treated together. Generally, non-methane
* Technically, electrically powered GSE do not emit any pollutants. The term emit, as used here, ascribes to the

electric GSE, the offsite increase in power generating station emissions due to increased airport electrical power to

support electric GSE recharging. In essence, the electrically powered GSE are treated as if they $emitted# the

increased power generating station emissions associated with their use.

-3-

hydrocarbon (NMHC), CO, NOx, PM, and CO2 emissions from LPG or CNG GSE are all

reduced relative to emissions from gasoline-powered GSE. Relative to diesel GSE,

emissions of NOx and PM are reduced, emissions of HC and CO are increased, and

emissions of CO2 can be either slightly increased or decreased depending on equipment

size. This section presents a review of important issues in determining the viability of

either conversion or replacement strategies.

Electric GSE - The majority of conventionally powered GSE can be either converted to

electric power or replaced with specially manufactured electrically powered counterparts. 

Although there is an increase in offsite power generating station emissions resulting from

the increased electrical demand required to recharge electric GSE, conversion to electric

power or replacement with electric GSE can be a very effective emission reduction

strategy. Even when the increased emissions from power generating stations are

considered, electric GSE usually emit* significantly less HC, CO, NOx, PM, and CO2

emissions than their fossil-fueled (i.e., gasoline, diesel, CNG, and LPG) counterparts. 

This section reviews issues for consideration in determining the viability of either

conversion or replacement strategies.

Emissions Aftertreatment - The majority of GSE continue to emit pollutants at

essentially uncontrolled rates. Certain emission species such as HC, CO, and, in some

cases, NOx can be substantially reduced by the installation of a catalytic converter in the

equipment exhaust system. Exhaust system traps (or filters) can perform similar

reduction functions for PM. Catalyst technology has been well proven in on-road vehicle

applications and particulate trap technology has advanced considerably over the last

several years. Both systems are commercially available for equipment in the off-road

sector, but long-term reliability and effectiveness have not been proven. This section

presents a review of the systems designed for application on gasoline, diesel, CNG, and

LPG engines.

Fixed Gate Support - While the majority of conventionally powered GSE can be either

converted to or replaced by GSE powered by alternative fuels such as LPG, CNG, or

electricity, a significant fraction of GSE can be eliminated entirely by incorporating fixed

point-of-use support equipment into aircraft gate design. Such design not only eliminates

all energy demands associated with moving displaced mobile GSE between aircraft gates

and maintenance/storage facilitates, but also facilitates the use of “hard-wired” electrical

power connections, thereby eliminating the need for a recharging infrastructure and

scheduling plan. Although, as with electrically powered GSE, there is an increase in

offsite power generating station emissions due to the increased demand for electrical

power, fixed equipment is likely to consume less power than equivalent mobile GSE due

to the elimination of the motive aspect of GSE operation. Even when the increased

emissions from power generating stations are considered, fixed electrically powered

support equipment usually emit significantly less HC, CO, NOx, PM, and CO2 emissions
-4-

than their mobile fossil-fueled (i.e., gasoline, diesel, CNG, and LPG) counterparts. This

section reviews issues related to the capital and operating cost and reliability of fixed

gate-based equipment.

GSE Control Strategy Summary - An evaluation of GSE and associated service

demands indicates that there are several control strategies that offer the potential to

reduce emissions over both the long and the short term. Such strategies include both

demonstrated and innovative technologies. All have associated issues that must be

considered prior to implementation, but a generalized classification includes the

following:

•The development of new engine emission standards for all currently unregulated

equipment;

•The replacement or conversion of gasoline or diesel powered GSE to LPG or CNG

fueling;

•The replacement or conversion of gasoline, diesel, LPG, or CNG powered GSE to

electric power;

•The replacement of mobile GSE with electrically powered fixed gate-based equipment;

•The retrofit of existing GSE with catalytic converters or particulate traps; and

•The preferential replacement of existing two-stroke gasoline engines.

As might be expected, both the feasibility and cost effectiveness of emission reductions

vary considerably across the potential control strategies. This section provides an

overview of the major issues associated with each strategy and, where possible, provides

an estimate of associated emissions reduction and cost effectiveness.

Estimating GSE Activity - Knowledge of the activity levels of airport GSE is critical to

accurately determining emissions performance. This is true regardless of whether one

considers the baseline emissions performance of today’s GSE fleet or the potential

emission reductions that can be derived through the implementation of various control

measures. Either assessment requires the detailed characterization of equipment

population and/or usage rates to derive accurate emission or emission reduction estimates. 

This section presents a review of the basic GSE emissions calculation methodology and

methods available to prepare population estimates. 

GSEModel Instructions - GSEModel is a personal computer spreadsheet-based analysis

tool that has been developed to quantify emission benefits and calculate the costeffectiveness of converting existing airport GSE to cleaner-burning fuels and engine

technologies. The model has been developed as a planning tool for use by metropolitan

planning organizations (MPOs), airports, and other agencies interested in evaluating

potential emission benefits and cost savings resulting from available GSE emission
-5-

control technologies. It has been designed with a mouse-enabled graphical user interface

to make it simple and easy to use.

The GSEModel tool is based upon the “best practice” methodologies and information

presented in the body of this report. It has been designed to utilize local (i.e., airportspecific) GSE usage and cost information coupled with best-available emission factor

data to perform the following functions using a consistent methodology:

&Estimate current and alternative technology GSE emissions by individual equipment

category (e.g., aircraft pushback tractors, baggage tugs, cargo loaders, etc.);

&Compute the emission benefits of the available alternative technologies;

&Quantify the incremental capital, operating, and life-cycle costs of converting GSE units

to these alternative technologies; and

&Calculate and compare the cost-effectiveness (cost per unit emissions reduced, e.g.,

$/ton) of these alternative technologies for each equipment category under airport-specific

operating and usage conditions.

A description of the model and operating instructions are presented in this section.

This project was conducted by Sierra Research, Inc. (Sierra) and Energy &

Environmental Analysis, Inc. (EEA) under Work Assignment 0-05 of U.S.

Environmental Protection Agency (EPA) contract #68-C7-0051. Sierra served in an

oversight capacity and had primary responsibility for preparing the spreadsheet model for

quantifying GSE model and related documentation. EEA had the lead responsibility for

preparing all of the other sections presented in this report.
AVAILABLE INFORMATION SOURCES
-1-

TABLE 1. GENERAL REPORTS

TITLE DATE SPONSOR DESCRIPTION 

Airline Industry Talking Points on SouthCoast/Sacramento FIPs

Nov-93 Air Transport Association Presents arguments from the airline industry regarding South

Coast/Sacramento FIPs. Airlines should be able to achieve greaemission reductions than estimated by EPA. 

Manual Calculation Methods for Air PollutionInventories

May-88 Guy T. Fagin, Capt., USAF, BSC, USAF

Occupational and Environmental HealthLaboratory, Human Systems Division,Brooks AFB, Texas 78235-5501

The purpose of this report was to provide guidelines for engineeto perform Air Emission Inventories. This report provided examcalculations, emission factors, and an example inventory.

Air Pollution Mitigation Measures for Airports andAssociated Activity

May-94 California Air Resources Board,

Sacramento, California

Provides a reference guide to emission mitigation techniques thaapplied to ground support equipment. Each measure is describewith guidelines for its use and constraints that may limit its effe

Electric Vehicles at Boston's Logan Airport (IN102438) 1994 Layla Sandell, Electric Power Research Institute, Palo Alto, California No abstract available

Airport Electrification Project: ConsolidatedResults and Analysis (TR-109041)

1997 Layla Sandell, Electric Power Research

Institute, Palo Alto, California

Studied electrification opportunities at seven airports and found all inventoried IC vehicles and equipment could be converted (dindirectly) to similar electric powered models. 

Power Quality Guidelines for Airport (TC-108418) 1997 Layla Sandell, Electric Power Research

Institute, Palo Alto, California

No abstract available

Document Control List not dated Environmental Protection Agency List of all documents used by EEA to perform tasks related to A

Emissions Control Regulatory Support, all work which was condunder an E.H. Pechan State Assistance Contract.

Methodology to Estimate Off-Road EquipmentPopulations

May-91 Environmental Protection Agency,

Office of Mobile Sources, Ann Arbor,MI

Discusses method used to estimate airport equipment populationaircraft departures as an activity indicator. Also collected populfrom airlines to compare to estimates.

Nonroad Engine and Vehicle Emission StudyReport

Nov-91 Environmental Protection Agency,

Office of Mobile Sources, Ann Arbor,MI

Quantifies the contribution of nonroad sources to ozone and carmonoxide air pollution and to other pollutants. Includes inventemissions estimates of Airport Service Equipment.

Airport Emission Inventories for FIP Areas May-93 Environmental Protection Agency,

Office of Mobile Sources, Ann Arbor,MI

Mobile source airport emissions inventory data for various nonaareas in CA. Report discusses data availability, recommendatiofurther data collection, a review of control strategies, and a forecaircraft fleet make up and activity.

Technical Support Document Civil and MilitaryAviation, California FIP NPRM

Mar-94 Environmental Protection Agency,

Office of Mobile Sources, Ann Arbor,MI

This document discusses technical information used by EPA dudevelopment of the proposed FIP control strategy for aircraft opIncludes operations and emissions data for various types of GSE
-2-

TABLE 1. GENERAL REPORTS (CONTINUED)

TITLE DATE SPONSOR DESCRIPTION 

GSE Population Data Sep-93 Air Transport Association Individual GSE population submissions from several ATA 

airlines as part of California FIP development process

Analysis of GSE Emissions Associated with AirportOperations in the South Coast Air Basin

Jul-94 Environmental Protection Agency Provides explanation of the analytical methodology used b

calculate air emissions from GSE in the California South CDescribes data sources, calculations, and results.

Memo - FIP Emission and Cost Forecasts for AviationSources: Controlled and Uncontrolled

May-95 Environmental Protection Agency Documents the final methodology used for evaluating the c

benefit of the California FIP after revisions were made. Infinal summary of uncontrolled and controlled emissions animpact forecasts for APUs and GSE in the FIP areas.

Memo - Refinement of FIP Ground SupportEquipment Calculations

Environmental Protection Agency Summarizes the calculation methodology of the refined GS

estimates presented in the previous referenced memo (May 

Technical Support Document Aircraft/Airports,California FIP Draft IFR 

Feb-95 Environmental Protection Agency, Office

of Mobile Sources, Ann Arbor, MI

This document discusses technical information used by EPdevelopment of the interim final FIP control strategy for aioperations. Includes operations and emissions data for vaof GSE and emission control measures. 

Technical Data to Support FAA's Advisory Circular onReducing Emissions from Commercial Aviation -Draft Final Report

Sep-95 Rich Wilcox - Environmental Protection

Agency; Bill Albee - Federal AviationAdministration

Presents technical data to support FAA's advisory Circular Reducing Emissions from Commercial Aviation. Data waand compiled in four main areas, including GSE.

Analysis of Techniques to Reduce Air Emissions atAirports

Jun-97 Environmental Protection Agency,

Washington, DC

Examines control strategies to reduce air emissions from msources at airports, as well as the potential for application ofour specific airports. Detailed emission inventories are cofor the four airports examined.

Airport Vehicle Fleets n/a Gas Research Institute Currently not available. According to synopsis, presents th

a survey of vehicle fleets at airports in the United States anInformation collected indicates a growing NGV population the busiest airports. 

Flying Off Course Oct-96 National Resources Defense Council

Washington, DC 20005

Determines the most important environmental issues conneairports and the best management techniques airports are umitigate them.

Airport Impact Mitigation and Management Study Jul-90 South California Association of

Governments Los Angeles, California

Outlines possible emission control measures that could be airports in the Southern California area. GSEs are not expdepth.
-3-

TABLE 2. JOURNAL ARTICLES

TITLE DATE SOURCE DESCRIPTION

airport rEVolution Oct-94 EPRI Journal Air Quality regulations are encouraging the use of electric vehicles at

urban airports across the US. Airports are ideal for EVs, given the short

distances and predictable routes that airport vehicles typically travel. 

Preparing for Takeoff Newsletter -

Volume 1, Number 2

1997 EPRI Discusses electrification projects at various airports, focusing specifically

on Southwest Airlines at Phoenix Sky Harbor Int’l airport, who is

replacing diesel-powered tugs and conveyor belt loaders with their

electric counterparts.

The Evolution of Preconditioned Air

and 400 Hz Central Systems

Feb-94 GSE Today Discusses why usage of preconditioned air (PCA) and 400 Hz central

systems has come about. Use of PCA and 400 Hz offers the aircraft

users opportunities to reduce costs, fuel usage, noise, and air pollution.

Selecting & Paying for Fixed Ground

Support Systems

Oct-96 GSE Today Discusses benefits of fixed power over APUs and presents a cost

analysis.

Why Use a Gas Turbine as a Main

Power Source for Air Start Units?

Jun-97 GSE Today New generation gas turbine engines are less complex than diesel engines

and more straightforward. Other benefits include lower maintenance

costs, lower fuel and oil costs, longer life span, lower emissions, and take

up less space.

California Clean Air Initiatives Bog

Down

Sep-97 GSE Today Discusses California’s effort to reduce airport emissions in Southern

California. Problem with figuring out what agency would have authority

for GSE. Discusses different approaches that could be taken to reduce

emissions from GSE. 

Clean Air Update: A Draft MOU for

Clean Air in California's SCAQMD

Apr-98 GSE Today Discusses draft MOU prepared by ATA to develop a system of voluntary

compliance to reduce emissions from GSE in SCAQMD. Still have to

figure out baseline emissions, where funds should come from, and getting

an accurate GSE operations inventory.

Cleared for Landing: NGVs Find a

Niche at Nation's Airports

Jun-95 Natural Gas Fuels Airports are ideal locations for NGVs due to the relatively restricted

travel range. Discusses the potential for alternative fueled vehicles at

airports with a spotlight on Denver International Airport and it's natural

gas vehicle fleet.

Airports: Models of Opportunity Sep-96 Natural Gas Fuels Discusses airports that have begun converting vehicles to natural gas as

well as funding opportunities available to airports to assist with

conversions.

In Plane Sight / A Flight Plan for the

Airport Market

Jan-98 Natural Gas Fuels Natural gas is gaining a solid reputation for its performance in airport

applications. NGVs account for the largest share of the increasing airport

AFV population. However, GSE conversions are not as popular for

several reasons. 

Ground Support Goes Electric Dec-95 Jane's Airport Review The market potential for electric GSE depends on legislative

requirements, perceived benefits, and the feasibility of converting.

Outlines the potential advantages or disadvantages of conversion to

electric GSE.

US Moves Closer to Towbarless

Concept

Dec-96 Jane's Airport Review Benefits of towbarless tractors (TLT) and possibilities of establishing a

market in the US.

Jane's Airports, Equipment and Services

1996-97, ATOS 3-R Towbarless Tractor

1996-

1997

Jane's Internet Web Site The idea of using the TLT for operational or dispatch towing to save fuel

seems to have disappeared and now these tractors are used for push-back

operations. It is likely TLTs will start replacing conventional tow-tractors

at a faster rate in the future.
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TABLE 3. AIRPORT REPORTS

TITLE DATE SOURCE DESCRI

Sacramento International Airport, 1995 Annual Air QualityReport

Apr-96 Jim Humphries, Air Quality Coordinator, Sacramento

International Airport, Sacramento, CA 95837

Includes emission estimates, mitigatioreductions.

Sacramento International Airport, Annual Air Quality Report- Fiscal Year 1992/1993

Sep-93 Jim Humphries, Air Quality Coordinator, Sacramento

International Airport, Sacramento, CA 95837

Includes emission estimates, mitigatioreductions.

Baltimore-Washington International (BWI) Airport, AirQuality Plan

Sept. 94 Barbara Grey Manager, Programming and

Environmental Services, Maryland AviationAdministration, BWI Baltimore, MD 21240

Explores primary sources of air emissestimates, projections for future emissmeasures. BWI noted that GSE are a could increase in future years.

Final Environmental Impact Statement - San DiegoInternational Airport, Lindbergh Field FacilitiesImprovements

Feb-94 William Johnstone, Federal Aviation Administration,

Los Angeles, CA 90009

Includes emission estimates and mitig

Phoenix Sky Harbor International Airport, Air PollutionReduction Measures 1991-1994

Aug-95 City of Phoenix Aviation Department, Environmental

Programs

Overview of emission reduction measHarbor - includes conversion of airpor

Maricopa Association of Governments, Aviation Air QualityStudy

Nov-96 Maricopa Association of Governments, Phoenix, AZ

85007

Purpose of this study is to develop an Also summarizes and evaluates emissifurther enhancement recommendation

Washington National Airport, New Terminal and RelatedFacilities Project - 400 Hertz Power System Study

Oct-91 Metropolitan Washington Airports Authority,

Washington, DC

The purpose is to compare the use of Aproviding required power, while consifuel costs to determine payback period

San Francisco International Airport Master Plan - Final EIR May-92 City of San Francisco, Department of City Planning,

San Francisco, CA

Includes emissions estimates for GSE 

Preconditioned Air and 400 HZ Study for San FranciscoInternational Airport, New Concourses "A" and "G"

Dec-94 Aviation Systems, Inc. for Hellmuth, Obata, and

Kassabaum

Purpose of this study was to analyze, bpreconditioned air and 400 Hz systemsystems. Includes cost analysis.

LAX Energy Study for Future Preconditioned Air, 400 Hz,and Battery Powered GSE Equipment

Aug-95 Aviation Systems, Inc. for LAX Determine the increase in electrical po

400 Hz systems to those gates which daddition, the power required if all or mbattery power

Environmental Studies - Boston-Logan International Airport 1994 and

1995

Massport, Boston, MA Includes emissions estimates for GSE

projections.
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TABLE 4. GSE MANUFACTURER PRODUCT INFORMATION

MANUFACTURER AND

GSE TYPE DESCRIPTION INFORMATION INCLUDED

Various "Advanced Transportation Vehicle Catalog" internet

website: www.calstart.org/cgi-bin/catalog.cgi

The CALSTART website provides specifications for

electric and alternative fuel GSE.

Krauss-Maffei - Towbarless Tractor Report - "The Super Tug Advantage" Includes technology information, emissions analysis at

LAX, and environmental aspects (fuel savings, noise

reduction, and exhaust gas reduction).

Krauss-Maffei - Towbarless Tractor Extract from a presentation of a paper concerning

"Towbarless Towing by Nose Gear Clamping"

Includes net fuel savings, advantages of towbarless

towing, fuel consumption comparisons, and emissions

savings.

Krauss-Maffei - Towbarless Tractor Product Brochure Includes technology information.

Krauss-Maffei - Towbarless Tractor Product Brochure Includes technology information, specifications, user

experience.

FMT - Aircraft Gate Support

Equipment

Product Brochure Technology information, time savings.

FMT - Aircraft Gate Support

Equipment

Product Presentation Material Includes advantages of vehicle free gate, products

offered by FMT, and cost comparisons.

Hobart - Ground Power Units Product Brochure Technology information.

FMC Jet Way Systems - PC Air and

400 Hz 

Report "400 Hz and PC Air Point of Use Systems" Includes two case studies done by Aviation Systems,

Inc. for LAX regarding feasibility of PC Air and 400

Hz systems. Also includes customer comments,

specifications, and cost comparisons.

FMC Jet Way Systems - PCA, 400 Hz,

potable water 

Product Brochure Capital costs, operating estimates, and approximate

payback time.
POTENTIAL CONTROL STRATEGIES
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TABLE 1. POTENTIAL HC REDUCTION STRATEGIES FOR AIRPORT GSE

Potential HC Emission Reduction (Percent Reductio

GSE Type Engine Type

Estimated

U.S.

Population

Fraction

of All

GSE

Fraction

of Type

Specific

GSE

Estimated

Fraction

of All

GSE HC

Convert

to LPG

Fueling

Convert

to CNG

Fueling

Replace

with LPG

Equipment

Replace

with CNG

Equipment

Replace2

with EV

Equipment

Retrofit

with Oxy

Catalyst

Diesel 2113 4.7% 76.6% 3.4% n/a n/a up 135 up 55 96 50 

2-str Gas 0 0.0% 0.0% 0.0%

Aircraft 4-str Gas 489 1.1% 17.7% 2.2% 50 65 50 65 99+ 90 

Pushback CNG 0 0.0% 0.0% 0.0%

Tractor LPG 63 0.1% 2.3% 0.1% n/a 35 n/a 35 98 70 

Electric 94 0.2% 3.4% 0.0% n/a n/a n/a n/a n/a n/a 

All 2759 6.1% 5.8%

Diesel 4399 9.8% 41.9% 4.6% n/a n/a up 105 up 35 97 50 

2-str Gas 0 0.0% 0.0% 0.0%

Baggage 4-str Gas 4863 10.8% 46.3% 26.6% 50 65 50 65 99+ 90 

Tug CNG 0 0.0% 0.0% 0.0%

LPG 973 2.2% 9.3% 2.7% n/a 35 n/a 35 99 70 

Electric 270 0.6% 2.6% 0.0% n/a n/a n/a n/a n/a n/a 

All 10505 23.3% 33.8%

Diesel 2429 5.4% 47.1% 1.7% n/a n/a up 45 up 5 98 50 

2-str Gas 0 0.0% 0.0% 0.0%

Belt 4-str Gas 2317 5.1% 45.0% 6.4% 50 65 50 65 99+ 90 

Loader CNG 0 0.0% 0.0% 0.0%

LPG 314 0.7% 6.1% 0.4% n/a 35 n/a 35 99 70 

Electric 94 0.2% 1.8% 0.0% n/a n/a n/a n/a n/a n/a 

All 5154 11.4% 8.5%

Diesel 31 0.1% 1.4% 0.0% n/a n/a up 375 up 215 98 50 

Carts 2-str Gas 612 1.4% 28.3% 2.0% 97 98 97 98 99.9+ 80 

4-str Gas 610 1.4% 28.2% 0.1% 45 65 45 65 99+ 90 

Tool & CNG 0 0.0% 0.0% 0.0%

Lavatory LPG 0 0.0% 0.0% 0.0%

Electric 910 2.0% 42.1% 0.0% n/a n/a n/a n/a n/a n/a 

All 2163 4.8% 2.1%

Diesel 146 0.3% 4.4% 0.0% n/a n/a up 105 up 35 98 50 

2-str Gas 0 0.0% 0.0% 0.0%4-str Gas 873 1.9% 26.1% 1.5% 65 75 65 75 99+ 90 

Forklift CNG 0 0.0% 0.0% 0.0%

LPG 1583 3.5% 47.4% 1.0% n/a 35 n/a 35 99 70 

Electric 737 1.6% 22.1% 0.0% n/a n/a n/a n/a n/a n/a 

All 3339 7.4% 2.5%
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TABLE 1. POTENTIAL HC REDUCTION STRATEGIES FOR AIRPORT GSE

(Continued)

Potential HC Emission Reduction (Percent Reductio

GSE Type Engine Type

Estimated

U.S.

Population

Fraction

of All

GSE

Fraction

of Type

Specific

GSE

Estimated

Fraction

of All

GSE HC

Convert

to LPG

Fueling

Convert

to CNG

Fueling

Replace

with LPG

Equipment

Replace

with CNG

Equipment

Replace2

with EV

Equipment

Retrofit

with Oxy

Catalyst

Diesel 2504 5.6% 82.0% 3.7% n/a n/a up 135 up 55 96 50 

2-str Gas 0 0.0% 0.0% 0.0%

Ground 4-str Gas 94 0.2% 3.1% 0.7% 50 65 50 65 99+ 90 

Power CNG 0 0.0% 0.0% 0.0%

Unit LPG 0 0.0% 0.0% 0.0%

Electric 455 1.0% 14.9% 0.0% n/a n/a n/a n/a n/a n/a 

All 3053 6.8% 4.3%

Service Diesel 409 0.9% 11.5% 0.2% n/a n/a up 155 up 70 96 50 

Trucks 2-str Gas 0 0.0% 0.0% 0.0%

4-str Gas 2905 6.4% 81.5% 8.4% 50 65 50 65 99+ 90 

Fuel, Food, CNG 0 0.0% 0.0% 0.0%

Lavatory, LPG 251 0.6% 7.0% 0.4% n/a 35 n/a 35 98 70 

Water, & Electric 0 0.0% 0.0% 0.0%

Other All 3565 7.9% 9.0%

Aggregate 30538 67.8% 66.1%

1 Unsigned and unqualified values signify emission reductions (in percent). Values preceded by the qualifier “up” signify emission increases (in percent).2 Emission reductions due to replacement with EV equipment can vary with the emissions performance of local power generating stations. The tabulated values repres

power generating station emission rates. For HC, the range of emissions variability across U.S. power generating stations is not dramatic and the tabulated emission affected by only a few percentage points regardless of local conditions.

3 In addition to the potential for direct replacement of some GSE services, fixed, gate-based systems such as electrical power and conditioned air also potentially reduc

(APU) emissions by 70-90 percent and emissions from (non-tabulated) GSE-based air conditioning service equipment by nearly 100 percent. Of the tabulated GSE, replacement is most feasible, with baggage tug and belt loader replacement quite difficult in retrofit applications.
-3-

TABLE 2. POTENTIAL CO REDUCTION STRATEGIES FOR AIRPORT GSE

Potential CO Emission Reduction (Percent Reductio

GSE Type Engine Type

Estimated

U.S.

Population

Fraction

of All

GSE

Fraction

of Type

Specific

GSE

Estimated

Fraction

of All

GSE CO

Convert

to LPG

Fueling

Convert

to CNG

Fueling

Replace

with LPG

Equipment

Replace

with CNG

Equipment

Replace2

with EV

Equipment

Retrofit

with Oxy

Catalyst

Diesel 2113 4.7% 76.6% 0.4% n/a n/a up 4000 up 4000 98 90 

2-str Gas 0 0.0% 0.0% 0.0%

Aircraft 4-str Gas 489 1.1% 17.7% 4.1% 40 40 40 40 99.9+ 90 

Pushback CNG 0 0.0% 0.0% 0.0%

Tractor LPG 63 0.1% 2.3% 0.3% n/a 0 n/a 0 99.9+ 90 

Electric 94 0.2% 3.4% 0.0% n/a n/a n/a n/a n/a n/a 

All 2759 6.1% 4.9%

Diesel 4399 9.8% 41.9% 0.4% n/a n/a up 4000 up 4000 98 90 

2-str Gas 0 0.0% 0.0% 0.0%

Baggage 4-str Gas 4863 10.8% 46.3% 37.9% 40 40 40 40 99.9+ 90 

Tug CNG 0 0.0% 0.0% 0.0%

LPG 973 2.2% 9.3% 4.7% n/a 0 n/a 0 99.9+ 90 

Electric 270 0.6% 2.6% 0.0% n/a n/a n/a n/a n/a n/a 

All 10505 23.3% 43.0%

Diesel 2429 5.4% 47.1% 0.1% n/a n/a up 3500 up 3500 98 90 

2-str Gas 0 0.0% 0.0% 0.0%

Belt 4-str Gas 2317 5.1% 45.0% 9.1% 40 40 40 40 99.9+ 90 

Loader CNG 0 0.0% 0.0% 0.0%

LPG 314 0.7% 6.1% 0.8% n/a 0 n/a 0 99.9+ 90 

Electric 94 0.2% 1.8% 0.0% n/a n/a n/a n/a n/a n/a 

All 5154 11.4% 10.0%

Diesel 31 0.1% 1.4% 0.0% n/a n/a up 5000 up 5000 98 90 

Carts 2-str Gas 612 1.4% 28.3% 0.1% 20 20 20 20 99.9+ 90 

4-str Gas 610 1.4% 28.2% 0.1% 40 40 40 40 99.9+ 90 

Tool & CNG 0 0.0% 0.0% 0.0%

Lavatory LPG 0 0.0% 0.0% 0.0%

Electric 910 2.0% 42.1% 0.0% n/a n/a n/a n/a n/a n/a 

All 2163 4.8% 0.2%

Diesel 146 0.3% 4.4% 0.0% n/a n/a up 4000 up 4000 98 90 

2-str Gas 0 0.0% 0.0% 0.0%4-str Gas 873 1.9% 26.1% 2.0% 55 55 55 55 99.9+ 90 

Forklift CNG 0 0.0% 0.0% 0.0%

LPG 1583 3.5% 47.4% 1.7% n/a 0 n/a 0 99.9+ 90 

Electric 737 1.6% 22.1% 0.0% n/a n/a n/a n/a n/a n/a 

All 3339 7.4% 3.8%
-4-

TABLE 2. POTENTIAL CO REDUCTION STRATEGIES FOR AIRPORT GSE

(Continued)

Potential CO Emission Reduction (Percent Reductio

GSE Type Engine Type

Estimated

U.S.

Population

Fraction

of All

GSE

Fraction

of Type

Specific

GSE

Estimated

Fraction

of All

GSE CO

Convert

to LPG

Fueling

Convert

to CNG

Fueling

Replace

with LPG

Equipment

Replace

with CNG

Equipment

Replace2

with EV

Equipment

Retrofit

with Oxy

Catalyst

Diesel 2504 5.6% 82.0% 0.5% n/a n/a up 4000 up 4000 98 90 

2-str Gas 0 0.0% 0.0% 0.0%

Ground 4-str Gas 94 0.2% 3.1% 1.2% 40 40 40 40 99.9+ 90 

Power CNG 0 0.0% 0.0% 0.0%

Unit LPG 0 0.0% 0.0% 0.0%

Electric 455 1.0% 14.9% 0.0% n/a n/a n/a n/a n/a n/a 

All 3053 6.8% 1.7%

Service Diesel 409 0.9% 11.5% 0.0% n/a n/a up 4500 up 4500 98 90 

Trucks 2-str Gas 0 0.0% 0.0% 0.0%

4-str Gas 2905 6.4% 81.5% 15.3% 40 40 40 40 99.9+ 90 

Fuel, Food, CNG 0 0.0% 0.0% 0.0%

Lavatory, LPG 251 0.6% 7.0% 1.0% n/a 0 n/a 0 99.9+ 90 

Water, & Electric 0 0.0% 0.0% 0.0%

Other All 3565 7.9% 16.4%

Aggregate 30538 67.8% 79.9%

1 Unsigned and unqualified values signify emission reductions (in percent). Values preceded by the qualifier “up” signify emission increases (in percent).2 Emission reductions due to replacement with EV equipment can vary with the emissions performance of local power generating stations. The tabulated values repres

power generating station emission rates. For CO, the range of emissions variability across U.S. power generating stations is not dramatic and the tabulated emission affected by only a few percentage points regardless of local conditions.

3 In addition to the potential for direct replacement of some GSE services, fixed, gate-based systems such as electrical power and conditioned air also potentially reduc

(APU) emissions by 70-90 percent and emissions from (non-tabulated) GSE-based air conditioning service equipment by nearly 100 percent. Of the tabulated GSE, replacement is most feasible, with baggage tug and belt loader replacement quite difficult in retrofit applications.
-5-

TABLE 3. POTENTIAL NOx REDUCTION STRATEGIES FOR AIRPORT GSE

Potential NOx Emission Reduction (Percent Reductio

GSE Type Engine Type

Estimated

U.S.

Population

Fraction

of All

GSE

Fraction

of Type

Specific

GSE

Estimated

Fraction

of All

GSE NOx

Convert

to LPG

Fueling

Convert

to CNG

Fueling

Replace

with LPG

Equipment

Replace

with CNG

Equipment

Replace2

with EV

Equipment

Retrofit

with Oxy

Catalyst

Diesel 2113 4.7% 76.6% 19.4% n/a n/a 75 75 97 0 

2-str Gas 0 0.0% 0.0% 0.0%

Aircraft 4-str Gas 489 1.1% 17.7% 1.0% 25 25 25 25 90 0 

Pushback CNG 0 0.0% 0.0% 0.0%

Tractor LPG 63 0.1% 2.3% 0.1% n/a 0 n/a 0 90 0 

Electric 94 0.2% 3.4% 0.0% n/a n/a n/a n/a n/a n/a 

All 2759 6.1% 20.5%

Diesel 4399 9.8% 41.9% 18.8% n/a n/a 80 80 97 0 

2-str Gas 0 0.0% 0.0% 0.0%

Baggage 4-str Gas 4863 10.8% 46.3% 7.9% 25 25 25 25 90 0 

Tug CNG 0 0.0% 0.0% 0.0%

LPG 973 2.2% 9.3% 1.2% n/a 0 n/a 0 90 0 

Electric 270 0.6% 2.6% 0.0% n/a n/a n/a n/a n/a n/a 

All 10505 23.3% 28.0%

Diesel 2429 5.4% 47.1% 2.6% n/a n/a 55 55 95 0 

2-str Gas 0 0.0% 0.0% 0.0%

Belt 4-str Gas 2317 5.1% 45.0% 1.9% 25 25 25 25 90 0 

Loader CNG 0 0.0% 0.0% 0.0%

LPG 314 0.7% 6.1% 0.2% n/a 0 n/a 0 90 0 

Electric 94 0.2% 1.8% 0.0% n/a n/a n/a n/a n/a n/a 

All 5154 11.4% 4.7%

Diesel 31 0.1% 1.4% 0.0% n/a n/a 80 80 96 0 

Carts 2-str Gas 612 1.4% 28.3% 0.0% up 135 up 135 up 135 up 135 55 0 

4-str Gas 610 1.4% 28.2% 0.0% 50 50 50 50 90 0 

Tool & CNG 0 0.0% 0.0% 0.0%

Lavatory LPG 0 0.0% 0.0% 0.0%

Electric 910 2.0% 42.1% 0.0% n/a n/a n/a n/a n/a n/a 

All 2163 4.8% 0.0%

Diesel 146 0.3% 4.4% 0.2% n/a n/a 80 80 97 0 

2-str Gas 0 0.0% 0.0% 0.0%4-str Gas 873 1.9% 26.1% 0.3% 20 20 20 20 90 0 

Forklift CNG 0 0.0% 0.0% 0.0%

LPG 1583 3.5% 47.4% 0.4% n/a 0 n/a 0 90 0 

Electric 737 1.6% 22.1% 0.0% n/a n/a n/a n/a n/a n/a 

All 3339 7.4% 1.0%
-6-

TABLE 3. POTENTIAL NOx REDUCTION STRATEGIES FOR AIRPORT GSE

(Continued)

Potential NOx Emission Reduction (Percent Reductio

GSE Type Engine

Type

Estimated

U.S.

Population

Fraction

of All

GSE

Fraction

of Type

Specific

GSE

Estimated

Fraction

of All

GSE NOx

Convert

to LPG

Fueling

Convert

to CNG

Fueling

Replace

with LPG

Equipment

Replace

with CNG

Equipment

Replace2

with EV

Equipment

Retrofit

with Oxy

Catalyst

Diesel 2504 5.6% 82.0% 20.9% n/a n/a 75 75 97 0 

2-str Gas 0 0.0% 0.0% 0.0%

Ground 4-str Gas 94 0.2% 3.1% 0.3% 25 25 25 25 90 0 

Power CNG 0 0.0% 0.0% 0.0%

Unit LPG 0 0.0% 0.0% 0.0%

Electric 455 1.0% 14.9% 0.1% n/a n/a n/a n/a n/a n/a 

All 3053 6.8% 21.3%

Service Diesel 409 0.9% 11.5% 1.4% n/a n/a 70 70 97 0 

Trucks 2-str Gas 0 0.0% 0.0% 0.0%

4-str Gas 2905 6.4% 81.5% 3.7% 25 25 25 25 90 0 

Fuel, Food, CNG 0 0.0% 0.0% 0.0%

Lavatory, LPG 251 0.6% 7.0% 0.3% n/a 0 n/a 0 90 0 

Water, & Electric 0 0.0% 0.0% 0.0%

Other All 3565 7.9% 5.3%

Aggregate 30538 67.8% 80.8%

1 Unsigned and unqualified values signify emission reductions (in percent). Values preceded by the qualifier “up” signify emission increases (in percent).2 Emission reductions due to replacement with EV equipment can vary with the emissions performance of local power generating stations. The tabulated values repres

power generating station emission rates. For NOx, the range of emissions variability across U.S. power generating stations is dramatic and emission reduction percen

local conditions, from: a 182 percent increase through a 91 percent reduction relative to 2-stroke gasoline emissions; a 40-90 percent reduction relative to 4-stroke gapercent reduction relative to LPG emissions; or a 60-99+ percent reduction relative to diesel emissions.

