Subject: Fire Prevention Date: 6/28/02

Initiated By: ANE-110

AC No: 33.17-1

Change:

1. PURPOSE. This advisory circular (AC) provides definitions, guidance, and

acceptable methods, but not the only methods, that may be used to demonstrate

compliance with the fire prevention requirements under Title 14 Code of Federal

Regulations (14 CFR), part 33. This AC may be incorporated into AC 33.2, Aircraft

Engine Type Certification Handbook at a later date.

2. RELATED CFR SECTIONS. Section 33.17 is the primary section addressed in this

AC, although other sections of part 33 that address fire prevention may also be

applicable. Other related parts and sections are listed in AC 20-135, Powerplant

Installation and Propulsion System Component Fire Protection Methods, Standards and

Criteria Appendix 1.

3. BACKGROUND. This AC is intended to provide guidance relating to these

requirements, and is considered a supplement to AC 20-135.

4. DEFINITIONS. For the purposes of this AC, the following definitions apply:

a. Hazardous Quantity: An amount of flammable fluid, vapor or other material

which could sustain a fire of sufficient severity and duration so as to significantly

increase the overall fire hazard or result in a hazardous condition.

b. External Lines, Fittings, and Other Components: Engine parts conveying

flammable fluids that are external to the main engine casings, frames, and other major

structure. These parts include, but are not limited to, fuel or oil lines, accessory gearbox,

pumps, heat exchangers, valves, and engine fuel control units.

c. Fireproof: Fireproof: The capability of a part or component to withstand, as well

as or better than steel, a 2000 0F average flame temperature (± 1500 F individual

thermocouple tolerance) for a minimum of 15 minutes, while still performing those

functions intended to be performed when exposed to a fire.
AC 33.17-1 6/28/02

d. Fire Resistant: The capability of a part or component to perform those functions

intended to be performed while exposed to the heat and other conditions that are likely to

occur at the particular location, and to withstand a 20000 F average flame temperature (±

1500

 F individual thermocouple tolerance) for a minimum of 5 minutes.

e. Hazardous Condition: Any condition which causes the engine to catch fire; burst

(release hazardous fragments through the engine case); generate loads greater then the

ultimate loads specified in § 33.23(a); lose the capability of being shutdown, or any other

result of exposure to fire which would preclude the continued safe operation or shutdown

of the engine.

f. Fire Hazard: The unintentional release or collection of flammable fluids; vapor or

other materials; a failure or malfunction which results in an unintentional ignition source

within a fire zone; the potential for a hazardous condition as the result of exposure to a

fire, or a sustained or non-self extinguishing fire.

g. Fire Zone: Fire zones are designated regions of a powerplant installation as

identified in Sections 23.1181, 25.1181 and 29.1181, and which are required to meet

specific powerplant fire protection requirements under the applicable aircraft subpart.

5. GENERAL.

a. Intent and Objective: The intent of § 33.17 is to ensure that the design, materials,

and construction methods utilized will minimize the occurrence and spread of fire. The

primary objectives are to contain, isolate and withstand a fire; prevent any source of

flammable material from feeding an existing fire; perform those engine functions

intended to be performed in the case of fire, and not result in a hazardous condition.

b. Fire Protection Capability - Determination of Fireproof vs. Fire Resistant:

(1) Section 33.17(b) requires that all flammable fluid conveying parts or

components are fire resistant, as a minimum, while § 33.17(c) requires flammable fluid

tanks and associated shutoff means to be fireproof. Therefore, it must be determined

which level of fire protection capability must be shown for each component requiring a

fire protection evaluation. In general, components which convey flammable fluids can be

evaluated to a fire resistant standard, provided the normal supply of flammable fluid is

stopped by a shutoff feature (also see § 33.71(c)(8)). For example, the fire resistant

criteria has been applied to engine fuel system components because the 5 minute

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6/28/02 AC 33.17-1

(2) exposure provides a reasonable time period for the flight crew to recognize a

fire condition, close the appropriate fuel shutoff valve(s), and shut down the appropriate

engine, thereby cutting off the fuel source. However, oil system components of turbine

engines may continue to flow oil after the engine has been shutdown due to continued

rotation (windmilling). These effects include the rotation of gearbox mounted oil

pump(s), and subsequent oil flow through the lubrication system. The supply of oil to the

fire might exist for as long as the continued rotation effects are present, or until the oil

supply is depleted. Therefore, oil system components may need to be evaluated from a

fire hazard perspective (e.g., quantity, pressure, flow rate, etc.) to determine whether the

fire resistant or fire proof standards should apply. Historically, it should be recognized

that most oil system components have been evaluated to a fireproof standard.

