Subject: TYPE CERTIFICATION OF

OXYGENATES AND OXYGENATED

GASOLINE FUELS IN PART 23

AIRPLANES WITH RECIPROCATING ENGINES

Date: 4/24/96

Initiated By: ACE-100

AC No: 23.1521-2

Change: 1

1. PURPOSE. This change transmits revised pages for the subject advisory circular

(AC).

2. EXPLANATION OF CHANGE. This change has been initiated to clarify the Scope

and to make the Scope and Purpose of this AC consistent with AC 23.1521-1B, Type

Certification of Automobile Gasoline in Part 23 Airplanes with Reciprocating Engines.

The asterisks (*) in the right and left margins indicate the beginning and end of the change.

3. DISPOSITION OF TRANSMITTAL. After filing the revised page, this change transmittal should be retained.

PAGE CONTROL CHART

Remove Page Dated Insert Page Dated

I and 2 1/21/93 1 and 2 4/24/96

Henry A. Armstrong

Acting Manager, Small Airplane Directorate

Aircraft Certification Service

FAA Form 1320-15 (4-82 Supersedes WA Form 132~2
AC 23.1521-2 1/21/93

Page intentionally left blank
Subject: TYPE CERTIFICATION OF

OXYGENATES AND OXYGENATED

GASOLINE FUELS IN PART 23

AIRPLANES WITH RECIPROCATING ENGINES

Date: 4/24/96

Initiated By: ACE-100

AC No: 23.1521-2

Change: 1

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),

applicable to approval procedures for certification of alternate fuels.

These procedures also apply to those airplanes approved/certificated

under previous regulations superseded by part 23. This material is

neither mandatory nor regulatory in nature and does not constitute a

regulation.

2. RELATED FAR SECTIONS. Sections 21.93(b), 23.23, 23.901(e),

23.903(a)(1), 23.903(f), 23.909, 23.951, 23.954, 23.955, 23.959, 23.961,

23.1041, 23.1093, 23.1521(d), 23.1541, and 23.1581.

3. RELATED READING MATERIAL. Related advisory circulars and other

reading material are listed in appendixes 1 and 2, respectively.

4. SCOPE.

 a. Fuels covered by this AC include oxygenates and oxygenated

gasolines. This includes methyl-tertiary-butyl ether (MTBE), methanol,

ethanol, other oxygenates, and gasolines containing these or other

oxygenates, except for automobile gasoline including those containing 11

percent or less alcohol by volume. Currently, gasolines are manufactured

to at least three different specifications available from the American

Society for Testing and Materials (ASTM). These specifications are ASTM

D 910 for aviation gasoline and D 439 and D 4814 for automobile gasoline.

D 4814 supersedes D 439. ASTM D 4814 is intended to describe both

straight gasoline and gasoline-oxygenate blends. Guidance for

certification of automobile gasoline including those containing 11

percent or less alcohol by volume is contained in AC 23.1521-1B Type

Certification of Automobile Gaoline in Part 23 Airplanes with

Reciprocating Engines or later revision.

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

fuel(s) that has (have) been substantiated during type certification of

the engine. The fuel must have a documented specification to define the

fuel properties.

 c. The product produced by mixing of alternate fuels is

considered a new alternate fuel and requires a separate approval for
AC 23.1521-2 1/21/93

2

use. Therefore, mixing of alternate fuels is prohibited unless the

mixture is separately certified and the mixture documented by

approved fuel specification. The alternate fuel specification should

include the fuel properties (including visibility of flame),

composition specification, and composition limits. An alternate fuel

should be compatible with 100LL aviation gasoline and/or other fuels

approved for use in the airplane.

