DOT/FAA/AR-05/25 

Office of Aviation Research 

Washington, D.C. 20591 

The Effect of Fuel on an Inert 

Ullage in a Commercial Transport 

Airplane Fuel Tank 

William M. Cavage 

July 2005 

Final Report 

This document is available to the U.S. public 

Through the National Technical Information 

Service (NTIS), Springfield, Virginia 22161. 

U.S. Department of Transportation 

Federal Aviation Administration
NOTICE 

This document is disseminated under the sponsorship of the U.S. 

Department of Transportation in the interest of information exchange. The 

United States Government assumes no liability for the contents or use 

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because they are considered essential to the objective of this report. This 

document does not constitute FAA certification policy. Consult your local 

FAA aircraft certification office as to its use. 

This report is available at the Federal Aviation Administration William J. 

Hughes Technical Center's Full-Text Technical Reports page: 

actlibrary.tc.faa.gov in Adobe Acrobat portable document format (PDF). 
Technical Report Documentation Page

1. Report No.

DOT/FAA/AR-05/25

2. Government Accession No. 3. Recipient's Catalog No.

5. Report Date

July 2005 

 4. Title and Subtitle 

THE EFFECT OF FUEL ON AN INERT ULLAGE IN A COMMERCIAL 

TRANSPORT AIRPLANE FUEL TANK

6. Performing Organization Code

ATO-P 

7. Author(s) 

William M. Cavage

8. Performing Organization Report No. 

DOT/FAA/AR-05/25 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

Federal Aviation Administration 

William J. Hughes Technical Center 

Airport and Aircraft Safety 

Research and Development Division 

Fire Safety Branch 

Atlantic City International Airport, NJ 08405

11. Contract or Grant No.

13. Type of Report and Period Covered

Final Report 

12. Sponsoring Agency Name and Address 

U.S. Department of Transportation 

Federal Aviation Administration

Office of Aviation Research 

Washington, DC 20591

14. Sponsoring Agency Code

ANM-112 

15. Supplementary Notes

16. Abstract

Recent Federal Aviation Administration research has illustrated that fuel tank inerting could be practical in the commercial fleet 

for the protection of center wing or body style tanks. The effect of pressure differences on the release of dissolved oxygen in a 

fuel load on an inert fuel tank ullage was studied. A test article was constructed and experiments were conducted to quantify the 

potential increase in oxygen concentration in an adjacent inert ullage as a result of gases in fuel during sea level stimulation, as 

well as at reduced atmospheric pressure. Different methods of stimulating the release of gases from the fuel were examined 

during laboratory experiments in an attempt to quantify the increase in oxygen concentration in an inert aircraft fuel tank ullage. 

This data was compared with flight test data in an attempt to gage the ability of laboratory tests and simple calculations to predict 

the resulting change in oxygen concentration of an inert commercial airplane fuel tank during a flight cycle. 

The oxygen evolution from different fuel loads was measured to determine the resulting oxygen concentration on an adjacent 

ullage. At sea level, the increase in oxygen concentration can be as great as 7 percent for an ullage inerted to 6 percent with an 80 

percent fuel load that was stimulated. Increasing altitude allows for an additional increase in the oxygen concentration of the 

ullage even if the ullage was at equilibrium at sea level. Inerting the ullage through fuel has the effect of scrubbing the fuel to 

some rudimentary level of protection that reduces or eliminates the increase in oxygen concentration due to fuel air evolution,

depending upon fuel load. Flight test data illustrated a relatively small amount of oxygen evolving from fuel compared to the 

theoretical amount available. Any effect of oxygen concentration increase due to fuel tended to be obscured by the more 

dominant effect of air entering the vent due to fuel consumption. Oxygen concentration flight test data with consumed fuel loads 

and inert ullages were best duplicated in laboratory experiments by not stimulating the fuel. Calculations of oxygen concentration 

increase have poor agreement with flight test data, but the flight test oxygen concentrations results fall within the band of 0 and 

100 percent oxygen evolution. 

17. Key Words 

Nitrogen enriched air, Ullage washing, Volume flow rate, Fuel 

tank inerting, Oxygen concentration, Volumetric tank 

exchange, Ostwald Coefficient

18. Distribution Statement 

This Document is available to the public through the National 

Technical Information Service (NTIS), Springfield, Virginia 

22161

19. Security Classif. (of this report)

 Unclassified 

20. Security Classif. (of this page)

 Unclassified

21. No. of Pages

 34

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
TABLE OF CONTENTS 

Page 

EXECUTIVE SUMMARY vii 

1. INTRODUCTION 1 

1.1 Background 1 

1.1.1 Fuel Scrubbing 1 

1.1.2 Previous Work 1 

1.2 Scope 2 

2. EQUIPMENT AND PROCEDURES 3 

2.1 Equipment 3 

2.1.1 Test Article 3 

2.1.2 Altitude Chamber 4 

2.1.3 Instrumentation and Data Acquisition 4 

2.2 Test Procedures 6 

2.2.1 Sea Level Tests 6 

2.2.2 Altitude Tests 7 

3. ANALYSIS 8 

3.1 Tank Air Entry 8 

3.2 Fuel Air Evolution 9 

4. DISCUSSION OF RESULTS 11 

4.1 Sea Level Experiments 11 

4.1.1 Stimulation Methods 11 

4.1.2 Resulting Ullage Effects 12 

4.1.3 Effects of Inerting Through Fuel 13 

4.2 Altitude Experiments 15 

4.2.1 Resulting Ullage Effects at Altitude 15 

4.2.2 Stimulation Comparison 16 

4.2.3 Flight Test Simulation 16 

iii
5. SUMMARY OF RESULTS 20 

6. REFERENCES 21 

APPENDICES 

A—Graph of Ostwald Coefficient for Oxygen and Nitrogen 

B—Calculation of Ullage Oxygen Concentration Increase Due to Adjacent Fuel Load

LIST OF FIGURES 

Figure Page 

1 Fuel Tank Test Article Installed in the Test Altitude Chamber 4 

2 Test Article and Instrumentation Diagram 5 

3 Time-Based Results of Oxygen Concentration Increase for Three Different 

Methods of Stimulating Air Release From Fuel 12 

4 Resulting Oxygen Concentration Increases vs Fuel Load for Three Different 

Starting Inert Ullages 13 

5 Amount of NEA Required to Inert an Ullage to 8 Percent Oxygen Concentration 

Through the Fuel vs Fuel Load 14 

6 Resulting Oxygen Concentration Increases vs Fuel Load for an 8 Percent Ullage 

Using Regular and Through Fuel Inerting 14 

7 Results Illustrating the Effect of a 60 Percent Fuel Load on an Ullage Inert to 8 

Percent Using Ullage Recirculation at Three Altitudes 15 

8 Results of Ullage Oxygen Concentration Increase for Four Different Fuel Loads 

Using Ullage Recirculation at Three Altitudes 16 

9 Results of Ullage Oxygen Concentration Increase for a 40 Percent Fuel Load 

Using Different Stimulation Methods at Three Altitudes 17 

10 Boeing Flight Test Simulation Ullage Oxygen Concentration Data Compared to 

Measured Flight Test Data for the Starting 40 Percent Fuel Load Test 18 

11 Boeing Flight Test Simulation Ullage Oxygen Concentration Data Compared to 

Measured Flight Test Data for the Starting 80 Percent Fuel Load Test 18 

iv
12 Flight Test Ullage Oxygen Concentration Data Compared to Calculated Bands of 

