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14 CFR Ch. I (1–1–19 Edition) 

§ 25.952 

or auxiliary power unit is permitted to 
be in operation. 

(b) Each fuel system must be ar-

ranged so that any air which is intro-
duced into the system will not result 

(1) Power interruption for more than 

20 seconds for reciprocating engines; or 

(2) Flameout for turbine engines. 
(c) Each fuel system for a turbine en-

gine must be capable of sustained oper-
ation throughout its flow and pressure 
range with fuel initially saturated with 
water at 80 


F and having 0.75cc of free 

water per gallon added and cooled to 
the most critical condition for icing 
likely to be encountered in operation. 

(d) Each fuel system for a turbine en-

gine powered airplane must meet the 
applicable fuel venting requirements of 
part 34 of this chapter. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Amdt. 25–23, 35 FR 5677, Apr. 8, 
1970; Amdt. 25–36, 39 FR 35460, Oct. 1, 1974; 
Amdt. 25–38, 41 FR 55467, Dec. 20, 1976; Amdt. 
25–73, 55 FR 32861, Aug. 10, 1990] 

§ 25.952

Fuel system analysis and test. 

(a) Proper fuel system functioning 

under all probable operating conditions 
must be shown by analysis and those 
tests found necessary by the Adminis-
trator. Tests, if required, must be made 
using the airplane fuel system or a test 
article that reproduces the operating 
characteristics of the portion of the 
fuel system to be tested. 

(b) The likely failure of any heat ex-

changer using fuel as one of its fluids 
may not result in a hazardous condi-

[Amdt. 25–40, 42 FR 15043, Mar. 17, 1977] 

§ 25.953

Fuel system independence. 

Each fuel system must meet the re-

quirements of § 25.903(b) by— 

(a) Allowing the supply of fuel to 

each engine through a system inde-
pendent of each part of the system sup-
plying fuel to any other engine; or 

(b) Any other acceptable method. 

§ 25.954

Fuel system lightning protec-


(a) For purposes of this section— 
(1) A critical lightning strike is a 

lightning strike that attaches to the 
airplane in a location that, when com-
bined with the failure of any design 

feature or structure, could create an 
ignition source. 

(2) A fuel system includes any compo-

nent within either the fuel tank struc-
ture or the fuel tank systems, and any 
airplane structure or system compo-
nents that penetrate, connect to, or are 
located within a fuel tank. 

(b) The design and installation of a 

fuel system must prevent catastrophic 
fuel vapor ignition due to lightning and 
its effects, including: 

(1) Direct lightning strikes to areas 

having a high probability of stroke at-

(2) Swept lightning strokes to areas 

where swept strokes are highly prob-
able; and 

(3) Lightning-induced or conducted 

electrical transients. 

(c) To comply with paragraph (b) of 

this section, catastrophic fuel vapor ig-
nition must be extremely improbable, 
taking into account flammability, crit-
ical lightning strikes, and failures 
within the fuel system. 

(d) To protect design features that 

prevent catastrophic fuel vapor igni-
tion caused by lightning, the type de-
sign must include critical design con-
figuration control limitations 
(CDCCLs) identifying those features 
and providing information to protect 
them. To ensure the continued effec-
tiveness of those design features, the 
type design must also include inspec-
tion and test procedures, intervals be-
tween repetitive inspections and tests, 
and mandatory replacement times for 
those design features used in dem-
onstrating compliance to paragraph (b) 
of this section. The applicant must in-
clude the information required by this 
paragraph in the Airworthiness Limi-
tations section of the Instructions for 
Continued Airworthiness required by 
§ 25.1529. 

[Doc. No. FAA–2014–1027, Amdt. 25–146, 83 FR 
47556, Sept. 20, 2018] 

§ 25.955

Fuel flow. 

(a) Each fuel system must provide at 

least 100 percent of the fuel flow re-
quired under each intended operating 
condition and maneuver. Compliance 
must be shown as follows: 

(1) Fuel must be delivered to each en-

gine at a pressure within the limits 
specified in the engine type certificate. 

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