(3) Other flammable fluid conveying components (i.e., except flammable fluid

tanks), such as hydraulic and thrust augmentation systems should be evaluated in a

similar manner. Flammable fluid tanks must be fireproof as discussed in paragraph 8(a)

of this AC.

NOTE: Sections 25.1189 (a)(2) and 23.1189(b)(2) require a shutoff valve in all

flammable fluid conveying components flowing into, within or through designated fire

zones, except that shutoff means are not required for, “Oil systems for turbine engine

installations in which all components of the system in a designated fire zone, including

oil tanks, are fireproof or located in areas not subject to engine fire conditions.”

c. Fire Test Pass/Fail Criteria: In general, the following fire test criteria have been

applied to the test article and found to be acceptable:

(1) Maintain the ability to perform those functions intended to be provided in the

case of fire. The functions intended to be provided in the case of fire will be determined

on a case by case basis. Some examples are as follows: Engine hydromechanical

controls must not cause a hazardous condition while continuing to operate, but must

allow or may cause a safe shutdown of the engine at any time within the required

exposure time period. A safe engine shutdown at any time during the fire resistant test is

an acceptable outcome for this type of component, provided the safe shutdown is

maintained until the end of the 5 minutes fire resistant test. For a flammable fluid tank

shutoff valve, the valve must be operable (to close after 5 minutes) or should default

closed, and be capable of maintaining this position without leakage of a hazardous

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AC 33.17-1 6/28/02

quantity for the full 15 minute fireproof test. The above examples are included to

illustrate the case by case nature of making this determination. The applicant, along with

the appropriate Federal Aviation Administration (FAA) Aircraft Certification Office

(ACO) should coordinate early in the program in this regard.

(2) No leakage of hazardous quantities of flammable fluids, vapors or other

materials. At no time during or at the end of the test should the test article leak a

hazardous quantity of flammable fluid in any manner. Pressurized lines should remain

pressurized during and following the test. Observation of the test article for a period after

the test flame is removed and with the test article still pressurized is generally needed to

determine whether leakage has occurred and to what extent. Hazardous quantity is

defined in Section 4 of this AC.

(3) No support of an existing fire event by the constituent material of the article

being tested or by flammable fluid leaking from the test article (e.g., rapid selfextinguishing and no re-ignition after test flame removal). Consideration must be given

to fires that continue to burn after removal of the test flame. This type of event could be

either combustion of the constituent material of the test article or combustion of

flammable fluid leaking from the component (firewalls are not considered in either case).

In general, these events should continue to be cause for failure of the test, unless it can be

shown that the residual fire will not significantly increase the overall fire hazard. The

acceptability of such a test result will be determined on a case by case basis, and will

consider the type and function of the component under test.

(4) No other hazardous condition should result. At no time during or at the end

of the test should a hazardous condition result. Hazardous condition is defined in Section

4 of this AC.

6. MATERIALS. Experience has shown that when using certain materials (e.g.,

magnesium and titanium alloys) appropriate design precautions may be required to

prevent an unacceptable fire hazard. Consideration should be given to the possibility of

fire because of rubbing or contact with hot gases. Any materials used for abradable

linings need to be assessed to ensure that fire or

explosion hazards are avoided. Consideration should also be given to the effects of

mechanical failure of any engine component, and to the effects of dimensional changes

(e.g., rotor/case clearances) resulting from thermal effects within the engine.

a. Use of Titanium: Many titanium alloys used in the manufacture of engine

components will ignite and sustain combustion under certain conditions. In general,

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6/28/02 AC 33.17-1

titanium fires burn very fast and are extremely intense. The molten particles in titanium

fires generate highly erosive hot sprays that have burned through compressor casings

with resulting radial expulsion of molten or incandescent metal. When showing

compliance with § 33.17(a), the applicant should assess the overall design for

vulnerability to titanium fires. If this assessment can not rule out the possibility of a

sustained fire, then it should be shown that a titanium fire does not result in a hazardous

condition. Additional information on the use of titanium parts in aircraft engines can be

found in Reference number 3 of this AC.

b. Use of Magnesium: Many magnesium alloys used in the manufacture of engine

components are highly combustible when in finely divided form, such as chips or

powder. Therefore, magnesium use should be carefully evaluated when used in thin

sections or where rubbing or high scrubbing speeds will be a consideration. Additional

information on the use of magnesium parts in aircraft engines can be found in Reference

number 9 of this AC. When showing compliance with § 33.17(a), the applicant should

assess the overall design for vulnerability to magnesium fires. If this assessment can not

rule out the possibility of a sustained fire, then it should be shown that a magnesium fire

will not result in a hazardous condition.