5. BACKGROUND.

 a. It is necessary to evaluate each engine and airplane model

for compliance with the applicable FAR and determine that it is safe

for operation. Therefore, a Supplemental Type Certificate (STC) or

Type Certification (TC) is required for each different series or

sub-series of engine and airplane models, for which approval is

requested. An STC or TC is issued to an applicant who demonstrates

to the FAA that the airplane meets the applicable regulations. The

STC does not automatically apply to other airplanes. The STC or TC

data developed by the applicant including drawings, reports, etc.,

are proprietary. A person wanting to operate an airplane on

alternate fuel may either obtain an STC, if one is available, from

the STC owner and comply with the STC requirements or the person may

request certification from the appropriate FAA certification office

and develop the data required for certification.

 b. Applicants have applied for STC's to allow for the use of

fuels other than aviation gasoline in part 23 reciprocating engine

airplanes. The FAA Technical Center conducted a study to determine

the conditions that are most likely to result in operational

problems, should fuels other than aviation gasoline be used in

general aviation reciprocating engine airplanes. The fuels studied

range from automotive gasolines to automotive and aviation gasolines

containing alcohol to straight alcohol fuels. The reports are

included in the related reading material.

6. CERTIFICATION PROCEDURES.

 a. It is mandatory that the engine (including

turbocharger/intercoolers) and engine/propeller combination be

approved for operation on the alternate fuel before it is eligible

for approval in the airplane. Operation of the engine with any

alternate fuel should be shown to meet the minimum design

requirements for the engine and be approved. Advisory Circular

20-24B, "Qualification of Fuel, Lubricants, and Additives for

Aircraft Engines, dated December 20, 1985, describes an acceptable

method of obtaining approval for the engine to operate on alternate

fuel. It may be possible for testing of the engine and airplane to

be conducted concurrently.

 b. The fuel for which approval is requested must be documented

by a specification that is written in sufficient detail to provide

at least the composition, physical properties, and limits by which

uniform quality and composition can be maintained. The applicant is
1/21/93 AC 23.1521-2

3

responsible for providing to the FAA certification office the fuel

specification and test methods and procedures to determine

compliance with the specification.

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

submit a report to substantiate that the fuel has undergone

sufficient test and development to show that, under the conditions

in which it will be used in the airplane, it 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. The data submitted to document the fuel used should

include the fuel composition and test methods used as well as

identify the test laboratory or organization performing the test.

 d. A material compatibility and fuel aging deterioration tests

should be performed. This test should be for 180 days. A fuel tank

representing a typical airplane fuel tank should be filled to threefourths full with the appropriate fuel. The tank should be placed

outdoors and exposed to sunlight for the 180 days. Samples of

material and components used in the airplane fuel system should be

placed partially in the fuel and partially out of the fuel. The

fuel and material and component samples should be evaluated after 90

and 180 days to determine deterioration. Standard tests conducted

in accordance with the procedures published by ASTM, Society of

Automotive Engineers (SAE), and other organizations may be used

instead of the 180-day test. Particular attention should be

directed to oxygenated fuel effects on bladder fuel cells and nonmetallic composite material fuel tanks.

 e. A description of the test program and equipment that the

applicant proposes to use in demonstrating the airworthiness of the

fuel to be approved should be submitted for approval. The engine(s)

and airplane fuel system(s), exhaust systems, 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 test (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.).

 f. Five hundred hours of airplane flight test should be

performed with the objective of showing that the subject fuel will

provide satisfactory performance and operation, will not cause an

unsafe condition, and will not cause deterioration or any other

unsatisfactory condition on or in any of the airplane fuel or oilwetted parts. The flight test performed under paragraph 8 of this

AC may be included in the 500 hours of flight test. It should be

possible to conduct these 500 hours of test concurrently with the
AC 23.1521-2 1/21/93

4

controlled flight test referenced in AC 20-24B, "Qualification of

Fuel, Lubricants, and Additives for Aircraft," dated December 20,

1985, paragraph 5b.

 g. If the 150 hour engine endurance test (Section 33.49) is

conducted in lieu of the 500 hour controlled flight test referenced

in AC 20-24B, paragraph 5b and the other requirements of AC 20-24B

are satisfactorily accomplished for engine certification with the

alternate fuel, then 150 hours of airplane flight test may be

performed in lieu of the 500 hour airplane flight test in paragraph

6f of this AC. The flight test performed under paragraph 8 of this

AC may be included in the 150 hour airplane flight test.

 h. At the completion of the airplane test, a report(s) should

be submitted which 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, these analyses should demonstrate minimum or

"worst case" properties.

