Subject: TYPE CERTIFICATION OF

 AUTOMOBILE GASOLINE IN PART 23

 AIRPLANES WITH RECIPROCATING

 ENGINES

Date: 3/2/95

Initiated By: ACE-100

AC No: 23.1521-1B

Change:

1. PURPOSE. This advisory circular (AC) provides information and

guidance concerning an acceptable means, but not the only means, of

compliance with part 23 of the Federal Aviation Regulations (FAR),

for approval/certification to use automobile gasoline with or

without oxygenates (alcohols, ethers, etc.) in part 23 airplanes.

These procedures also apply to those airplanes

approved/certificated under previous regulations superseded by part

23. Accordingly, this material is neither mandatory nor regulatory

in nature and does not constitute a regulation. This AC may be used

for certification of automobile gasoline that contains not more than

11 percent by volume of alcohol. Gasoline containing more than 11

percent by volume of alcohol should be certificated using the

guidance in AC 23.1521-2.

2. CANCELLATION. AC 23.1521-1A, Certification of Non-Oxygenated

Automobile Gasoline (Autogas) Instead of Aviation Gasoline (Avgas)

in Part 23 Airplanes with Reciprocating Engines, dated January 2,

1991, is canceled.

3. RELATED FAR SECTIONS.

 a. Section 23.1521(d) - Fuel grade or designation

 b. Section 23.961 - Fuel system hot weather operation

 c. Section 23.955 - Fuel flow

4. RELATED READING MATERIAL. Refer to appendix 1 for related

reading material.
AC 23.1521-1B 3/2/95

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5. BACKGROUND.

 a. In accordance with prior policy (Engine and Propeller

Directorate letter dated November 14, 1986), airplanes and engines

listed in the U.S. Department of Commerce Aeronautics Branch

document titled, Chapter XIII - Approved Aircraft Engines and

Accessories, dated May 1, 1931, may use any type of gasoline in the

airplanes. This does not include gasolines containing ethanol,

methanol, or other oxygenate additives except methyl-tertiarybutyl-ether (MTBE).

 b. In accordance with prior policy (Small Airplane Directorate

letter dated October 27, 1989), airplanes that have been previously

approved for operation on automobile gasoline, in combination with

an automobile gasoline approved alternate engine, can be fieldapproved provided there are no changes to the airplane or engine

and the engine fuel system originally approved for automobile

gasoline has a gravity type fuel system. Consideration must be

given to larger fuel flow requirements of larger engines. This

finding would require an engineering evaluation of the appropriate

airplane and engine Supplemental Type Certificates (STC's) and/or

corresponding data to ensure no changes were made to the original

fuel system. This decision to allow field approval must be made by

the appropriate aircraft certification office (ACO). This applies

only to the automobile gasoline portion of an engine change. There

may be other certification issues that must be addressed.

 c. Section 23.1521(d) requires the minimum fuel grade be

established so that it is not less than that required for the

operation of the engine within the limitations of § 23.1521(b) and

(c). The Type Certificate Data Sheet (TCDS) for the engine

specifies the minimum grade aviation gasoline that has been

established during type certification of the engine. Aviation

Grades 80/87, 100/130, and 100LL fuels are common aviation

gasolines approved for airplane engines.

 d. In recent years, some petroleum manufacturers have

discontinued the production of Grade 80/87 aviation fuel.

Therefore, several alternate fuels have been proposed for normally

aspirated or supercharged, fuel-injected, or carbureted lowcompression engines that were approved for operation on Grade

80/87 octane fuel.
3/2/95 AC 23.1521-1B

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 e. STC's may be issued to authorize use of unleaded or leaded

automobile gasoline in small airplanes approved under part 3 of the

CAR or part 23 of the FAR. Two STC's are required for each

different airplane--one for the engine and one for the airplane.

These STC's are issued when the applicant demonstrates to the FAA

that the modification meets the applicable FAR's. An STC issued to

authorize automobile gasoline in one airplane does not

automatically apply to other airplanes, regardless of whether it is

powered by the same engine. All STC data (drawings, reports, etc.)

developed by an applicant are proprietary. An airplane owner

seeking authorization to use automobile gasoline can develop his

own data to pursue STC approval (for engine and airplane) from the

FAA. Alternatively, if STC's have already been issued applicable

to his model engine or airplane, the airplane owner may purchase

these STC's from their owner and modify his engine or airplane in

accordance with the associated technical data. The airplane STC

should identify the engine STC as prerequisite under the

limitations.

 f. Specifications for automobile gasoline as well as aviation

gasoline have been developed by the American Society for Testing

and Materials (ASTM). These specifications are ASTM D 910 for

aviation gasolines and ASTM D 439 or ASTM D 4814 (latest revision)

for automobile gasoline. Automobile gasoline not containing

oxygenates conforming to D 439 and D 4814 are essentially identical

and may be used interchangeably. ASTM D 4814 is intended to

describe both straight gasoline and gasoline-oxygenate blends.

