Ground-Based Inerting of Commercial Transport Aircraft Fuel Tanks

North Atlantic Treaty OrganizationAVT Panel on Fire Safety and Survivability

Aalborg, DenmarkSeptember 23-26, 2002

Ground-Based Inerting ofCommercial Transport Aircraft

Fuel Tanks

William M CavageLead Engineer - Fuel Tank Inerting

FAA AAR-440, Fire Safety R&D Branch

AAR-440 Fire Safety R&D

Ground-Based Inerting Research___________________________________

Outline

• Background• Previous Research• Preliminary Research• Experimental Equipment• Analysis and Modeling• Results• Summary



• Three Accidents in Recent History– All Accidents had Explosions in Center Wing-Tank

– Explosions Occurred During/Just After Long Ground Operations on Hot Days with Empty (Residual Fuel) Center Wing Tanks

– Exact Ignition Source Not Found During Any Investigation

• FAA/Industry Seeking More Cost-Effective Inerting Methodologies– GBI is Inerting on Ground and then Continuing to Operate

– ARAC Committee Stated Could be Cost-Effective if Focused on Center-Wing Tanks (CWTs) Only

Background



• Macdonald and Wyeth - Fire and Explosion Protection of Fuel Tank Ullage, Circa 1970s

– Compares Different Methods of Reducing Flammability including Nitrogen Inerting

• Klueg, McAdoo, and Neese - Performance of a DC-9 Aircraft Liquid Nitrogen Fuel Tank Inerting System, 1972

– FAA Developed and Tested an Inerting System Using Stored LN2

• Stewart and Starkman - Inerting Conditions for Aircraft Fuel Tanks, 1955

– Comprehensive Work on Flammability Limits and [O2] Requirements

• Kuchta - Oxygen Dilution Requirements for Inerting Aircraft Fuel Tanks, 1970

Previous Research - Methods



• Ullage Washing Experiments (Inerting)– Quantitatively Determined Amount of Nitrogen Enriched Air

(NEA) Required to Inert a Fuel Tank Test Article• Simple Rectangular Tank with Single Deposit Nozzle and Single Vent

• Examined Different NEA % (Residual O2 Concentration) and Flow Rates as well as Examined Effects of Fuel Vapor and Temperature

• Ullage Washing is Term Describing Inerting by Ventilation

– Developed Nondimensional Relationships, Empirical Equation, and a Theoretical “Perfect Mixing” Solution

– Data Illustrated that a VTE of 1.5 to 1.6 is Needed to Inert an Ullage to 8% Oxygen by Volume with 95% NEA (5% [O2])

– Published Report DOT/FAA/AR-01/6

Preliminary Research



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0 0.5 1 1.5 2 2.5 3

Volumetric Tank Exchage

Tan

k O

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)

88% NEA

90% NEA

92% NEA

94% NEA

96% NEA

98% NEA

88 Cubic Ft Tank, 6 CFM Flow Rate

Rectangular Fuel Tank Inerting Data

VolumeTankFuel

RateFlowVolumeTimeExchangeTankVolumetric



• Fuel Effects on an Inert Ullage– Quantitatively Determined the Effect an Unscrubbed Fuel Load Can

Have on an Adjacent Inert Ullage• Simple Rectangular Tank with Single Deposit Nozzle and Single Vent

• Examined Fuel Loads of 20, 40, 60, and 80 Percent Full at Sea Level and Two Altitudes; Inerted to 6, 8, and 10 Percent

• Measured the Increase in Ullage [O2] due to Air Evolving from Fuel

– Circulated the Ullage Through the Fuel to Bring Tank to Equilibrium Quickly - Represents Maximum Fuel Effect on Ullage

– Study of Time Effects Showed Unless You Stimulate the Fuel, The Excess Air in Fuel had Small Effect on Ullage Compared to Air Entering Vent System (Fuel Burn)

– Report Pending Publication

Preliminary Research


Ground-Based Inerting Research___________________________________Inert Ullage Fuel Effects Data

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Altitude (ft)

Oxy

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ncr

ease

(%

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l) 20 % Fuel Load

40 % Fuel Load

60% fuel Load

80% Fuel Load

Ullage Recirculation at 3 Altitudes Inert to 8% Oxygen Concentration

Max Increase in Ullage [O2]Due to Adjacent Fuel Load



Preliminary Research• Lower Oxygen Content Study

– Performed Ignition Experiments with Model Fuel Tank in Pressure Chamber with both Propane and JP-8 at Reduced Oxygen Concentrations

• Tank Instrumented with Thermocouples and Sample Lines for Hydrocarbon and Oxygen Concentration Measurement.

