Policy
Statement
Subject: Fireproof Auxiliary Power Unit
(APU) Mounts
Date: Proposed
Policy No:
PS-ANM-112-11-003
Initiated By:
ANM-100
Summary
This policy statement clarifies that airplane manufacturers must show that the APU mounting
system, including the APU attachment points, is fireproof, or shielded so that it is capable of
withstanding the effects of fire in compliance with Title 14, Code of Federal Regulations (14
CFR) 25.865. Specifically, this policy addresses the installation of an APU that was not certified
as having fireproof mounting structure or attachment points under Technical Standard Order
(TSO) C77b.
Definition of Key Terms
In the text below the terms “must,” “should,” and “recommend” have a specific meaning that is
explained in Attachment 1.
Current Regulatory and Advisory Material
The regulation applicable to fireproofing structures and equipment within fire zones is § 25.865.
This section presents the regulatory basis for the installation and approval of essential flight
controls, engine mounts, and other flight structures located in designated fire zones or in adjacent
areas which would be subjected to the effects of fire in the fire zone, requiring that such
structures must be constructed of fireproof materials or shielded so that they are capable of
withstanding the effects of fire. As evidenced below in the section on relevant past practice, the
FAA has applied this section to APU mounts without controversy under the provisions of
§§ 25.901(d) and 25.1181(b). Section 25.901(d) requires each APU installation to meet the
applicable provisions of subpart E to part 25. Section 25.1181(b), which is located within
subpart E and is applicable to APU installations, requires each designated fire zone to meet the
requirements of a list of regulations that include § 25.865. The point of clarification expressed in
this policy statement regards what constitutes an “APU mount” for the purpose of showing
compliance with § 25.865 for APU installations.
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Section 25.865 originated in Notice of Proposed Rulemaking 68-18 (33 FR 11913, August 22,
1968). The following excerpt from the preamble to the Notice provides guidance on the intent of
this rule:
The current regulations do not require protection of engine mounts, or control systems
from the effects of fire. The need for this protection was recently highlighted when
control problems were experienced on a jet transport airplane after aluminum aileron rods
located outside of the fire zone became distorted due to heat from an engine fire. Engine
mounts and flight structures can also be affected by fire and a failure resulting from the
heat of a fire could cause a serious safety hazard.
Amendment 25-23 (35 FR 5665, April 8, 1970) followed Notice 68-18, and incorporated the
proposed change. The following excerpt from the preamble to the Amendment provides further
guidance on the intent of this rule:
In response to comments received, the proposed requirement of § 25.865 has been
changed to make it clear that it is only those flight controls, engine mounts, and other
flight structures which would be damaged by the “effects of fire” that need be constructed
of fireproof material or shielded. In response to another comment, § 25.865 has been
changed to make it clear that it is the “essential” flight controls with which the regulation
is concerned.
These components are also discussed in TSO-C77, TSO-C77a, and TSO-C77b.
Relevant Past Practice
In a review of airplanes certified since the adoption of § 25.865, the FAA has determined that
while applicants have shown that an APU airframe-mounting system up to the APU attachment
points is fireproof per the rule, the applicants have generally not specifically shown, and the
FAA has generally not specifically evaluated, the APU attachment points for compliance with
§ 25.865. For engine mounts, it is clear from the NPRM and final-rule preamble excerpts that the
objective of § 25.865 is to prevent structural failure caused by the effects of heat from any fire
within a designated fire zone. Failure of the engine mounts means the inability to retain the
engine in its proper position in the aircraft, or failure to carry engine-mount loads within the
aircraft certificated operating envelope, without permanent deformation. To meet this § 25.865
objective, any element of an engine-mount system, located within the designated fire zone,
including the engine itself, must be “constructed of fireproof material or shielded so that [the
engine-mount system located within the fire zone] is capable of withstanding the effects of fire”
as required by the rule.
Previous Policy
Previous policy only addresses § 25.865 in terms of how the requirement would apply to engine
mounts, not APU mounts and attachment points specifically.
Policy
Section 25.865 is an applicable requirement for all APU installations on transport airplanes.
Airplane manufacturers and modifiers installing APUs must show that the APU mounting system
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is fireproof, or shielded so that it is capable of withstanding the effects of fire, in compliance
with the regulation as specified for the APU airframe-mounting system. This policy applies to
installation of all APUs, including those produced under TSO authorization, for example, TSOC77, TSO-C77a, and TSO-C77b, as well as those produced without TSO authorization.
TSO-C77b authorization includes a finding that those features of the APU that form part of the
mounting structure or APU attachment points are fireproof. Therefore, as long as the installation
conditions assumed during TSO testing adequately cover the actual installation, an APU certified
to TSO C-77b should allow the installer to be able to show that new APUs meet the standards of
§ 25.865.
Effect of Policy
The general policy stated in this document does not constitute a new regulation. Agency
employees and their designees and delegations must not depart from this policy statement
without appropriate justification and concurrence from the FAA management that issued this
policy statement.
Whenever a proposed method of compliance is outside this established policy, the project aircraft
certification office has to coordinate it with the policy-issuing office using an issue paper.
Similarly, if the project aircraft certification office becomes aware of reasons that an applicant’s
proposal that meets this policy should not be approved, the office must coordinate its response
with the policy issuing office. Applicants should expect that certificating officials would
consider this information when making findings of compliance relevant to new certificate
actions. In addition, as with all guidance material, this policy statement identifies one means, but
not the only means, of compliance.
Implementation
This policy discusses compliance methods that should be applied to type certificate, amended
type certificate, supplemental type certificate, and amended supplemental type-certification
programs. The compliance methods apply to those programs with an application date that is on
or after the effective date of the final policy. If the date of application precedes the effective date
of the final policy, and the methods of compliance have already been coordinated with and
approved by the FAA or its designee, the applicant may choose to either follow the previously
acceptable methods of compliance or follow the guidance contained in this policy.
Conclusion
The FAA has concluded that it is necessary to clarify the applicability of § 25.865 to the APU
attachment points. Past practices that do not evaluate the fireproof capability of the APU
attachment points are no longer acceptable.
END
Attachment
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Attachment 1
Terms
Table A-1 defines the use of key terms in this policy statement. The table describes the
intended functional impact.
Table A-1 Definition of Key Terms
Regulatory
Requirements
Acceptable Methods of
Compliance (MOC)
Recommendations
Recommend
Language
Must
Should
Meaning
Refers to a regulatory
requirement that is
mandatory for design
approval
Refers to instructions for Refers to a
a particular MOC
recommended
practice that is
optional
Functional
Impact
No Design Approval
if not met
Alternative MOC has to
be approved by issue
paper.
None, because it is
optional