v. Engine starting characteristics may be affected by the use of alternate fuels. Tests should be conducted to verify the airplane engine restart capability at altitude. Also, engine starting capability at cold and hot ambient temperatures should be verified.
w. An alternate fuel that requires aviation gasoline for starting or other conditions thereby necessitating two fuel types should be evaluated for safe changeover operation. Altitude restart with the fuel should be addressed (see paragraph 8(11)).
x. If the fuel flame of the alternate fuel is not visible when the fuel is burning, then some means of making the flame visible should be provided. The flame should be as visible as that produced by ethanol burning in bright sunlight.
8. AIRPLANE CERTIFICATION EVALUATION.
a. Evaluation of an application for certification of an alternate fuel should include those items in paragraph 7 of this AC and the following items:
(1) The fuel must be approved by STC or TC for use in the engine (including turbocharger/intercooler).
(2) An analysis of the fuel used for each test should be accomplished. For certification testing, the analysis should demonstrate minimum or critical properties.
(3) A hot weather operation test (§ 23.961) should be conducted. The airplane should be tested with the fuel to the maximum altitude for which approval is requested (see paragraph 7t and u).
(4) Engine cooling (§ 23.1041) should be evaluated for compliance with the regulations.
(5) Evaluate for normal engine operation (§ 23.951) during all approved airplane maneuvers; e.g., takeoff and landing, balked landing, stall, spins, etc. Also, evaluate engine operation (§ 23.955(e)) when changing from one tank to another.
(6) Carburetor heat rise (§ 23.1093) should be evaluated for compliance with regulations. Keep in mind that some of the alternate fuels result in an unusually large temperature drop in carbureted fuel systems.
(7) Suction lift fuel systems are more critical than gravity (low pressure) feed systems with respect to vapor formation.
Both types of systems should be evaluated by analysis and/or flight test for operational problems.
(8) Evaluate the engine for proper operation and ensure that engine rated horsepower is within the limitations specified on the applicable Airplane Type Certificate Data Sheet. Airplanes with fixed pitch propellers may be evaluated by determining the static
r.p.m. and manifold pressure are within TCDS limits. If engine power is changed with use of the fuel, evaluation of these effects on airplane performance is necessary. Detonation testing with critical specification fuel should be accomplished (reference AC
33.47-1).
(9) Establish compatibility of airplane fuel wetted and oil wetted materials (elastomers, sealants, seals, liners, hoses, etc.) and components (including propeller system components and materials if engine oil is used for propeller operation) with the fuel. Industry standards such as Society of Automotive Engineers (SAE) procedures and ASTM specifications may be used to establish compatibility.
(10) Establish compatibility of the fuel quantity gauging system with the fuel and evaluate the unusable fuel quantity (§ 23.959).
(11) Establish the inflight restart envelope (§ 23.903(f)) with the fuel.
(12) Evaluate the compatibility of carburetor deicing fluid and the fuel and any effects on airplane operation.
(13) Evaluate the fuel flow (§ 23.955).
(14) Evaluate the effects of the alternate fuel weight per volume difference on load distribution limits (§ 23.23).
(15) If the alternate fuel will result in an increase in aircraft noise (due to changed performance or changed engine operating parameters), then an acoustical change in accordance with § 21.93(b) should be addressed.
b. Preparation of a Supplemental AFM or AFM Supplement (§ 23.1581), as applicable, is necessary to specify the airplane's proper operating procedures and limitations. A Supplemental AFM is used with airplanes that were originally certificated without an AFM, whereas an AFM Supplement is used with airplane models having an AFM. Procedures for determining that fuel is free of contamination (not dissolved water), does not contain unapproved materials, and meets the minimum specification for which approval was obtained, should be provided by the applicant and should be included in the AFM. It is the operator's responsibility to determine that the fuel used satisfies the approved fuel 13
requirements. Advisory Circular 20-43C, "Aircraft Fuel Control," dated October 26, 1976, contains advisory material concerning fuel handling procedures and methods to prevent contamination.
