# Presentation: 660637

## 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
- Model / Instrumentation
- Test Data
- Effect of Holds at 5K Feet
- Running System on Ground
- Effect of Deposit Schemes
- Starting Descent Altitude
- Blocking Vent System Dive Port
- 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 during a typical commercial flight profile
- Use modeling results to validate modeling methods with full-scale data
- 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

## 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 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 used same 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 during above mentioned mission
- All tests had right side vent system blocked
- Some tests had aft port on open vent side also blocked

## OBIGG System Model

## Effect of Hold at 5K Feet 

- Baseline case (no hold) repeated with two different hold times
- Hold at 5K feet
- 5 and 10 minute holds
- Results indicate that holds using high flow mode have little effect on both tank average oxygen concentrations and worst bay oxygen concentrations
- System is depositing NEA at approximate oxygen concentration as tank
- 5-10 minutes not that long to improve distribution for relatively small spread

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## Effect of Hold at 5K Feet (Cont’d) 

- Ideally like to decrease tank average oxygen concentration before you use high flow mode to improve distribution
- Use low flow mode during a hold at 5K feet (lower’s average tank oxygen concentration)
- Switch back to high flow for final descent (distributes)
- Results illustrate this flow methodology improves overall average tank oxygen concentration at touchdown but doesn’t increase in the worst bay oxygen concentration
- Using high flow mode to distribute gas has diminishing returns
- Effect of sample system minimal when checked

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## Running System After Touchdown 

- Repeated baseline case touchdown data with two tests that run system on ground after touchdown
- 15 and 30 minutes system run times
- one test only sampled beginning and end of ground sit time
- Running System in Low Flow Mode After Touchdown did little for worst bay oxygen concentration
- Gave consistent benefit over time for tank average oxygen
- Running System in High Flow Mode After Touchdown did decrease bay oxygen concentration spread
- Average oxygen concentration changed little
- Again, diminishing returns on reducing spread

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

**(High Flow Mode)**

## Deposit Scheme Comparison 

- Compared running system after touchdown with two different deposit scenarios
- Duel deposit method deposits low flow mode in bay 6 and high flow mode in bays 1 and 3
- Multi-deposit method deposits all flow into bay 6 and bay 2 (approximate equal split)
- Baseline used low flow mode during 10 minute hold pattern
- Fancy deposit schemes did little to improve touchdown average or worst bay oxygen concentration
- More work needed
- Repeatability check gave excellent results

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## Starting Altitude Comparison 

- Compared original baseline descent case (from 39K feet) with two additional starting altitudes (32K and 25K)
- Used same altitude profile with different starting point (same descent rates) with no hold
- Approximated system performance best possible
- Results were as expected
- Starting altitude will have dramatic effect on resulting average oxygen concentration provided the tank is consistently inert for all cases

## 747SP Scale Fuel Tank Inerting Data

## Effect of Blocking Dive Port 

- Existing venting scheme does not give optimal inerting efficiency
- Illustrated in constant inerting tests
- Would improve by blocking dive port (aft vent port) on open side
- Repeated baseline test data (no hold) with this venting configuration
- No effect on average tank oxygen concentration
- Had adverse effect on distribution with 2% higher spike in worst bay and a full 3% greater resulting worst bay oxygen concentration
- Compared  two deposit schemes with this venting config
- Use 10 min hold for comparison case
- Only small effect observed on bay 1 spike and on result

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## 747SP Scale Fuel Tank Inerting Data

## Summary

- System could be slightly undersized
- Using high flow mode only, decent hold buy you very little in terms of average of worst bay oxygen concentrations
- Running system on ground after touchdown in high flow mode will decrease oxygen concentration spread, but has diminishing returns (when average oxygen concentration is near 12%)
- More elaborate deposit schemes and system methodologies give relatively small performance benefit, but could improve inerting capability significantly of a marginal system