Oxygen Concentration Change 20 

LIST OF TABLES 

Table Page 

1 Calculations of Ullage Oxygen Concentration Increase for Fuel Usage and Air 

Evolution for Three Different Flight Tests 19 

v
LIST OF ACRONYMS 

FAA Federal Aviation Administration 

NEA Nitrogen-enriched air 

OBIGGS Onboard inert gas generation system 

CWT Center wing tank 

CFM Cubic feet per minute 

VTE Volumetric tank exchange 

vi
EXECUTIVE SUMMARY

Significant emphasis has been placed on fuel tank safety since the TWA flight 800 accident in 

July 1996. Recent Federal Aviation Administration (FAA) research has illustrated that fuel tank 

inerting could be practical in the commercial fleet if air separation modules, based on hollowfiber membrane technology, could be applied in an effective manner. The focus of the FAA fuel 

tank inerting initiative is on center wing or body style tanks, which are frequently not used by the 

aircraft operators and are likely to be empty on most domestic flight operations. When a center 

wing tank has a fuel load and is inerted, what effect the fuel load has on the inert ullage needs to 

be studied and understood to be taken into account in the design of inerting systems. Fuel tends 

to trap air, which evolves from the fuel when stimulated due to the differences in partial 

pressures of oxygen and nitrogen. Additionally, when and aircraft ascends to altitude, the 

pressure in the fuel tank ullage decreases causing a further pressure discrepancy between the 

ullage and dissolved fuel gases. This allows for additional oxygen evolution to potentially 

increase the oxygen concentration of an adjacent ullage. To counteract this, some military 

aircraft, maintaining an inert ullage, scrub the fuel of air. This is when a device is employed, 

usually during fueling, that displaces most of the air from the fuel and replaces it with nitrogenenriched air or nitrogen. 

To determine the effect of fuel on an adjacent inert ullage, a test article was constructed and 

experiments were conducted. The focus of the test was to quantify the potential increase in 

oxygen concentration of an adjacent inert ullage as a result of dissolved air in fuel during sea 

level stimulation as well as under reduced atmospheric pressure. Different methods of 

stimulating the release of oxygen from the fuel during laboratory experiments were examined in 

an attempt to quantify the exposure of a given inert aircraft fuel tank ullage to an increase in 

oxygen concentration. This data was compared with flight test data in an attempt to gage the 

ability of laboratory tests and simple calculations to simulate the resulting change in oxygen 

concentration of an inert commercial airplane throughout a flight regime. 

The oxygen evolution from fuel was measured to determine the resulting oxygen concentration a 

fuel load that is not scrubbed can have on an adjacent inert ullage. At sea level, the increase in 

oxygen concentration can be as great as 7 percent for an ullage inerted to 6 percent with an 80 

percent fuel load that is stimulated. Increasing altitude allows for an additional increase in the 

oxygen concentration of the ullage even if the ullage was at equilibrium at sea level. Inerting the 

ullage through fuel has the effect of scrubbing the fuel to some rudimentary level of protection 

that reduces or eliminates the increase in oxygen concentration due to fuel air evolution. This 

process had the effect of reducing the increase in oxygen concentration caused by a 60 percent 

full fuel load on an ullage at 8 percent oxygen from 3 to 1 percent, although this fuel scrubbing 

method illustrated diminishing returns as the fuel load increases. Flight test data illustrated a 

relatively small amount of oxygen evolving from fuel compared to the theoretical amount 

available. Also, any effect of air evolving from fuel tended to be obscured by the more dominant 

effect of air entering the vent due to fuel consumption. Oxygen concentration flight test data 

with consumed fuel loads and inert ullages were best duplicated in laboratory experiments by not 

stimulating the fuel. Calculations of oxygen concentration increase have poor agreement with 

flight test observations, but the resulting flight test oxygen concentrations fall within a calculated 

band based on ambient air replacing the consumed fuel and 100 percent exchange of oxygen and 

nitrogen in both the ullage and fuel. 

vii/viii
1. INTRODUCTION.

1.1 BACKGROUND. 

Significant emphasis has been placed on fuel tank safety since the TWA flight 800 accident in 

July 1996. Since the accident, the Federal Aviation Administration (FAA) has issued numerous 

Airworthiness Directives, enacted a comprehensive regulation to correct potential ignition 

sources in fuel tanks, and conducted research into methods that could eliminate or significantly 

reduce the exposure of transport airplanes to flammable vapors. The latter has been in response 

to a new FAA policy that strives to eliminate or reduce the presence or consequences of 

flammable fuel tank vapors. The approach has focused on fuel tank inerting, which is commonly 

used by the military. Recent FAA research has illustrated that fuel tank inerting could be 

practical in the commercial fleet. Fuel tank inerting would be accomplished by displacing the air 

in the existing empty tank space above the fuel (ullage) with nitrogen or nitrogen-enriched air 

(NEA). This could be accomplished with a ground source of NEA or more preferably with an 

onboard inert gas generation system (OBIGGS). The focus of the FAA fuel tank inerting 

initiative is on center wing or body style tanks in the commercial transport airplane fleet. These 

tanks are frequently not used by the aircraft operators and are likely to be empty on most 

domestic flight operations. When an inert fuel tank has a fuel load, the effect of the fuel load on 

the ullage needs to be quantified and accounted for in the design of an inerting system. 

1.1.1 Fuel Scrubbing. 

Air readily dissolves in fuel. Fuel, however, has a relatively large affinity for the oxygen in air, 

as compared to nitrogen, as given by a solution factor known as the Ostwald coefficient, which 

allows fuel to contain a larger percent of oxygen by volume then air. This can result in fuel 

having a dramatic effect on an adjacent inert ullage, particularly at high fuel loads, if the 

differences in oxygen partial gas pressures can equalized via some stimulation method. 

Commercial transport fuel tanks are vented to atmospheric pressure to maintain equal pressure 

between the ullage and ambient. When an aircraft ascends to altitude, the pressure in the fuel 

tank ullage decreases and dissolved gases will again evolve from fuel until the partial pressures 

of oxygen and nitrogen between the fuel and the ullage are equal. This will again result in an 

increase in oxygen concentration of an inert ullage. 

To counteract this effect, some military aircraft maintaining an inert ullage scrub the fuel of air. 

A device is employed, usually during fueling, that displaces most of the air from the fuel and 

replaces it with NEA or nitrogen. This scrubbing process can be accomplished by simply 

depositing NEA into the fuel to displace some of the dissolved air with NEA or with a more 

complex mixing system that displaces most of the oxygen. The resulting oxygen concentration 

of dissolved gases in the fuel will be dependent upon the oxygen concentration of the NEA and 

the thoroughness of the scrubbing process. This resulting oxygen concentration will determine 

the effect the fuel load will have on the adjacent inert ullage. 

1.1.2 Previous Work. 

Previous studies of fuel in conjunction with nitrogen inerting focused on the potential change in 

the oxygen concentration requirements for inerting, given a sloshing or vibrating tank [1]. This 

1
study made no mention of fuel scrubbing or the evolution of air from the fuel during sloshing, 

but rather focused on obtaining a reaction in a test article inert ullage after sloshing or vibrating 

the tank. Additionally, ullage oxygen concentration was not measured directly or tracked during 

testing. This is probably due to the relatively small fuel loads employed for the testing, which 

would have caused too little net change in ullage oxygen concentration to be tracked or 

measured. The Department of Defense, however, did an extensive study of OBIGGS in the 

1980s during which fuel scrubbing and ullage oxygen concentration effects were studied for 

simulated aircraft fuel tank missions [2]. The tests evaluated tanks at high fuel loads being 

inerted with a specified OBIGGS with varying degrees of scrubbed fuel. The results illustrated 

that a significant amount of fuel scrubbing is required to keep a small ullage (mostly full tank) 

below 10 % oxygen concentration during climb and the first parts of cruise. 