3 In addition to the potential for direct replacement of some GSE services, fixed, gate-based systems such as electrical power and conditioned air also potentially reduc

(APU) emissions by 70-90 percent and emissions from (non-tabulated) GSE-based air conditioning service equipment by nearly 100 percent. Of the tabulated GSE, replacement is most feasible, with baggage tug and belt loader replacement quite difficult in retrofit applications.
-7-

TABLE 4. POTENTIAL PM REDUCTION STRATEGIES FOR AIRPORT GSE

Potential PM Emission Reduction (Percent Reductio

GSE Type Engine Type Estimated U.S. Population Fraction of All GSE Fraction of Type Specific GSE Estimated Fraction of All GSE PM Convert to LPG Fueling Convert to CNG Fueling Replace with LPG Equipment Replace with CNG Equipment Replace2 with EV Equipment Retrofit with Oxy Catalyst 

Diesel 2113 4.7% 76.6% 21.6% n/a n/a 97 97 97 30 

2-str Gas 0 0.0% 0.0% 0.0%

Aircraft 4-str Gas 489 1.1% 17.7% 0.1% 15 15 15 15 20 10 

Pushback CNG 0 0.0% 0.0% 0.0%

Tractor LPG 63 0.1% 2.3% 0.0% n/a 0 n/a 0 5 10 

Electric 94 0.2% 3.4% 0.0% n/a n/a n/a n/a n/a n/a 

All 2759 6.1% 21.7%

Diesel 4399 9.8% 41.9% 26.8% n/a n/a 96 96 98 30 

2-str Gas 0 0.0% 0.0% 0.0%

Baggage 4-str Gas 4863 10.8% 46.3% 1.6% 15 15 15 15 45 10 

Tug CNG 0 0.0% 0.0% 0.0%

LPG 973 2.2% 9.3% 0.3% n/a 0 n/a 0 35 10 

Electric 270 0.6% 2.6% 0.0% n/a n/a n/a n/a n/a n/a 

All 10505 23.3% 28.8%

Diesel 2429 5.4% 47.1% 6.2% n/a n/a 96 96 97 30 

2-str Gas 0 0.0% 0.0% 0.0%

Belt 4-str Gas 2317 5.1% 45.0% 0.4% 15 15 15 15 45 10 

Loader CNG 0 0.0% 0.0% 0.0%

LPG 314 0.7% 6.1% 0.0% n/a 0 n/a 0 35 10 

Electric 94 0.2% 1.8% 0.0% n/a n/a n/a n/a n/a n/a 

All 5154 11.4% 6.6%

Diesel 31 0.1% 1.4% 0.0% n/a n/a 85 85 98 30 

Carts 2-str Gas 612 1.4% 28.3% 0.6% 98 98 98 98 99+ 10 

4-str Gas 610 1.4% 28.2% 0.0% 35 35 35 35 90 10 

Tool & CNG 0 0.0% 0.0% 0.0%

Lavatory LPG 0 0.0% 0.0% 0.0%

Electric 910 2.0% 42.1% 0.0% n/a n/a n/a n/a n/a n/a 

All 2163 4.8% 0.6%

Diesel 146 0.3% 4.4% 0.3% n/a n/a 96 96 98 30 

2-str Gas 0 0.0% 0.0% 0.0%4-str Gas 873 1.9% 26.1% 0.1% 35 35 35 35 55 10 

Forklift CNG 0 0.0% 0.0% 0.0%

LPG 1583 3.5% 47.4% 0.1% n/a 0 n/a 0 30 10 

Electric 737 1.6% 22.1% 0.0% n/a n/a n/a n/a n/a n/a 

All 3339 7.4% 0.5%
-8-

TABLE 4. POTENTIAL PM REDUCTION STRATEGIES FOR AIRPORT GSE

(Continued)

Potential PM Emission Reduction (Percent Reductio

GSE Type Engine

Type

Estimated

U.S.

Population

Fraction

of All

GSE

Fraction

of Type

Specific

GSE

Estimated

Fraction

of All

GSE PM

Convert

to LPG

Fueling

Convert

to CNG

Fueling

Replace

with LPG

Equipment

Replace

with CNG

Equipment

Replace2

with EV

Equipment

Retrofit

with Oxy

Catalyst

Diesel 2504 5.6% 82.0% 23.0% n/a n/a 96 96 97 30 

2-str Gas 0 0.0% 0.0% 0.0%

Ground 4-str Gas 94 0.2% 3.1% 0.0% 15 15 15 15 20 10 

Power CNG 0 0.0% 0.0% 0.0%

Unit LPG 0 0.0% 0.0% 0.0%

Electric 455 1.0% 14.9% 0.1% n/a n/a n/a n/a n/a n/a 

All 3053 6.8% 23.2%

Service Diesel 409 0.9% 11.5% 1.5% n/a n/a 96 96 97 30 

Trucks 2-str Gas 0 0.0% 0.0% 0.0%

4-str Gas 2905 6.4% 81.5% 0.5% 15 15 15 15 20 10 

Fuel, Food, CNG 0 0.0% 0.0% 0.0%

Lavatory, LPG 251 0.6% 7.0% 0.0% n/a 0 n/a 0 5 10 

Water, & Electric 0 0.0% 0.0% 0.0%

Other All 3565 7.9% 2.0%

Aggregate 30538 67.8% 83.4%

1 Unsigned and unqualified values signify emission reductions (in percent). Values preceded by the qualifier “up” signify emission increases (in percent).2 Emission reductions due to replacement with EV equipment can vary with the emissions performance of local power generating stations. The tabulated values repres

power generating station emission rates. For PM, the range of emissions variability across U.S. power generating stations is dramatic and emission reduction percentlocal conditions, from: an 80-99+ percent reduction relative to 2-stroke gasoline emissions; a 5000 percent increase through a 98 percent reduction relative to 4-strokpercent increase through a 98 percent reduction relative to LPG emissions; or a 100 percent increase through a 99+ percent reduction relative to diesel emissions.

3 In addition to the potential for direct replacement of some GSE services, fixed, gate-based systems such as electrical power and conditioned air also potentially reduc

(APU) emissions by 70-90 percent and emissions from (non-tabulated) GSE-based air conditioning service equipment by nearly 100 percent. Of the tabulated GSE, replacement is most feasible, with baggage tug and belt loader replacement quite difficult in retrofit applications.
-9-

TABLE 5. POTENTIAL CO2 REDUCTION STRATEGIES FOR AIRPORT GSE1

Potential CO2 Emission Reduction (Percent Reductio

GSE Type Engine

Type

Estimated

U.S.

Population

Fraction

of All

GSE

Fraction

of Type

Specific

GSE

Estimated

Fraction

of All

GSE CO2

Convert

to LPG

Fueling

Convert

to CNG

Fueling

Replace

with LPG

Equipment

Replace

with CNG

Equipment

Replace3

with EV

Equipment

Retrofit

with Oxy

Catalyst

Diesel 2113 4.7% 76.6% 10.2% n/a n/a up 5 0 45 up 3 

2-str Gas 0 0.0% 0.0% 0.0%

Aircraft 4-str Gas 489 1.1% 17.7% 1.8% 15 20 15 20 55 up 55 

Pushback CNG 0 0.0% 0.0% 0.0%

Tractor LPG 63 0.1% 2.3% 0.2% n/a 5 n/a 5 50 up 45 

Electric 94 0.2% 3.4% 0.2% n/a n/a n/a n/a n/a n/a 

All 2759 6.1% 12.4%

Diesel 4399 9.8% 41.9% 9.5% n/a n/a 5 10 50 up 3 

2-str Gas 0 0.0% 0.0% 0.0%

Baggage 4-str Gas 4863 10.8% 46.3% 15.2% 15 20 15 20 55 up 65 

Tug CNG 0 0.0% 0.0% 0.0%

LPG 973 2.2% 9.3% 2.5% n/a 5 n/a 5 50 up 45 

Electric 270 0.6% 2.6% 0.4% n/a n/a n/a n/a n/a n/a 

All 10505 23.3% 27.6%

Diesel 2429 5.4% 47.1% 2.8% n/a n/a up 10 up 2 55 up 3 

2-str Gas 0 0.0% 0.0% 0.0%

Belt 4-str Gas 2317 5.1% 45.0% 3.6% 15 20 15 20 55 up 60 

Loader CNG 0 0.0% 0.0% 0.0%

LPG 314 0.7% 6.1% 0.4% n/a 5 n/a 5 50 up 45 

Electric 94 0.2% 1.8% 0.1% n/a n/a n/a n/a n/a n/a 

All 5154 11.4% 6.9%

Diesel 31 0.1% 1.4% 0.0% n/a n/a up 15 up 10 65 up 3 

Carts 2-str Gas 612 1.4% 28.3% 0.1% 40 45 40 45 80 up 25 

4-str Gas 610 1.4% 28.2% 0.1% 15 20 15 20 75 up 45 

Tool & CNG 0 0.0% 0.0% 0.0%

Lavatory LPG 0 0.0% 0.0% 0.0%

Electric 910 2.0% 42.1% 0.0% n/a n/a n/a n/a n/a n/a 

All 2163 4.8% 0.2%

Diesel 146 0.3% 4.4% 0.1% n/a n/a 5 10 50 up 3 

2-str Gas 0 0.0% 0.0% 0.0%4-str Gas 873 1.9% 26.1% 0.8% 15 20 15 20 65 up 65 

Forklift CNG 0 0.0% 0.0% 0.0%

LPG 1583 3.5% 47.4% 1.2% n/a 5 n/a 5 60 up 35 

Electric 737 1.6% 22.1% 0.2% n/a n/a n/a n/a n/a n/a 

All 3339 7.4% 2.3%
-10-

TABLE 5. POTENTIAL CO2 REDUCTION STRATEGIES FOR AIRPORT GSE1

(Continued)

Potential CO2 Emission Reduction (Percent Reductio

GSE Type Engine

Type

Estimated

U.S.

Population

Fraction

of All

GSE

Fraction

of Type

Specific

GSE

Estimated

Fraction

of All

GSE CO2

Convert

to LPG

Fueling

Convert

to CNG

Fueling

Replace

with LPG

Equipment

Replace

with CNG

Equipment

Replace3

with EV

Equipment

Retrofit

with Oxy

Catalyst

Diesel 2504 5.6% 82.0% 11.2% n/a n/a up 5 2 45 up 3 

2-str Gas 0 0.0% 0.0% 0.0%

Ground 4-str Gas 94 0.2% 3.1% 0.5% 15 20 15 20 55 up 55 

Power CNG 0 0.0% 0.0% 0.0%

Unit LPG 0 0.0% 0.0% 0.0%

Electric 455 1.0% 14.9% 1.2% n/a n/a n/a n/a n/a n/a 

All 3053 6.8% 12.9%

Service Diesel 409 0.9% 11.5% 0.7% n/a n/a up 10 up 6 45 up 3 

Trucks 2-str Gas 0 0.0% 0.0% 0.0%

4-str Gas 2905 6.4% 81.5% 6.8% 15 20 15 20 55 up 55 

Fuel, Food, CNG 0 0.0% 0.0% 0.0%

Lavatory, LPG 251 0.6% 7.0% 0.6% n/a 5 n/a 5 50 up 45 

Water, & Electric 0 0.0% 0.0% 0.0%

Other All 3565 7.9% 8.1%

Aggregate 30538 67.8% 70.4%

1 Tabulated CO2 emissions and emission reductions are based on brake-specific fuel consumption estimates and are not adjusted for the relatively high CO emission ra

The table, therefore, reflects a scenario in which CO emissions are already controlled to a level that is minor relative to CO2. The one exception is for oxidation catal

tabulated increases in CO2 are primarily associated with the oxidation of large amounts of CO to CO2 (although an increase in exhaust system backpressure also cont

observed CO2 increases). Adjusting baseline GSE CO2 emission rates downward due to inherently high CO would negatively influence (by a substantial margin) the 

fuels with inherently low CO emissions. For example, correcting baseline 4-stroke gasoline CO2 emissions for high CO would alter the tabulated 15 percent CO2 emi

conversion or replacement with LPG equipment to a 15-30 percent CO2 emissions increase.

2 Unsigned and unqualified values signify emission reductions (in percent). Values preceded by the qualifier “up” signify emission increases (in percent).3 Emission reductions due to replacement with EV equipment can vary with the emissions performance of local power generating stations. The tabulated values repres

power generating station emission rates. For CO2, the range of emissions variability across U.S. power generating stations is dramatic and emission reduction percen

local conditions, from: a 75-90 percent reduction relative to 2-stroke gasoline emissions; a 40-85 percent reduction relative to 4-stroke gasoline emissions; a 30-85 peLPG emissions; or a 25-80 percent reduction relative to diesel emissions.

4 In addition to the potential for direct replacement of some GSE services, fixed, gate-based systems such as electrical power and conditioned air also potentially reduc

(APU) emissions by 70-90 percent and emissions from (non-tabulated) GSE-based air conditioning service equipment by nearly 100 percent. Of the tabulated GSE, replacement is most feasible, with baggage tug and belt loader replacement quite difficult in retrofit applications.
-11-

TABLE 6. ADDITIONAL GSE AND ASSOCIATED EMISSION IMPACT ESTIMATES

GSE Type Engine

Type

Estimated

U.S.

Population

Fraction

of All

GSE

Fraction

of Type

Specific

GSE

Estimated

Fraction

of All

GSE HC

Estimated

Fraction

of All

GSE CO

Estimated

Fraction

of All

GSE NOx

Estimated

Fraction

of All

GSE PM

Estimated

Fraction

of All

GSE CO2

Diesel 376 0.8% 78.5% 0.0% 0.0% 0.2% 0.2% 0.1%

2-str Gas 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

Conditioned 4-str Gas 41 0.1% 8.6% 0.0% 0.0% 0.0% 0.0% 0.0%

Air CNG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

Unit LPG 31 0.1% 6.5% 0.0% 0.0% 0.0% 0.0% 0.0%

Electric 31 0.1% 6.5% 0.0% 0.0% 0.0% 0.0% 0.0%

All 479 1.1% 0.0% 0.0% 0.2% 0.2% 0.1%

Diesel 771 1.7% 87.5% 1.0% 0.1% 5.6% 6.3% 2.8%

2-str Gas 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

Air 4-str Gas 63 0.1% 7.2% 0.1% 0.1% 0.0% 0.0% 0.1%

Start CNG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

Unit LPG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

Electric 16 0.0% 1.8% 0.0% 0.0% 0.0% 0.0% 0.0%

Turbine 31 0.1% 3.5% 0.0% 0.0% 0.2% 0.3% 0.1%

All 881 2.0% 1.1% 0.3% 5.9% 6.6% 3.0%

Diesel 157 0.3% 15.9% 0.2% 0.0% 0.9% 1.2% 0.4%

2-str Gas 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

4-str Gas 815 1.8% 82.5% 4.5% 6.3% 1.3% 0.3% 2.5%

Bobtail CNG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

LPG 16 0.0% 1.6% 0.0% 0.1% 0.0% 0.0% 0.0%

Electric 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

All 988 2.2% 4.7% 6.4% 2.2% 1.5% 3.0%

Diesel 1129 2.5% 77.4% 0.9% 0.1% 3.5% 5.0% 1.8%

2-str Gas 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

Cargo 4-str Gas 220 0.5% 15.1% 0.6% 0.9% 0.2% 0.0% 0.4%

Loader CNG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

LPG 110 0.2% 7.5% 0.2% 0.3% 0.1% 0.0% 0.2%

Electric 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

All 1459 3.2% 1.6% 1.3% 3.8% 5.1% 2.3%

Diesel 31 0.1% 6.1% 0.0% 0.0% 0.0% 0.0% 0.0%

2-str Gas 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

4-str Gas 475 1.1% 93.9% 0.1% 0.1% 0.0% 0.0% 0.1%

Deicer CNG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

LPG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

Electric 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

All 506 1.1% 0.1% 0.1% 0.0% 0.0% 0.1%

Diesel 47 0.1% 2.9% 0.0% 0.0% 0.1% 0.1% 0.1%

2-str Gas 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

4-str Gas 951 2.1% 58.1% 2.0% 2.9% 0.6% 0.1% 1.2%

Lift CNG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

LPG 324 0.7% 19.8% 0.3% 0.6% 0.2% 0.0% 0.3%

Electric 314 0.7% 19.2% 0.0% 0.0% 0.0% 0.0% 0.2%

All 1636 3.6% 2.4% 3.5% 0.9% 0.3% 1.7%

Diesel 16 0.0% 1.2% 0.0% 0.0% 0.0% 0.0% 0.0%

2-str Gas 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

Maintenance 4-str Gas 1259 2.8% 97.5% 2.9% 5.4% 1.3% 0.2% 2.4%

Truck CNG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

LPG 16 0.0% 1.2% 0.0% 0.0% 0.0% 0.0% 0.0%

Electric 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

All 1291 2.9% 3.0% 5.4% 1.3% 0.2% 2.4%
-12-

TABLE 6. ADDITIONAL GSE AND ASSOCIATED EMISSION IMPACT ESTIMATES

(Continued)

GSE Type Engine

Type

Estimated

U.S.

Population

Fraction

of All

GSE

Fraction

of Type

Specific

GSE

Estimated

Fraction

of All

GSE HC

Estimated

Fraction

of All

GSE CO

Estimated

Fraction

of All

GSE NOx

Estimated

Fraction

of All

GSE PM

Estimated

Fraction

of All

GSE CO2

Diesel 376 0.8% 21.6% 0.1% 0.0% 0.1% 0.2% 0.1%

2-str Gas 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

“Other” 4-str Gas 1195 2.7% 68.6% 0.9% 1.2% 0.2% 0.0% 0.4%

GSE CNG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

LPG 141 0.3% 8.1% 0.0% 0.1% 0.0% 0.0% 0.0%

Electric 31 0.1% 1.8% 0.0% 0.0% 0.0% 0.0% 0.0%

All 1743 3.9% 1.0% 1.2% 0.3% 0.3% 0.6%

Diesel 115 0.3% 39.9% 0.1% 0.0% 0.1% 0.6% 0.3%

2-str Gas 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

4-str Gas 173 0.4% 60.1% 0.6% 0.1% 0.1% 0.1% 0.5%

Bus CNG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

LPG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

Electric 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

All 288 0.6% 0.8% 0.1% 0.3% 0.7% 0.8%

Diesel 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

2-str Gas 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

4-str Gas 580 1.3% 100.0% 0.6% 0.0% 0.1% 0.0% 1.8%

Car CNG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

LPG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

Electric 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

All 580 1.3% 0.6% 0.0% 0.1% 0.0% 1.8%

Diesel 41 0.1% 1.5% 0.0% 0.0% 0.0% 0.0% 0.0%

2-str Gas 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

Pickup 4-str Gas 2595 5.8% 93.4% 10.0% 0.9% 2.2% 0.9% 7.8%

Truck CNG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

LPG 141 0.3% 5.1% 0.3% 0.0% 0.1% 0.0% 0.4%

Electric 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

All 2777 6.2% 10.3% 0.9% 2.3% 1.0% 8.2%

Diesel 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

2-str Gas 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

4-str Gas 1842 4.1% 96.7% 8.2% 0.7% 1.8% 0.8% 5.6%

Van CNG 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

LPG 63 0.1% 3.3% 0.1% 0.0% 0.0% 0.0% 0.2%

Electric 0 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

All 1905 4.2% 8.3% 0.8% 1.9% 0.8% 5.7%

Aggregate 14533 32.2% 33.9% 20.1% 19.2% 16.6% 29.6%
BASIS FOR GSE POPULATION ESTIMATES
* Examples include the following: (1) a 1994 California ARB report entitled Air Pollution Mitigation Measures for

Airports and Associated Activity estimated 2,500 GSE in California, implying a national population of about

26,000 units; (2) a 1995 ARB report entitled Documentation of Input Factors for the New Off-Road Mobile

Source Emissions Factor Model estimated 12,000 California GSE, implying a national population of about

125,000 units; (3) a 1995 U.S. EPA report entitled Technical Support Document: Aircraft/Airports estimated

2,500 GSE in California FIP areas, implying a national population of about 34,000 units; (4) a 1998 GRI report

entitled Survey of Airport Fleet Vehicles estimated a GSE population of between 24,000 and 34,000 at the top 48

airports, implying a national population of 33,000-47,000 units; (5) a 1991 U.S. EPA report entitled Nonroad

Engine and Vehicle Emissions Study - Report estimated a national GSE population of just under 85,000 units.

GSE Information Series 1 Page 1

Background: Airport ground support equipment (GSE) comprise a diverse range of vehicles

and equipment necessary to service aircraft during passenger and cargo loading and unloading,

maintenance, and other ground-based operations. The wide range of activities associated with

aircraft ground operations lead to an equally wide ranging fleet of GSE. For example, activities

undertaken during a typical aircraft gate period include: cargo loading and unloading, passenger

loading and unloading, potable water storage, lavatory waste tank drainage, aircraft refueling,

engine and fuselage examination and maintenance, and food and beverage catering. Airlines

employ specially designed GSE to support all these operations. Moreover, electrical power and

conditioned air are generally required throughout gate operational periods for both passenger and

crew comfort and safety, and many times these services are also provided by GSE.

Emissions Impact: The majority of GSE engines continue to be “uncontrolled” from an

emissions perspective. Although an increasing number of the larger vehicles used for GSE

operations such as water, fuel, lavatory, and aircraft maintenance services are powered by

emissions-certified on-road engines, a majority of similar older GSE and nearly all

specially-designed GSE continue to utilize engines that have not been designed for low

emissions. As a result, GSE do contribute significantly to the emissions of hydrocarbons (HC),

carbon monoxide (CO), oxides of nitrogen (NOx), particulate matter (PM), and carbon dioxide

(CO2) associated with airports. A recent U.S. EPA-commissioned study of four major U.S.

airports indicates that GSE are responsible for 15-20 percent of airport-related NOx and 10-15

percent of airport-related HC. Moreover, studies have shown that the availability of alternative

fueled and electric powered GSE as well as the availability of alternative means of providing, in

some cases, equivalent aircraft support services can lead to substantial reductions in GSE

emissions from current levels. This GSE information series is designed to provide both an

overview of these available emission reduction strategies and an estimate of the magnitude of

associated emission reductions.

GSE Population: Since there are no registration requirements for GSE or any national

organization charged with tracking GSE activity, there currently is no reliable database from

which accurate GSE populations can be determined. Numerous estimates of the national GSE

population have been made over the last several years using various statistical procedures, with

total population estimates ranging from as low as 10,000 units to 100,000 units or more.*

BASIS FOR GSE POPULATION ESTIMATES
GSE Information Series 1 Page 2

Available data on GSE is certainly sufficient to narrow this range considerably and, therefore, it

is likely that additional factors such as inconsistencies in considered equipment contribute to the

magnitude of the observed differential between previous national GSE population estimates.

Detailed data on GSE populations at specific airports were collected in support of two U.S. EPAand one California Air Resources Board-commissioned studies on airport emissions. As

indicated Table 1, these studies included the collection of detailed GSE inventories from 35

individual airlines at 10 U.S. airports, 6 of which are among the top 50 national airports in terms

of aircraft landing and take-off (LTO) cycles. In total, over 3,000 individual GSE were

represented in the reported inventory data and together they provide support for about 9 percent

of national annual LTO cycles. A simple extrapolation of this reported GSE population to total

national LTO activity readily indicates the inherent inaccuracy in national GSE population

estimates as low as 10,000 units and as high as 100,000 units.

Airport Airlines Providing Detailed GSE Data

Baltimore-Washington International Delta, Southwest, U.S. Airways

Boston Logan Business Express, Flagship, Northwest, Spirit,

U.S. Airways

Burbank Delta, Southwest, United

Long Beach Delta, United

Los Angeles International Alaska Air, American, America West,

Continental, Delta, Northwest, Skywest,

Southwest, TransWorld, United, U.S. Airways

Ontario Delta, Southwest, United

Orange County Delta, United

Palm Springs Delta, United

Phoenix Sky Harbor America West, Southwest

Sacramento Delta, Southwest

Table 1. Airports and Airlines Providing Detailed GSE Population Data

To provide a reliable estimate of national GSE population, a detailed statistical regression

analysis of the reported data was conducted. Examinations of scatter diagrams of airline- and

airport-specific GSE population by annual airline- and airport-specific LTO cycles as well as

preliminary regression statistics reveals distinct differences in the GSE inventory characteristics

of airlines along three major classifications. First, the GSE inventories of all major airlines

except Southwest (e.g., Delta, Northwest, United) show similar characteristics. Second, regional

carriers (e.g., Business Express, Skywest) show similar characteristics amongst themselves, but
GSE Information Series 1 Page 3

(not surprisingly) these characteristics are quite different than those of the major carriers. 

Finally, Southwest Airlines exhibits characteristics unique from those of both the regional and

other major carriers.

Stratifying the reported GSE population data in accordance with the three-level classification

scheme (i.e., major airlines other than Southwest, Southwest, and regional airlines) and

conducting detailed classification-specific regression analysis of GSE by annual LTO cycles

yields promising statistics, but a final adjustment for the major airlines (other than Southwest) is

required to obtain optimum results. Major airlines (especially those with international service)

operate both wide- and narrow-body aircraft, which possess significantly different demands in

terms of both the number of GSE required for gate support and the length of time those GSE are

required. Therefore, major airlines with a larger wide-body aircraft fleet and more frequent

wide-body LTO cycles tend to operate more GSE than major airlines with few wide-body LTO

cycles. Once this criteria is included in the regression analysis, satisfactory GSE statistics for all

three airline classifications are obtained. Table 2 presents the applicable statistical relationships.

Airline Classification Regression Equation

(GSE Population = )

Correlation

Coefficient

Major Airlines (other than Southwest) 0.0226 (LTOwb) + 0.0054 (LTOnb)

[t=5.2] [t=4.1]

0.79

Southwest Airlines 0.0022 (LTO)

[t=5.5]

0.83

Regional Airlines 0.0008 (LTO)

[t=4.0]

0.75

LTO indicates the total number of annual LTO cycles, LTOwb indicates the number of annual wide-body LTO cycles, and

LTOnb indicates the number of annual narrow-body LTO cycles.

Table 2. GSE Population Regression Statistics

As indicated by the correlation coefficients, none of the regression equations are excellent

predictors of the GSE population for a specific airline at a specific airport. However, all provide

reasonable airline- and airport-specific predictions and (since the t statistics are all significant at

over 99 percent confidence) highly significant representations of aggregate GSE population

across several airlines or several airports. Therefore, an evaluation of the three regressions using

national LTO data should provide for a good approximation of the national population of GSE. 

Such an evaluation yields a population estimate of just over 45,000 pieces of equipment

(approximately the midpoint of the range of previous population estimates).

A measure of the relative GSE “demand” of the various airline classifications can also be

obtained from the derived regression coefficients. A single GSE is required for every 44
GSE Information Series 1 Page 4

wide-body aircraft LTO cycles versus every 185 narrow-body LTO cycles for major airlines other

than Southwest. Southwest Airlines and regional carriers utilize substantially less GSE at the

ratio of a single GSE for every 455 and every 1,250 LTO cycles respectively. Major

narrow-body carriers utilize GSE at about 2.5 times the rate of Southwest Airlines, while

wide-body aircraft impose approximately 4 times the GSE demand of narrow-body aircraft.

GSE Population by Equipment and Engine Type: The same detailed airline- and

airport-specific data that was used to support the overall national GSE population estimate can be

used to disaggregate the estimate into its various type-specific components. As mentioned

above, over 3,000 individual GSE units are reflected in these data, representing approximately 7

percent of the estimated national GSE population. By aggregating the individual GSE

type-specific populations across the 35 airlines and 10 airports for which detailed GSE data is

available, an estimate of the national population type-specific equipment distribution can be

derived. Since the regression analysis described above indicated significant differences between

major airlines other than Southwest, Southwest, and regional airlines, this level of distinction

should be maintained in determining national GSE type distributions. The distribution of

national regional carrier GSE is developed by aggregating the data for the 5 regional carriers for

which detailed GSE population data was available. Similarly, the GSE type-specific equipment

distribution for national Southwest Airline GSE is based on the aggregation of data for the 6

airports for which detailed Southwest GSE populations were available and that of other major

airlines is based on data aggregated across the 24 available detailed populations. Table 3

presents the resulting type-specific national GSE population estimates.

Clearly, although GSE comprise myriad equipment types, several specific types are estimated to

dominate the overall population. In fact, baggage tugs and belt loaders constitute an estimated 35

percent of all GSE. Aircraft pushback tractors, forklifts, carts, ground power units, service

trucks, and pickup trucks constitute another 36 percent. Gasoline is estimated to be the dominant

engine type, powering an estimated 51 percent of GSE, while diesel engines are estimated to

power another 33 percent. The remaining 16 percent of GSE is powered in approximately equal

proportions by LPG and electricity.

References:

1. Analysis of Techniques to Reduce Air Emissions at Airports, Draft Final Report, prepared by

Energy and Environmental Analysis, Inc. for the U.S. Environmental Protection Agency,

September 1997.

2. Air Pollution Mitigation Measures for Airports and Associated Activity, prepared by Energy

and Environmental Analysis, Inc. for the California Air Resources Board, May 1994.

3. Technical Support Document for Civil and Military Aviation, prepared by Energy and

Environmental Analysis, Inc. for the U.S. Environmental Protection Agency in support of the

Notice of Proposed Rulemaking for the Federal Implementation Plan for California, March

1994.

4. Technical Support Document: Aircraft/Airports, prepared by Energy and Environmental
GSE Information Series 1 Page 5

Analysis, Inc. for the U.S. Environmental Protection Agency in support of the Interim Final

Rulemaking for the Federal Implementation Plan for California, February 9, 1995

5. GSE population data sheets submitted by member companies of the Air Transport

Association as part of the U.S. Environmental Protection Agency’s Federal Implementation

Plan development process for California, September 1993.

6. Documentation of Input Factors for the New Off-Road Mobile Source Emissions Factor

Model, prepared by Energy and Environmental Analysis, Inc. for the California Air

Resources Board, August 1995.

7. Survey of Airport Fleet Vehicles, Final Report, prepared by Edwards and Kelsey, Inc. for the

Gas Research Institute, April 1998.

8. Nonroad Engine and Vehicle Emissions Study - Report, 21A-2001, U.S. Environmental

Protection Agency, November 1991.

Equipment

Type

Engine

Type

Major1

Airlines

Split

Southwest

Airlines

Split

Regional

Airlines

Split

Major1

Airlines

Population

Southwest

Airlines

Population

Regional

Airlines

Population

All Carrier

Population

All Carrier

Split

Diesel 4.47% 12.94% 3.24% 1,914 169 30 2,113 4.69%

Aircraft Gasoline 1.06% 2.60% 0.00% 455 34 0 489 1.08%

Pushback Electric 0.22% 0.00% 0.00% 94 0 0 94 0.21%

Tractor LPG/CNG 0.15% 0.00% 0.00% 63 0 0 63 0.14%

All Engines 5.90% 15.54% 3.24% 2,526 203 30 2,759 6.12%

Diesel 0.88% 0.00% 0.00% 376 0 0 376 0.83%

Conditioned Gasoline 0.07% 0.00% 1.08% 31 0 10 41 0.09%

Air Electric 0.07% 0.00% 0.00% 31 0 0 31 0.07%

Unit LPG/CNG 0.07% 0.00% 0.00% 31 0 0 31 0.07%

All Engines 1.09% 0.00% 1.08% 469 0 10 479 1.06%

Diesel 1.72% 2.60% 0.00% 737 34 0 771 1.71%

Air Gasoline 0.15% 0.00% 0.00% 63 0 0 63 0.14%

Start Electric 0.04% 0.00% 0.00% 16 0 0 16 0.04%

Unit LPG/CNG 0.00% 0.00% 0.00% 0 0 0 0 0.00%

Turbine 0.07% 0.00% 0.00% 31 0 0 31 0.07%

All Engines 1.98% 2.60% 0.00% 847 34 0 881 1.95%

Diesel 9.34% 20.67% 13.95% 4,000 270 129 4,399 9.76%

Baggage Gasoline 10.62% 11.10% 18.27% 4,549 145 169 4,863 10.79%

Tug Electric 0.59% 1.45% 0.00% 251 19 0 270 0.60%

LPG/CNG 2.27% 0.00% 0.00% 973 0 0 973 2.16%

All Engines 22.81% 33.23% 32.22% 9,773 434 298 10,505 23.31%

Diesel 5.16% 16.62% 0.00% 2,212 217 0 2,429 5.39%

Belt Gasoline 5.09% 6.66% 5.41% 2,180 87 50 2,317 5.14%

Loader Electric 0.22% 0.00% 0.00% 94 0 0 94 0.21%

LPG/CNG 0.73% 0.00% 0.00% 314 0 0 314 0.70%

All Engines 11.20% 23.28% 5.41% 4,800 304 50 5,154 11.44%

1

 Excluding Southwest Airlines.