 (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 any deterioration, excessive seal

swelling shrinkage, hardness, or unsatisfactory condition on or in

any of the airplane (including turbocharger/intercooler) fuel,

exhaust, or oil-wetted parts that occurred as a result of the test.

7. CERTIFICATION CONSIDERATIONS. The FAR require that the critical

operating conditions for both the engine and airframe be considered

during airplane certification. Tests are required to demonstrate

the ability of the airplane fuel system to function under all

critical operating conditions. The following are some areas which

require special consideration as a consequence of using alternate

fuels in general aviation reciprocating engine airplanes.
1/21/93 AC 23.1521-2

5

 a. The energy per unit volume of the fuel affects the amount

of fuel which is metered to the engine in order for it to operate

satisfactorily. The lower the energy, the greater the amount of

fuel required to complete the same flight profile. In general, the

gasolines containing alcohols have less energy than aviation

gasoline, and the straight alcohols have less energy than the

gasolines which contain alcohols. The energy of MTBE lies between

gasoline and the alcohols.

 b. The fuel consumption varies with the energy density of the

fuel. Typically, the lower the energy density, the greater the fuel

consumption. Many of the alternate fuels have lower energy

densities than aviation gasoline. A means to set the fuel flow rate

for the specific fuel in use should be provided if the airplane is

intended to be operated on more than one type of fuel. Additional

fuel pumps (main and emergency) may be required to satisfy fuel flow

requirements due to changes in fuel density/heating value. Fuel

consumption effects, if any, on airplane range should be addressed.

 c. The octane rating of the fuel affects the tendency of the

fuel to detonate in the airplane engine. The higher the rating, the

lower the probability of encountering detonation. This in turn

means higher compression ratios may be used which increase the power

and efficiency of the engine. There are a number of octane ratings

established by the ASTM and the ratings of interest to the aviation

community are the Aviation Lean Octane rating and the Aviation Rich

Octane rating. The Motor Octane Number (MON) associated with

automobile gasolines is similar to the Aviation Lean Octane Number.

 d. The antiknock index is the average of the MON and the

Research Octane Number (RON) (an octane rating, which currently

applies to automobile gasoline only). This is the number associated

with regular or premium autogas (i.e., R+M/2 as displayed on the

fuel pump). Regular unleaded automotive gasoline typically has an

antiknock index of 87 (minimum MON of 82) and premium unleaded

automotive gasoline has an antiknock index of 91.0 or higher.

 e. Engine operations in high ambient temperatures and at low

pressure altitudes increase engine operating temperatures.

Likewise, power setting and the air-to-fuel ratio affects the

operating temperature of the engine. The higher the operating

temperature, the greater the probability of detonation or

preignition occurring.

 f. Abnormal combustion (detonation, preignition, knock, etc.)

characteristics of a fuel in an engine are affected by many

variables. These many variables include combustion temperature,

combustion pressure, and air-to-fuel ratio. High combustion

temperature and pressure are conducive to abnormal combustion such

as detonation or preignition. Engines operated at or near

stoichiometric air-to-fuel ratios are most likely to result in

detonation or preignition. Alternate fuels have different

combustion characteristics from gasoline, partly due to the oxygen
AC 23.1521-2 1/21/93

6

in the fuel, and the following should be considered in evaluating

these fuels. (See references 5 through 8 in appendix 2 for

discussion of abnormal combustion phenomena.)

 (1) Many alternate fuels burn with very lean air-to-fuel

ratios. If certification testing is conducted at only lean air-tofuel ratios, then substantiation should be provided to ensure that

stoichiometric air-to-fuel ratios are not attainable in that

installation. (Stoichiometric air-to-fuel ratios for some fuels

are: gasoline - 14.7 to 1; MTBE - 11.7 to 1; ethanol - 9.0 to 1;

and methanol - 6.45 to 1, from reference 5 in appendix 2.)