Care must be used in selecting automobile gasoline for aviation use

because automotive gasolines may not be manufactured in accordance

with these ASTM specifications and also may contain oxygenates that

are not included in the STC approval. These gasoline oxygenates

may be harmful to aviation fuel systems and engine operation and

should be avoided unless approved for use in the airplane. As

defined by ASTM specifications D 439 and D 4814, an oxygenate is an

oxygen-containing ashless organic compound such as an alcohol or

ether that may be used as a fuel or fuel supplement. The data

submitted to document the fuel used should include the fuel

composition (including oxygenates) and test methods as well as

identify the test laboratory or organization performing the fuel

tests. This data may be supplied by the fuel manufacturer if the

manufacturer has maintained control of the fuel. On December 14,

1992, the FAA approved (without additional testing) the use of MTBE

in automobile gasoline for those previously issued automobile

gasoline STC's (Small Airplane Directorate letter dated December

14, 1992).
AC 23.1521-1B 3/2/95

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 g. ASTM D 910, Standard Specification for Aviation Gasolines,

allows the use of isopropyl alcohol conforming to the requirements

of ASTM D 4171, Specifications for Fuel System Icing Inhibitor, as

a fuel system icing inhibitor. Accordingly, isopropyl alcohol

conforming to ASTM D 4171 may be used in concentrations up to 1

percent by volume, to benefit safety, as an icing inhibitor in

automobile gasoline. The addition of isopropyl alcohol may reduce

antiknock ratings. Therefore, tests performed to obtain approval

for the fuel should be accomplished with fuel containing 1 percent

by volume of the D 4171 isopropyl alcohol if the addition reduces

the antiknock rating and antiknock rating is critical to the test

being performed.

6. ACCEPTABLE MEANS OF COMPLIANCE.

 a. Engine STC. It is mandatory that the engine be approved

(by STC or type certificate (TC)) for operation on automobile

gasoline before automobile gasoline is eligible for approval in the

airplane. AC 20-24B describes an acceptable method of obtaining

approval for the engine to operate on automobile gasoline. Engines

that are approved for operation on 80/87 grade aviation fuel, or

normally aspirated engines that have a compression ratio of

approximately 7.2:1 or less, may operate satisfactorily on

automobile gasoline. Engines having a compression ratio greater

than 7.2:1 may experience detonation with associated engine

destruction when operating on low-octane automobile gasoline. In

either case, operation of the engine with any alternate fuel must

be shown to meet the minimum design requirements for the engine and

be approved. It may be possible for testing of the engine and

airplane to be conducted concurrently.

 b. A certification test plan and schedule should be prepared

by the applicant and presented to the appropriate FAA ACO for

acceptance. The test plan should include a description of the

test program and equipment that the applicant proposes to use in

demonstrating the airworthiness of the fuel to be approved. The

engine(s) and airplane fuel system(s) and oil system(s) components

should be subjected to a pretest and post-test inspection to

verify their conformity and condition prior to and after testing.

The test procedures should provide all the specific information

required to perform the tests (i.e., test fuel specification, test

location, engine model to be tested, specific test hardware and

instrumentation to be used, engine minimum and maximum operating

parameters, engine lubricant to be used, lubricant change

interval, list of all information to be recorded during the test

including changes to oil properties, intervals at which this data

is to be recorded, etc.).
3/2/95 AC 23.1521-1B

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 c. Prior to FAA authorization for test, the applicant should

submit a report to substantiate that the fuel under the conditions

in which it will be used in the airplane is compatible with the

applicable engine and airplane materials. The data should include

compatibility with materials, lubricants, and additives that are

approved for the engine, propellers (where applicable), and the

airplane.

 d. At the completion of the airplane tests, a report should be

submitted that includes at least the following:

 (1) A description of the airplane and engine(s) in which

the fuel was tested.

 (2) A chronological history of test conditions and engine

performance, including r.p.m., power or thrust levels achieved

during the test, fuel and oil consumption, oil changes, parts

replacement, and other pertinent test results.

 (3) An analysis of lubricating oil samples taken before

and after the test and before each oil change.

 (4) An analysis of material collected in the fuel and oil

filters after the test and when filters are changed.

 (5) An analysis of the fuel used during the test. For

substantiation testing, this analysis should demonstrate minimum

or "worst case" properties for the particular test, such as,

detonation, fuel system or carburetor icing, altitude restart, and

hot weather operation. Distinct fuels may be required to address

specific characteristics critical to these substantiation tests.