Previous FAA fuel tank inerting experiments focused on the amount of NEA gas needed to inert 

a commercial transport category airplane fuel tank ullage during ground operations. These 

experiments employed empty tanks that were simple rectangular test articles [3]. The results of 

these experiments illustrated that an empty tank can be inerted to 8 percent oxygen (by volume) 

with 1.5 volume exchanges of 5 percent NEA (5 percent oxygen). This is to say, to achieve an 

oxygen concentration in an ullage of 8 percent from ambient, which is 20.9 percent, a volume of 

5 percent NEA equal to 1.5 times the ullage volume must be deposited in the tank. These 

experiments are valid for a fuel tank ullage normally vented to atmosphere and assume the tank 

inert gas deposit is sufficiently far from the vent to allow for adequate mixing within the ullage. 

This reference also gives an analytical equation developed to calculate the resulting oxygen 

concentration of an ullage inerted with a given purity (oxygen concentration) and flow of NEA 

for a fixed amount of time. 

A proof of concept test of ground-based inerting was conducted by the FAA in conjunction with 

the Boeing company. This test was accomplished by inerting the center wing tank (CWT) of a 

Boeing 737-700 with NEA supplied from a ground source. The aircraft was then subjected to 

normal ground and flight conditions to determine the effect on the inert ullage. This test 

validated the inert gas requirements developed by the FAA and illustrated the change in oxygen 

concentration on an inert ullage due to the consumption of different fuel loads [4]. This test also 

examined the effects of ground fueling operations and cross-vented CWT venting systems on an 

inert ullage in an operational aircraft. 

1.2 SCOPE.

To determine the effect of fuel on an adjacent inert ullage, a test article was constructed and 

experiments were conducted. The focus of the test was to quantify the potential increase in 

oxygen concentration of an inert ullage adjacent to a fuel load as a result of the equalization of 

the partial pressures of dissolved gases in fuel and ullage during sea level stimulation as well as 

with reduced atmospheric pressure. The inert ullage was not continually supplied with inert gas 

in a attempt to isolate the effect of the fuel only on the ullage oxygen concentration. Different 

methods of stimulating the release of gases from the fuel during laboratory experiments were 

examined to quantify the exposure of a given inert fuel tank ullage to an increase in oxygen 

concentration. Laboratory tests were performed and compared with flight test data, as well as 

simple calculations, to gage the ability of these tools to predict the resulting change in ullage 

2
oxygen concentration of an inert commercial airplane fuel tank ullage, due to fuel air evolution 

and consumption throughout a flight regime. 

2. EQUIPMENT AND PROCEDURES.

2.1 EQUIPMENT.

The primary equipment used for the test was a fuel tank test article and altitude chamber with 

associated instrumentation. 

2.1.1 Test Article. 

The fuel tank test article was a welded rectangular aluminum tank roughly 3 by 3 by 2 feet high. 

It contained a drain at the bottom and vent on the top with an additional capped port on the top of 

the tank for refueling, defueling, and checking the fuel quantity. The vent in the top of the tank 

had a 4-inch extension with a small hole in the side and a one-way valve on top that only let 

ullage gas escape. When a pressure change forced ullage gas to escape, it could do so easily, but 

it was more difficult for chamber air to be deposited back into the ullage, with the ullage still 

being vented to atmosphere. Through experimentation, the fuel tank was determined to hold 128 

gallons. From that fuel quantity for any percentage, the fuel load could be calculated and a 

graduated stick was developed to determine fuel quantity after adding fuel. Also, a 12- by 8-inch 

rectangular hole was installed in the top of the tank to provide pressure relief, with a sheet of 

thick aluminum foil sandwiched between the rim of the rectangular hole and a retaining plate, in 

the event of a failure, which caused a reaction in the flammable fuel tank ullage. 

Additionally, an instrumentation panel was installed in the top of the tank. A 16- by 16-inch 

removable aluminum panel was fitted with various bulkhead fittings to allow access for gas 

sample tubing and thermocouples as well as allowing for the deposit of NEA and air for test 

purposes. Inside the tank, the NEA was directed down and away from the direction of the tank 

vent to maximize inert gas distribution in the ullage. Compressed air could be deposited into the 

tank to increase the oxygen concentration when needed. The tank also had a manifold mounted 

on the bottom, in the form of two U-shaped 1/4-inch tubes with small holes drilled along the 

length, to allow for the distribution of gas into the fuel. This could be inert gas for fuel 

scrubbing, air for reviving the fuel oxygen concentration to ambient conditions, or recirculated 

ullage gas into the fuel to equilibrate the ullage and dissolved fuel gases. Also, if necessary, fuel 

could be pumped out of the tank and back in through the manifold with a fuel pump to stimulate 

fuel air evolution. 

The industrial gas generator used to provide NEA to the test article was a general purpose, offthe-shelf hollow-fiber membrane gas separator with a skid-mounted compressor. The unit 

required 40 amps of 230 Vac three-phase power and was equipped with an oxygen analyzer and 

purity alarm. It contained two gas separation modules, each 2 inches in diameter, allowing the 

unit to generate as much as 10 cubic feet per minute (CFM) of 95 percent NEA (5 percent 

oxygen by volume). The NEA generator was equipped with a galvanic cell type oxygen analyzer 

to determine the oxygen concentration of the NEA being produced. When specific NEA oxygen 

concentrations were not required, bottled nitrogen roughly of a purity of 99.9% was used for 

inerting to save wear and tear on the NEA generator. 

3
2.1.2 Altitude Chamber. 

To simulate flight conditions, the rectangular fuel tank test article was tested in an altitude 

chamber to reduce the ambient pressure to desire levels. The chamber vacuum pump was 

continuously operated to decrease pressure to the desired altitude, while valves on the side of the 

chamber were opened and closed to meter in air and control the absolute pressure as well as the 

rate at which pressure changes. This allowed for not only static pressure testing to determine the 

effects of reduced pressure on an inert ullage adjacent to fuel, but also simulated flight profiles 

to compare flight test data with acquired laboratory data. The inside dimensions of the chamber 

are 72 by 71 by 93 inches. Figure 1 shows the fuel tank test article installed in the altitude 

chamber. 

FIGURE 1. FUEL TANK TEST ARTICLE INSTALLED IN THE TEST 

ALTITUDE CHAMBER 

2.1.3 Instrumentation and Data Acquisition. 

The primary instrumentation integrated with the test article was an oxygen analyzer for sea level 

gas sampling as well as a separate analyzer that sampled for altitude (reduced pressure) tests. 

Additionally, thermocouples were used to measure ullage and fuel temperatures, and a flow 

meter was used to meter NEA into the tank for experiments. The altitude chamber had a 

pressure transducer that measured absolute pressure in the chamber to determine altitude. Figure 

2 is a block diagram of the experimental test setup with instrumentation. 

