315 

Federal Aviation Administration, DOT 

§ 25.955 

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-
tion. 

[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-

tion. 

(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-
tachment; 

(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. 

(2) The quantity of fuel in the tank 

may not exceed the amount established 
as the unusable fuel supply for that 
tank under the requirements of § 25.959 
plus that necessary to show compliance 
with this section. 

(3) Each main pump must be used 

that is necessary for each operating 
condition and attitude for which com-
pliance with this section is shown, and 
the appropriate emergency pump must 
be substituted for each main pump so 
used. 

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