- continued -

Table 3a. Estimated National GSE Populations
GSE Information Series 1 Page 6

Equipment

Type

Engine

Type

Major1

Airlines

Split

Southwest

Airlines

Split

Regional

Airlines

Split

Major1

Airlines

Population

Southwest

Airlines

Population

Regional

Airlines

Population

All Carrier

Population

All Carrier

Split

Diesel 0.37% 0.00% 0.00% 157 0 0 157 0.35%

Gasoline 1.87% 0.38% 1.08% 800 5 10 815 1.81%

Bobtail Electric 0.00% 0.00% 0.00% 0 0 0 0 0.00%

LPG/CNG 0.04% 0.00% 0.00% 16 0 0 16 0.04%

All Engines 2.27% 0.38% 1.08% 973 5 10 988 2.19%

Diesel 2.64% 0.00% 0.00% 1,129 0 0 1,129 2.50%

Cargo Gasoline 0.51% 0.00% 0.00% 220 0 0 220 0.49%

Loader Electric 0.00% 0.00% 0.00% 0 0 0 0 0.00%

LPG/CNG 0.26% 0.00% 0.00% 110 0 0 110 0.24%

All Engines 3.41% 0.00% 0.00% 1,459 0 0 1,459 3.24%

Diesel 0.07% 0.00% 0.00% 31 0 0 31 0.07%

Gasoline 2.49% 0.77% 5.41% 1,067 10 50 1,127 2.50%

Cart Electric 2.12% 0.00% 0.00% 910 0 0 910 2.02%

LPG/CNG 0.00% 0.00% 0.00% 0 0 0 0 0.00%

All Engines 4.69% 0.77% 5.41% 2,008 10 50 2,068 4.59%

Diesel 0.07% 0.00% 0.00% 31 0 0 31 0.07%

Gasoline 1.06% 0.77% 1.08% 455 10 10 475 1.05%

Deicer Electric 0.00% 0.00% 0.00% 0 0 0 0 0.00%

LPG/CNG 0.00% 0.00% 0.00% 0 0 0 0 0.00%

All Engines 1.13% 0.77% 1.08% 486 10 10 506 1.12%

Diesel 0.33% 0.38% 0.00% 141 5 0 146 0.32%

Gasoline 2.01% 0.77% 0.00% 863 10 0 873 1.94%

Forklift Electric 1.72% 0.00% 0.00% 737 0 0 737 1.64%

LPG/CNG 3.66% 1.07% 0.00% 1,569 14 0 1,583 3.51%

All Engines 7.73% 2.22% 0.00% 3,310 29 0 3,339 7.41%

Diesel 0.18% 0.00% 1.08% 78 0 10 88 0.20%

Fuel Gasoline 0.92% 0.00% 0.00% 392 0 0 392 0.87%

Truck Electric 0.00% 0.00% 0.00% 0 0 0 0 0.00%

LPG/CNG 0.04% 0.00% 0.00% 16 0 0 16 0.04%

All Engines 1.13% 0.00% 1.08% 486 0 10 496 1.10%

Diesel 5.24% 4.06% 22.49% 2,243 53 208 2,504 5.56%

Ground Gasoline 0.22% 0.00% 0.00% 94 0 0 94 0.21%

Power Electric 1.06% 0.00% 0.00% 455 0 0 455 1.01%

Unit LPG/CNG 0.00% 0.00% 0.00% 0 0 0 0 0.00%

All Engines 6.52% 4.06% 22.49% 2,792 53 208 3,053 6.77%

Diesel 0.00% 0.00% 0.00% 0 0 0 0 0.00%

Lavatory Gasoline 0.04% 2.22% 5.41% 16 29 50 95 0.21%

Cart Electric 0.00% 0.00% 0.00% 0 0 0 0 0.00%

LPG/CNG 0.00% 0.00% 0.00% 0 0 0 0 0.00%

All Engines 0.04% 2.22% 5.41% 16 29 50 95 0.21%

Diesel 0.00% 0.00% 0.00% 0 0 0 0 0.00%

Lavatory Gasoline 1.58% 1.84% 1.08% 675 24 10 709 1.57%

Truck Electric 0.00% 0.00% 0.00% 0 0 0 0 0.00%

LPG/CNG 0.00% 0.00% 0.00% 0 0 0 0 0.00%

All Engines 1.58% 1.84% 1.08% 675 24 10 709 1.57%

Diesel 0.11% 0.00% 0.00% 47 0 0 47 0.10%

Gasoline 2.20% 0.00% 1.08% 941 0 10 951 2.11%

Lift Electric 0.73% 0.00% 0.00% 314 0 0 314 0.70%

LPG/CNG 0.73% 0.77% 0.00% 314 10 0 324 0.72%

All Engines 3.77% 0.77% 1.08% 1,616 10 10 1,636 3.63%

1

 Excluding Southwest Airlines.

- continued -
GSE Information Series 1 Page 7

Table 3b. Estimated National GSE Populations

Equipment

Type

Engine

Type

Major1

Airlines

Split

Southwest

Airlines

Split

Regional

Airlines

Split

Major1

Airlines

Population

Southwest

Airlines

Population

Regional

Airlines

Population

All Carrier

Population

Al Carrier

Split

Diesel 0.04% 0.00% 0.00% 16 0 0 16 0.04%

Maintenance Gasoline 2.89% 0.00% 2.16% 1,239 0 20 1,259 2.79%

Truck Electric 0.00% 0.00% 0.00% 0 0 0 0 0.00%

LPG/CNG 0.04% 0.00% 0.00% 16 0 0 16 0.04%

All Engines 2.97% 0.00% 2.16% 1,271 0 20 1,291 2.86%

Diesel 0.88% 0.00% 0.00% 376 0 0 376 0.83%

Other Gasoline 2.71% 2.60% 0.00% 1,161 34 0 1,195 2.65%

GSE Electric 0.07% 0.00% 0.00% 31 0 0 31 0.07%

LPG/CNG 0.33% 0.00% 0.00% 141 0 0 141 0.31%

All Engines 3.99% 2.60% 0.00% 1,709 34 0 1,743 3.87%

Diesel 0.66% 2.99% 0.00% 282 39 0 321 0.71%

Service Gasoline 3.55% 2.99% 4.32% 1,522 39 40 1,601 3.55%

Truck Electric 0.00% 0.00% 0.00% 0 0 0 0 0.00%

LPG/CNG 0.55% 0.00% 0.00% 235 0 0 235 0.52%

All Engines 4.76% 5.97% 4.32% 2,039 78 40 2,157 4.79%

Diesel 0.26% 0.38% 0.00% 110 5 0 115 0.26%

Gasoline 0.40% 0.00% 0.00% 173 0 0 173 0.38%

Bus Electric 0.00% 0.00% 0.00% 0 0 0 0 0.00%

LPG/CNG 0.00% 0.00% 0.00% 0 0 0 0 0.00%

All Engines 0.66% 0.38% 0.00% 283 5 0 288 0.64%

Diesel 0.00% 0.00% 0.00% 0 0 0 0 0.00%

Gasoline 1.32% 0.38% 1.08% 565 5 10 580 1.29%

Car Electric 0.00% 0.00% 0.00% 0 0 0 0 0.00%

LPG/CNG 0.00% 0.00% 0.00% 0 0 0 0 0.00%

All Engines 1.32% 0.38% 1.08% 565 5 10 580 1.29%

Diesel 0.07% 0.77% 0.00% 31 10 0 41 0.09%

Pickup Gasoline 5.93% 1.84% 3.24% 2,541 24 30 2,595 5.76%

Truck Electric 0.00% 0.00% 0.00% 0 0 0 0 0.00%

LPG/CNG 0.33% 0.00% 0.00% 141 0 0 141 0.31%

All Engines 6.33% 2.60% 3.24% 2,713 34 30 2,777 6.16%

Diesel 0.00% 0.00% 0.00% 0 0 0 0 0.00%

Gasoline 4.14% 0.00% 7.46% 1,773 0 69 1,842 4.09%

Van Electric 0.00% 0.00% 0.00% 0 0 0 0 0.00%

LPG/CNG 0.15% 0.00% 0.00% 63 0 0 63 0.14%

All Engines 4.29% 0.00% 7.46% 1,836 0 69 1,905 4.23%

Diesel 0.00% 0.00% 0.00% 0 0 0 0 0.00%

Water Gasoline 0.44% 0.38% 1.08% 188 5 10 203 0.45%

Truck Electric 0.00% 0.00% 0.00% 0 0 0 0 0.00%

LPG/CNG 0.00% 0.00% 0.00% 0 0 0 0 0.00%

All Engines 0.44% 0.38% 1.08% 188 5 10 203 0.45%

Diesel 32.47% 40.76% 61.41% 13,911 377 802 15,090 33.48%

All Gasoline 51.27% 59.24% 35.30% 21,963 548 461 22,972 50.97%

GSE Electric 6.85% 0.00% 1.45% 2,933 0 19 2,952 6.55%

Types LPG/CNG 9.34% 0.00% 1.84% 4,002 0 24 4,026 8.93%

Turbine 0.07% 0.00% 0.00% 31 0 0 31 0.07%

All Engines 100.00% 100.00% 100.00% 42,840 925 1,306 45,071 100.00%

1

 Excluding Southwest Airlines.

Table 3c. Estimated National GSE Populations
GSE EMISSIONS AND ACTIVITY ESTIMATES
* This differs from the measurement of emissions from on-road vehicles such as passenger cars and light trucks

where the vehicle engine is exercised over a well defined, but continuously changing (or transient) driving cycle

that includes a wide range of engine speed and load conditions. Such a test is more representative of the

performance of such vehicles since they undergo a wide range of continuously varying speed and load

combinations in actual use. GSE, however, generally do not undergo either frequent speed or load changes,

instead operating within relatively narrow speed/load ranges for extended periods. Accordingly, the emissions

performance of GSE can be accurately measured by testing the equipment (or engine) over an appropriate series of

such “steady state” conditions.

GSE Information Series 2 Page 1

Background: Airport ground support equipment (GSE) comprise a diverse range of vehicles

and equipment necessary to service aircraft during passenger and cargo loading and unloading,

maintenance, and other ground-based operations. The wide range of activities associated with

aircraft ground operations lead to an equally diverse fleet of GSE, each component of which has

its own emissions performance and activity characteristics. For example, activities undertaken

during a typical aircraft gate period include: cargo loading and unloading, passenger loading and

unloading, potable water storage, lavatory waste tank drainage, aircraft refueling, engine and

fuselage examination and maintenance, and food and beverage catering. Airlines employ

specially designed GSE to support all these operations. Moreover, electrical power and

conditioned air are generally required throughout gate operational periods for both passenger

comfort and safety, and many times these services are also provided by GSE.

The Necessity for Proper Activity Characterization: Accurately characterizing the emissions

performance of a particular GSE requires detailed knowledge in two specific areas: (1) the rate of

equipment emissions per unit of activity and (2) the amount of activity performed during the

period of interest. Generally, the unit activity emission rate can be either measured directly or

estimated from previous measurements taken for similar equipment or engines. Because

emission rates typically vary with engine speed and load (a measure of how “hard” the engine is

being worked), emission rates for GSE-type equipment are commonly measured over a broad

series of constant speed and load operating modes that, when weighted in accordance with the

amount of time the engine spends in each mode, can describe the average emission rate of the

equipment.* Two standard emission tests of this type are commonly employed for GSE-type

equipment. Engines rated at 25 horsepower (hp) or less are usually tested over the Society of

Automotive Engineers’ J1088 test cycle, while larger engines are tested over what is commonly

referred to as the “8 mode” test. Differences in the various test modes to which the engines are

subjected can be best defined through the amount of useful work that is performed by the engine

when operating in each mode and, therefore, emission measurements for GSE-type equipment

are usually measured in terms of grams of emissions per brake horsepower-hour of work

performed (g/bhp-hr).

Since GSE-type emission factors are measured in terms of work performed, a measurement of

the amount of work performed over the time period of interest is integral to the proper emissions

GSE EMISSIONS AND ACTIVITY

ESTIMATES
GSE Information Series 2 Page 2

GSE emissions

time period of interest

g

bhp - hr rated hp hours of operationtime period of interest

=×× × load factor

assessment of a piece of equipment. For short, single operating mode periods, this measurement

can be derived directly from the speed and load conditions placed on the engine during the

operating mode. However, over longer periods, equipment operations usually encompass a series

of differing operating conditions (e.g., extended idle, between gate transit) and the amount of

work performed is usually defined as a fraction of total possible work that can be performed

during the period. Commonly referred to as the load factor, this fraction is equal to the ratio of

actual work performed to work which could have been performed if the engine was operated

throughout the period at its manufacturer-rated speed and horsepower (those conditions over

which engine work output is a maximum). For example, the load factor for the aggregated J1088

emissions test is 0.39 (i.e., the derived emission rate over the J1088 cycle reflects an aggregated

series of test conditions in which 39 percent of maximum engine work is performed), while that

of the “8 mode” test for larger engines is 0.56.

Once a load factor has been established for a typical operating period, emissions performance

over that or any period of similar engine operation can be calculated from measured unit-work

emission rates. The amount of work performed over the period of interest is calculated by

multiplying the engine manufacturer-rated engine output (hp) by the total number of hours of

engine operation over the period to determine maximum possible time period work, and then

multiplying this maximum possible work by the representative load factor. The product of the

unit-work emission rate and work performed over the period of interest represents the overall

emissions estimate for the period of interest. Expressed algebraically, the generalized GSE

emissions calculation methodology is:

While conceptually simple, each of the components of the GSE emissions calculation equation

has inherent uncertainty with the exception of the rated engine horsepower (which is an element

of engine design). The unit-work emission rate is typically based on a standardized test with a

load factor of either 0.39 ( 25 hp engines) or 0.56 (> 25 hp engines). Equipment operated at

load factors substantially different than those of the test cycle may exhibit substantially different

emission rates. Experience has shown that emission rates over a load factor range of about ±0.2

of the test cycle load factor are fairly well represented. The hours of operation for the time

period of interest is certainly conceptually simple and for a single piece of equipment, relatively

easily tracked. But defining a “typical” hours per day operating rate over extended time periods,

across multiple units, across airlines, or across airports is inherently uncertain. The load factor is

perhaps the most uncertain element of the calculation. Although GSE-specific load factors have

been developed over the last several years, there is considerable uncertainty in these estimates

across units, airlines, and airports. For example, equipment idle time can vary considerably and

in extreme cases, where idle emissions are applicable for all or most of the time period of

interest, the GSE emissions calculation methodology “falls apart” as the load factor approaches

zero while the actual emission rate per unit work performed approaches infinity. Clearly each of

the factors upon which the generalized GSE emissions calculation methodology is dependent

must be accurately quantified to determine emissions with any degree of certainty.
GSE Information Series 2 Page 3

Emission Rate Data: Numerous studies over the last decade or so have attempted to compile

available emissions rate data on engines such as those used on GSE. Although the size of the

database continues to be relatively small, it is comprehensive in that engines of all applicable

sizes and configurations are represented. In many aspects, the state of the off-road engine

emissions database is comparable to that of on-road vehicles in the mid-1970’s. Interest in

off-road engine emissions became significant only in the late-1980’s and early 1990’s with the

advent of U.S. Environmental Protection Agency (EPA) and California Air Resources Board

(ARB) regulatory program development efforts. Emissions data collection has accelerated in

response to these efforts, but given the wide range of engine sizes and configurations found in the

off-road sector, the number of emission tests performed on most engine categories continues to

be limited. There are some exceptions, such as small lawn mower-type utility engines which

have been the focus of early regulatory attention and are thus fairly well represented, but in

general much data collection activity remains to be performed. To date, no comprehensive,

detailed GSE-specific emissions rate data collection program has been undertaken. This is

clearly an area where the development of a structured test program could not only improve the

quantity and quality of data collected, but also provide an indication of the degree of uncertainty

associated with current GSE emissions test data.

In the mid-1990’s, the ARB began development of an off-road emission factor model. A key

component of this development work was the collection and analysis of available emissions test

data for all types of off-road engines, including GSE. This emissions test data development

activity was performed at a very disaggregated level-of-detail and continues to represent the

“state-of-the-art” emissions rate “database” for GSE. Data was analyzed over relatively narrow

horsepower ranges as well as by fuel type (i.e., gasoline, diesel, etc.), valve configuration (i.e.,

overhead valve versus side valve), engine cooling design (i.e., air versus water cooled), fueling

system type (i.e., carburetion, indirect injection, direct injection), and air intake type (i.e.,

naturally aspirated versus turbocharged). In addition to emissions rate data, the ARB model

development efforts also included detailed reviews of available off-road engine (including GSE)

population (for California), load factor, and usage rate data (for California). In recognition of its

comprehensive nature, all data presented in this GSE information series, with relatively minor

exceptions where noted, are derived from the resulting model database.

Although all equipment are represented by the ARB data, it should be recognized that a targeted

GSE emissions data collection program would be invaluable in not only in augmenting and

quality assuring the existing database, but in also providing an important measure of emissions

variability. Moreover, any program based on the issuance of credits for GSE retirement or

conversion should be based on actual emission measurements for the specific equipment

involved. Only after sufficient data have been collected to provide a reasonable measure of

emissions rate uncertainty can surrogate engine data be expected to accurately represent the

emissions performance of any specific engine or equipment. The off-road emissions database,

and GSE database in particular, is not at this state of development.

Tables 1 through 11 present basic emission rate data for off-road engines by horsepower range as

extracted from the ARB off-road model. Tables 1 through 5 present new engine emission rates

for HC, CO, NOx, PM, and CO2 respectively. All emission rates, except those for CO2 are in

units of grams of emission per brake horsepower-hour of work performed (g/bhp-hr). CO2

emission rates are in grams per gallon of fuel consumed. Table 6 presents the brake-specific fuel
GSE Information Series 2 Page 4

Z X (X)(Y) X (1 Y) =+ = +

Z X (X)(0.5Y) X(1 0.5Y) 1/2 =+ + +

consumption (in units of pounds of fuel per brake horsepower-hour) and fuel density (in pounds

per gallon) estimates required to convert the CO2 emission rates presented in Table 5 into units of

g/bhp-hr. Specific estimates for CNG equipment are not included in the ARB model and there is

very little data available to construct detailed GSE CNG-specific emission rates. Nevertheless,

on the basis of limited data cited in previous GSE studies, CNG emission rates were estimated to

be: (1) for HC, 33 percent lower than corresponding LPG HC emissions, (2) for CO2, 8 percent

lower than corresponding LPG CO2 emissions, and (3) for CO, NOx, and PM, equivalent to

corresponding LPG emissions.

As is the case with all engines, emission rates usually increase with age due to engine and control

system deterioration, malperformances, or misadjustment. The ARB model includes estimates of

pollutant- and engine-specific emission deterioration rates, but both the magnitude and form of

these deterioration data should be viewed as even less developed than other aspects of off-road

(and specifically GSE) emission rate data since very little testing on in-use equipment has been

performed to date. As already discussed, a structured emissions testing program focused on GSE

(both new and in-use) is critical to both the validation and improvement of current emission rate

estimates. In the absence of such a test program, Tables 7 through 11 present the current best

estimates of pollutant-specific off-road engine emissions deterioration. The presented

deterioration data is expressed as the factor by which emissions increase over their full useful

life. For example, if an engine has a “when new” emission rate of X grams per brake

horsepower-hour and a full life deterioration factor of Y, its emission rate at full useful life (Z) is

equal to:

The emission impact estimates presented in this GSE information series are based on the

estimated emission rate at one-half of the applicable equipment’s useful life. If this emission rate

is defined as Z1/2, it is algebraically defined as:

The majority of GSE engines continue to be “uncontrolled” from an emissions perspective and

the emission rates presented in this GSE information series reflect such an assumption. Although

an increasing number of the larger vehicles used for GSE operations such as water, fuel, lavatory,

and aircraft maintenance services are powered by emissions-certified on-road engines, a number

of older GSE performing similar services and nearly all specially-designed GSE continue to

utilize engines that have not been designed for low emissions. As a result, the emission

estimates presented in this series will, in most cases, accurately reflect the potential emission

reductions due to control strategy implementation, but will in some cases, where newer GSE is

concerned, overstate potential reductions to varying extents. Actual emission reduction potential

must be based on data for the specific GSE targeted for control.

Evaporative emission estimates or evaporative emission reduction potential has not been

evaluated as part of this GSE information series. Generally, evaporative emissions are

significant 
GSE Information Series 2 Page 5

Horsepower

Range

Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG Diesel

0-X1 290.00 26.40 4.25

(X+1)-15 209.00 7.46 3.96 1.50

16-25 209.00 7.46 3.96 1.84

26-50 5.50 2.00 1.84

51-120 4.00 2.00 1.44

121-175 4.00 2.00 0.88

176-250 4.00 0.88

251-500 0.84

501-750 0.84

750+

1 X equals 2 for gasoline 2-stroke; 5 for gasoline 4-stroke and diesel equipment.

Table 1. Zero Hour HC Emission Rate (grams/brake horsepower-hour)

Horsepower

Range

Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG Diesel

0-X1 840.0 504.2 248.4

(X+1)-15 311.0 393.1 240.0 5.0

16-25 311.0 393.1 240.0 5.0

26-50 320.0 150.0 5.0

51-120 240.0 150.0 4.8

121-175 240.0 150.0 4.2

176-250 240.0 4.2

251-500 4.1

501-750 4.1

750+

1 X equals 2 for gasoline 2-stroke; 5 for gasoline 4-stroke and diesel equipment.

Table 2. Zero Hour CO Emission Rate (grams/brake horsepower-hour)
GSE Information Series 2 Page 6

Horsepower

Range

Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG Diesel

0-X1 0.36 2.12 1.99

(X+1)-15 0.90 3.48 1.77 10.00

16-25 0.90 3.48 1.77 6.92

26-50 3.75 3.00 6.92

51-120 4.00 3.00 13.00

121-175 4.00 3.00 11.00

176-250 4.00 11.00

251-500 11.00

501-750 11.00

750+

1 X equals 2 for gasoline 2-stroke; 5 for gasoline 4-stroke and diesel equipment.

Table 3. Zero Hour NOx Emission Rate (grams/brake horsepower-hour)

Horsepower

Range

Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG Diesel

0-X1 10.00 0.74 0.49

(X+1)-15 6.50 0.14 0.09 1.00

16-25 6.50 0.14 0.09 0.76

26-50 0.05 0.032 0.76

51-120 0.042 0.042 0.84

121-175 0.032 0.022 0.55

176-250 0.032 0.55

251-500 0.53

501-750 0.53

750+

1 X equals 2 for gasoline 2-stroke; 5 for gasoline 4-stroke and diesel equipment.

2 Values assumed to be zero in these size ranges in the ARB model, tabulated

values are based on 95 percent reduction from diesel for gasoline and 96

percent reduction from diesel for LPG.

Table 4. Zero Hour PM Emission Rate (grams/brake horsepower-hour)
GSE Information Series 2 Page 7

Horsepower

Range

Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG Diesel

0-X1 8932.77 8932.77 5768.71

(X+1)-15 8932.77 8932.77 5768.71 9797.23

16-25 8932.77 8932.77 5768.71 9797.23

26-50 8932.77 5768.71 9797.23

51-120 8932.77 5768.71 9797.23

121-175 8932.77 5768.71 9797.23

176-250 8932.77 9797.23

251-500 9797.23

501-750 9797.23

750+

1 X equals 2 for gasoline 2-stroke; 5 for gasoline 4-stroke and diesel equipment.

Table 5. Zero Hour CO2 Emission Rate (grams/gallon)

Horsepower

Range

Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG Diesel

0-X1 1.30 1.09 0.972

(X+1)-15 1.30 0.90 0.802 0.65

16-25 1.30 0.80 0.712 0.53

26-50 0.70 0.622 0.54

51-120 0.55 0.492 0.49

121-175 0.55 0.492 0.442

176-250 0.55 0.442

251-500 0.422

501-750 0.42

750+

Fuel Density (lb/gallon) 6.20 6.20 4.25 6.80

1 X equals 2 for gasoline 2-stroke; 5 for gasoline 4-stroke and diesel equipment.

2 Values encoded in the ARB model are incorrect. Tabulated LPG values are set to 89 percent

of 4-stroke gasoline values; tabulated diesel values are set according to available test data.

Table 6. Brake-Specific Fuel Consumption (pounds/brake horsepower-hour)
GSE Information Series 2 Page 8

Horsepower

Range

Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG Diesel

0-X1 0.00 1.09 1.09

(X+1)-15 0.00 1.59 1.59 0.00

16-25 0.00 1.59 1.59 0.00

26-50 1.38 1.38 0.51

51-120 1.38 1.38 0.28

121-175 0.37 0.37 0.28

176-250 0.37 0.28

251-500 0.44

501-750 0.44

750+

1 X equals 2 for gasoline 2-stroke; 5 for gasoline 4-stroke and diesel equipment.

Table 7. Full-Life HC Deterioration Factor

Horsepower

Range

Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG Diesel

0-X1 0.00 0.32 0.32

(X+1)-15 0.00 0.09 0.09 0.00

16-25 0.00 0.09 0.09 0.00

26-50 0.83 0.83 0.41

51-120 0.83 0.83 0.16

121-175 0.56 0.56 0.16

176-250 0.56 0.16

251-500 0.25

501-750 0.25

750+

1 X equals 2 for gasoline 2-stroke; 5 for gasoline 4-stroke and diesel equipment.

Table 8. Full-Life CO Deterioration Factor
GSE Information Series 2 Page 9

Horsepower

Range

Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG Diesel

0-X1 0.00 0.04 0.04

(X+1)-15 0.00 0.38 0.38 0.00

16-25 0.00 0.38 0.38 0.00

26-50 0.06 0.06 0.06

51-120 0.06 0.06 0.14

121-175 0.14 0.14 0.14

176-250 0.14 0.14

251-500 0.21

501-750 0.21

750+

1 X equals 2 for gasoline 2-stroke; 5 for gasoline 4-stroke and diesel equipment.

Table 9. Full-Life NOx Deterioration Factor

Horsepower

Range

Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG Diesel

0-X1 0.00 1.09 1.09

(X+1)-15 0.00 1.59 1.59 0.00

16-25 0.00 1.59 1.59 0.00

26-50 0.00 0.00 0.31

51-120 0.00 0.00 0.44

121-175 0.00 0.00 0.44

176-250 0.00 0.44

251-500 0.67

501-750 0.67

750+

1 X equals 2 for gasoline 2-stroke; 5 for gasoline 4-stroke and diesel equipment.

Table 10. Full-Life PM Deterioration Factor
GSE Information Series 2 Page 10

Horsepower

Range

Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG Diesel

0-X1 0.00 0.00 0.00

(X+1)-15 0.00 0.00 0.00 0.00

16-25 0.00 0.00 0.00 0.00

26-50 0.00 0.00 0.00

51-120 0.00 0.00 0.00

121-175 0.00 0.00 0.00

176-250 0.00 0.00

251-500 0.00

501-750 0.00

750+

1 X equals 2 for gasoline 2-stroke; 5 for gasoline 4-stroke and diesel equipment.

Table 11. Full-Life CO2 Deterioration Factor

Horsepower

Range

Gasoline

(2-Stroke)

Split

Gasoline

(4-Stroke)

Split

Diesel

Split

Aggregate

Split

0-X1 0.00 0.00 0.00 0.00

(X+1)-15 1.00 0.00 0.00 0.02

16-25 0.00 0.00 0.00 0.00

26-50 0.00 0.18 0.15 0.17

51-120 0.00 0.51 0.49 0.49

121-175 0.00 0.26 0.27 0.26

176-250 0.00 0.05 0.00 0.03

251-500 0.00 0.00 0.03 0.01

501-750 0.00 0.00 0.05 0.02

751+ 0.00 0.00 0.00 0.00

1 X equals 2 for gasoline 2-stroke; 5 for gasoline 4-stroke and diesel equipment.

Table 12. GSE Horsepower Distributions
GSE Information Series 2 Page 11

from gasoline powered equipment and associated emission reductions would accrue through the

imposition of such control strategies as alternative fueled or electric GSE conversion or

replacement. However, the lack of evaporative emissions data for most engines and equipment

types used in the GSE sector precludes an accurate assessment of the potential significance of

such emission reductions. Experience in the on-road vehicle sector indicates that evaporative

emissions can constitute a significant fraction of total HC emissions, but this significance is

measured on the basis of over two decades of exhaust emissions control. Evaporative emissions

are likely to constitute a much smaller fraction of total HC in the GSE sector. Nevertheless, the

quantification of evaporative emission rates is an area of needed research and emission reduction

potential should be reassessed once a reasonable GSE- or off-road engine-specific database

(including engines of the size used for GSE) has evolved.

GSE Engine Size Classification: As might be expected given the diverse nature of the various

tasks required of aircraft GSE, a wide range of equipment of varying engine size and fueling type

are encountered. The information series selection entitled Basis for GSE Population Estimates

provides a detailed breakdown of the various GSE equipment by fueling type. The equipment

and fueling type population breakdowns presented in this GSE information series differ from

those of the ARB off-road model for two reasons. First, the population data encoded in the ARB

model is California-specific. Second, the methodology for estimating GSE populations has been

improved since the development of the ARB model as described in the Basis for GSE Population

Estimates information series selection. Nevertheless, the breakdown of the various engine sizes

encountered within a specific equipment type, as required to accurately estimate GSE emissions,

is best performed using engine distribution data from the ARB model. While the population of

GSE equipment should be expected to vary from that of the ARB model, there is no reason to

expect that the distribution of engine sizes encountered within a specific equipment type

(regardless of population) will vary. Moreover, since the ARB model engine size distributions

incorporate all available data on GSE engine distribution, these data represent the current state of

GSE information.

While the ARB model continues to represent the best source of horsepower distribution data, it

should be recognized that even the basis for this data is limited, consisting of GSE inventory

information collected from several airlines during the California Federal Implementation Plan

(FIP) development process of the early-1990’s. There is no question that an expanded study of

GSE engine classifications is both appropriate and necessary to validate or augment existing data.

Table 12 presents the overall horsepower distribution of GSE engines. As indicated, all

two-stroke gasoline equipment is small at 15 horsepower or less. Four-stroke gasoline

equipment is found over a wide range of horsepower, ranging from 26 hp to as high as 250 hp. 

Diesel equipment spans an even larger range, from 26 hp all the way up to 750 hp. Neither LPG

or CNG GSE distributions can be derived from data encoded in the ARB model since it assumes

zero GSE populations for these fuels, but the range of engine horsepower should be very similar

to that of four-stroke gasoline equipment. Table 13 presents average horsepower data for the

various GSE equipment types represented in the ARB model. The variation across types is

dramatic as is expected from the wide range of horsepower observed in the GSE sector and, as a

direct result, emissions performance across GSE types (or even across fuels within a specific

GSE type) can varies considerably. Clearly, in cases where emission credits are being 
GSE Information Series 2 Page 12

Equipment Type Gasoline/LPG/CNG Diesel

Aircraft Pushback Tractor 130 216

Conditioned Air Unit 130 300

Air Start Unit 130 600

Baggage Tug 100 78

Belt Loader 60 45

Bobtail 100 100

Cargo Loader 70 76

Cart 12 12

Deicer 93 93

Forklift 50 52

Fuel Truck 130 180

Ground Power Unit 150 145

Lavatory Cart 12 12

Lavatory Truck 130 130

Lift 100 100

Maintenance Truck 130 130

Other 50 50

Service Truck 180 170

Water Truck 150 150

Table 13. Average Horsepower by Equipment and Fuel Type

determined on the basis of equipment retirements or conversions, emission impacts should be

calculated on the basis of actual equipment horsepower.

Conspicuously absent from Table 13 and other GSE emissions-related tables in this GSE

information series are estimates for “standard” cars, pickup trucks, vans, and buses that are used

as GSE for various general services such as personnel transport. Such vehicles are

emissions-certified on-road vehicles that have simply been adapted to GSE use. As such, the

emissions performance of these vehicles is equivalent to that of other on-road vehicles and is best

determined using the U.S. EPA’s MOBILE series of emission factor models developed

specifically for estimating on-road vehicle emissions. This is also the case for some unknown

fraction of the trucks used to provide fuel, food, water, and lavatory service. Generally, such

service trucks include special adaptations to facilitate use as GSE and many are of overseas

manufacture and uncertain emissions certification (since their use is restricted to off-road

applications, no specific emissions certification has historically been required). However, many

are undoubtedly certified on-road vehicles adapted for GSE use and as such, the emission

estimates presented for such vehicles in this GSE information series may be overstated.

Newer on-road certified vehicles already incorporate stringent emission controls and may not be
GSE Information Series 2 Page 13

good candidates for additional emission reduction through the imposition of the control strategies

discussed in this information series (which are slanted toward the reduction of uncontrolled

off-road engine emissions). Nevertheless, emission reduction strategies applicable to the general

on-road vehicle fleet (e.g., reformulated gasoline) are equally viable for on-road vehicles used as

GSE. Therefore, emission reductions for such vehicles should be evaluated in the same context

as that of the larger on-road vehicle fleet. The information series selection entitled Basis for GSE

Population Estimates does include an estimate of the population of on-road cars, pickup trucks,

vans, and buses in use as GSE, indicating that they may constitute as much as 12 percent of the

overall GSE fleet. The fraction of service trucks that are on-road emissions certified is less

certain and all such trucks are assumed to be uncontrolled from an emissions perspective in this

information series. Clearly, actual emissions performance should be assessed before any

emission reduction credits are granted for the imposition of emission controls on such vehicles.

GSE Load Factors: As was the case with other GSE attributes, the load factor for the various

equipment types used as GSE might be expected to vary considerably in accordance with the

range of services encountered in the sector. However, information on specific GSE load factors

is perhaps the most uncertain of all the activity indicators required to accurately estimate GSE

emissions performance. Very little detailed time-in-mode data exist and, as a result, GSE load

factors (like those for other off-road equipment types) have been estimated from actual fuel use

records and known brake-specific fuel consumption rates for GSE engines. While such an

approach is technically sound, there can be considerable uncertainty involved in associating a

specific operating time with recorded fuel use (except when concurrent equipment usage meter

readings are also available). As a result, there can also be considerable uncertainty in

determining precise load factor estimates from fuel consumption records. This uncertainty

generally increases as load factor decreases since fuel consumption rates are small relative to

rated consumption and the influence of operating time estimation errors becomes more

significant. Potential estimation errors can be reduced by increasing the size of the fuel

consumption database used to estimate the load factor, but to date the fuel consumption database

for GSE is limited and what data is available is reflected in the load factor data encoded in the

ARB off-road model. As a result, the ARB off-road model estimates have been used for the

emission estimates presented in this GSE information series. Table 14 presents a summary of the

load factor estimates by GSE type.

In reviewing the data presented in Table 14, the paucity of data on which to base GSE load

factors is obvious as several equipment types exhibit identical estimates due to the unavailability

of equipment-specific data. The 0.50 load factor estimate for small engines (i.e., carts) is

generally sufficiently close to the J1088 emissions test load factor of 0.39 so that the estimated

emission rates based on J1088 should be sufficiently representative. However, for larger

equipment (all other GSE), there are several cases where the estimated load factor is outside the

0.56 ±0.2 load factor range associated with the “8 mode” emissions test. Two (of 17) large

engine GSE exhibit estimated load factors above 0.76 and five exhibit estimated load factors

below 0.36. If these load factors are in fact accurate, then the use of “8 mode” emission test

results to accurately portray emission rates for these GSE is questionable.
GSE Information Series 2 Page 14

Equipment Type Gasoline/LPG/CNG Diesel

Aircraft Pushback Tractor 0.80 0.80

Conditioned Air Unit 0.75 0.75

Air Start Unit 0.90 0.90

Baggage Tug 0.55 0.55

Belt Loader 0.50 0.50

Bobtail 0.55 0.55

Cargo Loader 0.50 0.50

Cart 0.50 0.50

Deicer 0.95 0.95

Forklift 0.30 0.30

Fuel Truck 0.25 0.25

Ground Power Unit 0.75 0.75

Lavatory Cart 0.50 0.50

Lavatory Truck 0.25 0.25

Lift 0.50 0.50

Maintenance Truck 0.50 0.50

Other 0.50 0.50

Service Truck 0.20 0.20

Water Truck 0.20 0.20

Table 14. Average Load Factors by Equipment and Fuel Type

Without question, additional research into GSE load factors is both appropriate and necessary. 

Targeted studies using either detailed fuel consumption records and calibrated operating time

meters or automatic on-board dataloggers should be implemented at a number of airports of

varying size. These studies should include several units of each type of GSE that contributes

significantly to overall GSE emissions.

GSE Hours of Use: As indicated above, the overall emission rates for the various GSE depend

not only on emission rates, but the overall amount of time the equipment is used. However,

experience has shown that equipment usage varies considerably, not only across the various

airlines and airports, but by time-of-day, day-of-week, and season. Temporal variation is of

marginal concern in estimating long term (e.g., annual or multi-year) emission impacts, but

variation across airports is significant and prohibits the development of any generic usage rates

for GSE. For example, an airport with limited passenger service may use baggage tractors for

only a few hours per day while large, high traffic airports might use baggage tractors

continuously throughout an operating day. Based on this airport dependence, the emission

estimates presented in this GSE information series are expressed in terms of grams per operating

hour and should be equally applicable to any airport or airline employing GSE. However, due to
GSE Information Series 2 Page 15

the uncertainties discussed above, these emission estimates should be used for “screening”

purposes only (for example, to estimate the potential emission reduction impacts due to the

replacement of conventionally fueled GSE with electrically powered equipment) and supplanted

with actual site- and application-specific emission estimates in determining actual accrued

emission impacts.

Hourly-Specific GSE Emission Rate Estimates: Tables 15-19 present the various emission

rates, in units of grams per operating hour, derived using the various GSE data discussed above. 

As indicated above, these data should be used only for estimation purposes and supplanted with

actual site-specific data in determining actual emission impacts.

References:

1. Analysis of Techniques to Reduce Air Emissions at Airports, Draft Final Report, prepared by

Energy and Environmental Analysis, Inc. for the U.S. Environmental Protection Agency,

September 1997.