4
Altitude Chamber

Air Cleaner

Oxygen

Analyzer

DAS

Nitrogen

NEA Generator

Computer

T

Altitude O2

Analyzer

Pressure

Transducer

Sample Return

Fuel Tank

Test Article

ASM

Gas Sample

T

T

T

Compressed

Air

Selector

Valve

Fuel

Pump

Selector

Valve

Ullage

Pump

Gas Sample/Pressure Line

Instrumentation Wire

Legend

NEA/Air Deposit

Fuel Line

FIGURE 2. TEST ARTICLE AND INSTRUMENTATION DIAGRAM 

2.1.3.1 Oxygen Analysis. 

The oxygen concentration in the fuel tank ullage at sea level was measured with an oxygen 

analyzer that used a PolarographicTM oxygen sensor, temperature controlled sensor block, and 

sample flow bypass system. The sample was supplied at a flow rate between 2 and 4 liters per 

minute. The analyzer measured from 0 to 25 percent oxygen with a linear 0 to 5 Vdc analog 

output. Upscale calibration was frequently performed using 16.0 percent O2 calibration gas. The 

upscale calibration provided a more accurate measurement of lower oxygen concentrations while 

potentially sacrificing some accuracy at full scale (21 percent). To reduce the amount of 

hydrocarbon vapor potentially condensing in the oxygen analyzer, a sample gas condenser was 

used on the ullage gas sample. This consisted of a copper sample line winding through an icefilled box with a vertical T at the lowest point and a drain valve. This device removed fuel vapor 

from the gas sample, which was determined to have a negligible effect on the resulting measured 

oxygen concentration. 

At reduced pressures (increased altitude), a laboratory-based altitude oxygen analyzer developed 

by the FAA was used to measure the oxygen concentration of the test article ullage during the 

test. This analyzer employed a galvanic cell oxygen analyzer similar to the FAA Onboard 

Oxygen Analysis System and used the same sample train pressure control methodology [5]. This 

single channel analyzer did not employ the same liquid traps and flash arrestors necessary to 

ensure safety on an aircraft during flight operations. 

5
2.1.3.2 Temperature. 

To detect changes in temperature during the inerting process, the test article was instrumented 

with three K type thermocouples. Two thermocouples were located in the ullage of the tank, and 

one thermocouple was located in the fuel. 

2.1.3.3 Flow. 

The flow meter used to measure the flow of NEA to the test article at sea level was temperature 

compensating with a capacity of 0.7 to 6.1 CFM of air. The accuracy of the flow meter given by 

the manufacturer was ± 3 percent of full scale. 

2.1.3.4 Data Acquisition System. 

A computer data acquisition system continuously monitored and recorded the specified 

instruments during each test. Data acquisition was accomplished via two analog to digital 

converter boards configured in a standard desktop computer. A computer program was written 

to acquire the signals and convert each to engineering units with a specified calibration file. 

Each test was saved as an ASCII file and imported into a spreadsheet where the data was 

reduced, analyzed, and graphed. 

2.2 TEST PROCEDURES.

The fuel effects tests were performed in two primary research areas: Sea level testing and 

altitude testing. All tests required the same initial test procedures. The oxygen analyzer, data 

acquisition system, and NEA generator (when needed) were started and allowed to run for 

approximately 1 hour prior to testing to reach stable operating conditions. The oxygen analyzers 

were then calibrated before testing began. A fresh fuel load was added to the tank, as required 

for each test. After the fuel was added, the tank ullage was inerted to the appropriate oxygen 

concentration and the experiment was allowed to settle for approximately 15 minutes before 

starting. 

2.2.1 Sea Level Tests. 

Sea level tests were performed in the Aircraft Components Fire Test Facility at the William J. 

Hughes Technical Center, Atlantic City International Airport, New Jersey. The test apparatus 

was inside a climate-controlled, fire-proof test cell, allowing for consistent and relatively stable 

test conditions. 

2.2.1.1 Stimulation Methods. 

To study the most effective way of releasing excess air dissolved in fuel and equalizing the 

oxygen content of the fuel and ullage, several methods were applied to bring the fuel and ullage 

to equilibrium. The purpose of the stimulation method is to allow excess oxygen in the air 

dissolved in fuel to be released from the fuel into the ullage and to allow excess nitrogen in the 

ullage to be deposited in the fuel with little or no exchange of gases outside the fuel tank. 

6
Three methods were studied to equalize the fuel tank gases: ullage recirculation, tank sloshing, 

and radiant under heating. Ullage recirculation was performed by using a small pump to draft 

ullage gas and deposit it into the manifold at the bottom of the tank submerged in fuel. Tank 

sloshing was accomplished by elevating two cattycorner legs of the tank and rocking the tank 

from side to side. Radiant under heat was done by attaching a hot-plate heater to the underside 

of the tank and heating the bottom at maximum output. In each case, the experiment was 

prepared as stated above with a 40 percent fuel load and inerted to 8 percent oxygen by volume. 

Each method was then applied to the tank. 

2.2.1.2 Resulting Ullage Effects. 

To study the maximum effect a fuel load could have on an adjacent inert ullage, several tests 

were conducted with 20, 40, 60, and 80 percent fuel loads. The tank was inerted to 6, 8, or 10 

percent oxygen concentration for some or all of the fuel loads. The ullage recirculation 

stimulation method was applied to each experiment until the ullage oxygen concentration 

stabilized. The resulting ullage oxygen concentration was recorded and the increase in oxygen 

concentration calculated. 

2.2.1.3 Effects of Inerting Through Fuel. 

To study a simple way of scrubbing fuel and examine the resulting effects, several tests were 

conducted with 20, 40, 60, and 80 percent fuel loads. In each case, the ullage was inerted to 8 

percent, using 5 percent NEA, through the manifold at the bottom of the tank under the fuel level 

with the volume of NEA required being measured. After a settling period, the ullage 

recirculation stimulation method was applied to each experiment until the ullage oxygen 

concentration stabilized. The resulting ullage oxygen concentration was recorded. 

2.2.2 Altitude Tests. 

The altitude tests were performed in the altitude chamber located in the Airflow Induction Test 

Facility at the William J. Hughes Technical Center. 

2.2.2.1. Resulting Ullage Effects at Altitude. 

To study the effect of altitude on the ability of a fuel load to effect an adjacent inert ullage, 

several tests were conducted using the aluminum tank test article with a 20, 40, 60, and 80 

percent fuel load in the altitude chamber. As discussed in section 2.2.1.2, the experiment to 

determine the net ullage effect using ullage recirculation was repeated at sea level pressure for 

each fuel load, and the resulting ullage oxygen concentration was recorded. The chamber was 

then evacuated to a pressure altitude of 16,000 feet, the experiment was stabilized for 

approximately 10 minutes, and the ullage recirculation was repeated. After the ullage oxygen 

concentration stabilized, the chamber was further evacuated to 32,000 feet pressure altitude, the 

experiment was stabilized for approximately 10 minutes, and the ullage recirculation was 

repeated until the ullage oxygen concentration was stable at that altitude. The resulting oxygen 

concentration was noted after each stabilizing period. 

7
2.2.2.2. Stimulation Methods. 

To gage how efficient ullage recirculation releases inert gas from the fuel compared to other 

potential mechanisms on an existing aircraft, the results of the previous experiment measuring 

the net effects of fuel on an inert ullage at altitude were repeated with two other methods of 

stimulation. Specifically, the experiment was repeated with the 40 percent fuel load using no 

stimulation (altitude pressure change only), as well as using a fuel pump to draft fuel from the 

tank and redeposit it. The bellows type fuel pump tended to release gas from the fuel when used. 

