Appendix 4
TABLE 4-1 Example of Wing Fuel Tank Flight Test Results (Part 1 of 2)
Shown at 10 minute increments for this example to save space.
Should be recorded at 1 minute intervals Delta T
Estimated Tau Tank Temperature Error Time OAT Mach Numbe r TAT Altitude Fuel Load Fuel Temp Fuel Temp TAT Actual vs.
Predicted
Min
Deg.
F.
Deg.
F,
Ft.
%
Deg. F.
Deg. F.
Deg F.
Predicte d Temp based on Tau value
0 80 0.00 80.0 0 100 80.0 0.0 149.8 80.0 0.0 10 80 0.00 80.0 0 100 80.0 0.0 149.8 80.0 0.0 20 80 0.00 80.0 0 100 80.0 0.0 149.8 80.0 0.0 30 80 0.00 80.0 0 100 80.0 0.0 149.8 80.0 0.0 40 80 0.00 80.0 0 100 80.0 0.0 149.8 80.0 0.0 50 80 0.00 80.0 0 100 80.0 0.0 149.8 80.0 0.0 60 80 0.00 85.9 0 100 80.0 -5.8 149.8 80.0 0.0 70 42.5 0.43 58.2 8750 100 79.5 21.2 149.8 79.4 0.0 80 5 0.57 32.7 17500 100 77.4 44.6 149.8 77.1 -0.2 90 -32.5 0.71 8.5 26250 100 73.9 65.4 149.8 73.4 -0.5 100 -70 0.86 -18.7 35000 100 69.2 87.8 149.8 68.3 -0.8 110 -70 0.86 -18.7 35000 100 63.8 82.5 149.8 62.7 -1.2 120 -70 0.86 -18.7 35000 100 58.8 77.4 149.8 57.3 -1.4 130 -70 0.86 -18.7 35000 100 53.9 72.6 149.8 52.4 -1.6 140 -70 0.86 -18.7 35000 100 49.4 68.1 149.8 47.7 -1.7 150 -70 0.86 -18.7 35000 100 45.1 63.8 149.8 43.3 -1.8 160 -70 0.86 -18.7 35000 100 41.0 59.7 149.8 39.2 -1.8 170 -70 0.86 -18.7 35000 100 37.2 55.9 149.8 35.4 -1.8 180 -70 0.86 -18.7 35000 100 33.6 52.3 149.8 31.8 -1.8 190 -70 0.86 -18.7 35000 100 30.2 48.9 149.8 28.5 -1.7 200 -70 0.86 -18.7 35000 100 26.9 45.6 149.8 25.3 -1.6 210 -70 0.86 -18.7 35000 98 23.7 42.4 146.9 22.4 -1.3 220 -70 0.86 -18.7 35000 94 20.7 39.3 142.2 19.6 -1.1 230 -70 0.86 -18.7 35000 90 17.7 36.4 137.5 16.9 -0.8
A4-4
Appendix 4
TABLE 4-1 (Continued) Example of Wing Fuel Tank Flight Test Results (Part 2 of 2) Shown at 10 minute increments for this example to save space Should be recorded at 1 minute intervals Delta T Estimated Tau Predicte d Temp based on Tau value Tank Temperature Error Time OAT Mach Numbe r TAT Altitude Fuel Load Fuel Temp Fuel Temp- TAT Actual vs.
Predicted Min Deg.
F.
Deg F.
Ft. % Deg F. Deg F. Deg F.