2. Air Pollution Mitigation Measures for Airports and Associated Activity, prepared by Energy

and Environmental Analysis, Inc. for the California Air Resources Board, May 1994.

3. Technical Support Document for Civil and Military Aviation; prepared by Energy and

Environmental Analysis, Inc. for the U.S. Environmental Protection Agency in support of the

Notice of Proposed Rulemaking for the Federal Implementation Plan for California, March

1994.

4. GSE population data sheets submitted by member companies of the Air Transport Association

as part of the U.S. Environmental Protection Agency’s Federal Implementation Plan

development process for California, September 1993.

5. Documentation of Input Factors for the New Off-Road Mobile Source Emissions Inventory

Model, prepared by Energy and Environmental Analysis, Inc. for the California Air Resources

Board, August 1995.

6. California Off-Road Model (June 7, 1996 version) input files EMFAC.DAT, POP.DAT, and

ACTIVITY.DAT.
GSE Information Series 2 Page 16

Equipment Type Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG1 Diesel

Aircraft Pushback Tractor n/a 493.0 246.5 174.6

Conditioned Air Unit n/a 462.2 231.1 230.6

Air Start Unit n/a 554.6 277.3 553.4

Baggage Tug n/a 371.8 185.9 70.4

Belt Loader n/a 202.8 101.4 52.0

Bobtail n/a 371.8 185.9 90.3

Cargo Loader n/a 236.6 118.3 62.4

Cart 1,254.0 80.3 42.6 9.0

Deicer n/a 597.2 298.6 145.0

Forklift n/a 139.4 50.7 25.6

Fuel Truck n/a 154.1 77.0 45.1

Ground Power Unit n/a 533.3 266.6 109.1

Lavatory Cart n/a 80.3 42.6 9.0

Lavatory Truck n/a 154.1 77.0 32.6

Lift n/a 338.0 169.0 82.1

Maintenance Truck n/a 308.1 154.1 65.2

Other n/a 232.4 84.5 57.7

Service Truck n/a 170.6 85.3 34.1

Water Truck n/a 142.2 71.1 30.1

1 CNG emission rate equals 0.67 times LPG emission rate.

Table 15. Equipment-Specific HC Emission Rates (grams/operating hour)
GSE Information Series 2 Page 17

Equipment Type Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG1 Diesel

Aircraft Pushback Tractor n/a 31,949 19,968 788

Conditioned Air Unit n/a 29,952 18,720 1,038

Air Start Unit n/a 35,942 22,464 2,491

Baggage Tug n/a 18,678 11,674 222

Belt Loader n/a 10,188 6,368 136

Bobtail n/a 18,678 11,674 285

Cargo Loader n/a 11,886 7,429 197

Cart 1,866 2,465 1,505 30

Deicer n/a 30,004 18,752 458

Forklift n/a 6,792 3,184 81

Fuel Truck n/a 9,984 6,240 204

Ground Power Unit n/a 34,560 21,600 493

Lavatory Cart n/a 2,465 1,505 30

Lavatory Truck n/a 9,984 6,240 147

Lift n/a 16,980 10,613 259

Maintenance Truck n/a 19,968 12,480 295

Other n/a 11,320 5,306 151

Service Truck n/a 11,059 6,912 154

Water Truck n/a 9,216 5,760 136

1 CNG emission rate equals 1.00 times LPG emission rate.

Table 16. Equipment-Specific CO Emission Rates (grams/operating hour)
GSE Information Series 2 Page 18

Equipment Type Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG1 Diesel

Aircraft Pushback Tractor n/a 445.1 333.8 2,055.2

Conditioned Air Unit n/a 417.3 313.0 2,734.9

Air Start Unit n/a 500.8 375.6 6,563.7

Baggage Tug n/a 226.6 170.0 596.7

Belt Loader n/a 123.6 92.7 160.4

Bobtail n/a 226.6 170.0 765.1

Cargo Loader n/a 144.2 108.2 528.6

Cart 5.4 24.8 12.6 60.0

Deicer n/a 364.0 273.0 1,228.9

Forklift n/a 57.9 46.4 217.0

Fuel Truck n/a 139.1 104.3 529.7

Ground Power Unit n/a 481.5 361.1 1,280.0

Lavatory Cart n/a 24.8 12.6 60.0

Lavatory Truck n/a 139.1 104.3 382.5

Lift n/a 206.0 154.5 695.5

Maintenance Truck n/a 278.2 208.7 765.1

Other n/a 96.6 77.3 178.2

Service Truck n/a 154.1 115.6 400.2

Water Truck n/a 128.4 96.3 353.1

1 CNG emission rate equals 1.00 times LPG emission rate.

Table 17. Equipment-Specific NOx Emission Rates (grams/operating hour)
GSE Information Series 2 Page 19

Equipment Type Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG1 Diesel

Aircraft Pushback Tractor n/a 3.1 2.1 117.9

Conditioned Air Unit n/a 2.9 2.0 159.2

Air Start Unit n/a 3.5 2.3 382.1

Baggage Tug n/a 2.2 2.2 44.0

Belt Loader n/a 1.2 1.2 19.8

Bobtail n/a 2.2 2.2 56.4

Cargo Loader n/a 1.4 1.4 38.9

Cart 39.0 1.5 1.0 6.0

Deicer n/a 3.5 3.5 90.5

Forklift n/a 0.8 0.5 16.0

Fuel Truck n/a 1.0 0.7 30.2

Ground Power Unit n/a 3.4 2.3 73.0

Lavatory Cart n/a 1.5 1.0 6.0

Lavatory Truck n/a 1.0 0.7 21.8

Lift n/a 2.0 2.0 51.2

Maintenance Truck n/a 2.0 1.3 43.6

Other n/a 1.3 0.8 21.9

Service Truck n/a 1.1 0.7 22.8

Water Truck n/a 0.9 0.6 20.1

1 CNG emission rate equals 1.00 times LPG emission rate.

Table 18. Equipment-Specific PM Emission Rates (grams/operating hour)
GSE Information Series 2 Page 20

Equipment Type Gasoline

(2-Stroke)

Gasoline

(4-Stroke) LPG1 Diesel

Aircraft Pushback Tractor n/a 82,412 69,100 108,183

Conditioned Air Unit n/a 77,261 64,781 136,153

Air Start Unit n/a 92,714 77,737 326,766

Baggage Tug n/a 43,583 36,543 30,286

Belt Loader n/a 23,773 19,933 17,505

Bobtail n/a 43,583 36,543 38,829

Cargo Loader n/a 27,735 23,255 26,827

Cart 11,238 7,780 6,523 5,619

Deicer n/a 70,011 58,701 62,373

Forklift n/a 15,128 12,684 11,013

Fuel Truck n/a 25,754 21,594 28,527

Ground Power Unit n/a 89,148 74,747 68,941

Lavatory Cart n/a 7,780 6,523 5,619

Lavatory Truck n/a 25,754 21,594 20,603

Lift n/a 39,621 33,221 35,299

Maintenance Truck n/a 51,508 43,187 41,206

Other n/a 25,213 21,141 19,450

Service Truck n/a 28,527 23,919 21,554

Water Truck n/a 23,773 19,933 19,018

1 CNG emission rate equals 0.92 times LPG emission rate.

Table 19. Equipment-Specific CO2 Emission Rates (grams/operating hour)
LPG AND CNG CONTROL STRATEGIES
GSE Information Series 3 Page 1

Basic Control Strategy Summary: The majority of conventionally powered GSE can be

converted to either liquefied petroleum gas (LPG) or compressed natural gas (CNG) fueling or

replaced with a specially-manufactured LPG- or CNG-powered counterpart. The basic issues

surrounding the use of LPG or CNG as a GSE fuel are quite similar and, therefore, both fueling

strategies can be treated together. Generally, non-methane hydrocarbon (NMHC), carbon

monoxide (CO), oxides of nitrogen (NOx), particulate matter (PM), and carbon dioxide (CO2)

emissions from LPG or CNG GSE are all reduced relative to emissions from gasoline-powered

GSE. Relative to diesel GSE, emissions of NOx and PM are reduced, emissions of HC and CO

are increased, and emissions of CO2 can be either slightly increased or decreased depending on

equipment size. Important issues for consideration in determining the viability of either

conversion or replacement strategies include: the quality of equipment conversions, the cost of

conversion or replacement equipment, and the availability (or cost) of LPG or CNG refueling

facilities.

Potential GSE Equipment Affected: Generally, there are no technical limitations to the size or

type of GSE that can be converted to LPG/CNG power or replaced with equivalent LPG/CNG

equipment. For GSE powered by gasoline engines, both conversion and replacement are

technically feasible given the spark-ignition nature of both LPG/CNG and gasoline engine

designs. However, the compression-ignition design of diesel powered GSE generally restricts the

viability of LPG/CNG conversion strategies (due to the lack of an active ignition source),

rendering equipment replacement with specially manufactured LPG or CNG counterparts as the

only realistic LPG/CNG fueling strategy for diesel powered GSE. It is recognized that there have

been demonstration projects involving the passive ignition of gaseous fuels in

compression-ignition engines through such techniques as fumigation, but in general these

technologies are not considered to be sufficiently mature for mass market penetration at this time.

LPG/CNG Conversions: Although some aspects of the conversion process for LPG and CNG

vary, in general, both involve the same elements. Conversion of a spark-ignition engine (i.e., a

gasoline engine) essentially involves the installation of a new fuel tank, a new fuel delivery

system, and a new fuel metering system. To ensure low emissions operation, most advanced

conversion kits also include closed-loop performance feedback controllers that continuously

adjust the charge air/fuel mixture to promote optimum combustion. Previous generation

open-loop (i.e., non-feedback) conversion packages were prone to frequent combustion

“detuning” and large associated increases in emissions. Complete conversion packages for

specific engines are usually sold in kits available through a number of dealers nationwide. 

While, theoretically, diesel engines can also be converted to LPG or CNG fueling, the extensive

nature of the engine modifications required with current mass-market technology to turn these

compression-ignition engines into spark-ignition engines are prohibitive.

LPG/CNG Replacements: Most GSE manufacturers now sell either specially designed LPG

and CNG versions of their equipment or offer LPG and CNG fueling as an optional feature of

their gasoline-powered equipment. There are few, if any, GSE applications for which LPG- or

LPG AND CNG CONTROL STRATEGIES
GSE Information Series 3 Page 2

PctRed 1 1

PctRed

100

1

PctRed

100 100 overall

1 2 =−− 



 



 − 



 



 



 



 ×

CNG-powered equipment cannot be purchased.

Emission Impacts: The emission impacts of replacement LPG and CNG equipment and quality

LPG and CNG conversions are fairly well characterized. However, there are two issues of

consideration relative to any assessment of these characterizations. First, the emission

relationships presented in this information series are for unregulated (from an emissions

standpoint) GSE engines and assume that replacement or converted LPG and CNG systems do

not include any emissions aftertreatment devices (for example, catalytic converters) not found on

the GSE before replacement or conversion.

To the extent that any future emissions standards are imposed that impact the engines used in

GSE (generally large industrial-type engines), the relationship between emissions from GSE

powered by gasoline, diesel, LPG, and CNG will be fundamentally altered (potentially to the

point of equivalent emission limitations). Nevertheless, the conversion of GSE to LPG/CNG

will continue to be a viable control strategy even after the imposition of emission standards due

to the lag time associated with GSE retirement and replacement, but at some point (a decade or

so after regulatory effectiveness) the emissions benefits associated with such conversion will be

eliminated.

Conversely, should replacement or converted LPG/CNG equipment include emission control

aftertreatment equipment not present on the GSE before conversion or replacement, then

LPG/CNG emission reduction benefits will be larger than indicated in this information series. 

The combined emission reduction percentage (PctRedoverall) of the fundamental fueling system

replacement (generating percent emission reduction PctRed1) and the emissions aftertreatment

device (generating percent emission reduction PctRed2) can be estimated as follows:

The second issue for consideration is the quality of LPG and CNG conversions. Given the nature

of current gasoline GSE engines, there is no reason that a quality, properly installed conversion

kit cannot provide for emission performance similar to factory-produced LPG and CNG

equipment. However, experience has shown that conversion kits are not always certified

(emission certifications are currently only performed by the California Air Resources Board) or,

even if certified, properly installed and calibrated and, as a result, both operating and emissions

performance can be greatly affected. Frequent detuning observed with open loop conversion kits

indicates that such packages should either be exempted from or carefully monitored to ensure

compliance with stated emission reductions. As it stands today, the current “unregulated” state

of the aftermarket conversion industry can result in after-conversion equipment that generates

emissions (of pollutants that would normally be reduced) at the same or even greater rates than

the pre-conversion equipment. Since there is no way to quantify the fraction of conversions that

are of poor quality, the emission comparisons presented in this GSE information series assume

that a proper conversion has been performed and that emissions from that conversion are

equivalent to emissions from a replacement engine. However, in practice it will be necessary to

institute some type of certification and in-use compliance procedure to ensure that any emission
GSE Information Series 3 Page 3

reductions achieved in practice are both quantifiable and consistent with any ascribed credits.

Examples of possible conversion certification programs can be found in California, where the

State Air Resources Board (ARB) certifies aftermarket conversion kits to specific emission

standards which become enforceable under the California in-use vehicle compliance program. 

Jurisdictions such as the South Coast Air Quality management District rely on these certifications

before granting emission credits under their Mobile Source Emission Reduction Credit (MSERC)

program. However, in practice these certifications are only as effective as the in-use compliance

program that ensures that installers not only adhere to kit manufacturer requirements, but actually

achieve certification level emissions performance. One possible response is to require credit

emission reduction recipients to submit associated vehicles for periodic emission inspections to

ensure performance consistent with credit assumptions. There may be other equally effective

mechanisms, but clearly an effective program will require significant oversight and enforcement.

Using data developed for the off-road emissions model recently released by the California ARB,

detailed emission estimates for LPG, CNG, gasoline, and diesel GSE have been developed as

described in the information series selection GSE Emissions and Activity Estimates. Data used in

this emissions estimation analysis include zero hour emission rates, emissions deterioration rates,

equipment horsepower, equipment load factors, and brake-specific fuel consumption. Table 1

presents a comparison of derived emission rates for baggage tractors and belt loaders, which

together constitute over a third of all GSE. Emission rates for other GSE types will vary from

those presented in Table 1 due to variations in engine size and operating characteristics, but the

relative relationships between the various fueling types remain consistent. Therefore, the data in

Table 1 can be used to assess the relative degree of emission reduction to be expected by

converting or replacing conventionally fueled GSE. Table 2 presents these emission reduction

relationships and indicates the variability across different GSE types from those included in

Table 1.

Several aspects of the tables stand out. Relative to emissions from gasoline GSE, emissions from

both LPG and CNG equipment are reduced for all five emission species examined. Similar

relationships exist for LPG and CNG NOx and PM relative to diesel GSE emissions, where the

greatest emission reductions due to equipment conversion or replacement are observed. 

Although (apparently dramatic) emission increases are observed for both HC and CO relative to

diesel GSE emissions, these increases are not nearly as dramatic in absolute terms as the

percentage increase estimates imply. The large percentages simply result from the relatively low

HC and CO emission rates of diesel GSE engines. For example, while diesel baggage tractors

are estimated to emit over 200 grams of CO and about 70 grams of HC per hour, equivalent

LPG-powered equipment has estimated emission rates of over 9,000 and nearly 150 grams per

hour respectively (both well below the estimated emission rates of about 14,500 grams CO and

300 grams HC per operating hour for equivalently sized gasoline equipment (not shown in Table

1)). Finally, the variation of the estimates of LPG and CNG CO2 relative to diesel GSE around

zero are due to the sensitivity of diesel engine efficiency to engine size. However, the cases in

which diesel engine efficiency exceeds (and thus CO2 emissions are lower) than LPG and CNG

far outnumber the alternative. Finally, when considering CO2 emissions in the context of global

warming potential, it is also 
GSE Information Series 3 Page 4

Emissions from ICE-Powered Baggage Tractors of Gasoline Engine Size

HC CO NOx PM CO2

Gasoline (4-Stroke) 371.8 18678.0 226.6 2.2 43583.3

LPG 185.9 11673.8 170.0 2.2 36543.1

CNG 123.9 11673.8 170.0 2.2 33619.6

Emissions from ICE-Powered Baggage Tractors of Diesel Engine Size

HC CO NOx PM CO2

Gasoline (4-Stroke) 371.8 18678.0 226.6 2.2 43583.3

LPG 185.9 11673.8 170.0 2.2 36543.1

CNG 123.9 11673.8 170.0 2.2 33619.6

Emissions from ICE-Powered Belt Loaders of Gasoline Engine Size

HC CO NOx PM CO2

Gasoline (4-Stroke) 202.8 10188.0 123.6 1.2 23772.7

LPG 101.4 6367.5 92.7 1.2 19932.6

CNG 67.6 6367.5 92.7 1.2 18338.0

Emissions from ICE-Powered Belt Loaders of Diesel Engine Size

HC CO NOx PM CO2

Gasoline (4-Stroke) 52.0 135.6 160.4 19.8 17505.3

LPG 101.4 6367.5 92.7 1.2 19932.6

CNG 67.6 6367.5 92.7 1.2 18338.0

Table 1. Estimated Emission Rates for Baggage Tractors

Emissions Relative to Gasoline GSE Emissions Relative to Diesel GSE

LPG CNG LPG CNG

HC -50% to -65% -65% to -75% +95% to +140% +30% to +60%

CO -40% to -50% +4000% to +5000%

NOx -20% to -25% -75% to -80%

PM -20% -95%

CO2 -15% -20% -5% to +15% -10% to +10%

Table 2. Percent Emissions Reduction Due to LPG/CNG GSE
GSE Information Series 3 Page 5

important to recognize that emission species such as methane which are not estimated here can

play a critical role in determining the overall global warming potential associated with a specific

fuel. For example, CNG equipment will emit methane at a higher rate than either gasoline, LPG,

or diesel equipment.

Costs: The incremental cost of LPG conversions has been estimated to be in the range of $1,500

to $2,000 for systems capable of storing the LPG equivalent of about 20 gallons of gasoline. For

larger GSE, costs can range as high as $3,000. High pressure fuel storage requirements push

typical conversion costs to $4,000 to $4,500 per unit for CNG, even for systems sized to store the

equivalent of only about 15 gallons of gasoline. Locating space for adequate fuel tank storage

capacity can be an issue for CNG as well as LPG conversions. Most GSE do not have significant

unused cargo volumes in which to locate additional fuel storage tanks. Since safety concerns

preclude installation in exposed areas, fuel tanks must generally replace existing conventional

fuel tanks. However, the CNG’s lower fuel energy per unit volume can severely restrict

operating range, and thus conversion viability, in some applications.

For gasoline powered GSE, the incremental costs of specially designed LPG and CNG

replacement GSE equipment are similar to incremental conversion costs. However, for

replacement diesel equipment, incremental costs can be considerably higher given the generally

larger size and expected durability of diesel-type GSE. Cost premiums can be as high as 50

percent of the basic diesel GSE cost.

Some of the increased cost of equipment conversion or replacement can be offset by fuel cost

savings. Although fuel prices vary geographically, in general LPG and CNG cost from 30-50

percent less on an equivalent energy basis than retail gasoline or diesel (including the cost of

compression for CNG). However, most, and in some cases all, of this potential savings is lost in

airport applications since on-highway fuel taxes (that comprise 30-40 percent of retail fuel price)

are recouped as tax credits due to the off-road nature of GSE use. Potential fuel cost savings are

further eroded through an estimated loss in fuel efficiency of about 5 percent for LPG and 10

percent for CNG relative to gasoline GSE and losses of up to 30 percent relative to diesel GSE. 

Thus fuel cost savings, if any, will be modest. LPG systems could have fuel costs about $0.10

per gasoline equivalent gallon lower than their gasoline counterparts while CNG equipment fuel

costs will be about equal to those of their gasoline counterparts and possibly somewhat higher.

Both LPG and CNG GSE could have lower routine maintenance costs than either gasoline or

diesel GSE due to reduced particulate and associated deposit formation. However, several

airlines using such equipment have reported much higher non-routine maintenance costs for LPG

and CNG equipment. This may, however, be a result of early design problems associated with

early generation LPG and CNG systems and could decline as additional experience is gained.

Table 3 presents the results of a life cycle cost comparison for a baggage tractor under a high-use

operating scenario (i.e., the GSE is generally used to service aircraft continuously throughout an

operating day as typically occurs at high traffic airports). The tabulated costs represent the net

present value of the various expenditures required over the sixteen year useful life of the tractor. 

In all cases, the diesel-powered tractor exhibits the lowest life cycle costs by a substantial margin. 

This is consistent with an observed movement in the GSE industry towards greater 
GSE Information Series 3 Page 6

Fuel

Type

Purchase

Cost

Rebuild or

Replacement

Costs

Fuel

Costs

Reduced

LPG/CNG

Maintenance

Costs

Total Costs

if Reduced

LPG/CNG

Maintenance

Total Costs

if Same

LPG/CNG

Maintenance

Gasoline $17,000 $2,568 $59,481 $47,089 $126,139 $126,139 

Diesel $22,000 $1,351 $27,386 $47,089 $97,826 $97,826 

LPG $19,000 $2,568 $49,072 $37,176 $107,816 $117,729 

CNG $21,000 $2,568 $65,058 $37,176 $125,802 $135,715 

Assumptions: 16 year equipment life; 6 year engine replacement interval for gasoline, LPG, and CNG; 8 year engine

rebuild interval for diesel; $2,500 unit cost for all rebuilds/replacements; equipment used 8 hours per day for 350 days per

year; gasoline use is 3.2 gallons per hour at $0.75 (after tax credits) per gallon; diesel use is 1.7 gallons per hour at $0.65

(after tax credits) per gallon; LNG use is 3.3 gallons per hour at $0.60 per gallon; CNG use is 3.5 gallons per hour at $0.75

per gallon (including the cost of refueling facility operation and amortization); maintenance costs are $1.90 per hour for

gasoline and diesel; maintenance costs are $1.50 per hour for LPG and CNG under a reduced maintenance scenario or

$1.90 per hour under a “same maintenance” scenario.

Table 3. Life Cycle Costs for Baggage Tractors

diesel utilization in large engine applications. Relative to gasoline, both LPG and CNG tractors

show reduced or similar life cycle costs if one assumes reduced maintenance requirements for

LPG and CNG engines. However, if the assumption of reduced maintenance is eliminated, the

full life cycle costs favor gasoline over CNG and the savings associated with LPG are cut in half. 

Clearly, the maintenance issue is critical to the viability of LPG and CNG engines given the

current experience of many LPG- and CNG-powered GSE owners, where maintenance costs are

not only not reduced, but actually higher than those of comparable gasoline or diesel equipment.

It is also important to note that since both fuel and maintenance costs are a function of usage, the

range of life cycle costs narrows and any cost advantages of LPG and CNG engines are

diminished if GSE is used less frequently than assumed in the development of Table 3. For

example, if a baggage tractor is operated only half the time assumed (i.e., four hours per day

instead of eight), the range between diesel and gasoline life cycle costs narrows to about $12,000;

the range between gasoline and LPG narrows to about $8,000 under a reduced maintenance

scenario or $3,000 under a constant maintenance scenario; and CNG operation becomes about

$2,000 more costly than gasoline under a reduced maintenance scenario or $7,000 more costly

under a constant maintenance scenario. Clearly site- and application-specific calculations are

necessary before accurate airport and airline-specific cost relations can be determined.

Cost Effectiveness: It is difficult to provide detailed cost effectiveness estimates for either LPG

or CNG GSE because the impact of such equipment varies across the pollutants examined and

relative to whether gasoline or diesel GSE is being replaced. From the cost table presented

above, it is clear that from a simple operation and maintenance standpoint, diesel GSE are more

cost effective than gasoline, LPG, or CNG. The inclusion of an emissions valuation can shift this

relationship, but the shift is greatly dependent on the pollutants included in the valuation. Diesel

HC and CO are generally lower than those of gasoline, LPG, and CNG, so that credits assigned

to a reduction in either will continue to favor diesel GSE. Under a scenario in which only NOx
GSE Information Series 3 Page 7

reductions are assigned a marketable value, LPG becomes cost competitive with diesel at a

market value of $1,000-$2,000 per ton; CNG becomes cost competitive with diesel at a market

value of $3,000-$4,000 per ton; and gasoline becomes cost competitive with diesel at a market

value of about $3,500 per ton. A similar scenario that assigns marketable value to PM reductions

would make LPG competitive with diesel when PM is valued at $10,000-$20,000 per ton, CNG

competitive with diesel when PM is valued at $30,000-$40,000 per ton, and gasoline competitive

with diesel when PM is valued at about $30,000 per ton. The inclusion of CO2 emissions in an

evaluation program is somewhat more complex as the relationship between the various fuels

shifts slightly with engine size, but diesel engines generally emit lower CO2 than either gasoline,

LPG, or CNG so that under most scenarios in which value is granted to CO2 reductions,

diesel-powered GSE would be favored over the other three fuels.

Relative to gasoline GSE, emissions of all examined pollutants (HC, CO, NOx, PM, and CO2)

should decline through replacement with either LPG- or CNG-powered GSE. In fact, from the

table above it is clear that under a scenario in which LPG and CNG maintenance costs are

reduced relative to gasoline that these emissions reductions are derived for free (the life cycle

costs of both LPG and CNG are lower than those of gasoline) and any marketable credits

obtained through gasoline GSE replacement are essentially windfalls. (It should be noted that for

programs crediting HC, CO, or CO2, diesel GSE would also reflect a windfall status relative to

gasoline.) Even under an assumption of similar maintenance costs, LPG retains its windfall

status. However, CNG would lose its competitive edge under such an assumption unless HC

reductions are valued at about $2,000 per ton, CO reductions are valued at about $100 per ton, or

NOx reduction are valued at about $5,000 per ton (or some combination of all three).

References:

1. Analysis of Techniques to Reduce Air Emissions at Airports, Draft Final Report, prepared by

Energy and Environmental Analysis, Inc. for the U.S. Environmental Protection Agency,

September 1997.

2. Air Pollution Mitigation Measures for Airports and Associated Activity, prepared by Energy

and Environmental Analysis, Inc. for the California Air Resources Board, May 1994.

3. Documentation of Input Factors for the New Off-Road Mobile Source Emissions Inventory

Model, prepared by Energy and Environmental Analysis, Inc. for the California Air Resources

Board, August 1995

4. Advanced Transportation Vehicle Catalog, www.calstart.org/services/catalog/, CALSTART

internet website.

5. Survey of Airport Fleet Vehicles, Final Report, GRI-98/0064, prepared by Edwards and

Kelsey, Inc. for the Gas Research Institute, April 1998.
ELECTRIC GSE
* Technically, electrically powered GSE do not emit any pollutants. The term emit, as used here, ascribes to the

electric GSE, the offsite increase in power generating station emissions due to increased airport electrical power to

support electric GSE recharging. In essence, the electrically powered GSE are treated as if they $emitted# the

increased power generating station emissions associated with their use.

GSE Information Series 4 Page 1

Basic Control Strategy Summary: The majority of conventionally powered GSE can be either

converted to electric power or replaced with specially-manufactured electrically powered

counterparts. Although there is an increase in offsite power generating station emissions

resulting from the increased electrical demand required to recharge electric GSE, conversion to

electric power or replacement with electric GSE can be a very effective emission reduction

strategy. Even when the increased emissions from power generating stations are considered,

electric GSE usually emit*

 significantly less hydrocarbon (HC), carbon monoxide (CO), oxides of

nitrogen (NOx), particulate matter (PM), and carbon dioxide (CO2) emissions than their fossil

fueled (i.e., gasoline, diesel, compressed natural gas (CNG), and liquefied petroleum gas (LPG))

counterparts. Important issues for consideration in determining the viability of either conversion

or replacement strategies include: the capacity of the electric equipment to handle daily GSE

scheduling and load demands, the quality of equipment conversions, the cost of conversion or

replacement equipment, the availability (or cost) of electric recharging or battery replacement

stations, and the ability to schedule recharging or battery replacement around GSE service

demands.

Potential GSE Equipment Affected: Generally, there are no technical limitations to the size or

type of GSE that can be converted to or replaced with electrically powered equipment. 

Electrically powered versions of baggage tugs and belt loaders, which together account for over a

third of all GSE, are available and in use (although current usage constitutes only a minor

fraction of total activity). Additionally, electric powered versions of aircraft pushback tractors,

air start units, conditioned air units, forklifts, ground power units, lifts, general purpose vehicles

(cars, trucks, and vans), and other specialty GSE are currently available in the marketplace. 

Electric carts are already fulfilling about half of overall GSE cart demand.

GSE conversion generally requires the removal of the internal combustion engine and fuel

storage tanks to obtain sufficient room to install the necessary electric motor, motor controller,

and battery pack and, therefore, is equally viable on diesel, gasoline, CNG, and LPG fueled

GSE. However, some equipment types or configurations may not be able to store sufficient

battery capacity to fulfill required service demands within existing space limitations. Therefore,

the ability to convert a specific GSE to electric power, and the specifications for conversion,

require a detailed review of the candidate equipment.

Regardless of the availability of replacement or conversion equipment, a primary issue in

evaluating the potential acceptability of electrically powered GSE is the daily usage demand

placed on the equipment. Equipment that is in continuous or near-continuous service throughout

the day will require quick turnaround battery replacement facilities, quick recharge capability, or

ELECTRIC GSE
GSE Information Series 4 Page 2

the availability of fully charged backup equipment. Most GSE will require between one and five

charging cycles per day. GSE that can operate for a full day on a single charge are candidates for

off-peak charging, but most equipment will require all-hours recharging access, at least with

current battery technology. Battery storage advances could increase the fraction of GSE that can

operate throughout the day on a single charge, but existing technology can only be extended

through an increase in battery pack size (thus imposing additional storage space considerations). 

Clearly, the capability of scheduling GSE recharging within the constraints of aircraft service

demands is a key issue for those considering replacement or conversion of fossil fueled GSE.

Electric GSE Conversions: Conversion of fossil fueled GSE to electric power generally

involves the removal of the internal combustion engine and fuel tank and installation of an

electric motor, motor controller, and battery pack. Although the conversion process is generally

well established from a technical standpoint, the sizing of the electric motor, selection of an

appropriate controller, integration with (or replacement of) existing drivetrain components, and

selection and sizing of the battery pack within the volume constraints established by existing

GSE design, result in the need for a case-by-case design review. It is important to recognize that

design review and installation quality are important from a performance standpoint alone. Like

factory-designed replacement GSE, tailpipe emissions from converted electric GSE are

non-existent (unlike fossil fuel conversions such as CNG and LPG, for which emissions

performance can be quite dependent on conversion quality). Another limiting factor, in addition

to conversion design and packaging, is the location of qualified conversion facilities, which tend

to be concentrated in California.

Electric GSE Replacements: Specially designed electrically powered GSE are now available

from reputable GSE OEM s (original equipment manufacturers). The range of electric GSE now

available spans nearly the full range of aircraft service equipment. In general, the latest

generation of such equipment has attained a level of reliability equal to or better than that of

equivalent fossil fueled GSE. However, battery recharging capacity and scheduling continue to

represent major feasibility considerations given current battery technology.

Emission Impacts: The emission performance of replacement or converted electric GSE is well

understood since removal or retirement of the fossil fueled internal combustion engine (ICE)

results in zero emissions piece of equipment. Nevertheless, overall emission reduction impacts

are dependent on two key factors: (1) the emissions performance of the equipment being replaced

or converted, and (2) the specific power generating characteristics of the region in which the

airport is located.

Most GSE in operation today are unregulated from an emissions standpoint and, therefore, do not

incorporate emission reduction technology in their design. A definitive calculation of displaced

emissions due to conversion or replacement of equipment with electric GSE involves the

measurement of the average emission rate of the displaced fossil fueled GSE and the application

of the activity data associated with the displaced unit. A generalized expression of the necessary

calculation of displaced emissions takes the form:
* The tabulated emission rate data was developed using data from the off-road emissions model recently released by

the California Air Resources Board. Data used for this analysis included zero hour emission rates, emissions

deterioration rates, equipment horsepower, equipment load factor, and brake-specific fuel consumption. See the

information series selection entitled GSE Emission Rates and Activity for additional detail.

GSE Information Series 4 Page 3

unit time

hours of use

x hp x load factor x

brake hp - hr

grams = unit timeemissions

To calculate total displaced emissions, the emission rate per unit time (e.g., pounds per year)

should be aggregated over the total time such emissions would have accrued. For example, if a

unit had an expected additional service period of four years after displacement, then emissions

throughout this four year period would be displaced. Determining the emissions performance of

units which are displaced by attrition as part of a normal replacement cycle is more complex in

that reductions will only accrue in accordance with the emission rate of the newer-design

alternative fossil fueled GSE which could (and presumably would) have been purchased in the

absence of electric GSE purchase. In addition to fossil fueled GSE emission reductions due to

technology evolution, any future emission standards imposed on GSE will reduce the emission

reduction benefits of electric GSE by reducing the emission rate of displaced fossil fueled GSE. 

Under any emissions control scenario, the emission reduction benefits of switching GSE from

fossil fueling to electric power will depend on the specific fossil fuel being displaced since each

has its own emissions characteristics.

Airports served by relatively low emitting power generation facilities will produce larger

emission reduction benefits than will accrue in areas served by older, higher emitting generating

facilities. While HC and CO emission reductions are dramatic (approaching 100 percent)

regardless of local power generation characteristics, NOx, PM, and CO2 reductions can vary

considerably. Tables 1 and 2 demonstrate this sensitivity. Table 1 presents estimated hourly

emission rates for both fossil fueled baggage tractors*

 and the incremental electricity generation

required to power alternative electric baggage tractors. Table 2 presents the resulting emission

reductions associated with replacement of the various fossil fueled baggage tractors under

alternative power generation scenarios. Baggage tractors alone account for just under a quarter

of all GSE and the emission impacts associated with replacement of fossil fueled baggage

tractors are consistent with the impacts associated with replacement of other fossil fueled GSE.

As indicated, the dependence of emission impacts on power generation emissions can be

dramatic. For example, NOx reductions due to the replacement of gasoline baggage tractors can

range from 40 to 99+ percent and the potential range of CO2 reductions is nearly as wide at 40 to

80 percent. PM emissions can, as indicated, increase substantially under the highest emission

power generation conditions, but can also decline by as much as 90 percent under other power

generation conditions. Both the highest and lowest power generation scenarios are equally

unlikely to be applicable to any given electric GSE replacement scenario. The highest emission

scenario represents an uncontrolled coal-fired application while the lowest emission scenario

represents a maximum controls (regardless of cost effectiveness) natural gas application. 

There may be a few specific instances (e.g., California airports) where actual power generation

emissions approach those of the low emissions scenario, but generally emission reductions are

more likely to approximate those of the tabulated average scenario which is based on the

average of measured emission rates for six geographically diverse U.S. utilities. Therefore, 
GSE Information Series 4 Page 4

Emissions from ICE-Powered Baggage Tractors (grams/operating hour)

HC CO NOx PM CO2

Gasoline (4-Stroke) 371.8 18,678.0 226.6 2.2 43,583.3

LPG 185.9 11,673.8 170.0 2.2 36,543.1

Diesel 70.4 222.4 596.7 44.0 30,286.4

Incremental Utility Emissions from Electric Baggage Tractors Assuming

Utility Scenario1 HC CO NOx PM CO2

Best Case 0.5 1.9 4.4 0.2 9,645.2

Worst Case 6.8 10.2 139.4 75.4 25,619.9

Average Case 2.0 6.0 22.2 1.3 19,088.7

Incremental Utility Emissions from Electric Baggage Tractors Assuming

Utility Scenario1 HC CO NOx PM CO2

Best Case 0.4 1.5 3.4 0.2 7,523.2

Worst Case 5.3 7.9 108.7 58.8 19,983.6

Average Case 1.6 4.7 17.3 1.0 14,889.2

1 Utility (i.e., power generating station) emissions will vary in accordance with local

electricity generation practices. Major factors include boiler design, boiler fuel, and

emission controls in place. The best case scenario represents potential emissions if GSE

electrical demand is satisfied by a generating station firing natural gas and employing

maximum controls. Conversely, the worst case scenario represents potential emissions if

GSE electrical demand is satisfied by a generating station firing coal under essentially

uncontrolled (from an emissions standpoint) conditions. The average case represents a

more typical level of utility emissions and is based on actual emission rates for a

geographically diverse sample of utilities.

