# Presentation: 661363

## Inerting of a Scale 747SP Center Wing Fuel Tank During a Typical Commercial Flight Profile

- William CavageAAR-440 Fire Safety ResearchFederal Aviation Administration

## Outline

- Background
- Scale Tank Model
- Instrumentation
- Testing Scope
- Inerting Efficiency
- Test Data
- Summary

## Background

- FAA is Seeking to Improve Upon Existing Fuel Tank Safety in Fleet in the Wake of TWA800 Air Disaster
- Inerting of Fuel Tanks Could Provide Significant Fuel Tank Protection.
- Focus of the testing is to validate the ability of the FAA simplified fuel tank inerting system to inert the CWT of a 747SP
- Use modeling results to validate the modeling tools developed during GBI studies
- Study inert gas distribution during the commercial mission

## Description of Model

- Quarter-scale model of Boeing 747SP CWT was built from three-quarter inch plywood by scaling drawings from Shepherd report
- 24% length scale (1.4% Volume)
- Spars and spanwise beams simulated with quarter-inch plywood installed in slats with scaled penetration holes
- Vent system simulated with PVC tubing plumbed to an aluminum vent channel
- Removable lid to allow for model maintenance and modification
- Model in 6x6x7 altitude chamber
- Model inerted with manual NEA mixer

## Photo of Model

## 747 SP Bay Diagram with Volume Data

- Reported Volume   =  1775

- Percent Difference  = 4.47 %

## Instrumentation

- Onboard oxygen analysis system (OBOAS) acquired bay oxygen concentration data
- One sample port in each bay
- Sample returned to tank through manifold
- Thermocouple in chamber gave temperature
- Altitude measured by absolute pressure transducer
- NEA Flow metered/measured with mass flow controller and oxygen concentration determined with flow through galvanic cell type oxygen analyzer

## Onboard Oxygen Analysis System Block Diagram

## Scale Tank Testing Block Diagram

- Altitude Chamber

- NEA Mixer

- Oxygen
- Analyzer

- Flow
- Controller

- DAS

- Nitrogen

- Compressed
- Air

- NEA Generator

- Computer

- T

- OBOAS

- Pressure
- Transducer

- Sample Return

- Scale Tank Model

## Scope of Testing to Date 

- All testing involved single deposit in bay 6
- All testing used generic flight profile with different cruise times and different holds at 5K feet
- All testing uses same predicted system performance in terms of NEA flow and purity
- All tests had same venting configuration
- Right side (left wing tip) vent system blocked
- Plans to block aft port on open vent side for some tests

## OBIGG System Model

## Inerting Efficiency

- Based on perfect mixing solution

- Develop inerting efficiency equation

- Redimensionalize perfect mixing solution

- with

## Volume Inerting Results 

- Inerted tank twice with similar, constant inerting conditions at two different altitudes
- Compared nondimensional results
- Results were similar, but not identical
- Average Tank Inerting Close
- Bay inerting trends good, but values can differ greatly
- Some data trends unusual
- Inerting efficiency values very different

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## Inerting Efficiency Results 

- Constant inerting data inerting efficiency trends similar but values very different
- Tend toward same value?
- When examining a complete simulation, k values vary greatly
- Trends are not consistent
- Makes prediction of inerting with perfect mixing solution difficult
- Some calculations respectable with a constant k
- Good k value found by iteration
- Can be very accurate in a narrow data band
- Never examined descent data

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## Descent Inert Gas Distribution Results 

- All descent tests had similar results regardless of hold times which was better then intuitively expected
- Worst bays (bay 1 & bay 3) were typically 13-14% oxygen by volume
- Other bays typically between 10% and 12% oxygen
- Test showed that sample system has small effect resulting distribution

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## 5K Hold Results 

- Performed several descent tests
- Started at 39k feet, low tank oxygen concentration
- Examined the effect of holding at 5K on the resulting tank oxygen concentration as well as the worst bay oxygen concentration
- Results indicate short hold helps reduce tank oxygen concentration, but exhibits diminishing returns on longer holds
- The resulting oxygen concentration of the tank was generally around 12% , which is the predicted NEA purity at the 5k feet
- Holds tend to improve distribution

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## Run System After Touchdown Results 

- Performed several descent tests
- Started at 39k feet, low tank oxygen concentration
- Obtained resulting tank oxygen concentration/distribution
- Ran “system” for 15 and 30 minutes at sea level in low flow mode (5% NEA)
- Results indicate running system in low flow mode decreases the average tank oxygen concentration well, but does little for bay 1
- Bay 3 is usually worst bay at touchdown, but generally decreases oxygen concentration quickly

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## Summary

- Inerting efficiency calculations could be used to predict average tank inerting in limited cases only with manipulated k values
- Preliminary descent modeling illustrates good distribution of inert gas with resulting average tank oxygen concentrations of approximately 12%
- Short holds at 5K feet will improve the inert gas distribution in the tank and help lower the tank average oxygen concentration
- Running the system on the ground for 15 to 30 minutes in low flow mode will improve tank oxygen concentration but has little effect on the bay 1 oxygen concentration