The two tests with the alternate methods of stimulation were performed using the same test 

procedures and time line that was used for the 40 percent fuel load test with the ullage 

recirculation described in the previous section. This allowed for a fair comparison of the 

effectiveness of these methods to raise the ullage oxygen concentration. 

2.2.2.3 Flight Test Simulation. 

To gage how well these laboratory tests could simulate the oxygen concentration increase of an 

inert aircraft fuel tank ullage due to flight conditions, two tests were performed on the laboratory 

test article simulating two flight tests. The tank conditions (altitude, initial oxygen 

concentration, fuel load, and fuel consumption) observed during the FAA/Boeing ground-based 

inerting flight test demonstration were simulated in the test article and the resulting increased 

ullage oxygen concentration measured and compared to flight test data acquired. The test article 

was loaded with the same fuel load (volume percent), and the ullage was inerted to the same 

level prior to simulating the test flight profile. Fuel was pumped out of the tank at a rate that 

gave the same change in ullage volume percentage measured during the flight tests, and the 

ullage oxygen concentration was continuously measured during the test. The descent portion of 

the tests was not simulated and no fuel stimulation was used. 

3. ANALYSIS.

The primary calculations performed for comparison with measured data were the change in 

oxygen concentration from air entering an inert ullage due to fuel being consumed from the tank 

and the increase in oxygen concentration due to air evolving from the fuel. 

3.1 TANK AIR ENTRY.

The inerting equation developed in reference 3 is simply the general statement of the perfect 

mixing equation solved in terms of two dimensionless values known as the volumetric tank 

exchange (VTE) and the inerting ratio. The VTE is the exponent of the natural log function and 

describes the volume of gas deposited in terms of the volume of the tank. Substituting the 

variable parameters, this equation is stated below. 

Tank

NEA

VQt

Start

Start Ullage e

OO

OO − −= −

− 1

2 2

2 2

 (1) 

Where: = Oxygen Concentration of Ambient Air Start O2

8
O2 NEA = Oxygen Concentration of Inerting Gas 

O2 Ullage = Oxygen Concentration of the Ullage 

This equation was applied using the starting oxygen concentration as some number at or near 

ambient (20.9 percent oxygen by volume) and a low oxygen concentration inerting gas (NEA) 

was added, but the equation is valid for mixing any two gases of different oxygen concentrations 

in some volumetric ratio to obtain a resulting oxygen concentration. 

To calculate the increase in oxygen concentration of an inert ullage due to fuel consumption, it 

was assumed that the fuel consumption in volumetric terms is equal to the volume of air entering 

the tank. The assumption is only valid for tanks that are normally vented to atmospheric 

pressure, as is the case with virtually all modern commercial transport airplanes. This allows the 

VTE to be simplified as the volume percentage of fuel consumed. Assuming the starting oxygen 

concentration is low and the inerting gas added is air, the following equation can be simplified. 

)( )]1)(9.20[( (2) %

2 2 2

FL

Start Start OtO O e− −= −−

 With: FL% = Tank Percentage of Fuel Consumed 

This equation is valid for all size fuel tanks and assumes the fuel evolves no dissolved air that 

would change the oxygen concentration of the adjacent ullage. It should be noted that the 

percentage of fuel consumed is a percentage of the final volume of the ullage. If the final fuel 

load is not zero, the calculation assumes the final ullage space available is the zero percent full 

ullage. Also, volume inerting equations are only valid when the inert gas (or diluting gas in this 

case) has the same density, given the pressure altitude, as the ullage. 

3.2 FUEL AIR EVOLUTION.

The effect of a fuel load on an inert ullage can be determined by considering the mass of oxygen 

dissolved in fuel and the mass of oxygen in the ullage as it pertains to the partial pressure of 

oxygen for the given medium. At some preliminary state, the partial pressure of oxygen 

dissolved in the fuel is higher then the partial pressure of oxygen in the inert ullage. These 

partial pressures can be expressed in terms of mass, given the equation of state. 

U 1 UO = U 1 TRmVP UOO

O

OF FO

F C

TRm

VP FO

1

1 =

These equations can be restated as follows: 

UO

OU U

U TR

VP

mO

1

1 = (3) 

9
FO

OF OF

F TR

CVP

mO

1

1 = (4) 

Given: PO = Partial Pressure of Oxygen 

 V = Volume 

 MO = Mass of Oxygen 

 T = Temperature 

 RO = Universal Gas Constant of Oxygen 

 CO = Ostwald Coefficient 

The subscripts U and F denote ullage and fuel, respectively, with 1 indicating the initial state. 

The partial pressure of oxygen is defined as the static air pressure times the volume fraction of 

oxygen. The Ostwald coefficient is a fractional number that describes the solubility of a gas in a 

fluid and varies with fluid temperature (see appendix A). 

Given the mass of oxygen in the tank remains constant, the following equation is exact: 

OU 2 OF 2 OU 1 OF1 + mm = + mm

or 

2 1 1 2 ( ) OF OU OF OU m = + − mmm (5) 

Given the partial pressure of oxygen between the two medium within the tank are not equal, 

oxygen will evolve from the higher partial pressure fuel to the lower partial pressure ullage until 

some equilibrium state (denoted 2) is obtained. This can be stated as follows: 

OU 2 OF 2 = PP

Given the relationship between partial pressure of oxygen and mass of oxygen, this gives 

OF

OF FO

U

OU UO

CV

TRm

V

TRm 2 2 =

Combining with equation 5 and simplifying gives 

1( )

( ) 11

2

OUF

FU

OUF

FU

OU OF

U

CTV

TV

CTV

TV

mm

mO

+

+

=

 (6) 

Assuming static pressure remains constant, oxygen volume percent is calculated by determining 

the partial pressure of oxygen (from equation of state) and dividing by the static pressure. 

10
Atmo

O

P

P

O F 2 ][ 2 = (7) 

A more complete discussion of these calculations, including the calculation of partial pressure of 

nitrogen and true total pressure, is given as appendix B. 

4. DISCUSSION OF RESULTS.

The results of the tests discussed in section 2.2 are presented in two distinct sections. These are 

sea level results and results from altitude experiments. 

4.1 SEA LEVEL EXPERIMENTS. 

The results of the sea level experiments consisted of three areas of investigation: methods of 

stimulation, the resulting ullage effects, and the effects of inerting through fuel. 

4.1.1 Stimulation Methods. 

To examine the effect fuel evolving dissolved gases has on an adjacent inert ullage, the test 

article was filled with a 40 percent fuel load, and the ullage was inerted to 8 percent for three 

different tests. For each test, after a 10-15 minute settling period with no stimulation, the tank 

was stimulated with the three different methods, as discussed in section 2.2.1.1. This test was 

also performed with no stimulation (quiescent), during which no appreciable increase in oxygen 

concentration was observed during a 1-hour period. The results of the increase in oxygen 

concentration over time for the three different stimulation methods can be seen in figure 3. The 

data illustrates the superior effectiveness of the ullage recirculation method of stimulating 

oxygen from the fuel. Although the sloshing method illustrated some effectiveness, the 

consistency of the method was not as thorough as the ullage recirculation. The three distinctly 

different shapes of the sloshing curve are most likely due to three different people rocking the 

tank for different periods of time during the test. The difference from person to person is most 

likely due to subtle differences in the frequency and amplitude of the rocking motion. Heating 

the fuel from underneath did provide the release of excess air from the fuel and a resulting 

oxygen concentration increase in the ullage, but the method was not that effective. 

