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
2、from, is the sole responsibility of the user.” SAE reviews each technical report at least every five years at which time it may be reaffirmed, revised, or cancelled. SAE invites your written comments and suggestions. Copyright 2006 SAE International All rights reserved. No part of this publication m
3、ay be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: 724-776-4970 (outside USA)
4、 Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.org AIR790 REV. C AEROSPACE INFORMATION REPORT Issued 1964-04 Reaffirmed 1992-08 Revised 2006-08 Superseding AIR790B (R) Considerations on Ice Formation in Aircraft Fuel Systems RATIONALE The purpose of this proposed rev
5、ision is to organize and combine the useful information from AIR790B and ARP1401 into the AIR790C and to expand the document with additional information on icing, fuel and water management and testing. FOREWORD This AIR is intended to provide useful information for the consideration of ice formation
6、 in aircraft fuel systems and includes suggested test procedures to demonstrate the suitability of fuel systems, sub systems and components in environments having the potential for ice formation. It does not include consideration of ice formation in aircraft engines or fuel tank vent systems nor doe
7、s it include instructions for the use of anti-icing fuel additives. This report was initially based on conclusions reached at a combined Air Force-Navy-Industry conference held in 1959 and subsequently updated to reflect current industry consensus and practice by the SAE Committee AE-5. It represent
8、s a summary of contributions, based on personal experience, from aircraft fuel system engineering representatives from the industry. In the past, incidents and accidents occurred in the operation of military and civil aircraft which were attributed to the formation of ice in the fuel supply system r
9、esulting in intermittent or complete starvation of fuel flow. Considerable effort was devoted by many airframe companies, engine and accessory manufacturers, fuel system component suppliers and government agencies to study the problem of ice formation and to develop corrective measures. By its very
10、nature, the problem of ice formation is very complex and difficult to analyze. However, corrective measures were developed which, for many years, have virtually eliminated serious icing problems in aircraft fuel systems. Successful corrective measures are numerous and include, but are not limited to
11、, the use of anti-icing fuel additives, aircraft fuel heaters, improved in-flight fuel and ambient temperature monitoring, appropriate corrections in route or altitude or air speed and improved water management and drainage provisions in aircraft fuel tanks and ground storage and delivery systems. C
12、opyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR790 Revision C - 2 - 1. SCOPE Ice formation in aircraft fuel systems results from the presence of dissolved and undissolved water in the fuel. Di
13、ssolved water or water in solution with hydrocarbon fuels constitutes a relatively small part of the total water potential in a particular system with the quantity dissolved being primarily dependent on the fuel temperature and the water solubility characteristics of the fuel. One condition of undis
14、solved water is entrained water such as water particles suspended in the fuel as a result of mechanical agitation of free water or conversion of dissolved water through temperature reduction. Another condition of undissolved water is free water which may be introduced as a result of refueling or the
15、 settling of entrained water which collects at the bottom of a fuel tank in easily detectable quantities separated by a continuous interface from the fuel above. Water may also be introduced as a result of condensation from air entering a fuel tank through the vent system. Entrained water will settl
16、e out in time under static conditions and may or may not be drained, depending on the rate at which it is converted to free water. In general, it is not likely that all entrained water can ever be separated from fuel under field conditions. The settling rate depends on a series of factors including
17、temperature, quiescence and droplet size. The droplet size will vary depending upon the mechanics of formation. Usually the particles are so small as to be invisible to the naked eye, but in extreme cases can cause a slight haziness in the fuel. Free water can be drained from a fuel tank if low poin
18、t drain provisions are adequate. Water in solution cannot be removed except by dehydration or by converting it, through temperature reduction, to entrained, then to free water. Water strictly in solution is not a serious problem in aviation fuel so long as it remains in solution. Entrained and free
19、water are the most dangerous because of the potential of freezing on the surfaces of the fuel system. Further, entrained water will freeze in cold fuel and tend to stay in solution longer since the specific gravity of ice is approximately the same as that of hydrocarbon fuels. The elimination of und
20、issolved water, to the extent practicable, in fuel storage, handling and delivery systems as well as in aircraft fuel systems can reduce or eliminate the potential for icing problems. Appropriate testing of fuel systems, sub systems and components under controlled icing conditions can establish conf
21、idence in the safe operation of the aircraft fuel system in such icing conditions. Considerations for these measures to control potential icing problems are addressed herein. Several things happen to moisture laden fuel as the temperature is lowered, and an understanding of this helps to arrive at p
22、roper fuel conditioning procedures and subsequent testing for icing conditions. As the temperature of fuel is lowered, concentration of water droplets in the fuel begins to decrease in the vicinity of 40 to 50 F (4 to 10 C). Therefore, to get a reliable conditioning of fuel, samples should be taken
23、and mixing of fuel and water should be accomplished before lowering the temperature further. Ice crystals begin to form as the temperature nears the freeze point of water; however, due to impurities in the water, this normally takes place at slightly lower temperatures (27 to 31 F) (-3 to -1 C). As
24、the temperature is lowered further, the ice crystals begin to adhere to their surroundings in the form of ice. This is known as the critical icing temperature and occurs at about 12 to 15 F (-11 to -9 C). At temperatures below 0 F (-18 C), ice crystals tend to become larger and offer a threat to plu
25、gging small openings such as screens, filters, and orifices. The cooling rate and agitation or turbulence due to obstruction of flow have an effect on the type and size of ice formed, so it becomes important to test actual or closely simulated aircraft systems and to cool the fuel during tests at th
