# Presentation: 501755

## An Overview of Commercial

- Transport Airplane Fuel Tank
- Inerting Research

- William M CavageLead Engineer - Fuel Tank Inerting FAA AAR-440, Fire Safety R&D Branch

## Outline

- Background
- Previous Research
- Preliminary Research
- Full-Scale Fuel Tank Inerting
- Inert Gas Distribution Modeling
- Onboard Inert Gas Generation

## Background - Accident History

- 1990: 737-300 - Manila, Philippines
- 1996: 747-100 - New York, United States
- 2001: 737-400 - Bangkok, Thailand (Investigation Pending)
- 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

## Background - Recent Regulatory Activity

- 737, 747 Fuel Tank Wiring, 747 CWT Terminal Blocks, FQIS
- Scavenge Pump Inspections, Related Equipment
- Airworthiness Directives Issued
- Various Fuel Pump Wiring/Connectors Inspect/Replace
- Scavenge Pump and Surge Tank Flame Arrestor Installation
- FQIS Wiring, Terminals, and Connectors Inspect/Replace
- Special Addition to FARs (SFAR 88)
- Requires Re-Validation of all Fuel Tank Safety Analysis for Existing Large Commercial Transport Category Airplanes
- New Type Certificate Airplanes Must Minimize Flammability of Center-Wing Type Fuel Tanks to Be Equivalent to Wing Tanks

## Background - Proposed Flammability Rule

- Wrote Tasking Statement and Coordinated with JAA and ARAC and Published Notice in Federal Registry
- ARAC Committee Formed Working Group
- Congressionally Sanctioned Industry Oversight Committee that Advises FAA on Rulemaking
- ARAC Working Group is made up of Industry Experts, Government Regulators, and Consumer Advocates
- Focused on Analyzing Inerting Methods that could be Potentially Cost-Effective for the Commercial Fleet
- Performed Extensive Cost Benefit Analysis
- Report Pending Industry/Government Consensus

## Previous Research - Methods

- Compares Different Methods of Reducing Flammability including Nitrogen Inerting
- Emphasized Requirements for Low Weight and Reliability
- 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
- System was Big, Heavy, and Required Extensive Ground Support
- Yagle, et al. - Performance Tests of Two Inert Gas Generator Concepts for Airplane Fuel Tank Inerting, 1983
- Compared Pressure Swing Absorption (PSA) with Hollow Fiber Membrane (HFM)

## Previous Research - Requirements

- Most Comprehensive Work to Date on Flammability Limits and Oxygen Concentration Requirements
- Pre-Dates JP-8 (Jet A)
- Much of the Work is Theoretical with Limited Validation Points
- Kuchta - Oxygen Dilution Requirements for Inerting Aircraft Fuel Tanks, 1970
- Presents Minimum Oxygen Requirements for CO2 and N2 From a Large body of Spark Ignition Data
- No JP-8 Data (some Kerosene), Very Little Altitude Data

## Previous Research - Requirements (cont’d)

- Extensive Work on Ground Fires (Post Crash Scenarios) Causing Wing Fuel Tank Explosions
- Largely Destructive Work had Older Test Articles

## Preliminary Research

- 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

## Rectangular Fuel Tank Inerting Data

## Preliminary Research

- 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
- Report Pending Publication

## Inert Ullage Fuel Effects Data

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

## Preliminary Research

- Joint Project with Boeing that Evaluated the Concept of Ground-Based Inerting
- GBI is Inerting on Ground, Perform Ground Operations, and Fly
- Examined the Effects of Wind and Flight Conditions on Ullage Oxygen Concentration of a model 737-700
- Inerted Tank with Ground Supplied NEA to Approximately 8%
- Measured Oxygen Concentration at 8 Locations in CWT During “Normal” Ground and Flight Conditions
- Testing Illustrated that the Benefit from Inerting was Substantial
- Must Eliminate/Reduce CWT Cross Venting with Vent System Mod
- Some Benefit Even With Small Fuel Loads
- Published Report DOT/FAA/AR-01/63

## GBI Flight Test Data

## Preliminary Research

- 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
- Testing Illustrated that Equally Deposit NEA 95 will Allow for Comparable Results to the Simple Tank (VTE of 1.5-1.6 to 8%)
- More Importantly, Uneven Deposit of Inert Gas Resulted in More Efficient Inerting (Less Gas to Inert the Same), Particularly when Half the Vent System Was Blocked
- Published Report DOT/FAA/AR-02/51

## Boeing 747SP Scale CWT Model Inerting

## 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 Concentration
- 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

## Lower Oxygen Content Study Data

## Full-Scale Fuel Tank Inerting

- 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)
- Other Instrumentation
- Additional Thermocouples in Pack Bay, NEA Deposit, Cabin etc.
- 4 User Specified Oxygen Analysis Channels (Vent Channel, Dry Bay, etc.)
- Some Weather Data Acquired
- Full Complement of Ground Service Equipment

## Boeing 747SP in Photo

## 747SP Center Wing Tank Top Diagram

## Full-Scale Fuel Tank Inerting

- 747SP is Plumbed with a Single Deposit Manifold in Bay 3 Which was the Optimal Case (most efficient inerting), Developed from the Scale Tank Testing, with Half the Vent System Blocked
- Inerted Tank with Different NEA Purities and Different Flow Rates in an Attempt to Validate Existing Knowledge Base and Assumptions
- Limited Amount of Data to Date
- Preliminary Data Shows Deposit Methodology and Modeling Work was Sound and Productive
- Preliminary Data Looks Good as long as Mixing is Promoted by Operating the ACMs