c. Abradable Linings: Many fan, compressor, and turbine modules have abradable

linings between rotating blade tips and stator casings. Depending upon the material used

in the abradable lining, experience has shown that a fire or explosion can occur in the

presence of an ignition source if a significant amount of lining is removed during rubs

between rotor and stator. Under certain conditions, auto ignition can occur in the mixture

of small abradable particles and hot flowpath gases. These situations should be evaluated

by the applicant for each fan, compressor, and turbine stage with abradable linings.

d. Absorbent Materials: Absorbent materials should not be used in close proximity

to flammable fluid system components, unless they are treated or covered to prevent the

absorption of a hazardous quantity of such fluid.

e. Fiber and Resin Materials: Certain fiber and resin materials, such as aramid fiber

(e.g., Kevlar fabric) or carbon/graphite composites, may be combustible under certain

circumstances. In engines, aramid fabric is typically used as part of fan rotor

containment systems. Carbon/graphite composites have been used for fan blades, thrust

reverser components, and other parts. When showing compliance with § 33.17(a), the

applicant should assess the overall design for vulnerability to fires supported by these

materials. If the assessment can not rule out the possibility of these components

supporting a sustained fire, then it should be shown that such a fire will not result in a

hazardous condition.

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AC 33.17-1 6/28/02

7. CONDUCT OF FIRE TESTS.

a. Test Equipment: Guidance on acceptable burner types, burner configurations, and

other test hardware can be found in Reference numbers 1 and 2 of this AC. Pre and post

test calibrations of burner equipment are generally required. Measured burner flame

temperature fluctuations during the test are acceptable only when the pre and post test

calibrations are within the prescribed limits, and test burner controlling parameters are

constant during the test. Experience has shown that the measured temperature of the

flame could be affected by the presence of the component under test.

b. Flame Impingement Location: Generally, the test flame should be applied to the

test article feature(s), which are determined by analysis or test, to be critical with respect

to surviving the effects of the fire. For this approach, determination of the flame

impingement location(s) should consider, as a minimum, the following potential factors:

materials, geometry, part critical features, local torching effects, vibration, internal fluid

level/pressure/flow rate, surface coatings, fire protection features, wetting, etc. Other

factors not listed may also apply. Alternatively, the test plan may consider all potential

sources of fire in the intended installation when determining the test flame impingement

location requirements. The intent is to identify locations or features that can not be

directly impinged by fire and evaluate the critical features at locations that can be directly

impinged. If the applicant chooses this installation analysis approach, it should be based

on the actual intended installation, and should consider, as a minimum, the factors noted

above, and specifically the following potential installation factors: cowling and nacelle

structure, adjacent structure shielding, undercowl airflow, aircraft engine build up (EBU)

hardware, fuel sources, air sources, etc. Other factors not listed may also apply. Such

installation analyses should avoid simple generalities, such as “the most likely flame

direction is vertical assuming fuel collects at the bottom of the cowl," and should

generally be coordinated with the installer before the test plan is submitted. If this

approach is used, each new installation will need to be re-evaluated against the original

fire protection substantiation to confirm its applicability to the new installation. A

notation in the installation instructions may be necessary to explain this limitation.

Lastly, due consideration should be given to fire protection features such as fire shields,

fire protective coatings, or other methods, so as to not discourage or invalidate their use

for fire prevention purposes.

c. Operating Parameters for Test Articles: The operating characteristics and

parameters of the test article should be consistent, but conservative, relative to the

conditions that might occur during an actual fire situation in the type design product. For

example, where a high internal fluid flow increases the heat sink effect, and is less

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6/28/02 AC 33.17-1

conservative with respect to fire susceptibility, a minimum flow condition should be

specified for the test. The same is true for examples relating to internal fluid

temperatures, quantity, or other parameters. This evaluation primarily concerns critical

inflight operating conditions, including continued rotation (windmilling and propeller

feathering) after shutdown. Consideration of engine basic failure states (e.g.,

mechanically damaged component, or locked main rotor) are not generally under

consideration when evaluating test conditions. In addition, any facility slave hardware

used to establish boundary conditions for the test (e.g., simulated engine heat exchanger)

must do so in a manner representing type design operation.

d. Other Guidance: Other guidance on acceptable methods of conducting fire tests

can be found in References 1 and 2 of this AC.