26、e aircraft cooling rate or practical simulation to obtain more accurate results. Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR790 Revision C - 3 - 2. APPLICABLE DOCUMENTS The following publ
27、ications form a part of this document to the extent specified herein. The latest issue of SAE publications shall apply. The applicable issue of other publications shall be the issue in effect on the date of the purchase order. In the event of conflict between the text of this document and references
28、 cited herein, the text of this document takes precedence. Nothing in this document, however, supersedes applicable laws and regulations unless a specific exemption has been obtained. 2.1 SAE Publications Available from SAE International, 400 Commonwealth Drive, Warrendale, PA 15096-0001, Tel: 877-6
29、06-7323 (inside USA and Canada) or 724-776-4970 (outside USA), www.sae.org. ARP1401A Aircraft Fuel System and Component Icing Test 2.2 U.S. Government Publications Available from the Document Automation and Production Service (DAPS), Building 4/D, 700 Robbins Avenue, Philadelphia, PA 19111-5094, Tel
30、: 215-697-6257, http:/assist.daps.dla.mil/quicksearch/. MIL-F-17874B Fuel Systems: Aircraft, Installation and Test of 3. STORAGE, GROUND HANDLING AND DELIVERY SYSTEMS 3.1 Undissolved Water The fuel should be maintained with no detectable undissolved water at fuel ambient temperature. 3.2 Solid Conta
31、mination Solid contamination should not exceed 4.0 milligrams per gallon of fuel. 3.3 Control Techniques Procedures should be used to insure continuous compliance with the requirements of 3.1 and 3.2 at point of delivery to aircraft. Filtration to control the particulate contamination level of the f
32、uel, and water coalescing type equipment to separate and remove undissolved water from the fuel should be employed in storage,handling and delivery systems to accomplish this. 4. AIRCRAFT FUEL SYSTEMS 4.1 Anti-Icing Fuel Additive Icing inhibitor is included by specification requirement in some milit
33、ary aviation fuels. It is not included in commercial aviation fuels, however, it may be added by operators. The additive effectively lowers the freezing temperature of entrained and free water depending upon its percentage of concentration in the fuel. The additive is water soluble, therefore, its c
34、oncentration and effectiveness may be reduced if settled free water is removed from low point drains. Use of the additive may allow low point drainage of free water in cold ground operations which might otherwise be frozen. Copyright SAE International Provided by IHS under license with SAENot for Re
35、saleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR790 Revision C - 4 - 4.2 Fuel Heating Dedicated engine accessory fuel heaters have been successful in eliminating icing problems downstream of engine feed lines. Fuel can be heated by the use of circulation and transfer
36、pumps and heat exchangers making maximum use of available heat. In-flight corrections in route, altitude and air speed are commonly employed to control fuel temperature and avoid icing problems. 4.3 Water Management and Drainage Minimization of free water can be achieved with proper design considera
37、tions and drainage provisions and procedures. The tank bottom surfaces and associated ribs, stringers and features should provide passages for free water to migrate to low point drains in water sumps of adequate capacity. Drainage provisions should be located for maximum free water removal, accessib
38、ility and ease of operation and should display prominent clearly defined markings. Free water may also be routed to circulation or inter-tank transfer pump inlets or to engine feed pumps providing provisions exist for adequate mixing with fuel to prevent slugs of free water from entering the engine
39、feed system. Ejector pumps are often used to scavenge free water from trapped areas in fuel tanks to sumps or pump inlets. 4.4 System Components Filters and screens which are not necessary for safety of flight should be eliminated. Use of a reliable by-pass design around filters or screens which, if
40、 clogged, could result in engine flameout or other safety of flight hazard should be mandatory. By-pass elements should be located to prevent backwashing of sediment. Multiple by-pass elements in a principle filter may be considered. An impending by-pass or by-pass activation warning device on princ
41、iple filters or screens may be considered. Generally, No. 4 mesh screen or coarser is considered not subject to critical icing; however, this would depend on and should be demonstrated for the critical operating conditions such as mission profile, water content and environment. Filters and screens s
42、hould be selected for adequate filtration and capacity based on engine requirements and should be subject to maintenance inspections per aircraft and engine maintenance manuals. System components should be located, as practicable, in favorable environmental locations to best utilize available heat.
43、Insulation provisions may be considered where appropriate and practical. Components should be designed to be tolerant to water and ice with provisions for water run-off and drainage of water traps. Materials and coatings which are non-ice adhering should be used where appropriate. Low points in fuel
44、 lines where water can collect should be avoided wherever possible. Multiple vent system openings to atmosphere should be located such that no pressure differential exists between them to preclude continuous circulation of outside air which can introduce considerable quantities of water in some oper
45、ating conditions. 5. FUEL SYSTEM OR COMPONENT TESTING 5.1 Testing Considerations This section provides discussion and suggestions for the testing of fuel systems, sub systems and components and applies to all fuel flowing components and plumbing from fuel tank to engine, excluding engines. The basic
46、 test methods presented herein are derived from previously published methods recorded in specifications MIL-F-17874 and ARP1401 which have served the industry as a baseline from which specific test procedures have been developed for specific systems and components. Icing test procedures must be tail
47、ored for the system, sub-system or component being tested. Continuous or intermittent operation, flow rate and temperature schedules should be developed to simulate actual aircraft operating conditions as closely as practicable. Copyright SAE International Provided by IHS under license with SAENot f
48、or ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR790 Revision C - 5 - Icing tests conducted in the industry have varied widely in the requirements and procedures used. There has been a lack of agreement on certain aspects of conducting tests including but not limi
49、ted to general test set-ups, fuel conditioning, single pass test versus recirculating, use of anti-icing additive, test temperatures, test duration, water content analysis and post test requirements. With wide variations in test methods it is difficult to impossible to accurately compare test results from different sources and asses
