1、_ SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising there
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4、0 (outside USA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.org SAE values your input. To provide feedback on this Technical Report, please visit http:/www.sae.org/technical/standards/AIR5691 AEROSPACE INFORMATION REPORT AIR5691 Issued 2013-01 Guidance for the Desi
5、gn and Installation of Fuel Quantity Indicating Systems RATIONALE This guidance document has been produced in an effort to record best practice in fuel gauging system design and indicate those things which should be considered in the design of a new fuel gauging system in order to achieve an accurat
6、e and reliable fuel quantity indicating system (FQIS). It is also intended to capture changes in design practice since ARINC 611-1 was issued in 1999. FOREWORD Airlines and aircraft operators continue to experience an assortment of in-service problems caused by fuel gauging systems. The complex natu
7、re of the system and its importance to the operation of the aircraft tends to produce faults that need to be repaired prior to further flight. This is especially true on older systems where the probes in a tank were treated as a single input. These faults typically are inconvenient to repair and oft
8、en require an extensive amount of time to isolate and correct. With more advanced systems where each probe is connected individually to the processing electronics this can be less of an issue, and flights can continue with a single probe failure or even multiple probe failures in some cases, allowin
9、g failures to be addressed as a scheduled maintenance issue. Older systems often exhibited a large number of in-flight faults that can be difficult or impossible to reproduce on the ground. This, combined with the limited BITE (Built-In Test Equipment) facilities on older systems has led to many abo
10、rtive attempts at trouble-shooting and thousands of man-hours of unproductive effort spent in trying to determine the root cause of the fault. The primary problem with older FQIS systems has been associated with the electrical connections and connectors and the need to approach troubleshooting from
11、a fundamental physics approach (i.e., what could impact shifts in capacitance value). The fuel tank wiring harnesses with associated connectors are often the cause of the problem due to the size (gauge of wire), shielding, system complexity, routing of wire and moisture ingress. Problems in this are
12、a are brought about by the systems susceptibility to grounding anomalies on shielded cables (particularly applicable to older systems based on 400 Hz AC capacitance measuring technology). The problem most often observed is due to the wire harness shields no longer being properly grounded, and thus t
13、he noise rejection of the system is degraded and the harness capacitance can cause large errors in FQI reading. The replacement of a wiring harness requires typically up to 1 day of actual replacement time, once the replacement wire harness(es) are received, which generally equates to 3 days of airc
14、raft downtime. Often there is the necessity to replace the entire wiring harness, when frequently the fault is caused by just one faulty wire, as it is not always obvious where the intermittent failure is. Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproductio
15、n or networking permitted without license from IHS-,-,-SAE AIR5691 Page 2 of 63 Other shortcomings of capacitance systems, based on widespread experience with capacitance technology, are problems associated with the presence of water in the fuel. Water can be present in solution when the fuel is loa
16、ded (uplifted). The water solubility in fuel increases with temperature, conversely, as the fuel temperature decreases the water precipitates out of the fuel, leading to more free water in the tank, which ultimately changes to ice. The water settles at the bottom of the tank and can build up to such
17、 a level that it starts to rise between the concentric tubes of capacitance probes and may cause erroneous measurements. Liquid water has a very high dielectric constant when compared to aviation fuels (nominal 40 to 1 ratio) that introduces significant changes in measured capacitance for fuel senso
18、rs that are immersed in water, even if that immersion affects only the lower few millimeters of the probe. It must be noted that frozen water is not conductive and has a dielectric constant not far from fuel (1.4 to 1 ratio) and so does not have the same effect as free water. Traditional fuel system
19、s address water condensation by frequent sumping/water drain activities from the fuel tanks and placement of the fuel sensors sufficiently above the bottom of the tank to mitigate water accumulation effects. Fuel probes are designed to readily shed water from collecting on the inner surfaces of the
20、capacitive sensor. These measures do not solve the problem entirely, but reduce it to a practical level. Ultrasonic fuel quantity systems also have issues with water as there is a boundary layer between fuel and water, and reflection of the ultrasonic signal from this boundary can be read as fuel le
21、vel. Ultrasonic systems can have other issues such as temperature cycling extremes experienced by the fuel tanks putting thermal stress on the ultrasonic transducer, so the sensor quality is paramount. In addition, air bubbles in fuel will cause the sensor not to be able to read the fuel height, as
22、the lower surface of the bubble will reflect the ultrasonic signal, leading to a lower height reading for sensors affected by bubbles, This is particular problem during climb, where the fuel will out-gas rapidly producing many bubbles. A bigger problem area is the perceived poor accuracy of fuel qua
23、ntity systems (no matter what the base technology) from the viewpoint of operating crews. Discrepancies between the FQIS and the bowser meter on dispensing trucks during refueling can lead to delays where the loaded quantity must be checked. Crews may uplift more fuel than required as a contingency
24、factor to compensate for perceived inaccuracies. Declared system accuracy can have an effect on the required fuel load, with less accurate systems requiring extra fuel to be loaded. A more accurate system results in less fuel needed as a contingency factor. However, several surveys have shown that i
25、mproved accuracy of better than 1% would not allow a more refined fuel loading, and is not of economic benefit to the airlines. Even where extra fuel is not uplifted, there will be a direct cost of carrying additional fuel when the gauges are under-reading. It should be noted that the perceived inac
26、curacies in older gauging systems is often due to undetected or unannunciated failures rather than design inaccuracy. When determining the desired fuel system accuracy, be it 1%, 2% or greater, the aircraft design and operational considerations need to be considered to meet the market requirements.