8. OTHER CONSIDERATIONS.

a. Flammable Fluid Tanks:

(1) Flame Impingement Location: In the absence of an acceptable installation

assessment, the fire test flame should be applied to the tank location(s) or feature(s) that

have been determined by analysis or test to be critical regarding surviving the effects of

the fire (i.e., the location(s) or feature(s) least likely to survive the test conditions or meet

the test pass/fail criteria). In selecting the flame application location(s), all features of the

tank assembly must be considered. Typical tank installations include, but are not limited

to; tank body, inlet and outlet assemblies, sight-gauge, drain plug, magnetic chip detector,

quantity sender assembly, vent line assembly, fill cap and scupper, mounts, shutoff valve,

temperature sensor, and air/fluid separator assembly. Tanks can be designed and

manufactured with any combination of the above features, or other features not listed,

and of varying materials. Therefore, in some instances, compliance with § 33.17 may

need to be supported by data from other fire tests, multiple location testing,

subcomponent level tests, or service experience, to cover all tank assembly features.

Also, other aspects of determining impingement location should be considered, such as

vent system performance (e.g., oil tank fire tests have failed due to high internal pressure

and inadequate venting), the lack of heat sink effect for tank features at or above the

operating level (i.e., water line) of the tanks fluid contents; and the affect of any special

protective features (e.g., shields, coatings, feature placement, etc.) incorporated into the

design when developing the fire test plan.

(2) Other Test Parameters: With respect to fluid quantity, the tank quantity at

the start of the test should be no greater than the minimum dispatchable quantity minus

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AC 33.17-1 6/28/02

(3) the normal gulping volume, unless a greater quantity is more severe.

Relative to flow rate, the first 5 minutes of the test should be conducted at the most

critical operating condition (typically a minimum flight idle flow rate), and the

subsequent 10 minutes should be conducted at an engine shutdown flow rate (continued

rotation considered). The test may be run, at the applicant’s option, for 15 minutes at the

most critical condition (worst case of engine operating or inflight shutdown conditions).

(4) Fluid temperature should be at its maximum value (greater of steady-state or

transient limit established under § 33.7) at the start of the test, unless a lower temperature

is more severe. The tank internal pressure should be the normal working pressure for the

operating conditions at the start of the test. It is understood that these values may change

due to the test conditions.

The tank design and its intended application should be reviewed, and provide reasonable

assurance that the test set-up reflects the most critical flame impingement orientation and

operating conditions for the intended application. Note that the aircraft requirements of

parts 23, 25, 27, and 29 rely heavily on the fire prevention findings of part 33. Failure to

adequately test may result in aircraft installation issues.

b. Air Sources: In accordance with § 33.17(a), the applicant should evaluate the

effect of fire on components conveying bleed air, and evaluate whether failure of such

components could further increase the severity or duration of a fire within a fire zone.

c. Engine Mounts: The fire protection requirements for engine mount systems

(including engine type design) are governed by the aircraft regulations, and compliance is

shown as part of the aircraft certification. The engine manufacturer should coordinate

with the installer to minimize the possibility of installation issues affecting aircraft

certification.

d. Hot Surface Ignition (HSI): Information concerning hot surface ignition is

available in References 7 and 8 of this AC.

e. Firewalls: Firewall components have historically been evaluated at the aircraft

level (e.g., §§ 25.1191 & 25.1193). However, if a component whose primary function is

that of a firewall is (1) included in the engine type design; and (2) is needed to comply

with the requirements of § 33.17; then at no time during or at the end of a fire test should

the firewall component fail to contain the fire within the intended zone or area.

Acceptable evidence that the fire is contained would be that the firewall component does

not develop a burn through hole, does not fail at any attachment or fire seal point around

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6//28/02 AC 33.17-1

its periphery, does not cause backside ignition, and does not continue to burn after the

test flame is removed. Also, in no case should a hazardous quantity of fuel or fuel/air

mixture leak around or pass through the firewall. In addition, the firewall should contain

the fire without resulting in a hazardous condition. Lastly, the effects of pressure and

mechanical loading on the firewall structure and associated seals must be considered

when evaluating overall firewall capability and testing under § 33.17. Reference 1 (AC

20-135, Section 7) provides additional guidance for the testing and evaluation of firewalls

and fire seals. It should be noted that that nacelle cowlings are not generally considered

primary firewall components.

