AC 33.17-1B
9.5.4 Pressure and mechanical loading effects on the firewall structure and associated fire seals must be considered when evaluating overall firewall capability and testing under § 33.17. AC 20-135 provides additional guidance for testing and evaluating firewalls and fire seals.
9.5.5 Firewall components will also be evaluated under aircraft requirements when installed on the aircraft (e.g., §§ 23. 2440, 25.1191, 25.1193, 27.1191, 27.1193, 29.1191, and 29.1193).
9.5.6 Firewalls must be protected against corrosion per § 33.17(d)(3).
9.6 Shielding (§ 33.17(b)).
9.6.1 Section 33.17(b) requires that each component containing or conveying flammable fluid during normal engine operation be shielded or located to prevent ignition of leaking flammable fluid.
9.6.2 Ignition sources include:
x Hot surfaces with temperatures at or above the typical auto-ignition temperature for aviation fuels, oils, and hydraulic fluids or x Components producing an electrical discharge.
9.6.3 Compliance has been demonstrated through:
x Installation of drainage shrouds around flammable fluid lines or fittings, x Installation of spray shields to deflect leaking fuel away from ignition sources, and x Component placement on the engine that minimizes the likelihood of starting and supporting a fire.
9.6.4 The overall substantiation should show that leaked flammable fluid is unlikely to impinge on an ignition source to the extent that it starts and supports a fire.
9.6.5 For kerosene-type fuels, an auto-ignition temperature of 450°F has been accepted. However, fuel/air ratio, nacelle venting, and other factors may influence hot surface ignition risks for a given design. SAE Report No. 690436 and FAA Report No. FAA-RD-75-155 provide additional information related to hot surface ignition.
9.7 Drains and Vent Systems (§§ 33.17(b) and 33.17(f)).
9.7.1 Certain drain and vent systems may be exempt from § 33.17(b) requirements if they can be shown they do not typically contain or convey flammable fluids during normal engine operation.
9.7.2 Examples of drain and vent systems that may or may not require compliance:
x Requires compliance. A shrouded fuel manifold, considered a single assembly, cannot be separated into its main fuel line and outer drain line, which would flow if the main manifold failed.
x May not require compliance: A combustor drain line that typically drains residual fuel after an aborted engine start.
AC 33.17-1B
9.7.3 A drain or vent system that would convey a hazardous quantity of flammable fluid during continued rotation may need to meet a fireproof standard. Similarly, a drain collection reservoir that stores a hazardous quantity of flammable fluid would likely require fireproof evaluation.
9.7.4 Each drain or vent system component’s function should be carefully reviewed to determine compliance. Additionally, drain and vent system flow capacity should be at least equal to the maximum flow rate the system may need to convey.
9.8 Electrical Bonding (§ 33.17(g)).
9.8.1 Electrical bonding is a means to protect against the effects of unintentional electrical discharges or fault currents. Specifically, bonding can help minimize the risk of the ignition of flammable materials from static electrical discharges.
9.8.2 Section 33.17(g) requires that components, modules, or equipment that may cause or are susceptible to such effects must be electrically bonded (grounded) to the main engine reference.
Conductive parts are considered bonded when they are mechanically interconnected to maintain a common electrical potential.
9.8.3 This may be shown through:
x Examination of the type design drawings, x Electrical continuity checks, and x Physical inspection of a representative engine.
9.8.4 An additional benefit of good electrical bonding of engine components is minimizing electromagnetic interference of electrical equipment, especially controls.
9.9 Powerplant Designated Fire Zones.
Powerplant-designated fire zones are regions of a powerplant installation as identified in §§ 23.2440, 25.1181, and 29.1181.
10 CERTIFICATION TEST PLANS.
10.1 Required Information.
Certification test plans should include, but are not limited to, the following information:
x Component name(s), x Part number(s), x Part detail drawing(s) or sketches (e.g., denote critical features), x Installation drawing(s) or sketches (e.g., describe installation in an engine), x Description of component operation, x Hardware reconciliation to type design (test article and installation connections), x Definition and range of component operating parameters, 14
AC 33.17-1B x Flame direction/impingement analysis, x Fireproof/fire-resistant analysis, x Test equipment, test set-up, calibrations, and test fluids, x Test methods and procedures, x Test pass/fail criteria, x Data recording methods, x Applicable regulations, and x Time and place of test.
10.2 Submission and Approval.
The proposed certification test plan should include, at minimum, the information described in paragraph 10.1 of this AC. Before conducting the fire testing, the plan should be submitted to the applicable Certification Branch for coordination and approval.
11 COORDINATION WITH THE INSTALLER.
11.1 Coordination.
11.1.1 The Certification Branch and engine manufacturer should review the part 33 compliance plan to ensure the fire prevention intent and objective of each applicable section of part 33 are met.
11.1.2 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. Early coordination helps minimize potential installation problems after engine certification.
11.2 Additional Ground Capability.
11.2.1 The capability of some engine firewalls is affected by backside airflow driven by engine operation or aircraft in-flight speed. These conditions differ when the engine is shut down and not windmilling on the ground.
11.2.2 Ground conditions to consider include:
x Engine operation for five minutes, followed by x Engine shut down for 10 minutes (i.e., no scrubbing airflow).
11.2.3 Any additional tests necessary for a particular engine firewall to comply with the aircraft requirements discussed in paragraph 9.5 of this AC may be conducted in conjunction with § 33.17(d) engine certification tests. This is permissible if it can be shown that these tests are at least as severe as § 33.17(d) certification tests.
11.2.4 The following should be documented in the installation instructions required by § 33.5:
x The engine firewall, 15
AC 33.17-1B x Other engine components installed in a fire zone that require additional ground capability, and x Assumptions made during certification.
11.2.5 The applicant should provide the aircraft manufacturer with these assumptions made during engine certification for consideration in the installation design process.
12 AC FEEDBACK FORM.
For your convenience, the AC Feedback Form is the last page of this AC. Note any deficiencies found, clarifications needed, or suggested improvements regarding the contents of this AC on the Feedback Form.
DANIEL J.
Digitally signed by DANIEL J. ELGAS ELGAS Date: 2025.05.20 07:39:20 -04'00' Daniel -Elgas Director, Policy and Standards Division Aircraft Certification Service 16
OMB Control Number: 2120-0746 Expiration Date: 12/31/2027 Advisory Circular Feedback Form Paperwork Reduction Act Burden Statement: A federal agency may not conduct or sponsor, and a person is not required to respond to, nor shall a person be subject to a penalty for failure to comply with a collection of information subject to the requirements of the Paperwork Reduction Act unless that collection of information displays a currently valid OMB Control Number. The OMB Control Number for this information collection is 2120-0746. Public reporting for this collection of information is estimated to be approximately 20 minutes per response, including the time for reviewing instructions, searching existing data sources, gathering, and maintaining the data needed, completing, and reviewing the collection of information.
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