Table 1. Estimated Emission Rates for Baggage Tractors
GSE Information Series 4 Page 5

Percent Reduction1 from a Gasoline (4-Stroke) Baggage Tractor

Utility Scenario2 HC CO NOx PM CO2

Best Case 99.9 100.0 98.1 88.9 77.9

Worst Case 98.2 99.9 38.5 up 3328.4 41.2

Average Case 99.4 100.0 90.2 42.4 56.2

Percent Reduction1

 from an LPG Baggage Tractor

Utility Scenario2 HC CO NOx PM CO2

Best Case 99.8 100.0 97.4 88.9 73.6

Worst Case 96.3 99.9 18.0 up 3328.4 29.9

Average Case 98.9 99.9 86.9 42.4 47.8

Percent Reduction1

 from a Diesel Baggage Tractor

Utility Scenario2 HC CO NOx PM CO2

Best Case 99.5 99.3 99.4 99.6 75.2

Worst Case 92.4 96.4 81.8 up 33.8 34.0

Average Case 97.7 97.9 97.1 97.8 50.8

1 Unsigned and unqualified values signify emission reductions (in percent). Values

preceded by the qualifier $up# signify emission increases (in percent)

2 Utility (i.e., power generating station) emissions will vary in accordance with local

electricity generation practices. Major factors include boiler design, boiler fuel, and

emission controls in place. The best case scenario represents potential emission

reductions if GSE electrical demand is satisfied by a generating station firing natural gas

and employing maximum controls. Conversely, the worst case scenario represents

potential emission reductions if GSE electrical demand is satisfied by a generating

station firing coal under essentially uncontrolled (from an emissions standpoint)

conditions. The average case represents a more typical level of emission reductions and

is based on actual emission rates for a geographically diverse sample of utilities.

Table 2. Estimated Emission Reductions Due to Baggage Tractor Replacement
GSE Information Series 4 Page 6

replacement of fossil fueled GSE will, as indicated, generate substantial emission reductions at

most U.S. airports.

Costs: Initial purchase costs for electric GSE are high relative to their fossil fueled counterparts. 

The cost premium is almost entirely associated with the required battery pack and recharger. 

Table 3 presents a comparison of electric baggage tractor first costs relative to those of fossil

fueled GSE. As indicated, the cost premium ranges from about $8,000 relative to a diesel

powered tractor to about $13,000 relative to a gasoline-powered tractor. These purchase price

premiums are augmented by periodic battery replacement requirements (at about $4,500 every

5-6 years) that are 2 to 4 times higher on a life cycle basis than corresponding fossil fuel engine

rebuild or replacement costs. However, these cost premiums are counterbalanced by a

substantial reduction in fuel costs. Electric GSE use no fuel during idle periods and such periods

can comprise as much as 50 percent of typical GSE operation. Using an estimated electricity

cost of $0.045 per kilowatt-hour, the overall fuel savings associated with high-use GSE

operations such as baggage tractors can range from $2,500 per year relative to diesel equipment

to over $6,000 per year relative to gasoline and CNG equipment. While lower-use GSE fuel cost

savings will be smaller, it is clear that fuel savings alone can offset the entire electric GSE

purchase price premium in 2 to 3 years. Moreover, electric GSE fuel cost savings will increase

as more efficient electric motors and motor controllers continue to evolve.

In addition to reduced fuel costs, the latest generation of electric GSE have demonstrated

significantly reduced maintenance requirements. Costs have been estimated to be reduced by as

much as two-thirds relative to gasoline and diesel powered GSE. Table 3 presents the results of a

life cycle cost comparison for a baggage tractor under a high-use operating scenario (i.e.,

generally used to service aircraft continuously throughout an operating day such as occurs at high

traffic airports). The tabulated costs represent the net present value of the various expenditures

required over the sixteen year useful life of the tractor. Regardless of whether maintenance costs

are assumed to be reduced, the electric-powered tractor consistently exhibits the lowest life cycle

costs. Life cycle costs for the electric baggage tractor are estimated to be over 40 percent lower

than the next lowest cost diesel option under a reduced maintenance scenario and still 10 percent

lower even if maintenance costs are assumed to be identical to conventional gasoline and diesel

powered GSE maintenance costs.

Cost Effectiveness: It is difficult to provide precise cost effectiveness estimates for electric GSE

because the impact of such equipment varies across the pollutants examined and relative to the

fossil fuel equipment being replaced and the emissions performance of local utilities. However,

it is clear from the data presented in Table 3 that electric GSE represent the lowest cost option

relative to all fossil fuel GSE. Therefore, if an appropriate battery recharging schedule and

infrastructure can be established, all derived emission reductions accrue for free. Assuming local

utility emissions performance is not too different from average U.S. utility emission levels,

electric GSE are cost effective from an economic standpoint alone.
GSE Information Series 4 Page 7

Fuel

Type

Purchase

Cost

Rebuild or

Replacement

Costs

Fuel

Costs

Reduced

Maintenanc

e

Costs

Total Costs

If Reduced

Maintenance

Total Costs

If Same

Maintenance

Gasoline $17,000 $2,568 $59,481 $47,089 $126,139 $126,139

Diesel $22,000 $1,351 $27,386 $47,089 $97,826 $97,826

LPG $19,000 $2,568 $49,072 $37,176 $107,816 $117,729

CNG $21,000 $2,568 $65,058 $37,176 $125,802 $135,715

Electric $30,000 $5,147 $5,574 $15,696 $56,418 $87,810

Assumptions: 16 year equipment life; 6 year engine replacement interval for gasoline, LPG, and CNG; 8 year engine rebuild

interval for diesel; 5 year battery life for electric; $2,500 unit cost for all rebuilds; $4,500 unit cost for all battery replacements,

equipment used 8 hours per day for 350 days per year; idle is 40 percent of operating day; gasoline use is 3.2 gallons per hour at

$0.75 (after tax credits) per gallon; diesel use is 1.7 gallons per hour at $0.65 (after tax credits) per gallon; LNG use is 3.3 gallons

per hour at $0.60 per gallon; CNG use is 3.5 gallons per hour at $0.75 per gallon (including the cost of refueling facility operation

and amortization); electric use is 8.33 kilowatts per operating hour; maintenance costs are $1.90 per hour for gasoline and diesel;

maintenance costs are $1.50 per hour for LPG and CNG under a reduced maintenance scenario or $1.90 per hour under a “same

maintenance” scenario; maintenance costs are $0.63 per hour for electric under a reduced maintenance scenario or $1.90 per hour

under a “same maintenance” scenario.

Table 3. Life Cycle Costs for Baggage Tractors

References:

1. Analysis of Techniques to Reduce Air Emissions at Airports, Draft Final Report, prepared by

Energy and Environmental Analysis, Inc. for the U.S. Environmental Protection Agency,

September 1997.

2. Air Pollution Mitigation Measures for Airports and Associated Activity, prepared by Energy

and Environmental Analysis, Inc. for the California Air Resources Board, May 1994.

3. Documentation of Input Factors for the New Off-Road Mobile Source Emissions Inventory

Model, prepared by Energy and Environmental Analysis, Inc. for the California Air Resources

Board, August 1995.

4. Airport Electrification Project, Consolidated Results and Analysis, EPRI TR-109041, Electric

Power Research Institute, September 1997.

5. Off-Road Electric $Vehicle# Scoping Study, EPRI TR-101906, Electric Power Research

Institute, July 1993.

6. Electric Vehicles will Reduce Emissions at Boston s Logan Airport, Innovators with EPRI

technology, IN-102438, Electric Power Research Institute, June 1994.

7. Off-Road Electric Vehicles: An Existing Market, EPRI TR-103315, Electric Power Research

Institute, February 1994.
GSE Information Series 4 Page 8

8. Advanced Transportation Vehicle Catalog, www.calstart.org/services/catalog/, CALSTART

internet website.

9. Compilation of Air Pollutant Emission Factors, AP-42, Fifth Edition, Volume I: Stationary

Point and Area Sources, Chapter 1 (External Combustion Sources), as downloaded on July

29, 1998 from the U.S. EPA internet website www.epa.gov/ttn/chief/ap42.html.
EMISSIONS AFTERTREATMENT
GSE Information Series 5 Page 1

Basic Control Strategy Summary: The majority of GSE continue to emit pollutants at

essentially uncontrolled rates. Certain emission species such as hydrocarbons (HC), carbon

monoxide (CO), and, in some cases, oxides of nitrogen (NOx) can be substantially reduced by the

installation of a catalytic converter in the equipment exhaust system. Exhaust system traps (or

filters) can perform similar reduction functions for particulate matter (PM). Catalyst technology

has been well proven in on-road vehicle applications and particulate trap technology has

advanced considerably over the last several years. Both systems are commercially available for

equipment in the off-road sector, but long term reliability and effectiveness have not been

proven. Systems designed for application on gasoline, diesel, compressed natural gas (CNG),

and liquefied petroleum gas (LPG) engines are available, but important issues for consideration

in determining the practical application of these technologies include: installation costs, costs

associated with the use of catalyst-compatible (i.e., non-poisoning) fuel blends and lubricating

oils, exhaust system space constraints, increased exhaust and exhaust system temperatures, the

compatibility of GSE operating cycles with effective conversion temperature requirements, and

impacts on equipment reliability.

Potential GSE Equipment Affected: Generally, there are no theoretical limitations to the size

or type of GSE that can be equipped with a catalytic converter or particulate trap. Effective

converters have been designed for applications as small as lawn mower engines. In terms of

practicality, however, both catalytic converters and particulate traps require sufficient exhaust

system temperatures for high conversion efficiencies and, in the case of traps, effective

regeneration. Therefore, equipment with very low load factors may experience extended periods

of comparatively low exhaust temperatures, inhibiting both and catalyst and trap performance. 

Additionally, equipment that is operated only for brief periods between engine startup and

shutdown may accumulate a significant fraction of its operating time in conditions where the

catalyst has not yet attained the “light-off” temperature required for significant emissions

conversion. However, the lack of actual test data for either catalyst or particulate trap

performance on GSE (or other similarly sized and utilized equipment), renders it difficult to

accurately assess the potential impacts of GSE operating cycles on exhaust aftertreatment system

performance. The effectiveness of both certainly increases with operating temperature (up to the

point where thermal degradation of catalyst materials occurs). Given the current data deficiency,

the installation of catalyst or trap technology on GSE should be treated as developmental at this

time and instances of system installation should be closely monitored until demonstrated

performance data has been collected.

Oxidation Catalysts: Oxidation catalysts function by promoting the reaction of exhaust HC and

CO with oxygen to form water and carbon dioxide. The required oxygen can be obtained by

tuning the engine air/fuel mixture to the lean side of stoichiometry (i.e., the combustion air/fuel

mixture is set so that there is excess air relative to that required for complete fuel combustion) or

by introducing air into the exhaust system upstream of the catalyst. The latter approach,

commonly referred to as secondary air injection, was widely utilized in the on-road vehicle sector

during the 1970’s when oxidation catalyst technology was common. Although universally

EMISSIONS AFTERTREATMENT
GSE Information Series 5 Page 2

displaced by three-way catalysts in the on-road sector, oxidation catalyst technology is well

developed and has been shown to be effective, providing appropriate operational procedures are

observed.

In the typical oxidation catalyst, the water and carbon dioxide formation reactions are usually

promoted using the noble metal platinum. Platinum effectively lowers the temperature at which

the desired reactions take place. However, platinum (as well as other catalyst metals) is easily

poisoned if proper precautions are not observed. Fuels, lubricating oils, and additives containing

lead or phosphorus compounds should be avoided. Existing regulations should serve to ensure

appropriate fuel composition, but phosphorus-containing lubricating oils are still available and

should be replaced before any catalyst is installed.

Proper engine maintenance is also important in promoting both high catalytic conversion

efficiencies and long catalyst life. Engines tuned to run overly rich, or cylinder misfire, can

result in temperatures inside the catalytic converter shell (due to the temperature increases

associated with excessive conversion reactions) which exceed the durability tolerances of both

the catalyst and the substrate onto which it is coated. Prolonged exposure to excessive

temperatures can cause the catalyst and substrate to sinter, increasing exhaust system

backpressure, in extreme cases, to the point of engine failure. Proper engine maintenance is a

critical aspect of any emission reduction strategy designed around the catalytic conversion of

engine exhaust.

As already mentioned, optimum oxidation catalyst conversion efficiency is obtained when there

is a high concentration of oxygen in the exhaust entering the catalyst. Maximum engine power,

however, is obtained with combustion mixtures that are slightly rich (i.e., that have slightly less

air than required for complete combustion) and many off-road engines are set to run in a rich

mode to obtain maximum performance. For engines that are overpowered relative to operational

demands, enleanment of the air/fuel mixture may be an acceptable means of providing required

catalyst air, but enleanment will not be a viable approach for engines requiring full power to

fulfill operational demands. For such engines, secondary air injection will be required to achieve

high catalyst conversion rates.

Space is at a premium with all types of off-road equipment, including GSE. Equipment

configuration and sizing are usually designed around a specific engine and exhaust configuration

and, therefore, there is generally not sufficient “free” space to easily accommodate modifications. 

As a result, installing a catalytic converter even without an accompanying secondary air injection

system can be problematic. The requirement for secondary air simply compounds this difficulty. 

Aftermarket catalyst manufacturers have responded to these space restrictions by combining

muffler and catalyst functions into a single package that can be installed in place of the standard

exhaust system muffler. Moreover, manufacturers have designed effective air breather valves

and venturi nozzles into the upstream end of the catalyst/muffler shell to provide adequate

secondary air so that compact, fully complete oxidation catalyst systems are available for most

applications. The ability of these passive secondary air systems to provide adequate air for

effective conversion on larger engines (six and eight cylinder) with lower amplitude exhaust

pressure characteristics is potentially an issue, but no definitive conclusions can be reached as

performance test data for such applications is not currently available.
GSE Information Series 5 Page 3

Exhaust system temperatures can also be an issue of concern when considering the installation of

a catalytic converter. Catalysts produce substantial increases in exhaust system skin temperatures

and must be installed such that these increases are not a potential hazard, either through human

contact or proximity to combustible materials. Most catalyst installations require heat shielding

to ensure adequate safety, and space for such shielding can be an installation concern. Moreover,

to avoid the potential for human contact with hot catalyst surfaces, an under-body or

under-chassis installation location is generally preferred. Many currently marketed off-road

catalysts are designed to replace conventional mufflers which are already isolated for the same

heat-related concerns and, therefore, available installation locations may be acceptable (although

heat shielding may still be required given the higher skin temperatures of catalysts relative to the

mufflers they replace).

Oxidation catalysts can be purchased for gasoline, LPG, or CNG powered equipment. Specially

constructed versions can also be purchased for diesel engine application. Diesel converters are

generally larger than the mufflers they are designed to replace and, therefore, must be constructed

in recognition of the specific space allowances of the targeted equipment. HC and CO emissions

from diesel engines are generally quite low even without a catalyst. The primary diesel catalyst

design criteria is the reduction of the soluble organic portion of diesel PM. In some cases,

soluble organics can comprise up to 40 percent or more of total diesel PM.

Three-Way Catalysts: Three-way catalysts designed to promote the simultaneous oxidation of

HC and CO and reduction of NOx are also available for application in the off-road engine sector. 

These catalysts effectively promote reactions which add oxygen to HC and CO to form water and

carbon dioxide and remove oxygen from NOx to form molecular nitrogen and oxygen. Issues

relating to such installations are much the same as those for oxidation catalysts, with the added

issue of a required feedback system to control engine air/fuel ratio. Three-way catalysts only

function effectively over a narrow range of air/fuel ratios (around stoichiometry), and it is critical

that an effective air/fuel mixture control system be installed in conjunction with the catalyst if

high conversion efficiencies are to be obtained. Such systems, which are commonly known as

closed loop systems and are standard equipment on all light-duty on-road vehicles, function by

monitoring the amount of oxygen in the exhaust system and adjusting the engine air/fuel mixture

to maintain stoichiometry. Off-road engines and equipment generally do not incorporate closed

loop feedback and, therefore, effective three-way converter installations must include the

installation of such a system. Open loop systems (i.e., systems which do not monitor and adjust

engine air/fuel mixtures) are available, but only effective in situations where very frequent engine

checks and adjustments are the norm. Advanced aftermarket three-way catalyst systems include

all appropriate closed loop hardware (i.e., the exhaust oxygen sensor, electronic control module,

and throttle actuator) and detailed installation instructions.

Particulate Traps: The development of traps (or filters) designed to capture and burn the solid

particulate commonly emitted by diesel engines has been advancing since the late-1980’s, when

it appeared as if such devices might be required for on-road heavy duty vehicles to meet stringent

particulate standards. Although trap technology has never been required to meet PM standards in

the on-road sector (even after the imposition of two rounds of more stringent standards beyond

those originally thought to require traps for compliance - providing testament to the ingenuity of

diesel engine manufacturers), major technology advancements have continued to occur and

effective retrofit traps are available in the aftermarket. Early trap designs required an active
GSE Information Series 5 Page 4

element to combust collected particulate and regenerate trap capacity, but current designs

function by incorporating a catalyst that effectively lowers particulate combustion temperatures

into the filter material, thereby promoting filter regeneration during high load operating periods. 

As long as equipment duty cycles include periodic high load events, current generation

particulate traps can effectively reduce diesel PM emissions without need for an active filter

regeneration technology.

The issues surrounding the use of diesel PM traps on GSE are very similar to those associated

with the installation and use of catalytic converters as described above. Space limitations are

probably the most critical aspect of effective trap design because trap dimensions tend to be

substantially larger than those of gaseous catalysts, but operating cycle considerations can be as

important for some potential applications. The backpressure associated with particulate trapping

increases steadily as more particulate is collected. If equipment operating cycles do not include

sufficient periods of high load activity (promoting regeneration), this backpressure increase can

influence equipment performance. Ideal GSE applications are those with relatively high load

factors such that trap regeneration occurs routinely and backpressure is maintained at a low level. 

Particulate collection and combustion efficiencies of over 90 percent are attainable in properly

designed traps applications.

Emission Impacts: No operational data is available from GSE catalyst or trap installations to

assess the actual in-use emissions performance of these devices in the aircraft support

environment. Available data is limited to catalyst and trap manufacturer test data on similar

equipment such as industrial forklifts, and although there is no reason to expect that this data is

flawed, it undoubtedly does not represent data collected under average in-use operating

conditions. Moreover, these test data reflect conversion efficiencies immediately after converter

or trap installation and do not represent measurements that might be expected after catalyst or

trap aging has occurred. Nevertheless, there is sufficient evidence from the on-road sector to

indicate that converters can maintain very high efficiencies given adherence to proper engine

maintenance routines and vehicle operating procedures. If such procedures are followed, it is not

unreasonable to expect emission reduction impacts similar to those presented in Table 1.

Oxidation catalyst HC reduction potential declines for both LPG and CNG engines due to the

increasing concentrations of short chain hydrocarbons in the exhaust of such engines. Current

catalyst technology is most effective on long chain hydrocarbons and a substantial fraction of

compounds such as methane and ethane can pass through a catalyst without undergoing further

oxidation. The substantial increase in CO2 emissions results from an assumed backpressure

increase as well as the oxidation of HC and CO, both of which are observed in high

concentrations in uncontrolled GSE engines. However, increased backpressure accounts for only

about two percentage points of the estimated increase, with CO conversion accounting for nearly

all of the remainder.

Until such time as catalyst and trap performance has been consistently demonstrated in GSE

applications and sufficient test data on in-use engines is available to confirm generalized

emission reduction impacts, exhaust aftertreatment device installation and emission credits

resulting therefrom should be treated on a case-by-case basis. Individuals applying for such

credits should 
GSE Information Series 5 Page 5

Oxidation Catalyst Installation

GSE Fuel HC CO NOx PM CO2

Gasoline 90 90 0 10 up 60

Diesel 50 90 0 30 up 3

LPG 70 90 0 10 up 45

CNG 50 90 0 10 up 45

Three-Way Catalyst Installation

GSE Fuel HC CO NOx PM CO2

Gasoline 90 90 80 10 up 60

LPG 70 90 80 10 up 45

CNG 50 90 80 10 up 45

Particulate Trap Installation

GSE Fuel HC CO NOx PM CO2

Diesel 20 0 0 90 up 2

Unsigned and unqualified values signify emission reductions (in percent). Values preceded by the qualifier

“up” signify emission increases (in percent). CO2 emission increases result from increased exhaust system

backpressure and the oxidation of HC and CO, with CO oxidation accounting for the bulk of the estimated

increase.

Table 1. Potential Emission Reduction due to Aftertreatment Device Installation (percent)

be required to perform standardized emissions testing on a representative sample of target GSE

engines, both immediately before and after aftertreatment device installation as well as

periodically thereafter. Ideally, additional concurrent emissions testing would be performed on a

sample of otherwise identical control equipment which were not targeted for aftertreatment

device installation. To defray the costs of such database development testing and encourage

early aftertreatment device trials, a limited number of credits based on emission reduction

expectations might be offered to airlines undertaking the necessary demonstration efforts to

ensure that their pioneering work is recognized. In the interim, more detailed estimates of

exhaust aftertreatment-driven emission reductions are speculative at best.

Aftertreatment Device Costs: The current aftertreatment device market for off-road engines

such as those used in GSE is based on tailored device design and construction. Each potential

application is subjected to equipment-specific design review and evaluation to determine

potential device sizing restrictions and operating constraints. Only after such review is catalyst

or trap fabrication undertaken. As a result, costs for initial equipment installations can be quite

high. Even after initial design and construction, costs remain high relative to on-road aftermarket
GSE Information Series 5 Page 6

catalysts for two primary reasons. First, production volumes are limited due to the unique nature

of many off-road applications and limited catalyst demand (current demand results primarily

from Occupational Safety and Health Administration indoor air quality standards). Second,

catalyst loading (i.e., the amount of catalytic material present in the aftertreatment device) is

considerably higher than the typical loading for on-road catalysts. Limited catalyst size demands

increased catalyst loading for high conversion efficiency. For these reasons, off-road catalysts of

the type required in the GSE sector can cost on the order of $1,000 or more per unit (after initial

design and fabrication costs).

Cost Effectiveness: Given the current state of the off-road aftertreatment device market, it is not

possible to provide emission reduction cost effectiveness estimates. Such estimates will vary in

accordance with the pre-installation emission rates of the target equipment, the conversion

efficiency of the aftertreatment device, and the pollutants examined. Detailed cost effectiveness

calculations should be performed in conjunction with any emissions testing program

implemented to evaluate aftertreatment device potential.

References:

1. Analysis of Techniques to Reduce Air Emissions at Airports, Draft Final Report, prepared by

Energy and Environmental Analysis, Inc. for the U.S. Environmental Protection Agency,

September 1997.

2. Air Pollution Mitigation Measures for Airports and Associated Activity, prepared by Energy

and Environmental Analysis, Inc. for the California Air Resources Board, May 1994.

3. Documentation of Input Factors for the New Off-Road Mobile Source Emissions Inventory

Model, prepared by Energy and Environmental Analysis, Inc. for the California Air Resources

Board, August 1995

4. Emission Control Products: Diesel Exhaust Purifiers, Catalytic Mufflers, Catalytic Diesel

Filters, www.nett.ca, NETT Technologies internet website.

5. Clean Cat: Diesel Engine Catalytic Converters, clean-cat.com, Applied Diesel Technology,

Inc. internet website.

6. Personal communication on off-road catalyst design constraints and cost with NETT

Technologies, Mississauga, Ontario, Canada, July 1998.
FIXED GATE SUPPORT
* Technically, electrically powered GSE do not emit any pollutants. The term emit, as used here, ascribes to the

electric GSE, the offsite increase in power generating station emissions due to increased airport electrical power

demands to support electric GSE recharging. In essence, the electrically powered GSE are treated as if they

“emitted” the increased power generating station emissions associated with their use.

** During the period that aircraft main engines are shut down at the gate, aircraft still require power and often

conditioned air to ensure that control system operations as well as passenger and crew comfort and safety are

maintained.

GSE Information Series 6 Page 1

Basic Control Strategy Summary: While the majority of conventionally powered GSE can be

either converted to or replaced by GSE powered by alternative fuels such as liquid petroleum gas

(LPG), compressed natural gas (CNG), or electricity, a significant fraction of GSE can be

eliminated entirely by incorporating fixed point-of-use support equipment into aircraft gate

design. Such design not only eliminates all energy demands associated with moving displaced

mobile GSE between aircraft gates and maintenance/storage facilitates, but also facilitates the use

of “hard-wired” electrical power connections thereby eliminating the need for a recharging

infrastructure and scheduling plan. Although as with electrically powered GSE, there is an

increase in offsite power generating station emissions due to the increased demand for electrical

power, fixed equipment is likely to consume less power than equivalent mobile GSE due the

elimination of the motive aspect of GSE operation. Even when the increased emissions from

power generating stations are considered, fixed electrically powered support equipment usually

emit*

 significantly less hydrocarbon (HC), carbon monoxide (CO), oxides of nitrogen (NOx),

particulate matter (PM), and carbon dioxide (CO2) emissions than their mobile fossil fueled (i.e.,

gasoline, diesel, compressed natural gas (CNG), and liquefied petroleum gas (LPG))

counterparts. Important issues for consideration are essentially limited to the capital and

operating cost and reliability of fixed gate-based equipment.

Potential GSE Equipment Affected: Generally, the only technical limitations to the

replacement of mobile GSE with gate-based fixed support equipment are manifested in those

services which, by definition, require the manual movement of objects from one place to another. 

Cargo and baggage loading operations and, in some cases, passenger and service personnel

transport are among the few operations which cannot be easily reduced or eliminated through the

use of fixed gate-based equipment. The use of fixed gate-based power and conditioned air

services has become quite common both in the U.S. and worldwide. Gate-based electrical

connections eliminate not only the emissions associated with ground power units (GPU), but the

majority of aircraft-based auxiliary power unit (APU) emissions** as well. Similarly, gate-based

conditioned air support eliminates the use of equivalent traditional GSE. Other GSE that can be

either eliminated or curtailed through the use of fixed gate-based equipment include: lavatory,

fuel, water, food, and air start service equipment. In the most advanced cases, even mobile

baggage tugs and belt loaders can be eliminated through the installation of a centralized conveyer

belt-driven baggage distribution and delivery system that moves baggage directly from check-in

counters to the gate. However, these systems are expensive and usually cannot be retrofitted to

FIXED GATE SUPPORT
GSE Information Series 6 Page 2

existing terminal designs. Application of fixed baggage systems is really limited to new terminal

construction and is not usually a viable emission reduction option for existing airport operations.

Fixed Equipment Installation Issues: The installation of fixed airport service equipment at

new gates as well as retrofitting such equipment at existing gates is becoming more

commonplace. In the U.S., most such installations are limited to an electrical supply (which

either displaces a mobile GPU or curtails the use of an onboard aircraft APU) and possibly a

conditioned air supply for aircraft use while at the gate (which also curtails the use of an onboard

aircraft APU and displaces ground-based conditioned air GSE used in instances of

non-functional aircraft APU). However, fixed gate fuel, lavatory, food, water, and air start

services are also feasible.

Two general types of systems have been designed (and implemented) to provide fixed gate

aircraft power services. Central power systems are based on a large main electrical control center

within the airport facility that feeds individual aircraft gates through a distributed network. 

Conversely, point-of-use systems incorporate essentially independent controllers at each gate,

each connected independently to the airport electrical power system. The point-of-use systems,

due to easier, less intrusive, and less costly installation, as well as a reduced demand for

preventive maintenance (in the centralized system, all gates are dependent on the functionality of

the central power unit) have become the dominant choice for fixed gate-based power. Similar

approaches have been employed for gate-based conditioned air services. Large centrally located

chillers and control units with complex piping distribution systems have been operated, but have

generally fallen into disfavor relative to independently powered and operated point-of-use

systems.

In most gate-based installations, both aircraft power and conditioned air services are offered. 

However, in some cases only power services are available and, as a result, situations can arise

when aircraft APU must be operated regardless of gate power availability. If conditioned air is

demanded by weather conditions, the aircraft APU (or an on-ground, usually diesel fueled, GSE

equivalent) must be operated either in conjunction with or in place of the gate-based power

supply. Clearly, such a situation is not desirable and most airports opt to install both systems as a

package. Airports in colder climates sometimes provide both services at only a fraction of

available gates, based on the premise that conditioned air demands will be limited. However, it

is not clear whether aircraft gate routing is modified in accordance with conditioned air demands

or whether aircraft at non-equipped gates simply rely on APU usage during demanding weather

conditions.

The majority of point-of-use fixed power and air systems are installed on the underside of gate

passenger ramps and powered through electrical system cables designed to operate reliably in

conjunction with the telescoping movement of the ramp. Other systems have been installed as

“pop-up” units beneath surface of the gate apron. Either approach provides aircraft service

personnel easy access to both aircraft and service connections and allows for quick service

connection (and APU shutdown) upon aircraft arrival at the gate. APU use cannot be eliminated

completely as APU operation is required during pre-flight checks and main engine startup, but

use can be restricted to this period and APU usage for narrow body aircraft can be reduced from

40-45 minutes without fixed gate services to 5-7 minutes with fixed gate services. Wide body
GSE Information Series 6 Page 3

emissions

unit time

grams

brake hp - hr hp load factor 

hours of use

unit time

= ×× ×

aircraft APU usage reductions can be even more dramatic, with APU use declining from 60-90

minutes or more without fixed gate service to 5-7 minutes with gate service.

Fixed-gate fuel, water, and lavatory services are somewhat more complex in that each require the

availability of in-ground distribution systems or gate-based supply and storage tanks. Aircraft

refueling is already generally accomplished using an in-ground hydrant system in conjunction

with truck-mounted “hydrant-to-aircraft” equipment. This traditionally truck-mounted interface

equipment can be replaced with equivalent fixed gate equipment, with equipment storage,

reliability, and in-ground hydrant proximity being the only major technical concerns. Similar

concerns, along with the added issue of water supply connectivity, exist for gate-based potable

water service. Gate-based waste disposal concerns must be addressed in any installation of

lavatory service equipment at the gate as traditional mobile GSE will continue to be required for

any system based on simply installing gate-based transfer tanks.

Gate-based food and cabin supply service facilities can be relatively easily installed as add-on

storage rooms at existing gates. While these storage facilities will not eliminate mobile GSE

service entirely since supplies must be restocked and waste removed periodically, but gate

storage will allow for a substantial reduction in mobile GSE use as multiple aircraft can be

serviced through single re-supply and removal activities which can take place during periods of

low aircraft activity.

Emission Impacts: The emission impacts due to the installation and operation of fixed gate

services depends on three key factors: (1) the emissions performance of the equipment being

displaced or curtailed, (2) for curtailed equipment, the reduction in equipment usage, and (3) the

specific power generating characteristics of the region in which the airport is located.

Most GSE in operation today are unregulated from an emissions standpoint and, therefore, do not

incorporate emission reduction technology in their design. A definitive calculation of displaced

emissions due to equipment displacement or curtailment involves the measurement of the

average emission rate of the displaced (or curtailed) fossil fueled GSE and the application of the

activity data associated with the unit. A generalized expression of the necessary calculation of

displaced emissions takes the form:

To calculate total displaced emissions, the emission rate per unit time (e.g., pounds per year)

should be aggregated over the total time such emissions would have accrued. For example, if a

displaced unit had an expected additional service period of four years after displacement, then

emissions throughout this four year period would be displaced. Determining the overall

longevity of emission reductions is an issue to be addressed since the emissions performance of

displaced units may have changed during normal mobile equipment replacement cycles which

are discontinued once fixed equipment is in place. In addition to emission reduction influences

due to technology evolution, any future emission standards imposed on GSE will reduce the

emission reduction benefits of fixed gate services by reducing the emission rate of displaced

mobile GSE. Under any emissions control scenario, the emissions reduction benefits of
* The tabulated emission rate data was developed using data from the off-road emissions model recently released by

the California Air Resources Board. Data used for this analysis included zero hour emission rates, emissions

deterioration rates, equipment horsepower, equipment load factor, and brake-specific fuel consumption. See the

information series selection entitled GSE Emission Rates and Activity for additional detail.

GSE Information Series 6 Page 4

switching from mobile GSE to fixed gate-based (electrically powered) services will depend on

the specific fossil fuel being displaced since each has its own emissions characteristics.

Without definitive knowledge of exactly which mobile GSE are being displaced or curtailed for a

given application, it is difficult to estimate the overall emission reduction potential of fixed gate

services. Moreover, experience has shown that even when fixed gate services such as aircraft

power and conditioned air are available, they are not always used. Whether such non-use is a

result of airline practice (a recent EPA study found some airlines employing APU in place of

fixed gate services about 50 percent of the time) or other factors is unclear, but the overall

emissions reduction due to the installation of fixed equipment is certainly affected. Table 1

shows the estimated hourly-specific emission rates* for the various types of GSE that can be

displaced or curtailed by the installation of fixed gate-based equipment. Operators can estimate

the impacts at their specific facilities by selecting those specific equipment affected and applying

the number of hours of curtailed use.

Not shown in Table 1 are the emission rates for aircraft APU which can also be displaced

through the installation of fixed gate services. In fact, some of the largest emission reductions

due to fixed gate services will accrue from curtailed APU usage. APU emission rates vary

considerably across aircraft, averaging about 93 grams of HC, 1,055 grams of CO, and 542 grams

of NOx per operating hour but reaching as high as 360 grams per operating hour HC, 1,978 grams

per operating hour CO, and 542 grams per operating hour NOx. As indicated above, APU

operating time can be reduced by 85-95 percent, or 35-40 minutes per narrow body gate service

and 55-85 minutes per wide body gate service. Once these parameters have been established for

a particular airport application, total APU emission reductions can be calculated as:

emissions

unit tim e

g rams

o p eratin g minute

o p eratin g minutes reduced

aircraft service event

aircraft service events

unit tim e

=× ×

Since fixed gate equipment uses electricity to power required pumps, compressors, etc.,

incremental power generation emissions must be subtracted from the emission reductions due to

GSE displacement to derive net emission reductions. Therefore, airports served by relatively low

emitting power generation facilities will produce larger emission reduction benefits than will

accrue in areas served by older, higher emitting generating facilities. In the absence of definitive

power consumption estimates for fixed gate-based equipment, the emissions associated with

electrically powered GSE can be used to estimate net emission reductions. While fixed electrical

equipment will likely consume less electricity per service event than mobile electric GSE due to

the elimination of motive power demands, there are likely to be offsetting consumption losses

during non-service periods. Therefore, assuming power generation emissions equivalent to those 
GSE Information Series 6 Page 5

Gasoline (4-Stroke) Equipment

Type of GSE HC CO NOx PM CO2

Ground Power Units 533 34,560 482 3 89,148

Conditioned Air Units 462 29,952 417 3 77,261

Lavatory Carts 80 2,465 25 2 7,780

Lavatory Service Trucks 154 9,984 139 1 25,754

Fuel Service Trucks 154 9,984 139 1 25,754

Water Service Trucks 142 9,216 128 1 23,773

Food Service Trucks 171 11,059 154 1 28,527

Air Start Units 555 35,942 501 4 92,714

Gasoline (2-Stroke) Equipment

Type of GSE HC CO NOx PM CO2

Lavatory Carts 1,254 1,866 5 39 11,238

LPG Equipment

Type of GSE HC CO NOx PM CO2

Ground Power Units 267 21,600 361 2 74,747

Conditioned Air Units 231 18,720 313 2 64,781

Lavatory Carts 43 1,505 13 1 6,523

Lavatory Service Trucks 77 6,240 104 1 21,594

Fuel Service Trucks 77 6,240 104 1 21,594

Water Service Trucks 71 5,760 96 1 19,933

Food Service Trucks 85 6,912 116 1 23,919

Air Start Units 277 22,464 376 2 77,737

Diesel Equipment

Type of GSE HC CO NOx PM CO2

Ground Power Units 109 493 1,280 73 68,941

Conditioned Air Units 231 1,038 2,735 159 136,153

Lavatory Carts 9 30 60 6 5,619

Lavatory Service Trucks 33 147 383 22 20,603

Fuel Service Trucks 45 204 530 30 28,527

Water Service Trucks 30 136 353 20 19,018

Food Service Trucks 34 154 400 23 21,554

Air Start Units 553 2,491 6,564 382 326,766

Table 1. Emission Rates for Potentially Displaced GSE (grams/operating hour)
GSE Information Series 6 Page 6

associated with mobile electric GSE (see the information series selection entitled Electric GSE

for more information) should provide a reasonably accurate estimate of incremental utility

emissions due to the installation and operation of fixed equipment. These estimates will, of

course, need to be replaced with estimates derived from actual fixed gate-based equipment power

consumption data once fixed unit models and operating data have been established.