• Heaters Placed Underneath the Tank Control the Liquid Fuel Temperature (Flammability).

• Used Single High Power Spark to Ensure a Reaction if Probable

• Study at Sea Level and Altitude to 38K Feet

– Follow on Tests Validated Previous Testing that the Critical Oxygen Concentration is Approximately 12% at Sea Level

– Report Pending Publication



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Altitude (kft)

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Ignition

Non-Ignition

Lower Oxygen Content Study Data



• GBI Proof of Concept Ground/Flight Testing– Joint Project with Boeing that Evaluated the Concept of Ground-

Based Inerting

– Examined the Effects of Wind and Flight Conditions on Ullage Oxygen Concentration of a Model 737-700

• Tank Instrumented with Gas Sample Ports

• NEA Distribution Manifold Installed in Tank by Boeing

– Inerted Tank with Ground-Supplied NEA to Approximately 8%

– Measured Oxygen Concentration at 8 Locations in CWT During “Normal” Ground and Flight Conditions

• Examined Effects of Quiescent and Simulated Wind Conditions on Ground and Representative Flight Conditions in Air

Experimental Equipment/Procedures


Ground-Based Inerting Research___________________________________Boeing 737-700 Test Article

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• Boeing 747SP Test Article– Decommissioned from Airline Service and Purchased by the

FAA for Ground Testing Only• All Major Systems Fully Operational

• Has Independent Power for Test Equipment and Instrumentation

– Center-Wing Tank Fully Instrumented• Gas Sample Tubing for Oxygen and Total Hydrocarbon Analysis

• 32 Thermocouples in Tank (Ullage, Fuel, Walls, Floor, and Ceiling)

• Additional Thermocouples and User Specified Oxygen Analysis Channels

• Installed NEA Deposit Nozzle in Bay 3

– Inerted Tank with One Deposit Scheme Under a Variety of Conditions




747SP Center Wing Tank



• Scale Boeing 747SP Center Wing Tank – Quarter-Scale Model of Boeing 747SP CWT was Built from

Three Quarter Inch Plywood By Scaling Drawings from Shepherd Report (24% Length Scale)

• Variable Deposit Manifold Allowed for NEA to be Deposited at Any Rate in any Bay(s) Desired

• Measured Oxygen Concentration in Each of the 6 Bays

– Examined Various Distribution and Inert Gas Flow Scenarios• Varied Flow and Purity to Validate existing Nondimensional and

Empirical Relationships

• Examined Depositing at Different Locations with Different Venting Schemes




747SP Scale Center Wing Tank Model



Analysis• Volumetric Tank Exchange is the Ratio of the Volume of

Deposited Gas to the Volume of the Tank

• Average Tank Oxygen Concentration for 747SP

• Equation for Oxygen Concentration Given Inerting Efficiency k

VolumeTankFuel

RateFlowVolumeTimeExchangeTankVolumetric

]6[13.0]5[13.0]4[10.0

]3[10.0]2[23.0]1[31.][

222

2222

BayOBayOBayO

BayOBayOBayOOAverage

)]1)([()( *2222

VTEkeOOOtONEAAmbAmb



Inert Gas Distribution Engineering Model• Model Calculates Inert Gas Distribution in 6 Bay Tank,

in terms of Oxygen Concentration Evolution, Given NEA Purity and Bay Deposit Flow Rates– Based on Original Single Bay Inerting Model, by FAA CSTA for Fuel

Systems, which Tracks Oxygen In and Out of Each Bay Assuming Perfect Mixing During the Time Step

– Assumes an “Outward” Flow Pattern and Splits Flow into a Bay to Adjacent Bays Using Out Flow Area Relationships

• Basic Formula for Volume of Oxygen in a Bay:

Bay

OBay V

tVtO

)()]([ 2

2

BayOsumBaynnBayOnBayNEAOO VtVtQVtVtQIGOFtQtVtV /)1(/)1()1()(2222



Results - 737 Proof of Concept Flight Test• CWT Was Rendered Inert from External Source with