It is unclear if the heating and sloshing method provides the same resulting increase in oxygen 

concentration as the other method, as the equations for calculating the resulting oxygen 

concentration increase rely on an exchange of gases between the fuel and ullage (i.e., oxygen out 

of and nitrogen into the fuel). This illustrates the use of the phrase oxygen evolution is a 

misnomer. The process is an equalization of gases in and out of the fuel and ullage. It is 

unknown how effective methods that do not mix the ullage and fuel together are at obtaining 

consistent results. 

11
0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 10 20 30 40 50 60 70 80 90 10

Time (Minutes)

Oxygen Concentration Increase (% Vol)

0

 Ullage Recirculation

 Side-to-Side Slosh

 Radiant Under Heat

40% Fuel Load, Inert to 8%: 

Different Stimulation Methods

FIGURE 3. TIME-BASED RESULTS OF OXYGEN CONCENTRATION INCREASE FOR 

THREE DIFFERENT METHODS OF STIMULATING AIR RELEASE FROM FUEL 

4.1.2 Resulting Ullage Effects. 

The ullage recirculation method, illustrated in the previous test series, was repeated for different 

fuel loads and different starting ullage oxygen concentrations to obtain a series of curves 

(figure 4), giving the increase in oxygen concentration due to equalization of gases from the fuel 

and ullage at sea level pressure. The equations discussed in section 3.2 were used to calculate 

the resulting oxygen concentration given a varying fuel load. These calculations illustrate good 

agreement with the measured increases, although the results at 20 percent fuel load are difficult 

to characterize as they are less than 1 percent, near the error band of the analyzer. The resulting 

oxygen concentration represents an equilibrium state and illustrates a maximum increase in 

oxygen concentration given a fixed fuel load and ullage size at sea level in a vented tank. 

 

12
0%

2%

4%

6%

8%

10%

12%

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Fuel Load (% Full)

Change in Oxygen Concentration (% Vol)

 6% Inert Inert to 6%

 8% Inert Inert to 8%

 10% Inert Inert to 10%

Comparison of Test Data with O2 Mass 

Balance Calculations at Sea Level

Calculations Measured

FIGURE 4. RESULTING OXYGEN CONCENTRATION INCREASES VS FUEL LOAD 

FOR THREE DIFFERENT STARTING INERT ULLAGES 

4.1.3 Effects of Inerting Through Fuel. 

To examine the effect of fuel scrubbing, several tests were done inerting the tank ullage through 

a manifold beneath the fuel level. This has the effect of scrubbing some of the excess oxygen 

from the fuel, providing some protection against ullage oxygen concentration rise during 

operations that could stimulate the release of excess gas from the fuel. Figure 5 gives the amount 

of 5 percent oxygen NEA required, in terms of VTE, to inert the ullage to 8 percent oxygen 

through the fuel. The horizontal line corresponding to a VTE of 1.6 represents the VTE required 

to inert the actual ullage volume in the normal manner (not through the fuel), which illustrates 

the increased amount of NEA required to inert the ullage through the fuel. Although the amount 

of NEA needed per volume of ullage to inert the tank to 8 percent by volume increases, as 

illustrated by the circles, the total volume of NEA required decreases with increasing fuel load. 

This is seen by the triangles which illustrate the calculated VTE using the total fuel tank volume. 

Using the ullage recirculation method, each inert fuel load was stimulated to determine the 

resulting oxygen concentration increase as a result of inerting through the fuel. This data, given 

in figure 6, is compared with the 8 percent inert ullage data from figure 4. As expected, the 

inerting method reduced the increase in oxygen concentration of the ullage, but illustrated 

diminishing returns in terms of the decrease in the percentage of increase in oxygen 

concentration as fuel load increases. These are the fuel load cases that are the main focus of 

concern with large static fuel loads releasing air during operations. Although some of the 

measured increases seem significant, it should be noted that effect of evolving air from fuel on 

an ullage normally vented to atmospheric pressure would tend to be masked by the consumption 

of the fuel load, which has a much greater effect by allowing air to enter the ullage directly. 

13
0

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50 60 70 80 90

Fuel Load (% full)

Volumetric Tank Exchange to Inert

 VTE Based on Actual Ullage

 VTE Based on Total Tank Volume

Line Representing Standard Ullage Washing

VTE to Inert Ullage to 8%

[O2] Using 5% NEA

FIGURE 5. AMOUNT OF NEA REQUIRED TO INERT AN ULLAGE TO 8 PERCENT 

OXYGEN CONCENTRATION THROUGH THE FUEL VS FUEL LOAD 

-1

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60 70 80 90

Fuel Load (% full)

Oxygen Concentration Increase (% vol)

 Inerting Through Manifold

 Standard Ullage Washing

Total Increase in Ullage Oxygen 

Concentration from 8% at Sea 

FIGURE 6. RESULTING OXYGEN CONCENTRATION INCREASE VS FUEL LOAD FOR 

AN 8 PERCENT ULLAGE USING REGULAR AND THROUGH FUEL INERTING 

14
4.2 ALTITUDE EXPERIMENTS. 

The results of the altitude experiments consisted of three areas of investigation: the resulting 

ullage effects, stimulation comparison, and the flight test simulation. 

4.2.1 Resulting Ullage Effects at Altitude. 

The resulting effects of a fuel load with normal air content on an adjacent inert ullage was 

examined at reduced air pressure to simulate the effect of altitude on an inert ullage. The 

procedure for equalizing the fuel and ullage by recirculation was repeated to obtain data, as 

discussed in section 4.1.2. The tank, which was normally vented to atmospheric pressure, was 

then submitted to altitude conditions and stimulated again to illustrate any additional increase in 

ullage oxygen concentration, as discussed in section 2.2.2.1. The results of this experiment for a 

60 percent fuel load with an ullage inerted normally to 8 percent are illustrated in figure 7. All 

experiments are assumed to have a fuel load saturated with air. This experiment illustrates the 

effect of the Ostwald coefficient in that even though the ullage and fuel were at equilibrium at 

sea level at approximately 75 minutes into the test with an ullage oxygen concentration of 11 

percent, increasing altitude to approximately 16,000 feet pressure altitude resulted in another 1 

percent change in the ullage oxygen concentration after stimulation. Figure 8 gives the results of 

this experiment on the four fuel loads examined during the test series with the values calculated 

using the previously discussed theory. These values represent the maximum increase in ullage 

oxygen concentration a static fuel load can have at the indicated altitudes. 

0

2

4

6

8

10

12

14

0 50 100 150 200 250

Time (mins)

Oxygen Concentration (% vol)

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

Pressure Altitude (ft)

 Oxygen Concentration

 Pressure Altitude

Ullage Recirculation at 3 Altitudes

60% Fuel Load

FIGURE 7. RESULTS ILLUSTRATING THE EFFECT OF A 60 PERCENT FUEL LOAD ON 

AN ULLAGE INERT TO 8 PERCENT USING ULLAGE RECIRCULATION AT THREE 

ALTITUDES 

15
0

2

4

6

8

10

12

14

16

0 5000 10000 15000 20000 25000 30000 35000

Altitude (ft)

Oxygen Concentration Increase (% vol)

 20 % Fuel 20% Fuel

 40 % Fuel 40% Fuel

 60% Fuel 60% Fuel

 80% Fuel 80% Fuel

Measured Calculated

Comparison of Test Data with O2 Mass 

Balance Calculations at Sea Level

FIGURE 8. RESULTS OF ULLAGE OXYGEN CONCENTRATION INCREASE FOR FOUR 

DIFFERENT FUEL LOADS USING ULLAGE RECIRCULATION AT THREE ALTITUDES 

The calculated values for increase in ullage [O2] at altitude tend to have poor agreement with the 

measured results presented. One possible reason for this is the consistency of the initial 

dissolved air in the fuel, which is difficult to ensure from test to test. 

