# Presentation: 217622

## The Fuel Tank Flammability Assessment Method

**FRM Analysis **

## Program Overview

- In addition to performing a fuel tank flammability analysis, the program also allows the user to examine the impact that an FRM will have on tank flammability
- Tank flammability occurring when an FRM is in use is assumed to arise from one of two things:
**FRM Performance** – This refers to times throughout a flight that the FRM system is operating as expected, but is unable to protect the fuel tank due to limitations in its performance under certain operating conditions.
**FRM Reliability** – This refers to times throughout the operation of an aircraft that the FRM system in inoperable due to a failure of parts or systems.

## Program Overview – Main Calculations

## Performance Effects

- There are no direct inputs into the program regarding performance effects.  The user must instead model their FRM system in the program Visual Basic code.
- The user must ensure that the resulting FRM module accurately represents performance of the FRM system when operating as expected.
- Performance of the system under all flight conditions must be taken into account and must be shown to be accurate through testing and analysis of the FRM system.
- If additional data beyond what is provided by the program is required for the FRM module then the user is required to compute that information and add it into the program.

## Performance Effects

- The placement of the FRM module is identified in the program’s code with a comment statement that reads “TEST FRM ONLY”.
- The code following this statement is the default FRM performance code which replicates the effectiveness of a very basic inerting system.
- This code must be replaced by the user with code that reflects the FRM in use on the aircraft being analyzed.

## Performance Effects

- Depending on the type of FRM being utilized, there are numerous factors that need to be considered and addressed when modeling the performance effects of the FRM.
- Managing Heat Transfer to the Tank
- Cooling/Ventilation of Fuel Tanks
- Controlling Fuel Tank Pressure
- Limiting Oxygen Concentration
- Cross-Ventilation
- Oxygen Evolution
- Overnight Temperature Change

## Performance Effects – Oxygen Evolution

- When utilizing an inerting system, it is necessary to take into account the release of oxygen from the fuel.
- Air dissolves in fuel until the partial pressure of the dissolved air equals the local partial pressure of the ullage.
- Once the air is dissolved in the fuel, it will remain dissolved until the ullage partial pressure is changed.
- The resulting partial pressure of the dissolved gas can be significantly higher than the ullage partial pressure, potentially resulting in a large amount of oxygen being released into the ullage very quickly, thus affecting the resulting oxygen concentration in the ullage.

## Performance Effects – Oxygen Evolution

- The problem is approached by assuming total saturation of the fuel at the beginning of the flight, and determining the partial pressure of oxygen in the ullage and the fuel.
- It is then assumed that the oxygen release is driven by the difference of the two partial pressures and that there is some exponential mass transfer time constant to reach equilibrium.
- An effect for supersaturation, where the concentration of oxygen in the fuel has not reached equilibrium with that of the ullage can also be added by keeping the gas dissolved in the fuel (100% saturation) until a certain pressure differential is reached.

## Performance Effects – Oxygen Evolution

- This effect can be replicated by the user by assuming 100% saturation a normal atmospheric conditions during fueling (i.e. fuel is saturated with air at 21% O2).
- Under quiescent conditions, without altitude changes, the time constant used for gas transfer is 3500 minutes.
- For climb conditions, no gas transfer takes place until above 15,000 ft, and during the remainder of climb, the time constant used is 100 minutes.

## Performance Effects – Overnight Temperature Change

- As the airplane sits overnight, the temperature will change accompanied by an associated density change of the fuel tank ullage.
- The result is that the fuel tank will breathe in additional air as the ullage density increases.  The applicant must take this effect into consideration when modeling an FRM.
- This overnight ground period will occur for one flight per day out of an average number of flights per day.

## Performance Effects – Overnight Temperature Change

- To add this effect to the program, for any flight that will end with an overnight ground period, the model must choose a landing OAT as a random value from the following Gaussian distribution:
- The overnight temperature drop is then chosen as a random value from the following Gaussian distribution:

## Reliability Effects

- To determine the FRM reliabilty effects on tank flammability, additional user inputs are required:
**Mean Time Between Failure (MTBF)** - # of hours that the FRM is functional between failure events
**Failure Detection Capability (FDC) **- # of flights before a failure would be detected
**Minimum Equipment List (MEL) Assumption** – Average time, in flight hours, for FRM system to be restored once failure is detected

## Reliability Effects

- These reliability inputs are used by the program to calculate, for a period of 1,000,000 flights the percentage of time that the tank is at risk due to the inoperability of the FRM.
- The calculations involved in this determination are shown on the “Internal Calc 4” worksheet of the program, and are performed both for all flights involved and for warm day flights (ground ambient > 80 F)
- Some of the key things calculated on this worksheet include

## Reliability Effects

- The key calculations that go into determining the reliability effect are:
**Number of Failures** = 							Total flight hours/MTBF
**# of Flights/Hours with Undetected Failures** = 				Number of Failures/Flights x FDC
**Total # of Hours on MEL** =						Number of Failures x MEL
**Total # of Hours FRM Inoperable** = (b) + (c)
**Total # of Hours Tank is Flammable & FRM Inoperable**
**Reliability Contribution to flammability** = 				100 x (e)/(Total # of Flight Hours)

## Program Overview

## Slide 16

## Slide 17

## Program Overview – FRM Outputs

- Results of the FRM analysis are displayed in numerous tables on the FRM worksheet.
- Table displaying baseline (i.e., no FRM) flammability data and corresponding flammability data due to reliability issues of the FRM.
- Table displaying baseline flammability data and corresponding flammability data due to performance issues of the FRM for each phase of flight.
- Table showing overall FRM effectiveness (reliability and performance issues).

## Program Overview – FRM Outputs

## Program Overview – FRM Outputs