Few Problems – Slightly more NEA Required then Predicted

• Oxygen Concentration of the CWT Rose Very Little over Time Provided Cross-Venting was Eliminated

• CWT Stayed Inert During Flights with Small Fuel Loads and Provided Some Protection with Medium to High Fuel Loads



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Time (minutes)

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Vent Modified for No X Flow

No Vent Modification

Climb and Cruise, 0 Fuel Takeoff at Time 0

737NG CWT Oxygen Concentration



737NG CWT Oxygen Concentration Increase

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80% Fuel Load

40 % Fuel Load

20% Fuel Load

Zero Fuel

Climb and Cruise with No X VentingTakeoff at Time 0


Ground-Based Inerting Research___________________________________Results - Full Scale GBI

• Single Deposit of Inert Gas in the Compartmentalized Tank Worked Well– Provided Efficient Distribution of Inert Gas with Fair Bay-Bay

Mixing

• Quiescent Inerting of CWT Could Be Problematic– Vertical Mixing Problems Caused Inefficient Inerting in Some

Cases

– More Work Needed to Determine if Vertical Mixing is Easily Stimulated

– Calculated and Compared Efficiency Factors


Ground-Based Inerting Research___________________________________747SP Inerting Data with Single Deposit

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Time (minutes)

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Bay 1 Bay 2 Bay 3

Bay 4 Bay 5 Bay 6

Vent

Boeing 747SP Inerting DataSingle Deposit; 140 CFM of 95% NEA

8% Line


Ground-Based Inerting Research___________________________________747SP Inerting Data with Different Vertical Mixing

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Time (mins)

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APU Off - Fair Mixing

APU On - Good Mixing

Boeing 747SP Inerting DataMixing Comparison, 95% NEA, ~ 20 Min Inerting

Start Inerting

Stop Inerting

Start ACMs

Tank Mixing

Well Mixed

Well Mixed

Start ACMs Tank Mixing Well Mixed

8% Line

ACMs Running


Ground-Based Inerting Research___________________________________747SP Calculated Inerting Curves with Different Vertical Mixing

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VTE

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Quiescent Inerting - Bad Mixing

Quiescent Inerting - Fair Mixing

APU On - Good Mixing

Boeing 747SP Inerting DataCalculated Curves Using Full-Scale Data

8% Line


Ground-Based Inerting Research___________________________________Results - Inert Gas Distribution Modeling

• Engineering Model Data Modeled Full-Scale Test Article Oxygen Concentration Trends Well but Magnitudes Off Somewhat

• Scale Tank Data Modeled Full-Scale Test Article Very Well– Bay 4 Results Skewed in All Methods

• CFD Data Modeled Fairly Well, with Some Changes to Model


Ground-Based Inerting Research___________________________________Engineering Model Data Comparison

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Overall VTE

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Bay 1 Bay 1

Bay 2 Bay 2

Bay 3 Bay 3

Bay 4 Bay 4

Bay 5 Bay 5

Bay 6 Bay 6

747SP CWT Inerting, Single Bay DepositComparison with Engineering Model Data

747SP Data Engineering Model


Ground-Based Inerting Research___________________________________Scale Plywood CWT Model Data Comparison

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Overall VTE

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Bay 1 Bay 1 Bay 2 Bay 2 Bay 3 Bay 3 Bay 4 Bay 4 Bay 5 Bay 5 Bay 6 Bay 6

747SP CWT Inerting, Single Bay DepositComparison with Scale Tank Data

747SP Data Scale Tank


Ground-Based Inerting Research___________________________________CFD Model Data Comparison

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Overall VTE

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Bay 1 Bay 1 Bay 2 Bay 2 Bay 3 Bay 3 Bay 4 Bay 4 Bay 5 Bay 5 Bay 6 Bay 6

747SP CWT Inerting, Single Bay DepositComparison with CFD Data

747SP Data CFD Data


Ground-Based Inerting Research___________________________________Summary

• Inerting a Fuel Tank from a Ground Source of NEA is Simple and Practical Method of Providing Protection to Heated CWTs When Needed Most

• Limited Distribution of Inert Gas is Required to Inert a Compartmentalized Tank Provided Modeling is Performed and Vertical Mixing is Examined

• Modeling of Inert Gas Distribution in a Compartmentalized Tank Can be Achieved with a Limited Amount of Resources

Documents

Ground-Based Inerting of Commercial Transport Aircraft Fuel Tanks