Shielding: The overall intent of the § 33.17(b) requirement concerning the shielding and

location of components, is to minimize the possibility of leaking flammable fluids

contacting ignition sources and igniting. Ignition sources include hot surfaces with

temperatures at or above the typical auto-ignition temperature for aviation fuels, oils, and

hydraulic fluids, or any component that produces an electrical discharge. Compliance

with this requirement has been shown by installation of drainage shrouds around

flammable fluid lines or fittings; installation of spray shields to deflect leaking fuel away

from ignition sources, and general component location on the engine which minimizes

the possibility of starting and supporting a fire. Therefore, the overall substantiation

should show that leaked flammable fluid would not likely impinge on an ignition source

to the extent of starting and supporting a fire. For kerosene type fuels, an auto-ignition

temperature of 4500 F has been accepted, although fuel/air ratio, nacelle venting and

other factors may play a role in determining whether hot surface ignition is a hazard for a

given design. Information concerning hot surface ignition is available in References 7

and 8 of this AC.

f. Drains and Vent Systems: Certain drain and vent systems may be exempt from

the requirements of § 33.17(b) if it can be shown that they do not typically contain or

convey flammable fluids during normal engine operation. In this context, normal

operation is the taxi and flight portions of a typical flight. An example of a drain line that

might be exempt, is a combustor drain line that typically drains off residual fuel after an

aborted engine start. An example of a tube or line which would not be exempt is a

shrouded fuel manifold. Such a line is considered a single assembly that cannot be

separated into its main fuel line and its outer drain line (which would flow if the main

manifold failed). In the case of a drain and vent system line that would flow a hazardous

quantity of flammable fluid during continued rotation, then a fireproof standard may be

appropriate. A drain collection reservoir that stores a hazardous amount of flammable

fluid would likely be evaluated against a fireproof standard. The function of each drain

or vent system component should be carefully reviewed in making these determinations.

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AC 33.17-1 6/28/02

9. TEST AND COMPLIANCE PLANS.

a. Certification test plans should include, but are not limited to, the following

information:

(1) Component name(s),

(2) part number(s),

(3) part detail drawing(s) or sketches (e.g., denote critical features),

(4) installation drawing(s) or sketches (e.g., describe installation in an engine),

(5) description of component operation,

(6) definition and range of component operating parameters,

(7) flame direction/impingement analysis,

(8) est equipment, test set-up and test fluids,

(9) test methods and procedures,

(10) test criteria,

(11) data recording methods,

(12) industry standard references as applicable,

(13) applicable CFRs, and

(14) time and place of test.

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6//28/02 AC 33.17-1

b. The proposed certification test plan should contain as a minimum; the information

described in paragraph 9.a. of this AC, and should be submitted to the applicable FAA

ACO for coordination and approval before conducting the fire testing.

The cognizant ACO and engine manufacturer should review the part 33

compliance plan, ensuring that the fire prevention intent and objective of each part 33

section are met. Regarding the aircraft requirements of parts 23, 25, 27, and 29 listed in

AC 20-135, the applicant should be encouraged to review these sections with the installer

early in the program to minimize potential installation problems after engine certification.

/s/

Francis Favara,

Assistant Manager, Engine and Propeller Directorate

Aircraft Certification Service

Par 9 Page 11
REFERENCES

1. FAA Advisory Circular AC 20-135: “Powerplant Installation and Propulsion

System Component Fire Protection Methods, Standards, and Criteria", dated February 6,

1990.

2. FAA Powerplant Engineering Report No. 3A: “Standard Fire Test Apparatus and

Procedures", Revised March 1978.

3. FAA Advisory Circular AC 33-4: “Design Considerations Concerning the Use of

Titanium in Aircraft Turbine Engines”, dated July 28, 1983.

4. SAE AS1055B: “Fire Testing of Flexible Hose, Tube Assemblies, Coils, Fittings, and

Similar System Components”, dated March 1, 1978.

5. SAE AIR 1377: “Fire Test Equipment for Flexible Hose and Tube Assemblies”,

dated January 1980.

6. FAA Report No. FAA-RD-76-213: “Re-evaluation of Burner Characteristics for

Fire Resistance Tests”, dated January 1977.

7. SAE Report No. 690436: “Ignition of Aircraft Fluids on High Temperature Engine

Surfaces”, by W.T. Westfield of the FAA.

8. FAA Report No. FAA-RD-75-155: “Ignition and Propagation Rates for Flames in a

Fuel Mist”, dated October 1975, by C.E. Polymeropoulos

9. FAA Technical Report No. FAA-ADS-14: “A Study of the Flammability of

Magnesium,” by Paul Boris, Systems Research and Development Service, FAA dated

April 1964.

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