4.2.2 Stimulation Comparison. 

To gage the ability of excess fuel gases to increase the ullage oxygen concentration during 

normal conditions experienced in a commercial transport fuel tank, the 40 percent fuel load 

altitude test illustrated in the previous section was repeated using both fuel pump stimulation and 

no stimulation (quiescent). The test illustrated that the ullage recirculation was causing the 

fastest, most complete change in oxygen concentration as was previously discussed, although at 

altitude, the fuel pump became effective at releasing excess gas, and, thus, increasing the ullage 

oxygen concentration at altitude. Pressure changes seem to drive the rate at which gas is 

released for the quiescent case; however, changing pressure alone seems to be a poor stimulus 

for gas release. 

4.2.3 Flight Test Simulation. 

To judge the ability of simple scale laboratory experiments to predict the resulting oxygen 

concentration of an ullage, given an initial ullage oxygen concentration and a flight profile, the 

data in figure 9 was examined. Examining the flight test data, the quiescent fuel release had the 

16
resulting oxygen concentration that most typified the results observed, although oxygen 

concentration increase due to air evolution was never directly observed, as the fuel tank was 

constantly being consumed during each test, bringing air into the tank ullage via the vent system. 

0

2

4

6

8

10

12

0 50 100 150 200 250

Time (mins)

Oxygen Concentration (% vol)

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

Altitude (ft)

 Ullage Recirc

 Fuel Pump

 Quiescent

 Pressure Altitude

40% Fuel Load

FIGURE 9. RESULTS OF ULLAGE OXYGEN CONCENTRATION INCREASE FOR 

A 40 PERCENT FUEL LOAD USING DIFFERENT STIMULATION METHODS AT 

THREE ALTITUDES 

Two experiments were performed using the 17 cubic foot test article in the altitude chamber to 

duplicate the initial tank conditions, the fuel usage, and the flight profile of two flight tests 

performed in conjunction with the Boeing Company. Figures 10 and 11 illustrate ullage oxygen 

concentration data from the Boeing flight test [4] for the 40 and 80 percent starting fuel load, 

respectively, compared with flight test simulation data from the altitude chamber. The data 

illustrates good agreement for the taxi, takeoff, and cruise portions of the flight using only the 

change in pressure of the ullage as a driver for releasing excess oxygen in the fuel. For the 40 

percent fuel case, the laboratory experiment predicted an ullage oxygen concentration about 0.4 

percent higher than observed in the flight test or a 3 percent difference, while for the 80 percent 

fuel case, the laboratory experiment predicted about 1.1 percent less resulting ullage oxygen 

concentration or a 7 percent difference than measured in the flight test. 

17
0

5

10

15

20

25

0 20 40 60 80 100 120 140 160

Test Time (mins)

Oxygen Concentration (% vol)

0

5

10

15

20

25

30

35

40

45

Altitude (1000 ft) / Fuel Quantity (%)

 Oxygen Concentration Oxygen Concentration

 Fuel Quantity Fuel Quantity

 Altitude Altitude

Lab Test Flight Test

FIGURE 10. BOEING FLIGHT TEST SIMULATION ULLAGE OXYGEN 

CONCENTRATION DATA COMPARED TO MEASURED FLIGHT TEST DATA FOR THE 

STARTING 40 PERCENT FUEL LOAD TEST 

0

5

10

15

20

25

0 20 40 60 80 100 120 140 160

Test Time (mins)

Oxygen Concentration (% vol)

0

10

20

30

40

50

60

70

80

90

Altitude (1000 ft) / Fuel Quantity (%)

 Oxygen Concentration Oxygen Concentration

 Fuel Quantity Fuel Quantity

 Altitude Altitude

Lab Test Flight Test

FIGURE 11. BOEING FLIGHT TEST SIMULATION ULLAGE OXYGEN 

CONCENTRATION DATA COMPARED TO MEASURED FLIGHT TEST DATA FOR THE 

STARTING 80 PERCENT FUEL LOAD TEST 

18
The ullage oxygen concentration was calculated based on the volume of air entering the ullage 

being equivalent to fuel consumption. Also, the maximum increase in ullage oxygen 

concentration from the fuel during the tests discussed in figures 10 and 11 was calculated in 

terms of the equilibrium oxygen concentration in the ullage, assuming the fuel load at sea level 

caused the maximum increase, and the fuel load at takeoff created the maximum increase at 

cruise altitude. This was done using the methods described in section 3.2 and illustrated in 

figures 4 and 8. The resulting oxygen concentration due to air entering the vent system (fuel 

usage) represents the minimum increase in oxygen concentration expected during a flight test. 

The sum of that and the change in oxygen concentration due to air evolution from the fuel load 

represents the maximum increase in oxygen concentration expected during a flight test. Table 1 

illustrates the results of these calculations for the two previously illustrated flight tests. The table 

illustrates the maximum calculated amount of ullage oxygen concentration increase is much 

greater than the measured flight test results. 

TABLE 1. CALCULATIONS OF ULLAGE OXYGEN CONCENTRATION INCREASE FOR 

FUEL USAGE AND AIR EVOLUTION FOR THREE DIFFERENT FLIGHT TESTS 

Item 

Calculated 

Fuel Burn 

Resulting 

O2

Calculated 

Air 

Evolution 

Change 

Calculated 

Maximum 

Resulting 

O2

Measured 

Change 

40% Fuel Load Boeing Test 11.4 2.2 13.6 12.1 

80% Fuel Load Boeing Test 14.1 7.5 21.6 17.0 

These calculations make it possible to estimate the minimum and maximum change in oxygen 

concentration as a result of both fuel usage and fuel air evolution. These values can be used as a 

band to predict the potential change in ullage oxygen concentration of a particular flight profile 

and fuel usage. Figure 12 illustrates the measured ullage oxygen concentration for the two 

aforementioned flight tests with the predicted bands of resulting oxygen concentration. The 

figure illustrates that much of the oxygen in the fuel remains in the fuel even after long periods 

of flight at cruise with a significant amount of fuel still remaining in the tank, as is the case with 

the 80 percent fuel Boeing flight test. To be accurate, the calculation would need to be done in 

small time steps to blend the evolving gases, given some determined time constant, with the air 

entering the tank in the ullage to give the constantly changing ullage oxygen concentration. 

Currently, each separate calculation assumes the other does not exist, thus, the sum of the 

calculations is conservative and greater than what would result from a more accurate timedependent simulation. 