 (2) An additional condition that may result in detonation

or preignition is operation on a fuel with a lower energy density

immediately following operations on fuel with a higher energy

density. Detonation or preignition characteristics of an alternate

fuel should be evaluated by operating the engine on a lower energy

density fuel immediately following operation on a higher density

fuel. Detonation is possible even though the air-to-fuel ratio is

lean of stoichiometric.

 g. The Reid Vapor Pressure is a rough measure of the

volatility of the fuel. Higher values of Reid Vapor Pressure are

more conducive to vapor formation in fuel systems. Other

indicators are the initial boiling point of the fuel and the latent

heat of vaporization. A fuel with high volatility starts easier

and provides a more uniform fuel distribution in the engine. The

alcohols have low Reid Vapor Pressures, high initial boiling

points, and high latent heats of vaporization. All of these affect

the volatility of the fuel. The volatility of MTBE is close to

that of gasoline. Gasolines containing alcohols may have higher

vapor formation, and low Reid vapor pressure readings may not be

representative of vapor formation characteristics particularly when

water is dissolved in the fuel. The vapor liquid ratio may be a

good indicator of volatility.

 h. The Flash Point of a fuel is the temperature where

sufficient vapors are released to result in a fire if an ignition

source is present. The flash point of the alcohols is such that a

combustible mixture may be present in the fuel tanks at ambient

temperatures. Gasolines have flash points that are well below

common ambient temperatures; and typically, the fuel/air mixture in

the fuel tanks is too rich in fuel to support combustion. MTBE has

a flash point between aviation gasoline and the alcohol fuels.

 i. Since flammable fuel/air mixtures may be present in the

airplane tanks with some alternate fuels, particular attention

should be directed toward minimizing potential ignition sources.

Small amounts of gasoline (or other volatile substance) may prevent

the existence of these explosive fuel/air mixtures.
1/21/93 AC 23.1521-2

7

 j. Methanol is a strong solvent and is corrosive to aluminum,

copper, zinc, lead, and magnesium. Methanol is also poisonous and

toxic and regulated by the Federal Hazardous Substance Act.

Procedures of the American Industrial Hygiene Association should be

followed when handling methanol and other alcohol fuels.

 k. Material compatibility presents an area of concern. Many

of the materials currently found in general aviation airplanes are

susceptible to attack by alternate fuels. Examples include:

 (1) Corrosion of components in fuel systems may be a

problem.

 (2) The metals in the combustion chamber and exhaust

system may be attacked by some high acidic content by-products of

combustion that are associated with some of the alternate fuels.

 (3) Swelling, cracking, and failure of the synthetic and

natural rubbers, which are contained in the airplane fuel system,

may occur. This may be a problem when using alternate fuels

containing alcohol or other oxygenates. Oxygenated fuel may also

swell or soften plastics used in fuel systems.

 (4) Decomposition of fuel tank and access cover sealants

may occur over an extended period of time.

 l. Some alternate fuels dissolve the gums and residues that

build up over time and these gums may be deposited in filters and

metering orifices. This may present a problem for airplanes that

operate alternately on aviation gasoline then on an alternate fuel.

Fuels which contain oxygenates may exhibit this behavior.

 m. Existing engine oils may not be compatible with some

alternate fuels. Possible problems include large quantities of

unburned fuel in the oil, unusually acidic by-products of

combustion, and excessive water contamination. These problems may

be more severe if the engine is operated infrequently. Alternate

fuels may scavenge lead deposits which accumulate during operation

on aviation gasoline. These deposits may accumulate in the engine

oil. Fuel in the oil system may affect oil system components and

materials.

 n. Water contamination of an alternate fuel may affect the

characteristics of the fuel. Some examples are listed below:

 (1) Gasolines containing alcohols are subject to phase

separation. This may occur when liquid water is accidentally added

to the fuel during handling, when water is absorbed from the air, or

when the fuel is subject to temperature changes. Typically, the

precipitate (water, alcohol, and high octane components of the

gasoline) will not burn in the airplane engine. If the precipitate

contains a significant amount of the high octane components,
AC 23.1521-2 1/21/93

8

detonation may result. The usable fuel remaining should have an

octane rating higher than required by the airplane engine. Some

means for handling the precipitate or preventing the phase

separation should be provided.