27、Standards of acceptability of FQIS error, especially for large airplanes, have changed. Error magnitudes which were considered satisfactory in the era of low fuel costs, when many systems were designed, are no longer acceptable. “Normal“ variations in fuel characteristics such as permittivity/dielec
28、tric constant (K) and density (D) can cause unacceptable errors if these characteristics are not measured directly. The variations have been tolerable in the past when most systems did not make direct measurement of one or both these characteristics, but are no longer tolerable in higher accuracy sy
29、stems. The problem is further aggravated with the introduction of new sources of crude oil in the past decade, and the recent introduction of alternate fuels (“bio-fuels“ and “synthetic fuels“: fuels created from non-petroleum sources). Finally, assignment of inexperienced personnel to a particular
30、aircraft often results in “trial and error“ or “shotgun“ troubleshooting, partly as a result of the inherently ambiguous system BITE (Built-In Test Equipment). Frequently the most accessible components, which are not necessarily defective, are removed and replaced (normally the processor or indicato
31、r). Operating in this manner has several disadvantages. Maintenance costs are driven upwards because of testing unverifiable failures in the shop. Also, operators experience needless departure delays. Specialized test equipment addresses most of the troubleshooting concerns, but it requires costly e
32、quipment and personnel that have the necessary training/schooling to be able to utilize them to have an advantageous effect. Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR5691 Page 3 of 63 T
33、ABLE OF CONTENTS 1. SCOPE 6 1.1 Purpose . 6 2. REFERENCES 6 2.1 Applicable Documents 6 2.1.1 SAE Publications . 6 2.1.2 ARINC Publications 7 2.1.3 ASTM Publications 7 2.1.4 EASA Publications 7 2.1.5 ESA Publications . 7 2.1.6 FAA Publications . 8 2.1.7 RTCA Publications 8 2.1.8 U.S. Government Publi
34、cations 8 2.1.9 UL Publications . 8 2.1.10 CRC Publications 8 2.2 Related Publications . 9 2.3 Definitions, Symbols, and Terminology 9 2.3.1 Definitions . 9 2.3.2 Acronyms 10 2.3.3 Terminology 10 3. OVERVIEW OF GAUGING TECHNOLOGIES AND FUEL CHARACTERISTIC MEASUREMENTS . 12 3.1 General . 12 3.1.1 Cap
35、acitance Gauging . 12 3.1.2 Ultrasonic Gauging . 15 3.1.3 Comparison of Capacitance and Ultrasonic Measurement Techniques 16 3.2 Other In-Tank Sensors 19 3.3 Dielectric Constant (Permittivity) Measurement 19 3.3.1 Impact of Inerting 22 3.4 Velocity of Sound Determination . 22 3.5 Density Measurement
36、 . 23 3.5.1 Default Value . 24 3.5.2 Density Calculated From Permittivity 24 3.5.3 Density Calculated From Velocity of Sound . 25 3.5.4 Densitometer Density 26 3.5.5 Temperature Measurement 27 3.5.6 Effect of Synthetic Fuels on Permittivity and Density . 27 4. SYSTEM DESIGN CONSIDERATIONS . 27 4.1 G
37、eneral . 27 4.2 Accuracy Requirements 28 4.2.1 General . 28 4.2.2 Operational Accuracy Goals . 28 4.3 Accuracy Analysis and Error Sources 30 4.3.1 Scope of Accuracy Analysis 30 4.3.2 System Errors . 30 4.3.3 Random and Bias Errors . 30 4.3.4 Methods of Accuracy Analysis 31 4.3.5 Error Sources 32 4.3