19
0

5

10

15

20

25

30

0 20 40 60 80 100 120 140 160

Test Time (mins)

Oxygen Concentration (%Vol)

 80% Fuel Boeing Test

 40% Fuel Boeing Test

Flight Test Results Compared to 

Calculation Bands of Oxygen Evolution

Boeing 80% Fuel Test Calculated

Band with Air Evolution

Boeing 40% Fuel Test

Calculated Band with Air Evolution

Note: Calculation Band Represents Maximum and Minimum

 Resulting Oxygen Concentration Given 0% and 100% Air

 Evolution From Fuel 

FIGURE 12. FLIGHT TEST ULLAGE OXYGEN CONCENTRATION DATA COMPARED 

TO CALCULATED BANDS OF OXYGEN CONCENTRATION CHANGE 

5. SUMMARY OF RESULTS.

The ullage oxygen concentration will increase due to equalization of the partial pressures of 

dissolved oxygen in the fuel and ullage. The controlled experiments determined the resulting 

oxygen concentration change a fuel load can have on an adjacent ullage. At sea level, the 

increase in oxygen concentration can be as great as 7 percent for an ullage inerted to 6 percent 

with an 80 percent fuel load if the fuel is stimulated. Increasing altitude allows for an additional 

increase in the oxygen concentration of the ullage, even if the ullage was at equilibrium at sea 

level, due to the effect of the Ostwald coefficient and the reduced partial pressure of oxygen 

across the surface of the fuel. Inerting the ullage through fuel has the effect of scrubbing the fuel 

to some rudimentary level of protection that reduces or eliminates the increase in oxygen 

concentration due to fuel and ullage gas equalization. However, this fuel scrubbing method 

illustrates some significant increases in ullage oxygen concentration with higher fuel loads. 

Flight test data with fuel loads and inert ullages were best duplicated in laboratory experiments 

by not stimulating air to evolve from the fuel. Calculations of ullage and fuel gas equalization at 

altitude have poor agreement with presented results, but the resulting flight test oxygen 

concentrations fall within the calculated band based on ambient air replacement of consumed 

fuel and 100 percent partial pressure equalization of fuel and ullage gases. 

20
6. REFERENCES. 

1. Otte, Edwin E., Capt., and Lillie, Robert A., “Influence of Fuel Slosh upon the 

Effectiveness of Nitrogen Inerting for Aircraft Fuel Tanks,” AFAPL-TR-70-82, February 

1971. 

2. McConnell, P.M., Dalan, G., and Anderson, C.L., “Vulnerability Methodology and 

Protective Measures for Aircraft Fire and Explosion Hazards, Volume III Onboard Inert 

Gas Generation System (OBIGGS) Studies, Part 2 Fuel Scrubbing and Oxygen Evolution 

Tests,” AFWAL-TR-85-2060, January 1986. 

3. Burns, Michael and Cavage, William M., “Inerting of a Vented Aircraft Fuel Tank Test 

Article With Nitrogen-Enriched Air,” FAA report DOT/FAA/AR-01/6, April 2001. 

4. Burns, Michael and Cavage, William M., “Ground and Flight Testing of a Boeing 737 

Center Wing Fuel Tank Inerted With Nitrogen-Enriched Air,” FAA report 

DOT/FAA/AR-01/63, August 2001. 

5. Burns, Michael and Cavage, William M., “A Description and Analysis of the FAA 

Onboard Oxygen Analysis System,” FAA report DOT/FAA/AR-TN03/52, June 2003. 

21/22
APPENDIX A—GRAPH OF OSTWALD COEFFICIENT FOR OXYGEN AND NITROGEN

A-1
A-2
APPENDIX B—CALCULATION OF ULLAGE OXYGEN CONCENTRATION 

CHANGE DUE TO ADJACENT FUEL LOAD 

Fuel Level

Vu, Tu, POu

VF, TF, POf CO

Vent The effect of a fuel load on an inert ullage 

can be determined by considering the mass 

of oxygen dissolved in fuel and the mass of 

oxygen in the ullage as it pertains to the 

partial pressure of oxygen for the given 

medium. The partial pressure of oxygen is 

expressed in terms of the static pressure and 

the fraction of oxygen. 

O = S

*

2FractionOPP (B-1) 

In the case of fuel, the partial pressure of oxygen is a function of the Ostwald coefficient, which 

describes the amount of oxygen dissolved in fuel given a fuel temperature. 

O = SO

*

2FractionOPCP (B-2) 

At some preliminary state, the partial pressure of oxygen dissolved in the fuel is higher then the 

partial pressure of oxygen in the inert ullage. These partial pressures can be expressed in terms 

of mass given the equation of state. 

VP = mRT

U1 UO = U1 TRmVP UOO

O

OF FO

F C

TRm

VP FO

1

1 =

 

These equations can be restated as follows: 

UO

OU U

U TR

VP

mO

1

1 = (B-3) 

FO

OF OF

F TR

CVP

mO

1

1 = (B-4) 

 Given: PO = Partial Pressure of Oxygen 

 V = Volume 

 MO = Mass of Oxygen 

 T = Temperature 

 RO = Universal Gas Constant of Oxygen 

 CO = Ostwald Coefficient (see appendix A) 

B-1
Partial pressure of oxygen is denoted by the subscript O, and the subscripts U and F denote 

ullage and fuel respectively with 1 indicating the initial state. The Ostwald coefficient is a 

fractional number that describes the amount of gas that is dissolved in a fluid and varies with 

fluid temperature. 

Given the mass of oxygen in the tank remains constant, the following equation is exact. 

OU 2 OF 2 OU 1 OF1 + mm = + mm

or 

2 1 1 2 ( ) OF OU OF OU m = + − mmm (B-5) 

Given the partial pressure of oxygen between the two medium within the tank are not equal, 

oxygen will evolve from the higher partial pressure fuel to the lower partial pressure ullage until 

some equilibrium state (denoted 2) is obtained. This can be stated as follows: 

OU 2 OF 2 = PP (B-6) 

Given the relationship between partial pressure of oxygen and mass of oxygen, this gives 

OF

OF FO

U

OU UO

CV

TRm

V

TRm 2 2 =

Combining with equation B-5 and canceling RO gives the following equation: 

OUF

FU

O O O O CTV

TV

m mmm U U F U [( ]) ∗−+= 2 1 1 2

or 

OUF

FU

O

OUF

FU

O O O CTV

TV

m

CTV

TV

m mm U 2 U 1 F1 U 2 ( += ) −

Gathering MOu2 Terms and simplifying gives the following relation for the resulting mass of 

oxygen in an ullage adjacent to a fuel load. 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

+

=

OUF

FU

OUF

FU

O O

O

CTV

TV

CTV

TV

mm

m

U F

U

1

( ) 1 1

2 (B-7) 

B-2
Similarly, the equations for mass of nitrogen in the fuel and ullage can be derived with the 

subscript N denoting nitrogen in the following equations: 

UO

NU U

U TR

VP

mN

1

1 = (B-8) 

FO

NF NF

F TR

CVP

mN

1

1 = (B-9) 

Also the equations for the resulting mass of nitrogen in the fuel and ullage can be derived. 

2 1 1 2 ( ) NF NU NF NU m = + mm − m (B-10)

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

+

=

NUF

FU

NUF

FU

N N

N

CTV

TV

CTV

TV

mm

m

U F

U

1

( ) 1 1

2 (B-11) 

The resulting partial pressures of oxygen and nitrogen in the ullage can be determined 

U

OU OU

U V

RTm

PO

2

2 = (B-12) 

U

NU NU

U V

RTm

PN

2

2 = (B-13) 

By definition, the mole fraction of oxygen is equal to the partial pressure of oxygen divided by 

the resulting static pressure. 

2 ( ) 2 2

2 ][

OU NU

OU

PP

P

O + = (B-14) 

Assuming static pressure remains constant, oxygen volume percent is calculated by determining 

the partial pressure of oxygen (from equation of state) and dividing by the static pressure. 

Static

OU

P

P

O 2 ][ 2 =

B-3/B-4