 (6) A description of abnormal wear, deposits, metal

attack, or other harmful effects that occurred as a result of the

test.

 (7) A description of deterioration, excessive seal

swelling, shrinkage, hardness, or unsatisfactory condition on or

in any of the fuel or oil-wetted parts that occurred as the result

of the test.

 e. Airplane STC. The evaluation of an application for STC of

an airplane for operation on automobile gasoline should consider

the following items:
AC 23.1521-1B 3/2/95

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 (1) The fuel must be approved by STC or TC for use in the

engine.

 (2) An analysis of the fuel used for the test should be

accomplished. For certification testing, the analysis should

demonstrate minimum or critical properties and should include the

amount by type of oxygenates in the fuel.

 (3) A hot weather operation test in accordance with

§ 23.961 should be conducted. The airplane should be tested to

the maximum altitude for which approval is requested with the

critical fuel. The critical fuel is considered one having a

volatility Class E or winter grade from a northern geographical

class of the United States with an antiknock designation [(R+M)/2]

of 87. Refer to ASTM D 439 and/or D 4814 for location and

seasonal variations of automobile gasoline. This fuel should be

available on the open market for those regions of the United

States. The test fuel should be analyzed for compliance with the

limits of the ASTM D 439 and/or D 4814. The following tests

should be conducted (Test Procedure II and Test Procedure III

apply only to airplanes with suction lift fuel systems):

 (i) Test Procedure I.

 (A) Use fuel with an antiknock designation of

87 and a Reid Vapor Pressure of 12.5 minimum.

 (B) Keep fuel cooled from delivery at refinery

to installation in the test airplane at a temperature of 11oC

(52oF) or cooler in a nonvented container. Drain entire airplane

fuel system prior to placing the test fuel in the fuel tanks.

 (C) Fuel the aircraft with a minimum safe

amount of automotive gasoline and heat to a temperature of 43oC

(110oF+0-5oF). (Reference AC 23.961-1).

 (D) Prepare the airplane for takeoff in a

manner that will ensure the fuel temperature does not fall below

the values listed in (C) above.

 (E) Refer to AC 23.961-1 for test procedures.
3/2/95 AC 23.1521-1B

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 (ii) Test Procedure II.

 (A) Service experience indicates that the

following test should be conducted on airplanes proposed for

certification on automotive gasoline. Use fuel identified in

6e(3)(i)(A) and (B) above, fuel the airplane with a minimum safe

amount of fuel, and heat to 29oC (85oF +2oF). Prepare the airplane

for takeoff in a manner that will ensure the fuel temperature does

not fall below 28oC (83oF).

 (B) Refer to AC 23.961-1 for test procedures.

 (iii) Test Procedure III. Service experience indicates

that the following test should be conducted on airplanes proposed

for certification on automotive gasoline when the main fuel tanks

are at or below any pump on a suction lift fuel system. Subsequent

to completing 6e(3)(ii)(A) and (B) above, park the airplanes on the

ramp with ambient air temperature of 80oF or higher. Allow

airplane to sit on the ramp a minimum of 15 or a maximum of 30

minutes to heat soak. Without refueling, start engine, taxi,

complete normal preflight, takeoff, climb to 3000 feet AGL, return,

and land. Ensure that there are no abnormal engine operating

characteristics.

 (4) Evaluate the inflight restart envelope with the fuel

(§ 23.903(f)). Restart procedures should be placed in the flight

manual (paragraph 6e(12).

 (5) Engine cooling tests, in accordance with § 23.1041,

should be evaluated for compliance with the regulations if the

fuel flow rate or fuel net heat of combustion is different.

 (6) Evaluate for normal engine operation during all

approved aircraft maneuvers; e.g., takeoff and landing, balked

landing, etc. Also engine operation when changing from one tank

to another, in accordance with § 23.955(e), should be evaluated.

The stall test should be conducted first with aviation gasoline to

ensure that the test pilot is aware of the normal airplane

operation.

 (7) Carburetor heat rise test, in accordance with

§ 23.1093, should be evaluated for compliance with regulations if

the fuel freezing point or fuel icing inhibitors are different.

 (8) Suction lift fuel systems are more critical than

pressure feed systems with respect to vapor formation and should

be evaluated for operational problems.
AC 23.1521-1B 3/2/95

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 (9) Evaluate the engine for proper operation and confirm

that engine rated horsepower is within the limitations specified

on the applicable airplane TCDS. Airplanes with fixed pitch

propellers can be evaluated by determining the static r.p.m. and

manifold pressure are within TCDS limits. If engine power is

degraded with use of automobile gasoline, evaluation of these

effects on airplane performance is necessary. Detonation testing

with critical specification fuel should be accomplished on

turbocharged engines.