240 -70 0.86 -18.7 35000 87 14.9 33.6 132.8 14.3 -0.6 250 -70 0.86 -18.7 35000 83 12.2 30.9 128.1 11.9 -0.4 260 -70 0.86 -18.7 35000 79 9.7 28.3 123.4 9.5 -0.1 270 -70 0.86 -18.7 35000 76 7.2 25.9 118.7 7.3 0.1 280 -70 0.86 -18.7 35000 72 4.9 23.6 114.0 5.2 0.3 290 -70 0.86 -18.7 35000 68 2.7 21.3 109.3 3.2 0.5 300 -70 0.86 -18.7 35000 65 0.6 19.3 104.6 1.3 0.7 310 -70 0.86 -18.7 35000 61 -1.4 17.3 99.9 -0.4 1.0 320 -70 0.86 -18.7 35000 57 -3.3 15.4 95.2 -2.1 1.2 330 -70 0.86 -18.7 35000 54 -5.0 13.7 90.5 -3.7 1.3 340 -70 0.86 -18.7 35000 50 -6.6 12.0 85.8 -5.1 1.5 350 -70 0.86 -18.7 35000 46 -8.2 10.5 81.1 -6.4 1.7 360 -70 0.86 -18.7 35000 43 -9.6 9.1 76.4 -7.7 1.9 370 -70 0.86 -18.7 35000 39 -10.9 7.8 71.7 -8.8 2.1 380 -70 0.86 -18.7 35000 35 -12.1 6.6 67.0 -9.8 2.2 390 -70 0.86 -18.7 35000 31 -13.2 5.5 62.3 -10.7 2.4 400 -70 0.86 -18.7 35000 28 -14.1 4.5 57.6 -11.6 2.6 410 -70 0.86 -18.7 35000 24 -15.0 3.7 52.9 -12.3 2.7 420 -70 0.86 -18.7 35000 20 -15.8 2.9 48.2 -12.9 2.9 430 -70 0.86 -9.8 33906 17 -16.2 -6.5 43.5 -13.3 3.0 440 -20 0.67 22.1 22969 17 -10.4 -32.5 38.8 -8.1 2.3 450 30 0.48 55.6 12031 16 2.4 -53.2 34.1 4.0 1.7 460 80 0.28 80.0 1094 15 20.8 -59.2 29.9 23.5 2.7 470 80 0.00 80.0 0 15 36.2 -43.8 29.9 40.0 3.8 480 80 0.00 80.0 0 15 47.6 -32.4 29.9 52.3 4.7 490 80 0.00 80.0 0 15 56.0 -24.0 29.9 61.4 5.3
A4-5
Appendix 4
Comparison of Test and Calculated Fuel Temperatures -40 .0 -20 .0
0.0
20.0
40.0
60.0
80.0
100.0 0 200 400 600 Time -Mi nutes Fuel Tempe rature -De g F Tank Temp Deg F Predicted Temp based on Tau value Tank Temperature Error
FIGURE 4-1
Comparison of Test and Calculated Fuel Temperatures
5. Compliance Reporting. In order to show the fuel tank temperature modeling is acceptable using this method, the compliance report should include plots of the flight test data, as well as the predicted fuel tank temperatures and the associated error between the predicted temperature and those measured during the flight test.
A4-6
Appendix 5 APPENDIX 5
MEASUREMENT OF OXYGEN CONCENTRATION IN AIRPLANE FUEL TANKS
1. Introduction.
a. Background on Oxygen.
(1) Oxygen is the element most associated with life on our planet. It has an atomic number of 16 and is classified as an oxidizer, which means it reacts readily with other elements and is key to many reactions taking place. Oxygen is essential to every living thing. Air is a mixture made of 99.9 percent oxygen and nitrogen existing primarily on the molecular level in a single bond with itself, thus we use the chemistry term O2 when describing oxygen and N2 when describing nitrogen.
(2) Measuring oxygen in a given environment is accomplished by exposing a gas sample to a sensor designed to give feedback for a changing amount of oxygen in the sample. The various sensing technologies rely on one of several different natural properties of oxygen to change the voltage in a circuit, or more directly, the number of counts on an A/D converter. Virtually all oxygen sensors do not sense the volume percentage of oxygen in a sample, but rather, the partial pressure of oxygen in the gas sample in question. The analyzer is then calibrated for volume percent oxygen given a fixed sample pressure, flow, and temperature.
b. Types of Oxygen Sensors.
(1) Most sensing technologies work on some type of chemical reaction. A galvanic cell is a sensor containing a small amount of a liquid solution that reacts with oxygen, retained within a permeable membrane. The gas sample is exposed to the sensor membrane and oxygen from the sample enters or leaves the sensor solution to create equilibrium. The solution in these sensors is separated by an anode and cathode. A fixed current source changes the voltage across the anode and cathode given a changing amount of oxygen in the sensor solution (see Reference 1 of paragraph 4 of this appendix). This voltage can be converted to a fixed output with a voltage divider and some linearizing circuitry. Some sensors simply have a piece of chemical compound
(i.e., zirconium oxide) reacting between two electrodes. When a gas sample is exposed to the quantity of compound, the oxygen in the sample reacts with the compound and changes the voltage across the electrodes (see Reference 2 of paragraph 4 of this appendix).