 (2) An emulsion may be formed when liquid water is added

to an alternate fuel which consists primarily of one component (for

example, fuel grade methanol which contains small amounts of

gasoline to denature the fuel). Typically, these emulsions will not

burn in an airplane engine.

 (3) Fuels which are mutually miscible with water may

contain more water than an engine/fuel system can handle. If so,

some method of identifying the water content of the fuel should be

available to the pilot. Fuel tank and/or fuel system modifications

may be required.

 o. Switching from a tank containing one fuel to a tank

containing another may result in an engine stoppage. There are two

possible causes for this.

 (1) The resulting air-to-fuel ratio may be beyond either

the lean or rich combustion limit for the second fuel. Provisions

may be required to provide for the proper air-to-fuel ratio as the

fuel in use is changed.

 (2) Straight gasoline and fuels containing oxygenates may

not readily mix. As a consequence, the fuel flow may be interrupted

between the selector valve and the engine due to the formation of

fuel vapors at the interface of the fuels. Provisions to ensure the

flow of fuel may be required.

 p. Operation at the lean combustion limit may result in engine

stoppage when switching tanks if the fuel in the second tank does

not have at least the same energy density as the fuel in the first

tank.

 q. Fuel evaporating from the airplane tanks may entrain liquid

fuel. The airplane tank vents should be designed to separate the

liquid and return it to the tank, or the airplane range should be

adjusted to reflect the possible loss of up to 25% of the fuel

onboard. Fuel evaporating from the tanks may alter the octane

characteristics of the remaining fuel. The long-term effects of

aging should be evaluated.

 r. Adding fuel to a tank containing a different fuel, which is

not soluble in the first fuel, will probably result in an engine

stoppage or other operating problems. Provisions to prevent this

from occurring, or a means to drain the tank prior to refueling

should be provided.
1/21/93 AC 23.1521-2

9

 s. Airplane exposure to high ambient temperatures and direct

sunlight may result in fuel temperatures of 125°F (52°C), or higher.

These high temperatures increase the amount of vapor formed in the

fuel system which could then result in fuel interruption. The exact

effect depends on the type of fuel and the fuel system design.

 t. The hot weather (fuel) certification procedures (vapor

tolerance tests) are intended to establish margins for fuel supply

partial or total liquid flow discontinuities and depend upon the

fuel being evaluated (reference AC 23.961-1, "Procedures for

Conducting Fuel System Hot Weather Operation Tests," dated January

14, 1987). In general, the certification test for any fuel

should be conducted on days when the ambient temperature exceeds

30°C (850F), and when the engine is up to normal operating

temperatures. Normally, the critical fuel level is the minimum

amount of fuel required to complete the certification tests. To

ensure the most severe conditions are evaluated, the fuel being

tested should be handled and stored in sealed containers at or below

10°C (50°F), and the fuel should not be heated until it is in the

airplane tanks. Several methods of heating the fuel are available

including: heat exchangers placed in the airplane tank, exposure to

direct sunlight or heat lamps, and forcing hot air over the airplane

tanks. The fuel should not be agitated or handled during the

heating process. Below are some considerations for alternate fuels:

 (1) Gasolines containing alcohol may be more likely to

form vapor under the following conditions:

 (i) a fuel temperature in the tank of from 35° to

38°C (95° to 100°F);

 (ii) an alcohol concentration of 15% on a weight/weight

basis;

 (iii) a Reid Vapor Pressure at the maximum for which

approval is requested (or the maximum allowed for that class of

fuel); and,

 (iv) takeoff fuel flow.

If more than one alcohol is used in a gasoline alcohol blend, then a

total alcohol concentration of 15% on a weight/weight basis may be the

most likely to result in vapor formation. The gasoline blends made

with methanol are more prone to vapor formation than gasoline blends

made with ethanol.