c. Specify appropriate markings placards (§ 23.1541) to define the approved fuels and any operating limitations. The appropriate fuel specification should be included. Specify a placard to alert the pilot of a Supplemental AFM or AFM Supplement for proper operation.
d. The Flight Manual Supplement or Supplementary Flight Manual should contain a caution stating the following: "All airport, local, state, and federal regulations pertaining to airplane fueling operation must be complied with."
e. The applicant should provide for continued airworthiness (§ 23.1529). This should include all changed or added components of the engine, turbocharger/intercoolers, fuel system, oil system, and parts list.
MICHAEL GALLAGHER Manager, Small Airplane Directorate Aircraft Certification Service 14
Appendix 1 APPENDIX 1. ADVISORY CIRCULARS The publications listed below may be obtained from the U.S.
Department of Transportation, Utilization and Storage Section, M-443.2, Washington, D.C. 20590. Advisory Circular (AC) 23-8A can be obtained from the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C., 20402, or from any of the Government Printing Office bookstores located in major cities throughout the United States.
1. Qualification of Fuel, Lubricants, and Additives for Aircraft Engines, AC 20-24B, Federal Aviation Administration, December 20, 1985.
2. Aircraft Fuel Control, AC 20-43C, Federal Aviation Administration, October 26, 1976.
3. Protection of Airplane Fuel Systems Against Fuel Vapor Ignition Due to Lightning, AC 20-53A, Federal Aviation Administration, April 12, 1985.
4. Water in Aviation Fuels, AC 20-125, Federal Aviation Administration, December 10, 1985.
5. Flight Test Guide for Certification of Part 23 Airplanes, AC 23-8A, Federal Aviation Administration, February 9, 1989.
6. Substantiating Flow Rates and Pressures in Fuel Systems of Small
Airplanes, AC 23.955-1, Federal Aviation Administration,
June 10, 1985.
7. Unusable Fuel Test Procedures for Small Airplanes, AC 23.959-1, Federal Aviation Administration, January 14, 1985.
8. Procedures for Conducting Fuel System Hot Weather Operation Tests, AC 23.961-1, Federal Aviation Administration, January 14, 1987.
9. Certification of Non-Oxygenated Automobile Gasoline (Autogas) instead of Aviation Gasoline (Avgas) in Part 23 Airplanes with Reciprocating Engines, AC 23.1521-1A, Federal Aviation Administration, January 2, 1991.
10. Detonation Testing in Reciprocating Aircraft Engines, AC 33.47-1, Federal Aviation Administration, June 27, 1988.
11. Use of Alternate Grades of Aviation Gasoline for Grade 80/87, and Use of Automotive Gasoline, AC 91-33A, Federal Aviation Administration, July 18, 1984.
1 (and 2)
Appendix 2 APPENDIX 2. RELATED READING MATERIAL
1. Automotive Fuels Handbook, Society of Automotive Engineers, (SAE), 1990.
2. Alternate Fuels for General Aviation Aircraft with Spark Ignition Engines, DOT/FAA/CT-88/05, Federal Aviation Administration, March 1988.
3. The Performance of Alternate Fuels in General Aviation, DOT/FAA/OT-88/13, Federal Aviation Administration, July 1988.
4. Straight Alcohol Fuels for General Aviation Aircraft, SAE Paper 891038, General Aviation Aircraft Meeting and Exposition, April 11 13, 1989.
5. Alcohols and Ethers, A Technical Assessment of Their Application as Fuels and Fuel Components, API Publication 4261, Second Edition, July 1988.
6. Aviation Fuels, Maxwell Smith, G. T. Fouks & Co. Ltd., 1970.
7. Powerplants for Aircraft, Joseph Liston, McGraw-Hill Book Company, Inc., 1953.
8. Internal Combustion Engines and Air Pollution, Edward Obert, Intext Educational Publishers, 1973.
1 (and 2)