(2) Virtually all sensors have a finite life, due to the fact that the reactants and solutions degrade and dilute over time. It is possible to measure oxygen with a truly nonconsumable sensor. One commercially available oxygen analyzer uses the principals of paramagnetism of oxygen to measure the volume concentration of oxygen. Because A5-1
Appendix 5 oxygen becomes magnetic when exposed to a magnetic field and the other constituents of air (nitrogen, carbon dioxide, and water vapor) do not, it is possible to make a detector that is sensitive to the amount of paramagnetic oxygen in an otherwise diamagnetic mixture of gases. In the case of the Rosemount 755R paramagnetic oxygen analyzer, a dumbbell-shaped, nitrogen-filled, hollow glass sensing body, wrapped with a single platinum wire, is suspended on a platinum/nickel alloy ribbon. When the sensing body is surrounded by a gas sample, “magnetic buoyancy” occurs creating a torque on the dumbbell. The current required to counteract the torque and restore the dumbbell to its null position is directly proportional to the amount of paramagnetic gas (oxygen) in the sample (see Reference 3 of paragraph 4 of this appendix).
(3) Some sensors do not require a dedicated gas sample to be removed from the environment to be measured, and thus can be used in situ, or literally in place. Some of these sensors work on the fluorescence properties of light, which change with partial pressure of oxygen in a given environment. These sensors detect changing fluorescence properties (intensity, time decay, etc.) of the light reflected from a coherent source using a spectrometer (see Reference 4 of paragraph 4 of this appendix). This measurement method is best applied in a controlled environment as the process is very sensitive to changes in temperature and pressure and all in situ methods impose unique calibration problems. In general, sensors that are reacting with oxygen make poor in situ sensors due to their propensity to consume oxygen in the general area of the measurement causing a type of sensor drift during measurements of quiescent environments.
2. Choosing a Measurement Method. To choose a measurement method several considerations must be made. The quantity of data as well as the number of gas sample locations required will determine the method of acquiring oxygen concentration for a flight test. The primary methods of measuring the oxygen concentration in a fuel tank ullage applied to date are continuous sampling, in situ measurements, and discrete sample measurements. Each primary method of measurement has an analyzer/sensor technology that is best applied given the measurement requirements. Each sensor and analyzer combination will have specific gas sample requirements. In the case of in situ measurements, sensitive sensors and fiber optic cables may need special protection.
Discrete (individual) measurements made during any test always require the carefully choreographed and documented actions of test personnel during the test. This can lead to problems with post-test analysis of the data if information becomes lost or mishandled.
Virtually all instrumentation has an effect on the environment being measured.
Regardless of the method used to measure oxygen concentration, care should be taken to minimize the sampling system impact on the ullage oxygen concentration. Ullage samples drawn from the fuel tank for measurement should be returned if possible to the tank in locations that minimize the effect of the samples taken for measurement. The analysis presented for compliance should include the effect of the instrumentation on the results.
a. Continuous Sampling. The ideal measurement of oxygen concentration in a fuel tank environment is instantaneous and continuous. This, however, is not very practical as A5-2
Appendix 5 some finite time is required to sample ullage gas and excite a particular sensor. By keeping gas sample lines short and small (diameter), while maintaining a relatively high sample flow rate, a gas sample system with a relatively short response time can be developed. Traditional sensors generally have a response time between 500 ms and 5 seconds, depending upon the technology applied.
(1) Flow Through Sensor System.
(a) One such method of continuous sampling was developed and applied by the FAA using flow through galvanic cells and a pressure regulated sample train. The sample train uses a powerful pump to draw a gas sample from the ullage sample location in question, through a forward pressure regulator, which regulates the pressure on the input of the sample train to a vacuum pressure lower than the lowest expected pressure in the sample environment (2.73 psia or 141 Torr @ 40,000 feet). The gas sample is then pumped up to a pressure to that just above sea level (14.7 psia or 760 Torr), allowing the gas sample to flow through a flow meter, the galvanic sensor, and then a check valve.
Lastly, the gas sample passes through a pressure controller that regulates the back pressure to a fixed value that can be maintained throughout the flight cycle. This pressure is critical to maintaining the calibration of the analyzer with the galvanic cell sensor. The gas sample can then be returned to the ullage to allow for a minimal effect on the sample environment.
(b) Calibration of this measurement methodology is accomplished easily by having a selector valve in the sample train before the input pressure regulator. The sample train is completely pressure regulated, providing calibration gas to the system input, allowing time for the system to stabilize, and adjusting the analyzer gain allows for calibration. It is best to check for adherence of your oxygen measurement system to a wide variety of calibration gases at least once before and after a test program. This is accomplished by calibrating in the traditional manner, and then checking several other calibration gases with different oxygen concentrations within the measurement range of the testing.