38、.6 Error Sources in Accuracy Analysis 32 4.3.7 Error Sources in System Design . 41 4.4 System Architectures 44 4.4.1 General Architecture Considerations 44 4.4.2 Influence of AC25.981-1C and Composite Aircraft on FQIS Architecture 48 4.4.3 System Power Supply . 49 4.4.4 Sensor Isolation 49 Copyright
39、 SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR5691 Page 4 of 63 4.4.5 Use of Shielded Cable 49 4.4.6 In-tank Electronics 50 4.4.7 LRU Adjustments 50 4.4.8 Reliability and Redundancy: . 50 4.4.9 Effec
40、t of Faults 50 4.5 Fault Diagnosis and Maintainability 50 4.5.1 Built-In Test Equipment (BITE) . 50 4.5.2 Maintenance Considerations 51 4.6 System Integrity 51 4.6.1 Integrity . 51 4.6.2 Lightning Protection 51 4.7 Intrinsic Safety . 52 4.7.1 Maximum surface temperature . 52 4.7.2 Intrinsically Safe
41、 Signals . 52 4.7.3 Filament Heating Energy Limit 53 4.7.4 Wire Separation 53 4.8 Aircraft Data 54 4.8.1 Standard Data Labels . 54 4.8.2 Display of Fuel Quantity 54 4.8.3 Fuel Low Level Indication . 55 5. INSTALLATION CONSIDERATIONS . 55 5.1 Sensor Installation. 55 5.2 Wiring Harnesses 56 5.2.1 Harn
42、ess Installation 56 5.2.2 Electrical Wiring interconnect Systems - EWIS 56 5.2.3 Connections 57 5.2.4 Strain Relief and Routing 57 5.2.5 Tank Wall Connectors . 57 5.3 External Tank Component Installation 58 6. IN-TANK ENVIRONMENT 58 6.1 Fuel Types 58 6.2 Alternative Fuels . 58 6.3 Microbial Growth .
43、 58 6.4 Water . 59 7. LESSONS LEARNED . 60 7.1 Experience from Airframers 60 7.2 Experience from Gauging Suppliers . 60 7.3 Silver Oxide Deposits 60 8. NOTES 61 APPENDIX A COMMON JET FUEL TYPES . 62 APPENDIX B AVIATION JET FUEL ADDITIVES NOT COMPREHENSIVE - BASED ON ASTM D1655 . 63 FIGURE 1 CONCENTR
44、IC TUBE CAPACITOR IN AIR 13 FIGURE 2 PARTIALLY IMMERSED PROBE . 13 FIGURE 3 ULTRASONIC PROBE OPERATION . 16 FIGURE 4 LINE OF BEST FIT FOR JET A FUEL SAMPLES FROM ARINC 611 20 FIGURE 5 JET A PERMITTIVITY SPREAD FOR FUEL SAMPLES 21 FIGURE 6 VELOCITY OF SOUND VARIATION FROM ARINC 611 DATA 22 FIGURE 7 V
45、ELOCITY OF SOUND MEASUREMENT . 23 FIGURE 8 DENSITY VERSUS PERMITTIVITY BASED ON ARINC 611 DATA . 25 FIGURE 9 FUEL DENSITY VERSUS VELOCITY OF SOUND BASED ON ARINC 611 DATA 26 FIGURE 10 FUEL QUANTITY ACCURACY 29 FIGURE 11 SIMPLEX SYSTEM . 44 FIGURE 12 REDUNDANT SIMPLEX SYSTEM . 44 Copyright SAE Intern
46、ational Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR5691 Page 5 of 63 FIGURE 13 PARALLEL SYSTEM 45 FIGURE 14 DISSIMILAR FQIS SYSTEM 45 FIGURE 15 “BRICKWALL“ FQIS . 46 FIGURE 16 PARTITIONED SYSTEM 47 FIGURE 17 MONITOR
47、ED ARCHITECTURE . 47 FIGURE 18 ARCHITECTURE INCLUDING TSUS 48 FIGURE 19 ARCHITECTURE INCLUDING DATA CONCENTRATORS 49 FIGURE 20 TYPICAL INTRINSIC SAFETY BARRIER 54 TABLE 1 COMPARISON OF GAUGING TECHNIQUES . 17 TABLE 2 ERROR SOURCES AND CONSIDERATIONS - ERRORS DIRECTLY ACCOUNTED FOR IN ACCURACY ANALYS
48、IS . 33 TABLE 3 ERROR SOURCES AND CONSIDERATIONS - ERRORS SOURCES TO BE CONSIDERED DURING DESIGN . 36 TABLE 4 AIRCRAFT SYSTEM 54 Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR5691 Page 6 of
49、63 1. SCOPE This document is applicable to commercial and military aircraft fuel quantity indication systems. It is intended to give guidance for system design and installation. It describes key areas to be considered in the design of a modern fuel system, and builds upon experiences gained in the in