 (10) Establish compatibility of fuel systems material

(elastomers, sealants, seals, liners, hoses, etc.) with automobile

gasoline. Industry standards such as SAE procedures and ASTM

specifications may be used to establish compatibility.

 (11) Establish compatibility of fuel quantity gauging

system with automobile gasoline. Industry standards such as SAE

procedures and ASTM specifications may be used to establish

compatibility.

 (12) Preparation of a Supplemental Airplane Flight Manual

(AFM) or AFM Supplement, 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 the

fuel contains only approved materials such as oxygenates, and meets

the minimum specification for which approval was obtained, should

be provided by the applicant and should be included in the AFM.

Test methods contained in ASTM D 4814 may be used or alternate test

methods may be presented by the applicant to the FAA for

acceptance. It is the operator's responsibility to determine that

the fuel used satisfies the approved fuel requirements. AC 20-43C

contains advisory material concerning fuel handling procedures and

methods to prevent contamination. Procedures for mixing automobile

gasoline and aviation gasoline should be addressed.

 (13) Specify appropriate markings and placards (§ 23.1541)

to define the approved fuels and any operating limitations. The

appropriate fuel specification such as ASTM D 439 and/or D 4814 and

approved oxygenates should be included. Specify a placard to alert

the pilot of a Supplemental AFM or AFM Supplement for proper

operation.

 (14) The Flight Manual Supplement or Supplementary Flight

Manual should contain the following caution: "Care should be used

when selecting automobile gasoline for aviation use because the

automotive gasolines may not be produced in accordance with ASTM
3/2/95 AC 23.1521-1B

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specifications D 439 or D 4814. Gasolines conforming to the ASTM

specifications D 439 or D 4814 and that contain only approved

oxygenates or other additives should be used. Gasolines containing

unapproved oxygenates may be harmful to aviation fuel systems and

engine operation and should be avoided."

 (15) 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."

HENRY A. ARMSTRONG

Acting Manager, Small Airplane Directorate

Aircraft Certification Service
AC 23.1521-1B 3/2/95

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3/2/95 AC 23.1521-lB

Appendix 1

APPENDIX 1. RELATED READING MATERIAL

1. ADVISORY CIRCULARS (AC1s). The AC1s listed below can be obtained from the U.S. Department

of Transportation, General Services Section, M-443.2, Washington, D.C. 20590. AC 23-BA is for purchase

and can be obtained from the Superintendent of Documents, PC Box 371954, Pittsburgh, PA 15250-7954, or

from any of the Government Printing Office Bookstores located in major cities throughout the United States.

a. AC 20-24B, Qualification of Fuels, Lubricants, and Additives for Aircraft Engines, dated

December 20, 1985.

b. AC 20-43C, Aircraft Fuel Control, dated October 20, 1976.

c. AC 23-8A, Flight Test Guide for Certification of Part 23 Airplanes, dated February 9, 1989,

(SN 050-007-00817-1). AC 23-BA Change 1, dated August 30, 1993, (SN 05c-007-01013-3)

d. AC 23.955-1, substantiating Flow Rates and Pressures in Fuel Systems of Small Airplanes,

dated June 10, 1985.

e. AC 23.959-1, Unusable Fuel Test Procedures for Small Airplanes, dated January 14, 1985.

f. AC 23.961-1, Procedures for Conducting Fuel System Hot weather Operation Tests, dated

January 14, 1987.

g. AC 23.1521-2, Type Certification of Oxygenates and oxygenated Gasoline Fuels in Part 23

Airplanes with Reciprocating Engines, dated January 21, 1993.

h. AC 33.47-1, Detonation Testing in Reciprocating Aircraft Engines, dated June 27, 1988.

if AC 91-33A, Use of Alternate Grades of Aviation Gasoline for Grade 80/87 and Use of Automotive

Gasoline, dated July 18, 1984.

2. AMERICAN SOCIETY FOR TESTING AND MATERIALS (ASTM) STANDARDS. The

ASTM Standards listed below can be obtained from ASTM, 1916 Race

Street, Philadelphia, PA 19103:

a. ASTM D 910, Standard Specification for Aviation Gasolines, dated October 31, 1986, or later

revision.

b. ASTM D 439, Standard specification for Automotive Gasoline, dated October 31, 1988, or

later revision.

C. ASTM D 4814, Standard Specification for Automotive Spark-Ignition Engine Fuel, dated

October 31, 1988, or later revision.

1 (and 2)