(c) When applying this measurement method to an airplane flight test, care must be taken to preclude liquid fuel or vapors from entering the cabin from the fuel tank gas sample lines. Traditionally, all sample lines are double walled and the shroud (outer wall) space is vented overboard to the exterior of the cabin. Leak checks of both the line and shroud prior to testing are essential. The measurement system sample volume should be isolated from the fuel tank with flash arrestors in the event of a failure that creates an ignition within the measurement system itself. Most oxygen sensors operate on very low power; however, complete isolation of the sensor, and other sources of energy within the sample system, from the fuel tank is essential in the event of a failure that could create a reaction within any portion of the flammable sampling system.
(d) The sampling of liquid fuel should be precluded by use of float valves on the ends of each sample tube in the fuel tank. Also, in the event of a failure that allowed liquid fuel into the sample lines, fuel should be prevented from entering the A5-3
Appendix 5 measurement system with either an automatic float shut-off switch or a warning system that allows for the operator to shut-down the sample system. The measurement system itself should also be shrouded and ventilated to allow for protection of the cabin environment in the case of a sample train leak. During ground operation, when natural suction is not available from the cabin exterior, the measurement system housing should also be continuously purged.
(e) A complete description of the galvanic cell continuous sample system applied by the FAA for the purposes of studying in-flight fuel tank inerting is given in Reference 5 from section 4 of this appendix.
(2) Flow-by Sensor System.
(a) Some sensors react significantly with the sample, and therefore may not be appropriate for returning the sample to the tank. A method of applying this type of sensor to a fuel tank environment is to create a volume flow sample train similar to the one described in the previous section. A small amount of this sample is then removed from the loop, in the positive pressure portion of the flow train, allowed to flow through the reacting sensor in question, and then deposited overboard. Minimizing the flow to the sensor helps minimize the effect of the consumed sample and creates a safer environment should the reaction of the ullage sample with the sensor become hazardous.
It may be necessary to remove flammable fuel tank vapors from the sample gas to safely apply some sensors for measuring ullage gas.
(b) Calibration of this measurement methodology is accomplished easily by having a selector valve in front of the sensor housing, outside of the primary sample loop. Care must be taken to ensure that the calibration gases flow through the sensor at same pressure and flow rate as the flow by sample. The flow-by sensor should stabilize quickly, allowing for adjustment of the analyzer gain. Again, it is best to check for adherence of your oxygen measurement system to a wide variety of calibrations gases at least once before and after a test program.
(c) Ensuring safety of a flow-by sensor installation is done much in the same manner as described in the previous section. Special care may need to be taken to ensure the reaction between the sensor and the ullage gas sample remains completely benign to the measurement system and the aircraft in general. It is for this reason that high energy sensors have not been applied to in-flight measurement applications.
Whatever installation is chosen, careful analysis and application is required, within the confines of the regulations, to minimize risk in accordance with the individual program plan developed.
A5-4
Appendix 5
FIGURE 5-1
Block Diagrams of Flow Through and Flow Bypass Sample Systems
b. In Situ Measurements.
(1) Application of in situ sensors for measuring fuel tank ullage oxygen concentration has been applied and observed by the FAA on a limited basis. However, recent advances in application of these sensor technologies make them a viable option for future in-flight oxygen concentration measurement in a fuel tank ullage. Optical methods such as fluorescence quenching are attractive to apply to measurements in fuel tanks due to the intrinsically safe nature of the energy required for the sensor, and in situ methods in general alleviate the need to remove flammable ullage gas from the fuel tank for sampling.
(2) Each in situ sensor will have its own unique installation issues due to the sensor size and shape, as well as any environmental limitations such as temperature.
Routing of any fiber optic cables will need to be within the requirements for the cable.
(3) The main difficulty with applying in situ sensors is the need to calibrate the sensor in place. This is not practical on a daily basis in an airplane fuel tank. This is complicated by the fact that optical methods generally have a very nonlinear response and need to be calibrated for a changing ullage temperature. In previous applications of fluorescence quenching sensors, the calibration was done in a controlled environment using multiple gases at multiple temperatures. The system was then installed in the tank and each channel was then “normalized” to a baseline intensity using air at some middle calibration temperature. This proved to be problematic given the variable of the baseline intensity of the different channels between the lab and airplane.