Regardless of the incremental power generation emissions, HC and CO emission reductions will

be dramatic (approaching 100 percent). However, NOx, PM, and CO2 reductions can vary

considerably. Table 2 presents the sensitivity of electrical equipment emission reductions to the

range of power generating facilities in operation in the U.S. As indicated, the dependence of

emission reductions on power generation practices can be dramatic. For example, NOx

reductions due to the displacement of gasoline GSE can range from 40 to 99+ percent and the

potential range of CO2 reductions is nearly as wide at 40 to 80 percent. PM emissions can, as

indicated, increase substantially under the highest emission power generation conditions, but can

also decline by as much as 90 percent under other power generation conditions. Both the highest

and lowest power generation scenarios are equally unlikely to be applicable to any given GSE

displacement scenario. The highest emission scenario represents an uncontrolled coal-fired

application while the lowest emission scenario represents a “maximum controls” (regardless of

cost effectiveness) natural gas application. There may be a few specific instances (e.g.,

California airports) where actual power generation emissions approach those of the low

emissions scenario, but generally emission reductions are more likely to approximate those of the

tabulated “average” scenario which is based on the average of measured emission rates for six

geographically diverse U.S. utilities. Therefore, installation of fixed gate-based equipment

should generate substantial emission reductions at most U.S. airports.

Costs: Several previous studies of actual fixed gate-based power and conditioned air

installations have demonstrated the economic cost effectiveness of such systems. In general,

these systems, which carry installation costs ranging from about $50,000 for narrow body aircraft

gate service to about $130,000 for wide body aircraft gate service, have been shown to

completely pay for themselves in less than one year through savings in APU fuel and

maintenance costs. Net savings are estimated to be about $200 per day for narrow body aircraft

gate service and $400 per day for wide body aircraft service. Expanding fixed gate-based

services to include water, lavatory, fuel, and catering facilities has been estimated to add another

$20,000-$50,000 to installation costs (per gate based on a complete 12 gate terminal installation). 

Based on the operating and maintenance cost savings of GSE displacement alone, these services

are not as economically cost effective as gate-based power and air systems. However, additional

cost savings may accrue from reduced labor requirements for aircraft service, faster aircraft

cycling time allowing more aircraft to be serviced per gate, reduced gate congestion, and less

frequent GSE accidents.

Cost Effectiveness: It is difficult to provide precise cost effectiveness estimates for fixed

gate-based equipment because the impact of such equipment varies in accordance with the

pollutants examined, the scope of fixed services offered, the fueling characteristics of the

equipment being displaced, and the emissions performance of local utilities. However, as

discussed above, gate-based power and conditioned air systems have been demonstrated to be

costs effective on a purely economic basis and, therefore, all derived emission reductions due to
GSE Information Series 6 Page 7

the installation of these services accrue for free. The installation of additional services may be

cost effective, but a definitive determination will require airport-specific analysis.

Percent Reduction1

 from Gasoline (4-Stroke) GSE

Utility Scenario2 HC CO NOx PM CO2

Best Case 99.9 100.0 98.1 88.9 77.9

Worst Case 98.2 99.9 38.5 up 3328.4 41.2

Average Case 99.4 100.0 90.2 42.4 56.2

Percent Reduction1 from LPG GSE

Utility Scenario2 HC CO NOx PM CO2

Best Case 99.8 100.0 97.4 88.9 73.6

Worst Case 96.3 99.9 18.0 up 3328.4 29.9

Average Case 98.9 99.9 86.9 42.4 47.8

Percent Reduction1 from Diesel GSE

Utility Scenario2 HC CO NOx PM CO2

Best Case 99.5 99.3 99.4 99.6 75.2

Worst Case 92.4 96.4 81.8 up 33.8 34.0

Average Case 97.7 97.9 97.1 97.8 50.8

1 Unsigned and unqualified values signify emission reductions (in percent). Values

preceded by the qualifier “up” signify emission increases (in percent)

2 Utility (i.e., power generating station) emissions will vary in accordance with local

electricity generation practices. Major factors include boiler design, boiler fuel, and

emission controls in place. The best case scenario represents potential emission

reductions if GSE electrical demand is satisfied by a generating station firing natural gas

and employing maximum controls. Conversely, the worst case scenario represents

potential emission reductions if GSE electrical demand is satisfied by a generating station

firing coal under essentially uncontrolled (from an emissions standpoint) conditions. The

average case represents a more typical level of emission reductions and is based on actual

emission rates for a geographically diverse sample of utilities.

Table 2. Estimated Emission Reductions Due to the Use of Electric GSE
GSE Information Series 6 Page 8

References:

1. Analysis of Techniques to Reduce Air Emissions at Airports, Draft Final Report, prepared by

Energy and Environmental Analysis, Inc. for the U.S. Environmental Protection Agency,

September 1997.

2. Air Pollution Mitigation Measures for Airports and Associated Activity, prepared by Energy

and Environmental Analysis, Inc. for the California Air Resources Board, May 1994.

3. Documentation of Input Factors for the New Off-Road Mobile Source Emissions Inventory

Model, prepared by Energy and Environmental Analysis, Inc. for the California Air Resources

Board, August 1995.

4. “All the Power You Need,” product literature from Hobart Ground Power, 1996.

5. “JETLINK,” product literature from FMC/Jetway Systems.

6. “The Vehicle Free Ramp,” product literature from FMT.

7. Compilation of Air Pollutant Emission Factors, AP-42, Fifth Edition, Volume I: Stationary

Point and Area Sources, Chapter 1 (External Combustion Sources), as downloaded on July

29, 1998 from the U.S. EPA internet website www.epa.gov/ttn/chief/ap42.html.
GSE CONTROL STRATEGY SUMMARY
GSE Control Strategy Summary Page 1

GSE CONTROL STRATEGY SUMMARY

Background: Aircraft ground support equipment (GSE) represent one of three groups of

mobile emission sources at airports. Together with aircraft and ground access vehicles, GSE

contribute a small but significant share of the hydrocarbon (HC), carbon monoxide (CO), oxides

of nitrogen (NOx), and particulate matter (PM) emitted in metropolitan areas. Today, total

emissions from these three source categories comprise on the order of 2-3 percent of total

manmade emissions in a typical metropolitan area, but this share is expected to increase as air

travel continues to grow while emissions from other, non-airport sources are subject to

increasingly stringent controls. An evaluation of GSE and associated service demands indicates

that there are several control strategies that offer the potential to reduce emissions over both the

long and short terms. Such strategies include both demonstrated and innovative technologies

and all have associated issues which must be considered prior to implementation, but a

generalized classification includes:

• The development of new engine emission standards for all currently unregulated

equipment;

• The replacement or conversion of gasoline or diesel powered GSE to liquid petroleum

gas (LPG) or compressed natural gas (CNG) fueling;

• The replacement or conversion of gasoline, diesel, LPG, or CNG powered GSE to

electric power;

• The replacement of mobile GSE with electrically powered fixed gate-based

equipment;

• The retrofit of existing GSE with catalytic converters or particulate traps; and

• The preferential replacement of existing two-stroke gasoline engines.

As expected, both the feasibility and cost effectiveness of emission reductions vary considerably

across the potential control strategies. The following sections provide an overview of the major

issues associated with each strategy and, where possible, provide an estimate of associated

emissions reduction and cost effectiveness. Because all strategies will involve some application

and site specific dependencies, each ultimately needs to be evaluated in the context of the

specific conditions in place at the target airport(s). Moreover, to the extent that airlines or

airports are encouraged to undertake strategies on a voluntary basis in return for marketable

emission credits or alternative incentive rewards, the cost effectiveness of individual controls can

be greatly dependent on the (as yet undefined) value of the credit or incentive reward. 

Nevertheless, a general ranking of the likelihood of implementation is possible, and the

discussions below are presented in general order of increasing to decreasing likelihood.
GSE Control Strategy Summary Page 2

New Engine Emission Standards: The development of new engine emission standards is not a

control strategy that can feasibly be implemented on an airport-specific basis. Its inclusion here

is simply intended to place the remaining control strategies, which can be implemented locally,

in a proper context. Only the U.S. Environmental Protection Agency (EPA) and the California

Air Resources Board (CARB) have the authority to set new engine emission standards and both

have established standards, or are currently in the process of doing so, for several categories of

off-road engines used in GSE. Spark-ignition engines rated at 25 horsepower or less and

produced for California sale beginning in 1995 or national sale beginning in 1997 are subject to

CARB and EPA standards respectively. Although such engines are not found in a large fraction

of GSE, they do exist in small numbers in such applications as lavatory cart pumps. Of greater

impact on GSE are standards for off-road compression-ignition (i.e., diesel) engines. First round

standards adopted by EPA and CARB took effect in 1996 and 1995 respectively and a second set

of more stringent standards is slated to be phased in beginning in 1999. Ultimately, these

standards will affect all diesel GSE engines (currently about one-third of the total GSE engine

population), reducing emissions of HC, CO, and especially NOx and PM by substantial margins.

Currently, no emission standards have been adopted for off-road spark-ignition (e.g., gasoline)

engines rated at greater than 25 horsepower. About 60 percent of all GSE engines fall into this

category. Many such engines are derived from 1980’s-era automobile engines and, in

recognition, CARB is currently in the early development stages of regulatory standards for this

engine category based on a level-of-control equivalent to that associated with the installation of

closed-loop three-way catalyst aftertreatment systems. Initial plans cite a proposed phase-in of

such standards beginning in 2001. The EPA has not yet made any formal announcements of a

planning process for emission standards applicable to this engine category (CARB standards will

only affect engines built for sale in California), but it is likely that such a process will eventually

be undertaken.

Most, and probably all, new GSE engines will ultimately be covered by stringent emission

standards. Stringent standards affecting diesel GSE engines have been formally proposed and

standards affecting larger spark-ignition (i.e., gasoline, LPG, CNG, etc.) GSE engines are

probably not far off. However, since even the already-adopted off-road engine standards have

only recently taken effect, almost all in-use GSE engines not certified for on-road use (i.e.,

between 75 and 90 percent of all GSE) are uncontrolled from an emissions standpoint. 

Considering the rate of GSE engine turnover, it is likely that uncontrolled engines will continue

to dominate the GSE fleet at least through 2010. Clearly, new engine emission standards are a

longer term approach to reducing GSE emissions. To achieve short term emission reductions,

control strategies affecting the GSE engines already in use must be explored. Such short term

approaches are the focus of this summary document and the companion GSE Information Series

documents.

In considering the emission reduction and cost effectiveness estimates presented in this and

companion documents, it is critical that the reader recognize that baseline GSE emission rates are

assumed to be uncontrolled. Relative to engines certified to emission standards in effect either

now or in the future, the emissions reduction potential and cost effectiveness estimates presented

for the short term control strategies will be greatly overstated. These estimates are valid only in

the context of targeting reductions from GSE engines not certified to meet the evolving new
GSE Control Strategy Summary Page 3

engine emission standards. Over the short term, this restriction poses few if any concerns. 

However, over time, controlled engines will enter the GSE fleet and gradually (over the next 10

to 15 years) reduce or even negate the effectiveness estimates presented below. As this occurs,

revised estimates of control strategy effectiveness relative to the controlled engine emission

levels will be required.

Replacement or Conversion to LPG/CNG Power: Both LPG and CNG offer the opportunity

to reduce emissions from conventionally powered GSE. While both gasoline and diesel GSE are

candidates for replacement equipment powered by LPG or CNG, the conversion of existing

equipment is more suitable to gasoline powered GSE. For the most commonly available LPG

and CNG technologies, an active ignition system, as found on gasoline engines, is required to

initiate combustion. As a result, the conversion of compression-ignition diesel equipment is

more complicated (and expensive) than the similar conversion of gasoline equipment. 

Technologies such as pilot injection/fumigation systems have demonstrated the ability to

combust gaseous fuels in diesel engines without active ignition systems, but these technologies

are generally recognized as developmental and do not reflect typical diesel conversion

technology, which includes the installation of an active ignition system.

Both CNG and LPG are currently available as factory options for most GSE. These options

generally reflect high quality design and installation and carry full manufacturers warranties. 

However, conversions of existing engines can vary considerably in terms of quality and

performance. In principle, the conversion to LPG or CNG power is fairly simple, requiring in

the simplest cases only the installation of a new fuel tank and new fuel delivery and metering

systems. But achieving good performance and low emissions simultaneously can be quite

challenging and, as a result, a large percentage of conversions never achieve the emissions

reduction potential of either LPG or CNG. In some cases, emissions performance actually

declines relative to pre-conversion emissions. To aid in overcoming such problems, advanced

low emissions conversion kits include a closed-loop combustion control system capable of

continuously adjusting the combustion charge air-fuel ratio in accordance with exhaust gas

characteristics. Even so, the calibration of equipment conversions is as much art as science and

should only be performed by qualified, experienced personnel to ensure low emissions while

maintaining good engine performance. Such quality conversions can achieve emissions

performance equivalent to factory installed replacement equipment.

Under a scenario in which emission reduction credit (either marketable or State Implementation

Plan credit) is granted for GSE conversion, it will be necessary to impose certain restrictions to

ensure claimed emission reductions are actually achieved. Both the EPA and CARB have

established certification procedures for on-road engine conversion kits, but the application of

these procedures to off-road GSE engines will be difficult until such time as emission standards

for such engines are in place (generally the EPA and CARB procedures demonstrate compliance

with an emission standard not quantify a specific emission level). Nevertheless, a variation of

the on-road process designed to establish a "maximum warranted emission rate" could be adapted

to the certification of off-road engine conversion kits. Warranty requirements for kit

manufacturers and installation requirements for kit installers, as well as non-compliance

penalties, would add necessary incentives to ensure that claimed reductions are achieved. 

Periodic emission testing requirements might also be imposed throughout the life of any
GSE Control Strategy Summary Page 4

emission reduction credit. In the absence of a formal certification program, certain conversion

kit requirements might be established to provide a minimally acceptable guarantee of low

emissions. For example, kits which do not include closed-loop combustion control systems

might be excluded from any program assigning emission reduction credits. For purposes of this

document, it is assumed that an appropriate low emissions compliance program is in place and

that converted equipment emissions performance is similar to that of factory installed

replacement equipment. If such performance levels are not achieved in practice, the emission

reduction impacts presented below will be overstated.

Table 1 presents an estimate of the emission impacts associated with the replacement or

conversion of GSE to LPG or CNG power. Relative to gasoline GSE, emissions of HC, CO,

NOx, and PM as well as carbon dioxide (CO2) are all expected to decline. Relative to diesel

GSE, significant reductions in both NOx and PM are expected, while minor CO2 changes and

significant increases in both HC and CO occur. These latter increases are a function of the

inherently low HC and CO emissions associated with diesel engines. While the overall mass

emissions increase of both pollutants is modest relative to the quantity of HC and CO emitted by

gasoline GSE (as indicated, both LPG and CNG are expected to emit lower levels of HC and CO

than gasoline GSE), the percentage change as measured from a very small baseline is substantial. 

For example, with new (uncontrolled) engine CO emission rates of about 5 grams per brake

horsepower-hour (g/bhp-hr) for diesel GSE, 150 g/bhp-hr for LPG GSE, and 250 g/bhp-hr for

gasoline GSE, it is easy to see that while the 145 g/bhp-hr CO increase between LPG and diesel

engines represents a 2,900 percent increase, the absolute emissions increase still places the LPG

CO emission rate at only about 60 percent of that of gasoline GSE. In-use deterioration further

expands the difference between diesel and LPG engines, but does not significantly alter the

LPG/gasoline relationship. 

Therefore, relative to gasoline, a switch to LPG or CNG power will result in net emission

reductions regardless of the particular pollutant of interest. Relative to diesel, LPG and CNG are

effective emission reduction strategies for NOx and PM, but those reductions come at the cost of

increases in HC and CO.

Emissions Relative to Gasoline GSE Emissions Relative to Diesel GSE

LPG CNG LPG CNG

HC -50% to -65% -65% to -75% +95% to +140% +30% to +60%

CO -40% to -50% +4000% to +5000%

NOx -20% to -25% -75% to -80%

PM -20% -95%

CO2 -15% -20% -5% to +15% -10% to +10%

Table 1. Emission Impacts of LPG and CNG GSE
GSE Control Strategy Summary Page 5

The replacement cost premium for LPG GSE relative to gasoline GSE is estimated to be similar

to the cost of conversion at $2,000-$3,000 per unit. For CNG, first cost premiums are higher due

to the need for high pressure fuel storage, at $4,000-$4,500 per unit. Because off-road equipment

users are provided a one-for-one tax credit against gasoline and diesel fuel taxes, the fuel cost

savings associated with LPG and CNG usage in the on-road sector are substantially diminished. 

In fact, while LPG is estimated to provide a net savings of about $0.10 per gasoline equivalent

gallons of fuel used, the cost of CNG is not expected to be significantly different than the cost of

an equivalent quantity of gasoline. Over the long term, maintenance costs for both LPG and

CNG are expected to be 20-25 percent lower than conventional fuel maintenance costs, but this

improvement has yet to be observed in-use due to a high rate of occurrence of LPG/CNG

equipment problems. This trend should disappear as experience is gained and equipment

becomes more reliable but observational support is lacking at this time.

Table 2 presents an estimate of life cycle costs for baggage tractors (which constitute up to 25

percent of all GSE) powered by the various fuel options. While absolute costs will vary

somewhat for other types of GSE, the relative relationships across fuel types will be similar. 

Regardless of whether maintenance costs are reduced or not, LPG equipment is cost competitive

with gasoline GSE. Conversely, CNG is only cost competitive under a reduced maintenance,

high equipment usage scenario. In all cases, diesel GSE are estimated to be the least cost

alternative when no definitive value is assigned for emissions performance.

Although emissions valuation can alter life cycle cost relations, it is difficult to place a generic

value on emissions reduction. Emission reductions of high value in one area can be of marginal

or negative value in another. Therefore, a specific emissions valuation strategy must be

implemented before a definitive assessment of the impact on life cycle costs can be made. 

Nevertheless, some 

Gasoline Diesel LPG CNG

High Equipment Usage Scenario (Hub-Type Airport)

Reduced Maintenance1 $126K $98K $107K $126K

Same Maintenance $126K $98K $118K $136K

Average Equipment Usage Scenario (Non-Hub Airport)

Reduced Maintenance $53K $47K $49K $56K

Same Maintenance $53K $47K $52K $59K

1 The "reduced maintenance" scenario assumes LPG and CNG maintenance costs are 20

percent lower than gasoline and diesel maintenance costs. The "same maintenance" scenario

assumes all four GSE types possess similar maintenance costs.

Table 2. Life Cycle Costs for Baggage Tractors
GSE Control Strategy Summary Page 6

general observations are possible. First, under any scenario in which a positive value is assigned

to either HC or CO reductions, the life cycle cost advantage of diesel GSE will be enhanced due 

to the inherently low HC and CO emission rates of diesel engines. If NOx emission reductions

alone are valued, a value of $1,000-$2,000 per ton will render LPG life cycle costs competitive

with diesel GSE costs. A value of $3,000-$4,000 per ton of NOx will render CNG GSE cost

competitive with diesel. To achieve diesel cost competitiveness on the basis of PM reductions

alone requires a substantially higher per ton valuation, on the order of $10,000-$20,000 per ton

for LPG and $30,000-$40,000 per ton for CNG. Of course, the PM reductions are derived for

free if NOx is also valued at the rates previously indicated.

In summary, it is apparent that replacement or conversion of both gasoline and diesel GSE to

LPG or CNG can provide significant emission reductions relative to current uncontrolled

baseline emission rates and be cost competitive under reasonable emissions valuation scenarios. 

In fact, LPG GSE is cost competitive with gasoline GSE regardless of emissions reduction value. 

However, in cases where emissions reduction is not assigned a marketable value (as is the case

under current market conditions), diesel GSE is overwhelmingly the least cost option relative to

gasoline, LPG, and CNG. This is consistent with recent movement in the GSE sector toward a

greater utilization of diesel equipment. It is, therefore, likely that some definitive incentive will

need to be instituted before large scale shifts in the GSE fleet toward either LPG or CNG will be

observed.

Replacement or Conversion to Electric Power: Virtually all GSE, regardless of current

fueling option, are candidates for conversion to electric power or replacement with electrically

powered equipment. GSE manufacturers currently offer an electric power option on a wide

range of equipment types, while several firms (most centered in California) offer aftermarket

conversions to electric power. The issues of range and performance which continue to dampen

the acceptability of electric vehicles in the on-road sector are largely eliminated or controlled in

the operating environment of GSE. Clearly, such operations are restricted to a small geographic

area so that proximity to battery recharging or replacement centers (once in place at the airport)

is not an issue. Additionally, most GSE duty cycles consist of short periods of high load

operation followed by extended periods of idle or engine-off. Since electric GSE consume no

power under either mode, both the total time between battery discharge cycles and the amount of

time available for opportunity charging can be significant. On-road performance concerns such

as top speed and grade handling ability are usually not of concern in airport settings where grade

is limited and speed restrictions are present regardless of fuel.

The emission impacts associated with the use of electric GSE can vary with local power

generating characteristics. While electric GSE emit no pollutants in the conventional sense, they

do place an additional demand on local power generating stations and the emissions associated

with this additional demand are a direct result of electric GSE use. Table 3 presents estimates of

the emission impacts of electric GSE. To provide an indication of the dependence of emission

reductions on local power generating characteristics, Table 3 presents minimum, maximum, and

typical emission reduction estimates. The minimum reductions reflect impacts assuming

uncontrolled coal-fired power generation, while the maximum reductions reflect impacts

assuming a "maximum controls" (regardless of cost) natural gas-fired power generation scenario. 

Typical 
GSE Control Strategy Summary Page 7

Relative to Diesel Relative to Gasoline Relative to LPG/CNG

HC -90% to -99% (-98%) -98% to -99.9% (-99%) -96% to -99.9% (-99%)

CO -96% to -99% (-98%) -99.9% to -99.9% (-99.9%) -99.9% to -99.9% (-99.9%)

NOx -80% to -99% (-97%) -40% to -98% (-90%) -20% to -97% (-85%)

PM +30% to -99% (-98%) +3500% to -90% (-40%) +3500% to -90% (-40%)

CO2 -25% to -70% (-45%) -40% to -80% (-55%) -30% to -75% (-50%)

Table 3. Emission Impacts of Electric GSE

emission impacts are based on average emission rates observed across a sample of

geographically diverse power generating stations and should reflect a good estimate of the

emission impacts expected in most circumstances. Neither the minimum or maximum reductions

are reflective of likely impacts in any, except the most atypical, area. Therefore, while actual

impacts will require a local analysis for confirmation, the tabulated typical impacts should

provide a good indication of the emission reduction potential of electric GSE.

Clearly, the emission reduction potential associated with the use of electric GSE is dramatic

compared with all four internal combustion engine (ICE) fuels. Under any scenario, HC and CO

emissions are nearly eliminated. The same is typically true for NOx and PM relative to diesel

GSE. The variation in potential NOx reductions relative to gasoline, LPG, and CNG GSE is a bit

wider, but typical reductions approach 90 percent for all three fuels. Because PM emissions

from the three fuels are substantially lower than those of diesel GSE, electric vehicle PM

reductions are typically smaller than HC, CO, and NOx, but still significant. In all but the most

extreme cases, replacement or conversion of ICE-powered equipment can be expected to

generate large emission reductions for all pollutants, including CO2.

Electric GSE carry a significant purchase or conversion price premium of $10,000-$15,000

relative to gasoline GSE or $8,000-$10,000 relative to diesel GSE. This initial price premium is

further compounded by periodic battery replacement requirements (every five years or so) that

are twice as expensive as the alternative engine replacement or rebuild requirements associated

with ICE-powered GSE. However, electric vehicle fuel savings are significant, not only because

of lower unit fuel costs, but because no power is consumed during equipment idle periods. Fuel

savings relative to diesel GSE can reach $2,500 per year under a high equipment usage scenario

and approach $1,000 per year for average usage. Fuel savings relative to gasoline and CNG can

be twice that relative to diesel, with LPG savings between the two extremes. Electric GSE, are

expected to have reduced maintenance costs relative to ICE-powered GSE, but like CNG and

LPG equipment, first generation electric GSE were prone to high maintenance and were also

found to be inefficient and under-powered. Users of second generation electrics have reported

significant improvements in all three areas and expectations of long term maintenance cost

reductions on the order of 70 percent now appear viable. Nevertheless, electric GSE batteries
GSE Control Strategy Summary Page 8

require frequent and careful maintenance checks to ensure long battery life and keep replacement

costs to a minimum.

Life cycle cost estimates for the various ICE-powered GSE were previously presented in Table 2. 

Table 4 reviews these estimates and adds similarly calculated life cycle cost estimates for electric

GSE. Once again, while the tabulated costs are specifically calculated for baggage tractors, the

relative relationships across fuels will be similar for other GSE types. Strikingly, electric GSE

possess the lowest life cycle costs under all but the same maintenance/average usage scenario

and even then electric GSE costs are competitive with the other fueling options. For a highly

trafficked airport, electric GSE are clearly the lowest cost option. This is also true for lesser

trafficked airports if electric GSE durability expectations are achieved in-use. Moreover, these

relations are based on calculations which do not include any valuation for emission reductions. 

In most cases, electric GSE are cost effective on a purely economic basis alone.

Based on these relationships, a greater utilization of electric GSE is a reasonable future

expectation and any such increase will provide significant emission reductions. Nevertheless,

there are institutional barriers which must be overcome before large scale penetration can be

expected. The poor reputation inspired by inefficient, under-powered, and high maintenance first

generation electric GSE must be overcome though a demonstrated service reliability by the new

second generation equipment. If the new electric GSE can demonstrate an in-use reliability

equivalent to that of conventional GSE, greater penetrations will be observed. Additionally,

recharging infrastructure enhancements will be required at airports to support significant electric 

Gasoline Diesel LPG CNG Electric

High Equipment Usage Scenario (Hub-Type Airport)

Reduced Maintenance1 $126K $98K $107K $126K $56K

Same Maintenance $126K $98K $118K $136K $88K

Average Equipment Usage Scenario (Non-Hub Airport)

Reduced Maintenance $53K $47K $49K $56K $42K

Same Maintenance $53K $47K $52K $59K $52K

1 The "reduced maintenance" scenario assumes LPG and CNG maintenance costs are 20

percent lower and electric GSE maintenance costs are 70 percent lower than gasoline and

diesel maintenance costs. The "same maintenance" scenario assumes all five GSE types

possess similar maintenance costs. For electric GSE, both maintenance scenarios assume

frequent, effective battery maintenance checks to ensure long battery life.

Table 4. Life Cycle Costs for Electric Baggage Tractors
* Generally, two types of gate-based services are available. Point-of-use systems rely on dedicated equipment

installed at each aircraft gate. Centralized systems use large centrally located equipment, supported by a network

distribution system to transfer power and conditioned air to individual airport gates. The point-of-service systems

GSE Control Strategy Summary Page 9

GSE penetrations. Should these barriers be overcome, it is not unreasonable to expect significant

movement toward an electric GSE fleet. Providing additional incentive by placing a definitive

value on associated emission reductions may be sufficient to accelerate this movement.

Replacement of GSE with Electrically Powered Fixed Gate-Based Equipment: A

substantial number of the services currently provided by mobile GSE can be performed by

electrically powered equipment installed in a fixed position at each aircraft gate. Initial

movements in this direction have already taken place at many U.S. airports where fixed

gate-based equipment has been installed to handle the electrical power and conditioned air

requirements of aircraft. This equipment displaces not only GSE ground power and conditioned

air units, but high emission rate aircraft auxiliary power units (APU) as well. Additional GSE

types which are also candidates for replacement include: fuel, water, catering, and compressed air

service as well as somewhat more difficult to replace baggage and aircraft pushback tractors.

Many airports have already recognized the cost effective nature of fixed gate-based power and

conditioned air systems. Gate-based units capable of providing the 400 Hz electrical service

demanded by aircraft are being installed at an increasing number of airports. At gates without

such systems, the power necessary to operate critical aircraft systems is usually provided by an

on-board APU (a small jet engine) or, when the APU is out-of-service, a mobile GSE ground

power unit. Due to the length of time a typical aircraft spends at the gate (relative to the time

spent approaching, landing, taxiing, and taking off from an airport), APU emissions can be

responsible for 10-30 percent of the HC, 40-80 percent of the CO, and 30-60 percent of the NOx

emitted by aircraft during a typical landing and takeoff (LTO) cycle (which, from an airport

emissions standpoint, begins and ends when the aircraft enters and leaves the tropospheric

mixing zone). Although APU emissions cannot be completely eliminated due to the need for

APU operation during engine startup, overall APU emissions can be reduced by up to 90 percent. 

As a result, the emission reductions derived from the installation of fixed gate-based electrical

power units far exceed those derived from the replacement of ICE-powered GSE ground power

units with mobile electric GSE.

In most cases, fixed gate-based conditioned air service is installed in conjunction with gate-based

electrical power service. These two services go hand-in-hand, because in the absence of ground

service both will be supplied using power generated by the aircraft APU. Therefore, if

climatological conditions demand conditioned air service during aircraft gate periods, the aircraft

APU (or alternative GSE conditioned air unit) will be operated even if gate-based electrical

power is available. In such situations, the emissions reduction effectiveness of gate-based

electrical power is completed negated.

As might be expected, gate-based electrical power and conditioned air service have substantial

initial costs. Such costs can range from $50,000-$130,000 per gate (for the most popular

point-of-service systems*) depending on the type of aircraft serviced (i.e., narrow body versus
generally have lower capital costs and can be installed with less service disruption.

GSE Control Strategy Summary Page 10

wide body), but because of the very high operating costs of aircraft APU, associated cost savings

can reach $30-$50 per hour. At highly trafficked airports, such operating cost savings result in

payback periods of less than one year. While the payback period might be several years for less

highly trafficked airports, fixed gate-based power and conditioned air systems are cost effective

from a purely economic standpoint and as recognition of this cost effectiveness continues to

increase, more and more airports are adopting fixed gate-based power and conditioned air

service. Placing a marketable value on the associated emission reductions can only serve to

accelerate a process that is already underway.

"Vehicle free" gate systems represent a longer term goal. Stationary fuel, water, and lavatory

hookups can be installed at airport gates, but each require a fairly complex distribution system to

completely eliminate the GSE currently providing these services. The installation of gate-based

food and supply storage rooms can greatly reduce GSE catering service demand. Centralized

baggage conveyors which distribute baggage from check-in points to aircraft gates can displace

GSE baggage tractors (which alone represent up to 25 percent of all GSE), but such systems are

very difficult to retrofit into existing airports. Automated aircraft pushback systems offer the

potential to displace GSE pushback tractors. All of these services are technically feasible at this

time and, in fact, have been installed at a few European airports such as Arlanda in Sweden. 

However, these systems are much less cost effective than the gate-based power and air systems

described above since operating cost savings do not approach those associated with the shutting

down of aircraft APU. Nevertheless, actual cost effectiveness must be determined on the basis of

local airport conditions and the specific services targeted. Marketable emission credits may

accelerate the installation of vehicle free gate demonstration technologies, but are more likely to

influence replacement of ICE-powered mobile GSE with mobile electric alternatives due to

similar emission reductions and much lower capital cost requirements.

Installation of On-Road Type Emissions Aftertreatment Devices: As discussed above, many

current GSE engines are similar to those used in 1980’s-era automobiles. In recognition of this,

CARB is in the early stages of proposing new engine emission standards equivalent to the

level-of-control possible through the use of three-way catalytic converters. Catalysts have been

in use in the on-road vehicle sector for 25 years and have demonstrated the capability to maintain

high emissions conversion efficiencies throughout the useful lives of passenger cars and light

trucks. However, it must be recognized that one reason catalyst effectiveness and durability have

advanced to their current state in the on-road sector is that vehicle manufacturers have designed

complete combustion control systems around the aftertreatment equipment. Unleaded gasoline,

highly effective stoichiometric air-fuel mixture control systems, and current discussions to limit

fuel sulfur content are but examples of the advances made (or proposed) to accommodate

emissions aftertreatment. Notwithstanding the fact that current GSE engines and engine controls

are not manufactured to support today’s sophisticated automotive aftertreatment devices, retrofit

of devices such devices or their less sophisticated predecessors is possible. For gasoline, LPG,

and CNG GSE, potential aftertreatment devices include both three-way and oxidation catalysts

while diesel aftertreatment, due to excess air combustion characteristics, are limited to oxidation

catalysts and particulate traps 
GSE Control Strategy Summary Page 11

While there are no theoretical limitations to the retrofit of catalyst or particulate trap technology

on GSE, there are several practical issues regarding both installation and operation. Off-road

engine powered GSE are open-loop systems in that they do not have combustion control

feedback systems to adjust combustion air-fuel mixture as necessary to promote aftretreatment

efficiency. Therefore, the simplest catalyst installations will be limited to oxidation-only

catalysts which do not require near-stoichiometric combustion for effective operation. However,

most spark-ignition GSE are tuned rich, at least under high load conditions, necessitating the use

of secondary air injection to promote effective oxidation. Closed-loop combustion control

systems could be included as an integral component of the retrofit, thereby opening the

possibility for the use of three-way oxidation/reduction catalysts, but a switch to stoichiometric

operation may limit critical maximum power for some GSE. Perhaps most importantly, most

GSE have operating cycles that consist of short engine-on periods interrupted by extended

engine-off periods. Therefore, a substantial portion of GSE operation may occur before exhaust

catalysts have reached effective conversion temperatures (i.e., light-off) or particulate traps have

reached effective regeneration temperatures. Finally, installation location and space will be

restricted on most GSE so that aftertreatment packages designed to replace stock mufflers are

likely to be the most feasible approach. Neither the durability or effectiveness of such units have

yet been demonstrated over extended usage periods.

Table 5 presents the potential emission reduction effectiveness of exhaust aftertreatment devices. 