 (2) Fuels which consist primarily of one constituent

(e.g., MTBE or methanol), present a special condition. For this

class of fuels, the worst case for vapor formation may be takeoff

fuel flow and a fuel temperature near the boiling point of the

principal constituent. The results of a temperature survey

conducted by the FAA Technical Center indicate that the maximum fuel

temperature one could expect to experience is 52°C (125°F). Since
AC 23.1521-2 1/21/93

10

the boiling point of this class of fuel is typically higher than

52°C, a fuel temperature between 52° to 55°C (125° to 131°F) may be

the most critical and may provide realistic certification criteria.

 (3) Service experience with automotive gasoline has

indicated that a critical condition for high vapor pressure fuels

with a hydrocarbon base may be fuel at 85°F.

 (4) Some other conditions may be the most critical for

vapor formation dependent upon the fuel composition. The applicant

should investigate and propose the most critical conditions for the

hot fuel testing.

 u. During the flight test for hot weather (fuel) certification,

the following criteria should be considered:

 (1) The takeoff and climb should be made as soon as

possible after the fuel in the tank reaches the desired temperatures

determined in paragraph 7t. The airspeed should be the same as that

used to meet the requirements of § 23.65 with the exception the

airplane weight should be the weight with the critical fuel level,

minimum crew, and ballast as necessary to maintain center of gravity

limits.

 (2) Power settings should be maintained at the maximum

approved levels for takeoff and climb to provide for the maximum

fuel flow.

 (3) The climb should be continued to the maximum operating

altitude desired or previously approved for the airplane.

Appropriate limitations should be noted in the Airplane Flight

Manual (AFM) Supplement or Supplemental Airplane Flight Manual.

 (4) Tests should be conducted with the fuel system

operating in accordance with the normal procedures outlined in the

AFM.

 (5) The following data should be recorded for the flight

test:

 (i) Fuel temperature in the tank.

 (ii) Fuel pressure at the start of the test and

continuously (recording device or manually) during climb noting any

pressure failure, fluctuation, or variations.

 (iii) Main and emergency fuel pump operation, as

applicable.

 (iv) Pressure altitude.

 (v) Ambient air temperature.
1/21/93 AC 23.1521-2

11

 (vi) Airspeed.

 (vii) Engine power, cylinder head temperature,

turbocharger temperature, torque, r.p.m., exhaust gas temperature,

manifold pressure, and fuel flow, as appropriate.

 (viii) Comments on engine operations.

 (ix) Fuel quantities in the fuel tank(s) during

takeoff.

 (x) Fuel vapor pressure of the fuel to be used

during the test.

 (xi) Fuel grade or designation, determined prior to

test.

 (6) A fuel pressure failure is considered to occur when

the fuel pressure decreases below the minimum prescribed by the

engine manufacturer or the engine does not operate satisfactorily.

 (7) The emergency fuel pumps should be inoperative if

they are being considered for use as backup pumps. This test may be

used to establish the maximum pressure altitude for operation with

the pumps off.

 (8) It should not be necessary to provide additional heat

for the fuel system after the original fuel sample is heated to

temperature during the hot weather tests.

 (9) If significant fuel pressure fluctuations occur

during testing of the critical flight condition but pressure failure

does not occur, additional testing should be considered to determine

that pressure failure does not occur during any expected operating

mode. Also, the fuel system should be evaluated for vapor formation

during cruise flight at maximum approved altitude in smooth air at

low to moderate power setting and low fuel flow and during idling

approach to landing. It is possible for vapor lock or fuel boost

pump cavitation to be critical at a reduced power and fuel flow

rate.

(10) Appropriate AFM instructions may be necessary

concerning proper use of fuel pumps during hot weather operations.

Also, any limitation on the outside air temperature as a result of

hot weather tests should be included in the AFM.

(11) The hot weather tests may have to be repeated if the

critical fuel tank for the particular installation cannot be

positively identified.
AC 23.1521-2 1/21/93

12

 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.
1/21/93 AC 23.1521-2

13

 (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
AC 23.1521-2 1/21/93

14

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
1/21/93 AC 23.1521-2

Appendix 1

1 (and 2)

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.
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1/21/93 AC 23.1521-2

Appendix 2

1 (and 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.