(4) To alleviate this, a sleeve assembly could be fabricated for the sensor that allows the sensor to be immersed in calibration gas through the sleeve, deposited from the outside of the fuel tank, to allow for daily checking of the calibration for several A5-5
Appendix 5 gases. At the very least, the sensors would need to be validated at a single point before every flight. This has been accomplished in the past by having the inert gas generation system create a fixed oxygen concentration gas for an extended period of time. This will eventually stabilize the tank at a single oxygen concentration and temperature to allow for validation of each sensor in the tank. It is easy to see that this could quickly become expensive and time consuming if the calibration had to be checked multiple times and/or with multiple gases on a daily basis.
(5) Safety requirements for equipment located within a fuel tank such as in situ measurement systems are already established in the existing § 25.981 and related sections (See AC 25.981-1B (or latest revision). These safety requirements establish limitations on power required for instruments installed and places limitations on materials for use in fuel tanks. Part 25 addresses analysis of failure modes in general (§ 25.1309) and sets general installation requirements (subpart F). The power required for any sensor installed in a fuel tank needs to be calculated to be less than the minimum ignition energy prescribed by the FAA for fuel tank installations. Any cables that need to be installed should be routed in accordance with standard industry practices within the requirements of all airworthiness regulations. Each individual installation needs to be analyzed and evaluated for risk within the confines of the regulations and agreed upon by all project stakeholders.
c. Discrete Sampling . Although not desirable, it is possible to evaluate the capability of a particular inerting system to allow for protection of a given fuel tank without acquiring continuous oxygen concentration data throughout a complete flight cycle. This may be because of advances in ullage oxygen concentration modeling or because of the existence of a large quantity of previously acquired data for a similar inerting system and application. Regardless, collection of discrete samples at several critical times for a few different ullage locations can provide a complete picture of capability of an inerting system to maintain an inert ullage in a specified fuel tank during a flight cycle for some applications. Discrete methods may provide a cost advantage over continuous sampling measurements and can be considerably less complex if large quantities of gas samples are not required. The primary methods of acquiring discrete samples are through vacuum bottle sampling or through single point calibration and sampling.
(1) Vacuum Bottle Sampling.
(a) One method of acquiring discrete ullage gas samples at various times is by the use of vacuum sample bottles, which are previously cleaned, evacuated, and hermetically sealed. Vacuum bottles are generally used by plumbing several bottles in series. Each bottle has an individual hand operated valve to let in a gas sample, to the desired sample line, which is located within the ullage of the fuel tank in question. Each vacuum sample bottle in the series will provide a single, discrete gas sample in time at that particular ullage sample location. A vacuum sample bottle generally has a second vacuum bottle adjacent to it which is deployed (valve opened) before deploying the A5-6
Appendix 5 actual vacuum sample bottle. This is done to “update” the sample line with a fresh ullage gas sample before opening the sample bottle to acquire the gas sample.
(b) Each sample location will have an associated series of vacuum sample bottles, depending upon the number of samples required and the individual test setup.
Typically these bottles are analyzed after completion of the test for oxygen content and whatever additional gas analysis is desired, by a laboratory that specializes in this kind of gas sample. It is critical to have a simple, well documented, plan for gas sampling to ensure that test personnel acquire the required gas samples at the appropriate time and allow for adequate documentation. As is always the case with human/test interaction, a series of procedures and checks are required to minimize the possibility of human error.
FIGURE 5-2
Vacuum Bottle Sample System Installed on an NTSB Test Aircraft
(c) The primary safety concerns with a vacuum bottle sample system are similar to those for a continuous sample system. All sample lines penetrating the pressurized bulkhead should be leak checked and shrouded to preclude the possibility of ullage gas or fuel entering the cabin. Additionally, each sample bottle series should be housed in a shroud which is ventilated overboard to the cabin exterior. The advantage of analyzing the gas samples after the flight test is that no additional safety features are required to protect the aircraft fuel tank from the potential ignition source of a sensor or associated gas sample regulation equipment.
A5-7
Appendix 5
(2) Single Point Calibration and Sampling.
(a) It is possible to use an analyzer and sample train to obtain discrete oxygen concentration measurements during a flight test. This can be accomplished by rapidly calibrating an analyzer sensor with a calibration sample gas, and then exposing the sensor to the desired ullage gas sample. This method would probably only be effective for measuring the oxygen concentration at times and tank locations with a relatively stable pressure, temperature, and oxygen concentration within the fuel tank ullage.