## 747SP Center Wing Tank Inerting Data

## 747SP Center Wing Tank Inerting Data

## Full-Scale Fuel Tank Inerting

- Quiescent Full-Scale Testing has Indicated that Inerting of the 747SP in the Optimal Deposit Method Developed with the Plywood Model can be Problematic (No ACMs Operating)
- Inerting Efficiency Different from Test to Test
- Measured Strong “Vertical Effects” in one Full Length Bay
- Bottom of Bay [O2] Lags Behind Top of Bay on Quiescent Tests
- Complicated by the Fact that Sample System only Samples from Top
- Preliminary Data Shows Effect is Not NEA Temperature Dependant but is Effected by the NEA Temperature
- Developed Inerting Efficiency Coefficient to Compare Tests
- Back Calculate VTEs between 1.2 and 2.0 to reach 8% [O2] with 95% NEA

## 747SP Inerting Data with Different Vertical Mixing 

## 747SP Calculated Inerting Curves with Different Vertical Mixing 

## Inert Gas Distribution Modeling

- Preliminary Models
- Original Simple Inerting Model Developed by Ivor Thomas (FAA CSTA for Fuel Systems)
- Tracks the Volume of Oxygen In and Out of a Tank and Calculates Oxygen Concentration Given the Tank Volume
- Uses a Basic Spreadsheet Layout and Runs Instantaneously Given the Volume of the Tank, The Flow Rate and Purity of the NEA
- Basic Formula for Model
- Model Results Compared Well with Ullage Washing Data

## Simple Inerting Model Results Compared with Ullage Washing Data 

## Inert Gas Distribution Modeling

- Scale CWT Model
- Original Plywood Model Results Encouraging, But NEA Distribution Did Not Compare Well with Full-Scale Data
- Made Several Improvements to Model
- Better Modeling of Vent System Cross Sectional Flow Areas
- Ensured Lid Did Not Leak Around Bay Tops which Would Alter Flow Pattern
- Performed Additional Testing
- Scale Tank Inert Gas Distribution Results Compared Very Well to Data Acquired on 747SP full-scale test article
- Additional Testing Planned for Different Deposit Methods (Onboard System)

## Scale Plywood CWT Model

## Scale Plywood CWT Model Data Comparison

## Inerting Model Development

- Multiple-Bay Inerting 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 Inerting Model by Ivor Thomas 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
- Presently Does Half Blocked Venting Case Only
- Compared with Full-Scale Test Article
- Must Run ACMs to Obtain Data that Agrees with Engineering Model (Which Assumes Perfect Mixing)
- Agrees Best for Single Deposit Case Compared with Scale Tank Data

## Engineering Model Assumed Flow Pattern

- Bay 1

- Flow Out

- Bay 2

- Flow In

- Bay 3

- Bay 4

- Bay 5

- Bay 6

- Flow Out

## Engineering Model Data Comparison

## Full-Scale Data Compared with Modeling Methods

## Onboard Inert Gas Generation System

- Popular Methods of Generating NEA or N2
- Pressure Swing Absorption (PSA)
- System Pressurizes a Separation Bed with Air and Allows to Dwell
- Can Generate High Purity N2 and O2 (OBIGGs/OBOGs)
- Systems can be Designed to Have High Output Volumes
- Needs High Pressure Bleed Air / Many Complex Moving Parts
- Hollow Fiber Membrane Gas Separation
- Hollow Polymeric Membranes Are Wove and Bundled in Canisters  -Pressurized Bundle Ends Separates Fast and Slow Gases in Air
- Can Operate at Low Pressures and Systems Have very Few Moving Parts
- Can Not Generate O2, Inefficient at Producing NEA above 99% Nitrogen
- Distillation / Cryogenic
- New Technology Allows for Small Scale Liquefaction of Air Components
- Low Output so Storage is Needed

## Onboard Inert Gas Generation System

- Commercial Transport Onboard System Development
- Optimized a System for Inerting Boeing 747SP CWT on Ground
- System Designed to Inert Empty CWT below 12% in less then 90 Minutes with Compressor.  Can Inert in Half that time with Bleed Air
- System Installed in Pack Bay Area with Crashworthy Mounting
- System Weight Approx. 240 lbs Plus Deposit Plumbing and Vent Modification
- System Designed to Provide Some Cargo Bay Fire Protection
- Replace a Bleed System for 747SP
- Specified all Airworthy, Commercially Available Parts
- Some Parts will be for Ground Test Only
- Compressor is Modification of Existing Design

## OBGI System Block Diagram

## 3-D Rendering of System Installation

## Closing Remarks

- Addition Information
- Contact Me @
- AAR-440 Fire Safety Branch, Building 204
- Wm. J Hughes Technical Center
- Atlantic City Int’l Airport, NJ  08405
- (609) 485 - 4993
- william.m.cavage@faa.gov
- Fire Safety Web Site @ www.fire.tc.faa.gov
- Inerting Work is Under *Systems Fire Group* / *Fuel Tank Safety*
- More Information on Projects and Facilities
- Download Reports and Presentations **(Including This One!!!)**
- International Systems Fire Protection Working Group
- Open Forum to Discuss FAA/Industry R&D in Area of Aviation Fire Protection (Including Fuel Tank Safety) - Register on Web Site