It is critical to recognize that these estimates are not based on any demonstrated experience in the

GSE sector, but instead rely on performance observed for on-road vehicles. Until such time as

the appropriate demonstrations have been made on off-road equipment such as GSE, the

reduction estimates presented in Table 5 should be interpreted as upper-bound reduction

potentials. Catalytic HC reduction potential declines for both LPG and CNG engines due to the

increasing concentrations of short chain hydrocarbons in the exhaust of such engines. Current

catalyst technology is most effective on long chain hydrocarbons and a substantial fraction of

compounds such as methane and ethane can pass through a catalyst without undergoing

oxidation. The substantial increase in CO2 emissions results from an assumed backpressure

increase as well as the oxidation of HC and CO, both of which are observed in high

concentrations in uncontrolled GSE engines. Increased backpressure accounts for only about two

percentage points of the estimated increase, with CO conversion accounting for nearly all of the

remainder.

Until such time as catalyst and particulate trap performance has been consistently demonstrated

in GSE applications and sufficient test data on in-use engines is available to confirm generalized

emission reduction impacts, exhaust aftertreatment device installation and any emission credits

granted for such installation should be treated on a case-by-case basis, supported by on-going

emissions testing. Given catalyst and trap sizing and design limitations, costs for aftertreatment

device installation will be substantially higher than the cost of similar devices for on-road

equipment. Initial catalyst costs of $1,000 or more per unit are likely. Given the lack of

practical experience, it is not possible to accurately estimate cost effectiveness at this time, but

clearly emissions reduction potential is significant and the institution of marketable emission

credits may be sufficient incentive to undertake at least the necessary demonstration programs.
GSE Control Strategy Summary Page 12

Oxidation Catalyst Installation

GSE Fuel HC CO NOx PM CO2

1

Gasoline -90% -90% 0% -10% +60%

Diesel -50% -90% 0% -30% +3%

LPG -70% -90% 0% -10% +45%

CNG -50% -90% 0% -10% +45%

Three-Way Catalyst Installation

GSE Fuel HC CO NOx PM CO2

Gasoline -90% -90% -80% -10% +60%

LPG -70% -90% -80% -10% +45%

CNG -50% -90% -80% -10% +45%

Particulate Trap Installation

GSE Fuel HC CO NOx PM CO2

Diesel -20% 0% 0% -90% +2%

1 CO2 emission increases result from increased exhaust system backpressure and the oxidation

of HC and CO, with CO oxidation accounting for the bulk of the estimated increase.

Table 5. Potential Emission Impacts of Exhaust Aftertreatment

Preferential Replacement of Two-Stroke Gasoline Engines: Uncontrolled two-stroke

gasoline engines emit about 7 times the HC, 1.5 times the CO, and 10 times the PM of similarly

sized uncontrolled four-stroke engines, while two-stroke NOx is only about 20 percent of that of

four-stroke engines. While the population of two-stroke GSE is declining, there remains a

number of such units in use in applications such as pumps on lavatory carts. In general, all such

equipment is rated at less than 25 horsepower and, therefore, subject to recent new engine

emission standards established by both the EPA and CARB. Since both two-stroke and

four-stroke engine standards are identical under the EPA and CARB programs, high emission

rate two-stroke engines will disappear from the GSE fleet over time. Nevertheless, near term

preferential replacement of existing two-stroke engines can generate significant reductions in HC

and PM. While, because of relative low two-stroke engine populations, these reductions are not

of the magnitude possible through the previously discussed control strategies, they can be cost

effective as the cost and reliability of small four-stroke engines become competitive with

similarly sized two-strokes.
GSE Control Strategy Summary Page 13

References:

`

1. Basis for GSE Population Estimates, GSE Information Series 1, prepared by Energy and

Environmental Analysis, Inc. for the U.S. Environmental Protection Agency, September

1998.

2. GSE Emissions and Activity Estimates, GSE Information Series 2, prepared by Energy and

Environmental Analysis, Inc. for the U.S. Environmental Protection Agency, September

1998.

3. LPG and CNG Control Strategies, GSE Information Series 3, prepared by Energy and

Environmental Analysis, Inc. for the U.S. Environmental Protection Agency, September

1998.

4. Electric GSE, GSE Information Series 4, prepared by Energy and Environmental Analysis,

Inc. for the U.S. Environmental Protection Agency, September 1998.

5. Emissions Aftertreatment, GSE Information Series 5, prepared by Energy and

Environmental Analysis, Inc. for the U.S. Environmental Protection Agency, September

1998.

6. Fixed Gate Support, GSE Information Series 6, prepared by Energy and Environmental

Analysis, Inc. for the U.S. Environmental Protection Agency, September 1998.

7. Analysis of Techniques to Reduce Air Emissions at Airports, Draft Final Report, prepared by

Energy and Environmental Analysis, Inc. for the U.S. Environmental Protection Agency,

September 1997.

8. Air Pollution Mitigation Measures for Airports and Associated Activity, prepared by Energy

and Environmental Analysis, Inc. for the California Air Resources Board, May 1994.

9. Technical Support Document for Civil and Military Aviation; prepared by Energy and

Environmental Analysis, Inc. for the U.S. Environmental Protection Agency in support of

the Notice of Proposed Rulemaking for the Federal Implementation Plan for California,

March 1994.

10. Documentation of Input Factors for the New Off-Road Mobile Source Emissions Inventory

Model, prepared by Energy and Environmental Analysis, Inc. for the California Air

Resources Board, August 1995.

11. Airport Electrification Project, Consolidated Results and Analysis, EPRI TR-109041,

Electric Power Research Institute, September 1997.
GSE Control Strategy Summary Page 14

12. Off-Road Electric "Vehicle" Scoping Study, EPRI TR-101906, Electric Power Research

Institute, July 1993.

13. "Electric Vehicles will Reduce Emissions at Boston’s Logan Airport," Innovators with EPRI

technology, IN-102438, Electric Power Research Institute, June 1994.

14. Off-Road Electric Vehicles: An Existing Market, EPRI TR-103315, Electric Power Research

Institute, February 1994.

15. Advanced Transportation Vehicle Catalog, www.calstart.org/services/catalog/, CALSTART

internet website.

16. Compilation of Air Pollutant Emission Factors, AP-42, Fifth Edition, Volume I: Stationary

Point and Area Sources, Chapter 1 (External Combustion Sources), as downloaded on July

29, 1998 from the U.S. EPA internet website www.epa.gov/ttn/chief/ap42.html.

17. "All the Power You Need," product literature from Hobart Ground Power, 1996.

18. "JETLINK," product literature from FMC/Jetway Systems.

19. "The Vehicle Free Ramp," product literature from FMT.

20. Emission Control Products: Diesel Exhaust Purifiers, Catalytic Mufflers, Catalytic Diesel

Filters, www.nett.ca, NETT Technologies internet website.

21. Clean Cat: Diesel Engine Catalytic Converters, clean-cat.com, Applied Diesel Technology,

Inc. internet website.

22. Personal communication on off-road catalyst design constraints and cost with NETT

Technologies, Mississauga, Ontario, Canada, July 1998.
ESTIMATING GSE ACTIVITY
Estimating GSE Activity Page 1

unit GSE emissions

tim e period of interest

g

bhp - hr rated hp hours of operationtim e period of interest

=× × × load factor

Knowledge of the activity levels of airport ground support equipment (GSE) is critical to

accurately determining emissions performance. This is true regardless of whether one considers

the baseline emissions performance of today’s GSE fleet or the potential emission reductions that

can be derived through the implementation of various control measures. Either assessment

requires the detailed characterization of equipment population and/or usage rates to derive

accurate emission or emission reduction estimates.

The basic GSE emissions calculation methodology for a particular piece of equipment can be

summarized algebraically as:

where: g/bhp-hr” represents an equipment-specific emission factor expressed in grams of emissions

per brake horsepower-hour of work,

“rated hp” indicates the maximum power output of the equipment expressed in horsepower,

“hours of operation/time period of interest” indicates the total accumulated time (expressed in

hours) the equipment’s engine is in operation during any particular period of interest, and

“load factor” indicates the ratio of actual expended work to maximum possible work during the

same time period. 

Both the “g/bhp-hr” and “rated hp” parameters are elements of engine design. However, the

former is generally dependent on the latter (at least across discrete power ranges) and, therefore,

the specific horsepower demands of a given airline’s or airport’s support services can influence

the emission rates of its GSE. In this context, the horsepower distribution of GSE can be viewed

as an indicator of GSE activity (i.e., is the activity being conducted by large, high-powered GSE

or smaller, lower-powered counterparts). The remaining two parameters, “hours of

operation/time period of interest,” and “load factor” are obviously indicators of GSE activity and

can vary with both GSE application and airport design.

While the four parameters defined above are sufficient to determine emission rates for a

particular equipment type, total GSE emissions can only be determined once the overall

distribution and population of GSE is known. Expressed algebraically, aggregate GSE emissions

are calculated as:

ESTIMATING GSE ACTIVITY
Estimating GSE Activity Page 2

aggregate GSE emissions

time period of interest

unit GSE emissions

time period of interest GSE population

i

i

i=1

GSE types

= 



 



 ×















 ∑

where: “unit GSE emissions/time period of interest” are calculated as above,

“GSE population” indicates the number of individual units of a particular equipment type in

operation, and

“GSE types” indicates the total number of different types of equipment in operation.

Both the population of individual equipment and the number of equipment types in service will

vary across airlines and airports. Therefore, even given a detailed database of GSE-specific

emission factors, it is still necessary to accurately define at least five specific GSE activity

parameters (rated horsepower, load factor, usage rate, equipment-specific population, and

number of specific GSE types in use) to estimate overall GSE emissions. Unfortunately, detailed

knowledge of each of these parameters is complicated by several factors. First, GSE includes a

diverse range of vehicles and equipment of differing sizes, usage rates, and load factors. Second,

usage rates and load factors can vary substantially across time so that GSE activity, and therefore

emissions, can be temporally dependent. Third, GSE demands can vary substantially across

airports in accordance with (among other factors) airport design and aircraft activity. For

example, a busy airport may require dedicated GSE for each gate to meet service demands while

a low volume airport may be able to use the same equipment to service multiple gates. Fourth,

and perhaps most importantly, very little information has been collected on any of these

parameters so that databases sufficient to distinguish temporal and airport- or airline-specific

influences remain to be developed.

Given this situation, GSE emissions estimation must be considered as a developing exercise at

this time. Initial estimates for all the parameters required to calculate emissions have been

developed, but the uncertainties inherent in these parameter estimates are unknown as the robust

databases necessary to evaluate such uncertainty are non-existent. An overview of current GSE

emissions estimation parameter values as well as important associated background information is

presented in the GSE Information Series 1 and 2 selections, entitled Basis for GSE Population

Estimates and GSE Emissions and Activity Estimates respectively. This abbreviated document

serves to both bring together these parametric values in a single document and provide an initial

assessment of some of the uncertainty inherent in key GSE emissions estimation parameters.

Before presenting current “best estimates” for each of the required GSE activity parameters, it is

important to place the limitations imposed by the relative infancy of the GSE emissions

estimation field in a proper perspective. Basically, such perspective involves the recognition that

the majority of emissions estimation uncertainty comes into play only when aggregated estimates

are required, be they across equipment types, airlines, or airports. Emissions estimation

parameters for specific GSE units can be readily determined from site-specific analysis. 

Population estimation methods become irrelevant when treating individual units and equipment

horsepower, usage rates, and load factors can be readily determined either through direct
Estimating GSE Activity Page 3

observation or analysis of associated data such as fuel use records. Therefore, in instances where

emission reduction estimates due to such control measures as fueling system conversion are

required, most of the emissions estimation uncertainty can be controlled simply by replacing the

“average” parametric estimates used for aggregate GSE emissions estimation with site-specific

equivalent estimates derived specifically for the affected equipment.

GSE Population Estimates: Ideally, GSE population data by equipment type would be

compiled and maintained through a national database so that accurate population estimates for

any given airport, airline, or equipment type could be readily developed. However, no such

database exists and no standard GSE tracking procedures have been developed across airports. 

Therefore, alternative mechanisms for estimating GSE population must be derived. Two

possible approaches involve so-called “top-down” and “bottom-up” estimation. Under the

top-down approach, aggregate (i.e., national or state-specific) GSE populations are estimated and

allocated to individual airports on the basis of some activity indicator. For example, scrappage

algorithms can be applied to annual GSE sales data to estimate aggregate GSE populations. 

These GSE can then allocated to individual airports through the use of an activity indicator such

as the number of aircraft landing and take-off (LTO) cycles. Such an approach was employed in

the U.S. Environmental Protection Agency’s (EPA’s) 1991 Nonroad Engine and Vehicle

Emissions Study (NEVES). Alternatively, under a bottom-up approach, GSE populations are

estimated for individual airports and aggregated as necessary. In the absence of comprehensive

airport-specific data, such an approach typically involves the statistical analysis of known GSE

population data for a given sample of airports in order to relate observed GSE populations to one

or more explanatory parameters that are readily available for all airports (e.g., LTO cycles). 

Once such a relationship has been defined, it is a relatively simple matter to apply the regression

equation to other airports and develop airport-specific GSE population estimates.

Both approaches are theoretically sound, but both also have inherent weakness and potentially

large uncertainties. Generally, however, the bottom-up approach tends to more readily

incorporate airport-specific information into the derived population estimates. Moreover,

uncertainties with potential top-down approach sources of error, such as the use of standard

scrappage algorithms, are inherently addressed in the derived bottom-up regression relations. For

these reasons and given that a small sample of airport-specific GSE population data are available

to undertake the necessary regression analysis, the bottom-up approach generally reflects a more

robust GSE population estimation approach. The current best estimate bottom-up regression

equation approach is presented in GSE Information Series 1 selection entitled Basis for GSE

Population Estimates. Basically, the approach is based on aircraft LTO cycles as the predictive

GSE population parameter and the resulting regression equation is expressed algebraically as

follows:

GSE = 0.0226 (LTOnswwb) + 0.0054 (LTOnswnb) + 0.0022 (LTOsw) + 0.0008 (LTOprop)
* In total, GSE populations for 35 individual airlines at 10 airports, comprising nearly 2,500 GSE, are incorporated

in the regression analysis. Together, these airline/airport combinations account for about 9 percent of national

LTO cycles.

Estimating GSE Activity Page 4

where: “GSE” represents the calculated GSE population,

“LTOnswwb” indicates the number airport LTO cycles accumulated by wide-body jets, exclusive

of those operated by Southwest Airlines,

“LTOnswnb” indicates the number airport LTO cycles accumulated by narrow-body jets,

exclusive of those operated by Southwest Airlines,

“LTOsw” indicates the number airport LTO cycles accumulated by all jets operated by

Southwest Airlines, and

“LTOprop” indicates the number airport LTO cycles accumulated by non-jet aircraft.

As described in GSE Information Series 1, the regression equation yields a national GSE

population estimate of about 45,000 units. This estimate is consistent with several estimates

derived over the last several years using alternative approaches, but considerably lower than the

estimate derived using the top-down approach employed for the NEVES (about 85,000 units).

Although the regression equation is based on a significant sample of observed GSE population*

and the relationships with the selected predictive parameters (i.e., the various LTO cycle

parameters) are significant at over 99 percent confidence, the variability observed across airlines

and airports is, nevertheless, significant (correlation coefficients for component regressions are

generally around 0.80). Therefore, a review of the ability of the regression equation to accurately

forecast individual airport GSE populations is important in assessing the absolute utility of the

population predictions.

Airlines at several airports were contacted directly to obtain comparative GSE inventory

information, but unfortunately this survey was not successful as airline and airport managers at

airports including Atlanta’s Hartsfield International, Indianapolis International, Norfolk

Regional, Albuquerque International, and Portland (Oregon) International declined to provide the

requested information. Of the airports surveyed, only Sacramento Metropolitan and Madison

County Airport in Huntsville, Alabama provided comprehensive GSE population data which can

be compared with regression predictions. As a alternative source of comparative data, survey

responses included in Appendix A of the Gas Research Institute’s (GRI’s) 1998“Survey of

Airport Vehicle Fleets: Final Report” were examined and those that included sufficient data to

estimate airport GSE populations were identified. Of the 38 survey responses included in the

GRI report, the 12 airports presented in Table 1 included sufficient information to estimate

airport GSE populations (note, Table 1 also includes statistics for the Huntsville and Sacramento

airports which provided GSE population data in response to direct requests).
Estimating GSE Activity Page 5

Airport 1996

LTO’s

NSWWB

LTO’s

NSWNB

LTO’s

SW

LTO’s

PROP

LTO’s

Observed

GSE

Predicted

GSE

Percent

Error

Huntsville 5,792 0 5,792 0 0 37 31 -16%

Orange County 40,628 709 34,964 4,955 0 66 216 227%

Sacramento 42,778 437 20,349 18,366 3,626 128 163 28%

San Antonio 45,301 317 27,410 15,142 2,432 112 191 70%

Kansas City 66,243 78 48,152 17,082 931 169 300 78%

Baltimore 73,917 161 56,092 10,873 6,791 325 336 3%

San Diego 77,447 1,511 43,835 27,069 5,032 356 335 -6%

Salt Lake City 100,029 9,527 72,969 15,408 2,125 554 645 16%

Miami 127,880 27,265 92,336 1 8,278 2,819 1,121 -60%

Detroit Metro 170,980 9,767 151,640 6,163 3,410 661 1,055 60%

St. Louis 229,259 6,221 142,626 31,022 49,390 1,196 1,019 -15%

Los Angeles 247,388 40,005 139,292 38,047 30,044 2,200 1,764 -20%

Dallas/Ft. Worth 373,263 14,070 262,531 1 96,661 600 1,812 202%

O'Hare 377,096 22,825 300,658 1 53,612 2,894 2,181 -25%

Aggregate Data

1,978,001 132,893 1,398,646 184,130 262,332 12,117 11,169 -8%

Aggregate Data Exclusive of Miami and Dallas/Ft. Worth

1,476,858 91,558 1,043,779 184,128 157,393 8,698 8,236 -5%

Table 1. Predicted Versus Observed GSE Populations

LTO cycle data, as required to evaluate the GSE population regression equation, was extracted

from the U.S. Department of Transportation, Bureau of Transportation Statistics’ (BTS’) report

entitled “Airport Activity Statistics of Certificated Air Carriers” for the calendar year 1996. 

LTO cycle data for each airport was disaggregated into the four required predictive parameters

(i.e., wide-body jet LTO’s exclusive of Southwest Airlines, narrow-body jet LTO’s exclusive of

Southwest Airlines, Southwest Airlines LTO’s, and non-jet LTO’s) using the aircraft type- and

airline-specific LTO data published in the BTS report. The single limitation associated with the

use of the BTS report data is that it does not include LTO cycles accumulated by foreign air

carriers. As a result, airports at which foreign carrier LTO’s comprise a significant fraction of

total LTO cycles can be expected to exhibit some under-prediction of GSE population.

Foreign carrier LTO data can be requested from the FAA, but requires special processing by

agency staff and, therefore, such data were not requested for this basic analysis. Previous

analyses of aircraft operations have, however, demonstrated that foreign carriers can comprise as

much as 15 percent of LTO cycles at high traffic international airports such as Los Angeles

International, with smaller contributions of 0-5 percent at non-hub international airports such as
* Although the reliability of “observed” GSE populations (on which the regression prediction error is based) is not

assured given the simple extraction of such populations from published surveys of uncertain consistency and

quality, the general magnitude of most such “observations” appears reasonable. Therefore, while additional

efforts to collect and validate accurate GSE population data will be beneficial, it is quite probable that the

extracted observations provide a good assessment of regression accuracy, especially in terms of differences across

airports.

Estimating GSE Activity Page 6

Boston Logan and Baltimore/Washington International. Since foreign carrier LTO’s tend to be

more heavily weighted toward wide-body aircraft, which (as indicated by regression coefficients)

carry about 2.5 times the GSE demand of narrow-body aircraft, GSE under-prediction due to

non-consideration of foreign carrier LTO’s can reach 25 percent or more at foreign hub airports

such as Los Angeles International, Chicago O’Hare, and Miami International. Lesser

under-predictions of 0-10 percent would be expected at foreign carrier non-hubs such as St.

Louis Lambert, Detroit Wayne, and Kansas City International.

Table 1 indicates observed (domestic carrier) LTO cycles, observed GSE population, the

regression-predicted GSE population, and the percent error of prediction for each of the 14

airports at which observed GSE population data was available for comparison. Figure 1 presents

a corresponding graphical illustration of the observed and predicted GSE populations. As

indicated, the error of prediction ranges from -60 to +227 percent, with an aggregate error over

all 14 airports (representing about 25 percent of all domestic carrier LTO’s) of about -8 percent. 

Unfortunately, due to the third-party nature of 12 of the 14 observed GSE populations, it is not

possible be certain that all of the indicated error is, in fact, associated with the regression

predictions. For example, as shown in Figure 1, Miami and Dallas/Ft. Worth exhibit observed

GSE populations that are vastly out-of-line with other airports of similar LTO activity. Miami

exhibits an “observed” GSE population as large as airports with three times as many LTO cycles,

whereas Dallas/Ft. Worth exhibits a population that is as small as airports with two to three times

less LTO activity. Clearly, it is possible that the GSE populations reported in the GRI survey for

these airports are not correct. In fact, if these two airports are eliminated from the analysis, the

regression predictions are within about ±25 percent of observed for eight of the twelve airports

investigated, with an aggregate error of -5 percent.

The largest band of prediction error appears to fall in the 40,000-70,000 LTO range, where the

regression equation appears to over-predict GSE populations by a substantial margin. Three of

the four airports with errors of prediction outside the ±25 percent range (excluding Miami and

Dallas/Ft. Worth) fall within this LTO region. Conversely, for those seven airports with greater

than 70,000 LTO cycles (again, excluding Miami and Dallas/Ft. Worth), four: San Diego, St.

Louis, Los Angeles, and O’Hare, all exhibit under-predictions right-in-line with expectations

given the exclusion of foreign carrier LTO’s and two: Salt Lake City and Baltimore, are only

slightly over-predicted. Only Detroit’s Wayne County Airport shows evidence of substantial

over-prediction.

Given the estimated prediction errors, several conclusions can be drawn regarding the use of the

GSE population regression*

. First, additional regression development may improve predictions.
Estimating GSE Activity Page 7

0

500

1,000

1,500

2,000

2,500

3,000

0 50,000 100,000 150,000 200,000 250,000 300,000 350,000 400,000

LTO Cycles

GSE Population

Observed

Predicted

Miami

Detroit

St.Louis

Los Angeles

Chicago Dallas

Salt Lake City

BaltimoreSan Diego

Huntsville Orange County

SacramentoSan Antonio

Kansas City

Figure 1. Predicted Versus Observed GSE Populations

For example, the current regression already includes a distinct treatment of Southwest Airlines. 

Perhaps, a singular treatment of other airlines might reveal additional unique characteristics

across carriers. Given the over-prediction observed for Detroit, investigation of Northwest

Airlines GSE practices might provide a good initial focus. Additionally, some characterization

of airport layout and aircraft traffic intensity may be beneficial. Parameters indicating the degree

of simultaneous (multi-gate) aircraft service demand and inter-gate GSE impedances may help to

define otherwise unconsidered differences between airports.

Second, despite uncertainty in the observed GSE populations, it is clear that the regression

predicts GSE population fairly well across an aggregate grouping of airports with differing

characteristics. Estimated national and regional GSE population predictions are likely to be quite

reasonable. Third, given the significant error observed for individual airports, especially those

with less than 70,000 or so annual LTO cycles, regression predictions should be used only for

emissions estimation screening purposes at individual airports. Such predictions can provide a

reasonable first estimate of potential emission loads associated with GSE or potential emission

reductions due to GSE controls, but detailed emission estimates should be reserved until actual

GSE counts are available or until enhanced regression parameters have been developed.

Equipment Type-Specific GSE Population Estimates: The problems inherent in estimating

the total GSE population in-use at airports are magnified when one considers the individual types

of equipment which are used for aircraft ground support. Various types of equipment can be
Estimating GSE Activity Page 8

used to support various service demands and not all airlines address the same service

requirements with the same equipment. Certain units such as aircraft pushback tractors, baggage

tugs, and belt loaders are fairly common across carriers and airports, but others such as forklifts,

carts, and ground power units vary considerably in usage rates and population, depending on both

airline and airport practices. Even developing a specific list of GSE types that apply at all

airports is not straight-forward. However, over the last several years, a somewhat “standard” list

of 23 GSE types has been in-use in the airport analysis community and, in the absence of

alternatives or obvious deficiencies, this list represents a convenient GSE categorization scheme. 

These 23 equipment types, as listed in Table 2, provide an adequate level-of-detail to characterize

the important differences in design and usage of component GSE.

Given the current paucity of detailed GSE population data, there is little alternative at this time

but to base estimates of specific equipment type populations on the aggregate equipment

distributions for those carriers which have provided detailed GSE data. As described in GSE

Information Series 1, such data consist of detailed population distributions for 35 airlines at 10

U.S. airports. Table 2 presents the equipment type distributions derived from this data, at both

the level-of-classification associated with the overall GSE population regression parameters (i.e.,

jet aircraft exclusive of Southwest Airlines, Southwest Airlines, and non-jet aircraft) and at an

aggregate level for all observed sample distributions.

At this point in the development of GSE databases, the equipment type-specific distributions

presented in Table 2 reflect the “best methodology” for estimating specific equipment

populations at airports for which site-specific data is unavailable. GSE population estimates for

any selected airport can be developed by applying tabulated distribution data to aggregate

population estimates predicted using the GSE population regression described above. While this

approach inherently incorporates all of the uncertainty already described for regression-predicted

aggregate GSE populations, as well as any additional uncertainty inherent in the equipment

type-specific distributions, it nevertheless incorporates the most complete data currently

available.

Figures 2 and 3 present a comparison of equipment type distributions estimated using the “best

methodology” regression/disaggregation approach to distributions derived from data for GSE at

two airports not reflected in the database used to develop the underlying estimation statistics. 

The data for St. Louis was extracted from the GRI airport fleet vehicle survey already described,

while that for Sacramento was provided directly by airport personnel. Note, the comparisons

presented focus solely on error in the equipment type-specific distributions and ignore any

aggregate GSE population prediction error (due to the use of the GSE/LTO regression equation)

which has already been discussed above. Also, the default (or predicted) GSE distributions were

developed by applying the aircraft-specific GSE distributions presented in Tables 2a-2c to

airport-specific LTO data for 1996. The aggregate GSE distributions presented in Table 2d are

not considered in these comparisons, but would deviate substantially from those indicated. As

expected, the “best methodology” approach to determining individual equipment subpopulations

includes substantial error when applied to any given airport. Nevertheless, in the absence of

alternative site-specific data, the default (i.e., predicted) equipment type-specific distributions do

provide a reasonably accurate means of estimating of equipment populations. In most cases, the 
Estimating GSE Activity Page 9

Equipment

Type

Population

Fraction

Diesel

Share

Gasoline

Share

Electric

Share

LPG/CNG

Share

Turbine

Share

Aircraft Pushback Tractor 0.0590 0.7577 0.1801 0.0372 0.0249 0.0000

Conditioning Air Unit 0.0109 0.8017 0.0661 0.0661 0.0661 0.0000

Air Start Unit 0.0198 0.8701 0.0744 0.0189 0.0000 0.0366

Baggage Tug 0.2281 0.4093 0.4655 0.0257 0.0996 0.0000

Belt Loader 0.1120 0.4608 0.4542 0.0196 0.0654 0.0000

Bobtail 0.0227 0.1614 0.8222 0.0000 0.0164 0.0000

Cargo Loader 0.0341 0.7738 0.1508 0.0000 0.0754 0.0000

Cart 0.0469 0.0154 0.5314 0.4532 0.0000 0.0000

Deicer 0.0113 0.0638 0.9362 0.0000 0.0000 0.0000

Forklift 0.0773 0.0426 0.2607 0.2227 0.4740 0.0000

Fuel Truck 0.0113 0.1605 0.8066 0.0000 0.0329 0.0000

Ground Power Unit 0.0652 0.8034 0.0337 0.1630 0.0000 0.0000

Lavatory Cart 0.0004 0.0000 1.0000 0.0000 0.0000 0.0000

Lavatory Truck 0.0158 0.0000 1.0000 0.0000 0.0000 0.0000

Lift 0.0377 0.0291 0.5823 0.1943 0.1943 0.0000

Maintenance Truck 0.0297 0.0126 0.9748 0.0000 0.0126 0.0000

Other GSE 0.0399 0.2200 0.6793 0.0181 0.0825 0.0000

Service Truck 0.0476 0.1383 0.7464 0.0000 0.1153 0.0000

Bus 0.0066 0.3887 0.6113 0.0000 0.0000 0.0000

Car 0.0132 0.0000 1.0000 0.0000 0.0000 0.0000

Pickup Truck 0.0633 0.0114 0.9366 0.0000 0.0520 0.0000

Van 0.0429 0.0000 0.9657 0.0000 0.0343 0.0000

Water Truck 0.0044 0.0000 1.0000 0.0000 0.0000 0.0000

All Equipment 1.0000 0.3247 0.5127 0.0685 0.0934 0.0007

Table 2a. Jet Aircraft GSE Distribution, Exclusive of Southwest Airlines
Estimating GSE Activity Page 10

Equipment

Type

Population

Fraction

Diesel

Share

Gasoline

Share

Electric

Share

LPG/CNG

Share

Turbine

Share

Aircraft Pushback Tractor 0.0324 1.0000 0.0000 0.0000 0.0000 0.0000

Conditioning Air Unit 0.0108 0.0000 1.0000 0.0000 0.0000 0.0000

Air Start Unit 0.0000 n/a n/a n/a n/a n/a

Baggage Tug 0.3222 0.4329 0.5671 0.0000 0.0000 0.0000

Belt Loader 0.0541 0.0000 1.0000 0.0000 0.0000 0.0000

Bobtail 0.0108 0.0000 1.0000 0.0000 0.0000 0.0000

Cargo Loader 0.0000 n/a n/a n/a n/a n/a

Cart 0.0541 0.0000 1.0000 0.0000 0.0000 0.0000

Deicer 0.0108 0.0000 1.0000 0.0000 0.0000 0.0000

Forklift 0.0000 n/a n/a n/a n/a n/a

Fuel Truck 0.0108 1.0000 0.0000 0.0000 0.0000 0.0000

Ground Power Unit 0.2249 1.0000 0.0000 0.0000 0.0000 0.0000

Lavatory Cart 0.0541 0.0000 1.0000 0.0000 0.0000 0.0000

Lavatory Truck 0.0108 0.0000 1.0000 0.0000 0.0000 0.0000

Lift 0.0108 0.0000 1.0000 0.0000 0.0000 0.0000

Maintenance Truck 0.0216 0.0000 1.0000 0.0000 0.0000 0.0000

Other GSE 0.0000 n/a n/a n/a n/a n/a

Service Truck 0.0432 0.0000 1.0000 0.0000 0.0000 0.0000

Bus 0.0000 n/a n/a n/a n/a n/a

Car 0.0108 0.0000 1.0000 0.0000 0.0000 0.0000

Pickup Truck 0.0324 0.0000 1.0000 0.0000 0.0000 0.0000

Van 0.0746 0.0000 1.0000 0.0000 0.0000 0.0000

Water Truck 0.0108 0.0000 1.0000 0.0000 0.0000 0.0000

All Equipment 1.0000 0.4076 0.5924 0.0000 0.0000 0.0000

Table 2b. Southwest Airlines GSE Distribution
Estimating GSE Activity Page 11

Equipment

Type

Population

Fraction

Diesel

Share

Gasoline

Share

Electric

Share

LPG/CNG

Share

Turbine

Share

Aircraft Pushback

Tractor

0.1554 0.8325 0.1675 0.0000 0.0000 0.0000

Conditioning Air Unit 0.0000 n/a n/a n/a n/a n/a

Air Start Unit 0.0260 1.0000 0.0000 0.0000 0.0000 0.0000

Baggage Tug 0.3323 0.6221 0.3341 0.0438 0.0000 0.0000

Belt Loader 0.2328 0.7138 0.2862 0.0000 0.0000 0.0000

Bobtail 0.0038 0.0000 1.0000 0.0000 0.0000 0.0000

Cargo Loader 0.0000 n/a n/a n/a n/a n/a

Cart 0.0077 0.0000 1.0000 0.0000 0.0000 0.0000

Deicer 0.0077 0.0000 1.0000 0.0000 0.0000 0.0000

Forklift 0.0222 0.1724 0.3448 0.0000 0.4828 0.0000

Fuel Truck 0.0000 n/a n/a n/a n/a n/a

Ground Power Unit 0.0406 1.0000 0.0000 0.0000 0.0000 0.0000

Lavatory Cart 0.0222 0.0000 1.0000 0.0000 0.0000 0.0000

Lavatory Truck 0.0184 0.0000 1.0000 0.0000 0.0000 0.0000

Lift 0.0077 0.0000 0.0000 0.0000 1.0000 0.0000

Maintenance Truck 0.0000 n/a n/a n/a n/a n/a

Other GSE 0.0260 0.0000 1.0000 0.0000 0.0000 0.0000

Service Truck 0.0597 0.5000 0.5000 0.0000 0.0000 0.0000

Bus 0.0038 1.0000 0.0000 0.0000 0.0000 0.0000

Car 0.0038 0.0000 1.0000 0.0000 0.0000 0.0000

Pickup Truck 0.0260 0.2941 0.7059 0.0000 0.0000 0.0000

Van 0.0000 n/a n/a n/a n/a n/a

Water Truck 0.0038 0.0000 1.0000 0.0000 0.0000 0.0000

All Equipment 1.0000 0.6141 0.3530 0.0145 0.0184 0.0000

Table 2c. Non-Jet Aircraft GSE Distribution
Estimating GSE Activity Page 12

Equipment

Type

Population

Fraction

Diesel

Share

Gasoline

Share

Electric

Share

LPG/CNG

Share

Turbine

Share

Aircraft Pushback Tractor 0.0612 0.7659 0.1772 0.0341 0.0228 0.0000

Conditioning Air Unit 0.0106 0.7850 0.0856 0.0647 0.0647 0.0000

Air Start Unit 0.0195 0.8751 0.0715 0.0182 0.0000 0.0352

Baggage Tug 0.2331 0.4188 0.4629 0.0257 0.0926 0.0000

Belt Loader 0.1144 0.4713 0.4496 0.0182 0.0609 0.0000

Bobtail 0.0219 0.1589 0.8249 0.0000 0.0162 0.0000

Cargo Loader 0.0324 0.7738 0.1508 0.0000 0.0754 0.0000

Cart 0.0459 0.0150 0.5450 0.4400 0.0000 0.0000

Deicer 0.0112 0.0613 0.9387 0.0000 0.0000 0.0000

Forklift 0.0741 0.0437 0.2615 0.2207 0.4741 0.0000

Fuel Truck 0.0110 0.1774 0.7903 0.0000 0.0323 0.0000

Ground Power Unit 0.0677 0.8202 0.0308 0.1490 0.0000 0.0000

Lavatory Cart 0.0021 0.0000 1.0000 0.0000 0.0000 0.0000

Lavatory Truck 0.0157 0.0000 1.0000 0.0000 0.0000 0.0000

Lift 0.0363 0.0287 0.5813 0.1919 0.1980 0.0000

Maintenance Truck 0.0286 0.0124 0.9752 0.0000 0.0124 0.0000

Other GSE 0.0387 0.2157 0.6856 0.0178 0.0809 0.0000

Service Truck 0.0479 0.1488 0.7422 0.0000 0.1089 0.0000

Bus 0.0064 0.3993 0.6007 0.0000 0.0000 0.0000

Car 0.0129 0.0000 1.0000 0.0000 0.0000 0.0000

Pickup Truck 0.0616 0.0148 0.9345 0.0000 0.0508 0.0000

Van 0.0423 0.0000 0.9669 0.0000 0.0331 0.0000

Water Truck 0.0045 0.0000 1.0000 0.0000 0.0000 0.0000

All Equipment 1.0000 0.3348 0.5097 0.0655 0.0893 0.0007

Table 2d. Sample Aggregate GSE Distribution
Estimating GSE Activity Page 13

default distributions are able to identify both the type and relative presence of the various

equipment types in-use at each airport. Most importantly, estimates for aircraft tugs (pushback

tractors), baggage tugs, belt loaders, and on-road vehicles, which together comprise 60-75

percent of observed equipment populations, are generally accurate. While the comparative data

used to generate Figures 2 and 3 does not include the fuel-type statistics, one can reasonably

expect substantial error in fuel-specific equipment population predictions given individual air

carrier and airport practices regarding alternative fuel use, etc.