(b) All of the safety considerations associated with single point calibration and sample methods are identical to those of continuous sampling methods. For this reason alone, it may not be desirable to apply this methodology to a flight test scenario.
Most of the cost and complexity of a gas sample system installation for a flight test environment is attributable to the safety features (sample line shrouds, box ventilation system, and system safety analysis). To apply these methods and not install the additional equipment to continuously sample, at a relatively small cost and complexity, does not seem cost effective. Again, every installation and application needs to be analyzed for certification requirements, data ranges, and program risk, within the confines of the regulations, to ensure a successful, cost effective flight test.
3. Selections of Sample Locations. The primary consideration when selecting the location and number of gas sample ports is the physical geometry of the fuel tanks, as well as the compartmentalization of the fuel tank. The two types of fuel tanks considered are single bay tanks and multiple bay tanks.
a. Single Bay Fuel Tanks.
(1) Single bay fuel tanks have a single volume, broken up only by structural members, which do not impede the flow of gases inside the tank. It is possible to select a single gas sample location that is representative of the entire ullage average oxygen concentration, but without prior measurements during similar inerting testing or extensive computation calculations, it is difficult to ensure a single measurement location is representative of the entire ullage. For fuel tanks where air enters the fuel tank during descent, a continuous measurement of ullage oxygen concentration during a specified aircraft descent profile is essential to determining the level of protection afforded the tank by the inerting system and is the best measure of the sizing of inerting system for the specified fuel tank. The critical sampling location would typically be located near where outside air enters the tank. Therefore if a single sampling location is proposed, substantiation that the selected sampling location will provide oxygen measurements that demonstrate the tank is inert must be provided.
(2) Measurements too close to the airplane vent (air deposit) or the NEA deposit nozzle can provide a biased measure of the average ullage oxygen concentration.
It is possible that during the mixing of NEA and vent air into the tank, brief pockets of A5-8
Appendix 5 high or low oxygen concentration ullage gas may exist within the ullage. Without compartmentalized bay walls, these pockets will be transient and difficult to identify.
Mixing can be problematic in the vertical direction (between the top and bottom of the tank), particularly when considering low flow rates of air or NEA entering a tall thin fuel tank. If a fuel tank has associated radiant heat on the bottom (heated CWT), this problematic mixing will be small in nature and short lived.
(3) At the end of every test the aircraft fuel tank ullage will have a measured average ullage oxygen concentration after the aircraft has come to a complete stop and the inerting system has been shut off. By continued examination of this average over a period of 20-60 minutes during the operation of the aircraft air cycle machines (bottom heat), good vertical mixing can be validated. Any change in the average ullage oxygen concentration after the test is complete, with no additional air or NEA is entering the tank, can only be caused by changing oxygen concentration readings due to the mixing of the resulting ullage constituency. Again, without prior knowledge, the only way to ensure the inerting system is appropriate for the aircraft fuel tank is by the continuous measure of oxygen concentration at several strategic locations within the ullage during an appropriate descent flight test.
b. Multiple Bay Fuel Tanks.
(1) Multiple bay tanks are fuel tanks broken into several smaller volumes by solid or nearly solid web structures. These structures can be somewhat limited in their partitioning with large openings and extensive gaps between the web and the tank structure as is the case with many wing fuel tanks. These web structures can also be very solid in nature, with the only large openings being doors removed during maintenance and small web gaps for equalizing fuel and ullage gas, making a very compartmentalized tank.
(2) Fuel tanks with relatively open partitions may only require a few gas sample locations across many bays to allow for the accurate measurement of the overall fuel tank ullage average. Previous research highlighting inert gas distribution behavior during ground inerting may be beneficial to determining the number and location of gas sample ports. However this is not a guarantee of good air redistribution given the different nature of the NEA deposit and the vent opening. In the end, several more oxygen concentration sample locations may be necessary to allow for validation of inert gas distribution and ullage gas mixing during certain flight tests. These additional sample locations can be utilized when the mixing of the tank needs to be examined or validated using oxygen measurement channels redundant at that time.
(3) In the case of very compartmentalized fuel tanks, inert gas distribution may be problematic, depending on the sizing of the inerting system and the NEA deposit scheme selected. This generally requires at least one gas sample in each compartment, with long thin compartments perhaps requiring two gas sample locations. Bays that have a large vertical height, while still remaining relatively thin, may require an additional sample port at the bottom of the bay. Again, any differences in the oxygen concentration A5-9