Given airline- and airport-specific influences on equipment type distributions, error in the use of

default GSE distributional data is not unexpected. As indicated above in the discussion of

aggregate population predictions, the default distributional data should only be used for screening

purposes only. Default distributions can provide a reasonable first estimate of potential emission

loads associated with GSE or potential emission reductions due to GSE controls, but detailed

“official” emission estimates should be reserved until actual site-specific GSE counts are

available. For determining the emission reduction associated with control measures which focus

on one or several specific pieces of equipment, all default distribution uncertainty becomes

irrelevant as data specific to the targeted equipment must be known and can easily displace

default statistics.

GSE Horsepower Classification: As described in the GSE Information Series 2 selection

entitled GSE Emissions and Activity Estimates, the basis for current “best estimates” of GSE

horsepower, load factor, and usage rates all derive from data developed for the California Air

Resources Boards’ (ARB’s) off-road emission factor model. The aggregate data presented below

differs somewhat from that of the ARB model only because the California-specific equipment

population data included in the ARB model has been replaced with more robust national data to

develop aggregate individual equipment data. Extractions from the ARB model remains the

“best estimate” methodology for determining GSE horsepower for several reasons. The model

continues to incorporate all GSE horsepower data currently available. Moreover, the model

treats the horsepower data in very disaggregated format so that subsequent aggregations using

alternative population data are quite easy. However, while the ARB model continues to represent

the best source of horsepower distribution data, it should be recognized that the basis for this data

continues to be limited, consisting of GSE inventory information collected from only a handful

of airlines during the California Federal Implementation Plan (FIP) development process of the

early-1990’s. There is no question that an expanded study of GSE engine horsepower

distribution is both appropriate and necessary to validate and augment existing data.

Table 3 presents average horsepower data for the various GSE equipment types. As expected,

the variation across equipment types is dramatic, in accordance with the wide range of engine

power demands in the GSE sector. As a result, the emissions performance of specific GSE types

(or even across fuels within a specific GSE type) can vary considerably. As described above for

GSE population estimation, these data should only be used for emissions screening purposes and

should be displaced with actual equipment horsepower in all instances of emission reduction

estimates for specific equipment controls. Note also that Table 3 does not include estimates for

on-road vehicles used in GSE applications since emission estimates for such equipment should

be derived using the appropriate on-road emission factor model (i.e., MOBILEx, PARTx, or

EMFACx). It should similarly be recognized that an unknown fraction of the trucks used to 
Estimating GSE Activity Page 14

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Aircraft Tug

A/C Unit

Air Start Unit

Baggage Tug

Belt Loader

Bobtail

Cargo Loader

Cart

Deicer

Forklift

Fuel Truck

GPU

Lavatory Cart

Lavatory Truck

Lift

On-Road Vehicles

Other GSE

GSE Type

Fraction of All GSE

St. Louis Actual Population Fractions

St. Louis Default Population Fractions

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Aircraft Tug

A/C Unit

Air Start Unit

Baggage Tug

Belt Loader

Bobtail

Cargo Loader

Cart

Deicer

Forklift

Fuel Truck

GPU

Lavatory Cart

Lavatory Truck

Lift

On-Road Vehicles

Other GSE

GSE Type

Fraction of All GSE

Sacramento Actual Population Fractions

Sacramento Default Population Fractions

Figure 2. Comparative Equipment Type Distributions for St. Louis

Figure 3. Comparative Equipment Type Distributions for Sacramento
Estimating GSE Activity Page 15

Equipment Type Gasoline/LPG/CNG Diesel

Aircraft Pushback Tractor 130 216

Conditioned Air Unit 130 300

Air Start Unit 130 600

Baggage Tug 100 78

Belt Loader 60 45

Bobtail 100 100

Cargo Loader 70 76

Cart 12 12

Deicer 93 93

Forklift 50 52

Fuel Truck 130 180

Ground Power Unit 150 145

Lavatory Cart 12 12

Lavatory Truck 130 130

Lift 100 100

Maintenance Truck 130 130

Other 50 50

Service Truck 180 170

Water Truck 150 150

Table 3. Average Horsepower by Equipment and Fuel Type

provide fuel, food, water, and lavatory service are also on-road certified vehicles. Generally,

such service trucks do include special adaptations to facilitate use as GSE and many are of

overseas manufacture and uncertain emissions certification (since their use is restricted to

off-road applications, no specific emissions certification has historically been required). 

However, many are undoubtedly certified on-road vehicles adapted for GSE use and as such,

associated emission estimates should also be determined using on-road emission factor models.

GSE Load Factors. As described above for GSE horsepower, the basis for current “best

estimates” of GSE load factors derives from data developed for the ARB’s off-road emission

factor model. As was the case for GSE horsepower data, the aggregate data presented below

differs somewhat from that of the ARB model because California-specific equipment population

data has been replaced with more robust national data to aggregate individual equipment

estimates. Nevertheless, extractions from the ARB model continue to incorporate all GSE load

factor data currently available. Like horsepower, the model treats load factor data in a very

disaggregated format so that subsequent aggregations using alternative population data are quite
Estimating GSE Activity Page 16

easy to accomplish. However, it must be recognized that the basic data used to develop the ARB

model continues to be limited in scope, consisting primarily of GSE inventory information

collected from only a handful of airlines during the California Federal Implementation Plan (FIP)

development process of the early-1990’s. An expanded study of GSE load factors is both

appropriate and necessary to validate and augment existing data.

Table 4 presents a summary of current “best estimate” GSE load factors by equipment type. As

was the case with GSE horsepower, on-road equipment types have been excluded from Table 4

as associated emissions estimation should be conducted using the appropriate on-road emission

factor models. Again, some fraction of service trucks will also be on-road certified vehicles and

should also be modeled as such.

Equipment Type Gasoline/LPG/CNG Diesel

Aircraft Pushback Tractor 0.80 0.80

Conditioned Air Unit 0.75 0.75

Air Start Unit 0.90 0.90

Baggage Tug 0.55 0.55

Belt Loader 0.50 0.50

Bobtail 0.55 0.55

Cargo Loader 0.50 0.50

Cart 0.50 0.50

Deicer 0.95 0.95

Forklift 0.30 0.30

Fuel Truck 0.25 0.25

Ground Power Unit 0.75 0.75

Lavatory Cart 0.50 0.50

Lavatory Truck 0.25 0.25

Lift 0.50 0.50

Maintenance Truck 0.50 0.50

Other 0.50 0.50

Service Truck 0.20 0.20

Water Truck 0.20 0.20

Table 4. Average Load Factors by Equipment and Fuel Type
Estimating GSE Activity Page 17

GSE Hours of Use: Like all GSE emissions estimation parameters, equipment usage rates vary

considerably from airport to airport. Moreover, use hours even at a single airport can vary

considerably by time-of-day, day-of-week, and season. Little investigation into such variation

has been undertaken for GSE and, therefore, usage rates have generally focused on long term

(e.g., annual) activity. Even so, variation across airports (e.g., due to variations in aircraft LTO

cycles) is significant and limits the utility of generic usage rates for GSE. Nevertheless,

estimates of annual equipment usage have been developed for such applications as the ARB

off-road model. Like other estimates extracted from the ARB model, usage rates are based on

limited data, consisting primarily of GSE information collected from only a handful of airlines

during the California Federal Implementation Plan (FIP) development process of the

early-1990’s. An expanded study of GSE activity (and its variability across airports) is both

appropriate and necessary to validate and augment existing data.

Table 5 presents the average annual GSE usage rates derived from the ARB off-road model. As

with all other GSE activity parameters, the use hour estimates should be supplanted with actual

site-specific data whenever possible. For use hours, this caution is amplified. Large,

highly-trafficked airports may experience GSE usage rates much higher than those presented. 

For example, baggage tugs might operate as much as eight hours per day for 350 days or more

per year, yielding an annual usage rate of 2,800 hours or more. This is over three times the usage

rate indicated in Table 5. Similar, but opposite, errors can be evidenced at very low-traffic

airports. Since emission estimates are directly proportional to usage rate and airport-specific

differences can be large, substantial emissions estimation errors can accrue due to a failure to

account for airport-specific usage rates. As with all other GSE activity estimation parameters,

“default” usage rates should be limited to emissions screening analysis only.

Equipment Type Use Hours/Year

Aircraft Pushback Tractor 551

Conditioned Air Unit 22

Air Start Unit 135

Baggage Tug 876

Belt Loader 810

Bobtail 876

Cargo Loader 719

Cart 150

Deicer 22

Forklift 726

Fuel Truck 22

Ground Power Unit 796

Lavatory Cart 183

Lavatory Truck 1212
Estimating GSE Activity Page 18

Lift 376

Maintenance Truck 449

Other 183

Service Truck 1299

Water Truck 310

Table 5. Average Annual Hours-of-Use by Equipment Type

References:

1. Survey of Airport Vehicle Fleets, Final Report, prepared by Edwards and Kelsey, Inc. for the

Gas Research Institute, April 1998.

2. Airport Activity Statistics of Certificated Air Carriers for twelve months ending December

31, 1996, U.S. Department of Transportation, Bureau of Transportation Statistics.

3. Breakdown of GSE Equipment for Sacramento Metropolitan Airport, provided by Jim

Humphries to Bob Dulla of Sierra Research via fax, August, 1998.

4. Analysis of Techniques to Reduce Air Emissions at Airports, Draft Final Report, prepared by

Energy and Environmental Analysis, Inc. for the U.S. Environmental Protection Agency,

September 1997.

5. Air Pollution Mitigation Measures for Airports and Associated Activity, prepared by Energy

and Environmental Analysis, Inc. for the California Air Resources Board, May 1994.

6. Technical Support Document for Civil and Military Aviation; prepared by Energy and

Environmental Analysis, Inc. for the U.S. Environmental Protection Agency in support of the

Notice of Proposed Rulemaking for the Federal Implementation Plan for California, March 1994.

7. GSE population data sheets submitted by member companies of the Air Transport

Association as part of the U.S. Environmental Protection Agency’s Federal Implementation Plan

development process for California, September 1993.

8. Documentation of Input Factors for the New Off-Road Mobile Source Emissions Inventory

Model, prepared by Energy and Environmental Analysis, Inc. for the California Air Resources

Board, August 1995.

9. California Off-Road Model (June 7, 1996 version) input files EMFAC.DAT, POP.DAT, and

ACTIVITY.DAT.
APPENDIX A

GSE MODEL INSTRUCTIONS
GSEMODEL OPERATING INSTRUCTIONS

Introduction

GSEModel is a personal computer spreadsheet-based analysis tool that has been developed to

quantify emission benefits and calculate the cost-effectiveness of converting existing airport

ground support equipment (GSE) to cleaner-burning fuels and engine technologies. The model

has been developed as a planning tool for use by metropolitan planning organizations (MPOs),

airports, and other agencies interested in evaluating potential emission benefits and cost savings

resulting from available GSE emission control technologies. It has been designed with a mouseenabled graphical user interface to make it simple and easy to use.

The GSEModel tool is based upon the “best practice” methodologies and information presented

earlier in the body of this report. It has been designed to utilize local (i.e., airport-specific) GSE

usage and cost information coupled with best-available emission factor data to perform the

following functions using a consistent methodology:

& Estimate current and alternative technology GSE emissions by individual equipment

category (e.g., aircraft pushback tractors, baggage tugs, cargo loaders, etc.);

& Compute the emission benefits of the available alternative technologies;

& Quantify the incremental capital, operating, and life-cycle costs of converting GSE units

to these alternative technologies; and

& Calculate and compare the cost-effectiveness (cost per unit emissions reduced, e.g.,

$/ton) of these alternative technologies for each equipment category under airportspecific operating and usage conditions.

Nevertheless, as with any analysis tool, the results computed by the model retain the inherent

uncertainties of the data and estimates upon which they are based. As described in the operating

instructions that follow, the model provides “default” values for a number of inputs to enable the

user to quickly develop GSE emission reduction and cost-effectiveness estimates associated with

alternative technologies. Where actual local data are available, the user is encouraged to utilize

them to provide more accurate results. 
*

 The GSEModel application should theoretically also be able to run under Windows NT 4.0, but

it has not been tested under that operating system.

Following this introduction, the remaining sections of the GSEModel documentation are

organized as follows:

& Required Operating Environment - describes the software and hardware needed to run

the GSEModel application;

& Quick-Start Installation and Instructions - provides an overview on installing and

executing GSEModel to get started quickly; and

& Detailed Operating Instructions and Guidance - contains step-by-step instructions for

operating the application and basic guidance for inputting data and interpreting the

results.

Required Operating Environment

Software Requirements - GSEModel is a spreadsheet-based application that runs on personal

computers using Microsoft’s Windows 95 or Windows 98 operating systems.*

 The model was

written in Microsoft Excel Visual Basic for Applications (VBA), an extension of the Visual

Basic programming language that allows applications to be designed with a user-friendly

interface around a series of Excel spreadsheet calculations. As a result, users must have

Microsoft Excel 95 (i.e., Excel 7.0) or Excel 97 installed on their computer in order for the

GSEModel application to run.

Hardware Requirements - The minimum hardware requirements listed by Microsoft to run

Windows 95 and Excel 7.0 are sufficient to use the GSEModel program. The application has

been satisfactorily tested at 640 x 480, 800 x 600, and 1,024 x 768 display resolutions, at both

256-color and 16-bit (HiColor) depths, and should run at any available video display setting. 

The GSEModel program requires 500 KB of disk capacity. Each saved scenario file (described

later) occupies 14 KB of disk space.

“Quick-Start” Installation and Operating Instructions

Installation of the application simply consists of downloading or copying the GSEMODEL.XLS

file to any valid directory chosen by the user. However, it is suggested that a separate, initially

empty directory be created for the GSEModel application (e.g., C:\GSEMODEL) and the

GSEMODEL.XLS file copied into it. As explained later in more detail, a key ease-of-use feature

of the application is its ability to save and re-load “input scenario” files. Thus, it is

recommended that a new directory be created to store the GSEMODEL.XLS application and the
*

 If the Windows Explorer has been replaced by Internet Explorer (e.g., in Windows 98), Internet

Explorer can be configured to launch programs with a single mouse click instead of a doubleclick.

user-created scenario files (suffixed “.gse”) that it uses in order to separate them from other files

on your computer.

Running the Program - From Windows 95 or Windows 98, the GSEModel application can be

executed in several ways familiar to Windows users:

& Create and launch a shortcut to the application (creating a shortcut is discussed in the

next section);

& Double-click*

 the GSEMODEL.XLS file from within the Windows Explorer or Internet

Explorer window (assuming Excel has been installed and associated with .XLS file

types);

& Right-click the GSEMODEL.XLS file, then click “Open” from the pop-up menu; or

& Click the Start button at the bottom left of Windows 95 and Windows 98 desktop,

select “Documents” from the pop-up menu, and then click the GSEMODEL.XLS file

from the pop-up list of documents.

(Note that this latter method works only when the GSEMODEL.XLS file has been recently

opened and appears in the “Recent Documents” list.)

Inputting Data - Once the program finishes loading (signaled by a “Ready” indicator at the

bottom left of the application window), the user is placed at the top of the first of three data input

screens, the SCENARIO INPUTS screen. Using the graphical “point-and-click” interface, much

of the input data required by the application can be easily entered by clicking drop-down lists and

selecting specific items or clicking various check-boxes to utilize default information supplied by

the model. Table A-1 provides a brief summary of the data inputs required by the application.

The buttons near the bottom of each input screen allow the user to navigate from screen to

screen.

Viewing Results - After entering all required inputs (or selecting default values), GSEModel

automatically performs the alternative technology emission reduction and cost-effectiveness

calculations. To view the results, simply click the “View Results” button (the right button at the

bottom of each input screen). The user can scroll down the tabular summary to examine the

calculated results.
Table A-1

Summary of GSEModel Data Inputs

Scenario Inputs

IC Technology Activity 

& Costs Inputs Electric Technology Inputs

& A scenario title (optional)

& The GSE equipment category

being evaluated

& The “base” or current

technology used in that type of

equipment

& The number of units of that

type and technology being

considered for conversion

& The alternative technologies to

be assessed

& Annual hours of operation (of

the selected equipment type)

& Expected equipment and

engine life

& Purchase, replacement,

operating and maintenance

costs

& Discount rate assumed (for Net

Present Value cost analysis)

& The weighting scheme

(optional) used to combine

pollutant emission reductions

in calculating costeffectiveness

& The amount of time (in %) that

the current technology GSE

equipment being evaluated is

operated at idle

& Expected electric GSE

equipment life and battery life

& Electric GSE purchase,

replacement, operating and

maintenance costs

& Power-generation utility

emission rates reflective of

local conditions

(Note that if complete input data have not been entered, when the View Results button is clicked

an “Incomplete Input” message box is displayed that indicates which data must still be entered.)

To return to the input screen after viewing the results calculated by GSEModel, click the “Edit”

item on the main menu near the top of the application window. Then click either of the three

“Return to ...” items to go back to one of the input entry screens.

IMPORTANT - Other than exiting the program, this is the only way to navigate out of

the Results screen!

Printing Inputs and Results - To print both the input data and the tabulated emission reductions

and cost-effectiveness results for the current scenario being evaluated, click the “Print” item on

the main menu bar, then click the “Print Scenario” item on the drop-down menu. A two-page

report is then printed on the current “default” printer selected from Windows. The first page

shows a summary of the data inputs; results are displayed on the second page.

Saving and Loading Scenario Files - To make repeated use of the GSEModel tool quicker and

easier, the application includes the capability to save a complete set of analysis inputs and re-use

them later as a modifiable template for evaluating other GSE scenarios. (This avoids having to
Figure 1

type in complete data from scratch for each analysis.) After inputting data for a specific analysis

case, the information can be saved to a “scenario file” (indicated by a “.gse” file extension) by

clicking “File” on the main menu and then clicking “Save Scenario.” A dialog box is then

displayed, allowing the file to be saved to disk for later use. Click the arrow to the right of the

“Save in” box to select the destination directory, then type a file name for the scenario to be

saved in the “File name”box, using the .GSE suffix. (It is suggested that the “.gse” scenario files

be saved to the same directory where the application program was stored, e.g., C:\GSEMODEL.)

Similarly, to load and re-use a saved scenario file, click “File” on the main menu, then click

“Load Scenario.” Select the directory and file to be loaded into the application.

Exiting the Program - To exit the GSEModel application, click “File” on the main menu, then

click “Exit GSEModel.” The application will then close and exit. Remember to save the current

inputs in a scenario file as described above if you plan to re-use them later.

IMPORTANT - Do not close the GSEModel application using the Close Box in the

upper right-hand corner of the application window such as the one shown. 

If this happens inadvertently, you may be prompted as shown below in Figure 1 to save

changes to the GSEMODEL.XLS file. 

ANSWER “NO” TO THIS PROMPT, OR ELSE YOU MAY CORRUPT THE

PROGRAM!

Detailed Operating Instructions and Guidance

This section of the documentation provides step-by-step descriptions and guidance for operating

and inputting data to the GSEModel application and understanding the calculated results.

Creating a Shortcut to Launch GSEModel - Prior to running the application for the first time, the

user may want to create a shortcut to the application, enabling it to be launched directly from the

Windows desktop with a single mouse click. To do so, perform the following steps:
Figure 2

Scenario Inputs Screen

& Open the Windows Explorer and locate the GSEMODEL.XLS file in the

folder/directory it was installed into;

& Right-click the GSEMODEL.XLS file and click “Copy”on the pop-up menu;

& If necessary, size the Explorer window so that part of the desktop is visible, then rightclick the mouse on any visible portion of the desktop and click “Paste Shortcut” from

the pop-up menu.

Windows then places a shortcut to the application on the desktop called “Shortcut to

GSEMODEL.XLS.” To rename the shortcut (e.g., to simply “GSEModel”), select the shortcut

icon, right-click and select “Rename” from the pop-up menu. Type in the new name and hit the

Enter key on the keyboard when finished.

Inputting Data - Scenario Inputs Screen - After being launched, the GSEModel program takes

several seconds to load. When loading is finishing (denoted by a “Ready” indicator in the lower

left corner of the application window), the user is placed at the first of three inputs screens, the

Scenario screen, shown in Figure 2. The user can navigate from screen to screen using the

buttons located toward the bottom of each input screen.
On the Scenario screen, basic information about the analysis case being evaluated are entered,

including the GSE equipment category and the current and alternative emission control

technologies being considered. White-colored areas indicate where user inputs are provided. 

Each of the inputs on this screen is discussed below.

Scenario Title (optional) - If desired, the user can enter a specific title for the scenario being

evaluated.

Equipment Category - Using a drop-down list, accessed by clicking the down arrow to the right

of this input box, one of 19 available GSE equipment categories must be selected for analysis. 

(As stated earlier, the GSEModel application is set up to analyze emission reductions and costeffectiveness for one category at a time.) Table 2 lists the available GSE equipment categories.

Table 2

GSE Equipment Categories

Aircraft Pushback Tractor

Air Conditioning Unit

Air Start Unit

Baggage Tug

Belt Loader

Bobtail

Cargo Loader

Cart

Deicer

Forklift

Fuel Truck

Ground Power Unit

Lavatory Cart

Lavatory Truck

Lift

Maintenance Truck

Other GSE

Service Truck

Water Truck

Current Technology - A similar drop-down list feature is used to select the technology currently

used in the GSE equipment being evaluated. The following choices are available:

& Gas-2 - Two-stroke gasoline engines;
*

The GSE Information Series 1 (Basis for GSE Population Estimates) document also prepared

under this EPA Work Assignment provides a detailed discussion of how GSE equipment unit

counts can be derived from air carrier LTO data.

& Gas-4 - Four-stroke gasoline engines;

& LPG - Liquid Petroleum gas-fueled engines;

& CNG - Compressed natural gas-fueled engines;

& Diesel - Diesel-designed and fueled engines; and

& Electric - Electrically powered equipment.

Note that for a given GSE equipment category, any one of these options can be selected. The

GSEModel program does not include logic to identify “valid” choices for each individual

equipment category. That responsibility is left up to the user.

Input Units Method/Number of Units - By clicking either of the two radio buttons, the user

indicates which of two methods will be used to input the number of GSE units (for the selected

category and current technology) to be evaluated:

1. Direct Entry - The user simply enters the number of units being considered in the cell

below (e.g., 30 baggage tractors); or

2. Calculate from LTOs - If the number of units of a particular equipment category is

unknown, annual LTO (landing and takeoff operation) information can be entered and

the GSEModel program will estimate the number of units of the equipment type and

current technology selected.

If option 2 is selected, annual LTO data for four commercial air carrier categories are required:*

& Wide-body jets from all carriers except Southwest Airlines; 

& Narrow-body jets from all carriers except Southwest Airlines;

& Southwest Airlines jets; and

& Non-jet regional/commuter aircraft (e.g., turboprops).

Alternative Technologies - Using a group of check-boxes, the user can select alternative

technology categories to be evaluated by the program as indicated. (Depending upon the current

technology selected earlier, one of the alternative technology check-boxes may be disabled and

shaded out.) The “All Above” check-box can be used to select or de-select all available

technologies.
Figure 3

I-C Technology Activity & Cost Inputs Screen

Inputting Data - I-C Technology Activity & Cost Inputs Screen - Figure 3 shows the layout of the

I-C Technology Activity & Cost Inputs screen. 

This screen is used to enter activity and cost data for the current and alternative technologies for

the GSE equipment category selected earlier. (Information for all internal combustion (I-C)

technologies is entered on this screen. Electric technology-specific data are entered on the next

screen.) Each input area is discussed below.

Activity Inputs - Three “activity”-related data elements are entered in this table for both the

current and alternative technologies being modeled:

& Annual Usage - the number of hours per year that the equipment is operated (for I-C

engines, this consists of the entire time the engine is on and running, including idling);

& Equipment Life - the expected useful life, in years, of the GSE equipment (typically 16

years); and

& Engine Life - the estimated interval, in years, before an engine replacement or rebuild

is needed.

As Figure 3 shows, check-boxes at the left of each of these activity data inputs can be clicked to
*

This is normal. The GSEModel program uses logic that fixes the columns in these tables to

specific technologies. Depending on which technology has been selected as the current

technology and the alternative technologies chosen, the columns in these tables corresponding to

non-selected technologies will be deactivated. 

**Most maintenance cost data are reported in these units. If only annual costs are available for a

number of GSE units, simply divide these costs by the product of the number of units and their

annual usage level, i.e., Unit Maint. Cost ($/hr) = Annual Maint. Cost for Multiple GSE Units /

[# Units x Usage (hrs/year)].

have the program supply default values for each element. However, the user is encouraged to

utilize actual data for the specific airport being modeled, especially for the annual usage inputs.

Also note that depending upon which current and alternative technologies have been selected

earlier, one or more of the columns in this table (and the Cost Inputs table discussed below) will

be disabled and shaded out.*

Cost Inputs - A similarly displayed table is used to enter required cost information as follows:

& Purchase Price - the initial purchase cost of that equipment category and technology, in

dollars;

& Replacement/Rebuild Cost - the cost of rebuilding or replacing the engine of the

equipment category and technology being evaluated at each interval specified above in

the Engine Life inputs, in dollars;

& Fuel Use - the estimated amount of fuel used for each specific technology, in Gasoline

Gallon Equivalent (GGE) gallons per hour of operation;

& Unit Fuel Cost - the cost of each technology-specific fuel, in dollars per gallon; and

& Unit Maintenance Cost - the maintenance-related costs of each equipment/technology

combination, in dollars per hour of equipment use.**

Reliable, verified data for these costs are not currently available. Therefore, no defaults have

been set up within GSEModel for these inputs. The GSE Information Series documents123456 also

compiled under this Work Assignment provide rough estimates for these inputs.

Discount Rate - Below these tables is a cell to enter the discount rate (in percent) to be used in

the Net Present Value (NPV) cost calculations (discussed in detail later). A typical discount rate

used in NPV analysis is 6%.

Pollutant Cost-Effectiveness Weighting Scenario (optional) - Finally, using a drop-down 
*

The California Air Resources Board typically uses a HC-NOx-CO weighting scheme of 1-1-1/7,

respectively to assess ozone-related control strategies for which multi-pollutant reductions are

anticipated.

**This input is used by the model to account for the fact that many I-C engine-powered GSE are

left on for prolonged periods and continue to idle without being used. Conversely, it assumes

that electric GSE are shut off when not actually in use.

list, the user can optionally select a weighting scheme to calculate combined-pollutant costeffectiveness in addition to the simple, individual pollutant cost-effectiveness estimates always

produced by the program. For example, for ozone-related control-strategy analysis, planning

agencies often calculate cost-effectiveness by combining HC, NOx, and CO reductions, where

CO is discounted relative to the ozone-formation potential of HC and NOx.* In addition to

ozone, default weighting schemes have been set up for PM10 and CO-only control strategy costeffectiveness calculations. (To bypass this option, simply leave the “(none)” item selected in the

drop-down list.)

Inputting Data - Electric Technology Inputs Screen - If the electrically powered GSE alternative

technology was selected for evaluation in the Scenario Screen described earlier, additional

electric technology-specific inputs must be entered on this screen, shown in Figure 4. The inputs

required in this screen are defined below.

Activity Inputs - The activity-related data required for electric technology is similar but not

identical to IC engine inputs. These input distinctions are as follows:

& Base Tech. Operation at Idle - the frequency at which the “base” or current technology

engine being modeled operates at idle when turned on**, in percent;

& Battery Life - the expected life of the battery as regularly charged and used to power

electric GSE, in years.

The model provides default battery life estimates by GSE category if the check-box to the left of

the table is clicked.

Cost Inputs - As with the activity data, electric GSE cost inputs required by the model are similar

to their IC engine counterparts, with the following differences:

& Electric Use - the power consumption rate of the electric GSE equipment, in kilowatts

per operating hour; and
Figure 4

Electric Technology Inputs Screen

& Unit Electricity Cost - the energy rate charged by the local electric utility, in dollars per

kilowatt-hour (airports are typically charged an industrial rate that is cheaper than

residential rates). 

The program contains default values for both of these inputs based on data contained in the GSE

Information Series 4 - Electric GSE guidance cited earlier. That guidance also includes estimates

for the other electric technology cost data required by the program for selected equipment

categories.

Utility Emission Rates Scenario - Finally, using a drop-down list, the user must select one of

three utility emission rate scenarios contained in the program. This input is used to account for

the emissions generated at the utility power plant due to the incremental power demand that

results from converting IC engine GSE to electric technology. Table 3 lists these scenarios and

shows the utility emission rates assumed by the model.
Table 3

Electric Utility Emission Rate Scenarios and 

Power Plant Emission Factors (g/hp-hr)

Scenario HC CO NOx PM

Minimum 0.008 0.035 0.080 0.004

Typical 0.037 0.109 0.403 0.023

Maximum 0.124 0.185 2.534 1.371

Once all of the inputs required in each of the screens described above are entered (or default

values are selected), the GSEModel program automatically performs emission reductions and

cost-effectiveness calculations. The “View Results” button located at the bottom of each of these

screen can then be clicked to examine the results, or the “Print” item on the main menu can be

clicked to print them.

Output Results - Emissions and Cost-Effectiveness Calculations - This final sub-section of the

GSEModel documentation briefly discusses how the emission reduction and cost calculations are

performed and presents results from a sample calculation. The methodologies (and default data)

used by the program are based entirely upon the GSE Information Series guidance that is

summarized in the body of the report to which this documentation is appended. 

Using equipment category-specific “average life” emission factors contained in that guidance

multiplied by the annual usage inputs described in the preceding sub-sections, GSE emissions (in

tons/day) are estimated for current and alternative technologies selected.

Emission reductions (from the current technology emission levels) are then computed on both an

absolute (tons/day) and relative (%) basis.

Lifetime costs (based on the equipment useful life, not the engine replacement interval) are then

forecasted for engine/battery replacement, operation (i.e., fuel or electricity use), and

maintenance by combining the various cost inputs with the equipment usage inputs, and the

initial capital cost.

A Net Present Value (NPV) calculation is then performed on these cash flows, using the input

discount rate, to compute NPV life-cycle costs for each cost component (initial, replacement,

operating and maintenance), and in total under each technology evaluated. The NPV

methodology provides a basis to compare a series of uneven future cash flows for difference

scenarios (i.e., GSE technologies as used in this application) on an equivalent present value basis,

given an assumed discount rate.

Once the NPV costs are computed, individual pollutant cost-effectiveness ($/ton) is computed by

dividing incremental NPV costs (alternative technology costs relative to the current technology)

by incremental emission reductions for each alternative technology, where the emission
reductions are also expressed on an NPV basis. If selected by the user, combined pollutant costeffectiveness is similarly calculated by dividing incremental NPV costs by the summed

weighted-pollutant reductions.

A sample GSEModel Results report showing these outputs is provided in Figure 5.
GSE MODEL RESULTS

SCENARIO TITLE: Cost-Effectiveness Calculation Example (Baggage Tug)

EQUIPMENT CATEGORY: Baggage Tug

CURRENT TECHNOLOGY: Gas-4

NUMBER OF UNITS: 1

Emissions (tons/year) Current Alternative Technologies

Gas-4 LPG CNG Diesel Electric

HC 1.148 - 0.574 0.382 0.217 0.006

CO 57.648 - 36.030 36.030 0.686 0.019

NOx 0.699 - 0.525 0.525 1.842 0.068

PM 0.007 - 0.006 0.006 0.136 0.004

Emission Reductions (tons/year) Current Alternative Technologies

Gas-4 LPG CNG Diesel Electric

HC n/a - 0.57 0.77 0.93 1.14

CO n/a - 21.62 21.62 56.96 57.63

NOx n/a - 0.17 0.17 -1.14 0.63

PM n/a - 0.00 0.00 -0.13 0.00

Emission Reductions (%) Current Alternative Technologies

Gas-4 LPG CNG Diesel Electric

HC n/a - 50.0% 66.7% 81.1% 99.4%

CO n/a - 37.5% 37.5% 98.8% 100.0%

NOx n/a - 25.0% 25.0% -163.3% 90.2%

PM n/a - 16.3% 16.3% -1758.9% 46.5%

NPV Lifetime Costs ($) Current Alternative Technologies

Gas-4 LPG CNG Diesel Electric

Purchase Cost $17,000 - $19,000 $21,000 $22,000 $30,000

Replacement Cost $2,568 - $2,568 $2,568 $2,568 $6,566

Fuel Cost $59,481 - $49,072 $65,058 $27,386 $5,576

Maintenance Cost $47,089 - $37,176 $37,176 $47,089 $15,614

Total Cost $126,139 - $107,816 $125,802 $99,044 $57,756

NPV Lifetime Emissions (tons) Current Alternative Technologies

Gas-4 LPG CNG Diesel Electric

HC 10.157 - 5.079 3.381 1.924 0.056

CO 510.265 - 318.916 318.916 6.076 0.164

NOx 6.190 - 4.643 4.643 16.302 0.606

PM 0.065 - 0.054 0.054 1.201 0.035

Weighted Total: Ozone 89.243 55.281 53.584 19.094 0.685

NPV Incremental Cost Savings ($) Current Alternative Technologies

Gas-4 LPG CNG Diesel Electric

Purchase Cost n/a - -$2,000 -$4,000 -$5,000 -$13,000

Replacement Cost n/a - $0 $0 $0 -$3,997

Fuel Cost n/a - $10,409 -$5,576 $32,095 $53,905

Maintenance Cost n/a - $9,914 $9,914 $0 $31,475

Total Cost n/a - $18,323 $337 $27,095 $68,383

NPV Emission Reductions (tons) Current Alternative Technologies

Gas-4 LPG CNG Diesel Electric

HC n/a - 5.079 6.776 8.233 10.101

CO n/a - 191.349 191.349 504.189 510.101

NOx n/a - 1.548 1.548 -10.112 5.584

PM n/a - 0.011 0.011 -1.136 0.030

Weighted Total: Ozone n/a 33.962 35.659 70.149 88.557

Incremental Cost-Effectiveness ($/ton) Current Alternative Technologies

Gas-4 LPG CNG Diesel Electric

HC n/a - $3,608 $50 $3,291 $6,770

CO n/a - $96 $2 $54 $134

NOx n/a - $11,839 $218 -$2,680 $12,245

PM n/a - $1,742,065 $32,057 -$23,842 $2,278,120

Weighted Total: Ozone n/a $540 $9 $386 $772

Figure 5

Sample GSEModel Results Report
1. GSE Information Series 1, “Basis for GSE Population Estimates,” Energy and

Environmental Analysis, Inc., September 1998.

2. GSE Information Series 2, “GSE Emissions and Activity Estimates,” Energy and

Environmental Analysis, Inc., September 1998.

3. GSE Information Series 3, “LPG and CNG Control Strategies,” Energy and Environmental

Analysis, Inc., September 1998.

4. A. “GSE Control Strategy Summary,” Energy and Environmental Analysis, Inc., September

1998.

B. “Estimating GSE Activity,” Energy and Environmental Analysis, Inc., September 1998.

5. GSE Information Series 5, “Emissions Aftertreatment,” Energy and Environmental

Analysis, Inc., September 1998.

6. GSE Information Series 6, “Fixed Gate Support,” Energy and Environmental Analysis, Inc.,

September 1998.

4. REFERENCES