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318 

14 CFR Ch. I (1–1–18 Edition) 

§ 25.977 

(b)  Carburetor vapor vents. Each car-

buretor with vapor elimination connec-
tions must have a vent line to lead va-
pors back to one of the fuel tanks. In 
addition— 

(1) Each vent system must have 

means to avoid stoppage by ice; and 

(2) If there is more than one fuel 

tank, and it is necessary to use the 
tanks in a definite sequence, each 
vapor vent return line must lead back 
to the fuel tank used for takeoff and 
landing. 

[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as 
amended by Docket No. FAA–2014–0500, 
Amdt. No. 25–143, 81 FR 41207, June 24, 2016] 

§ 25.977

Fuel tank outlet. 

(a) There must be a fuel strainer for 

the fuel tank outlet or for the booster 
pump. This strainer must— 

(1) For reciprocating engine powered 

airplanes, have 8 to 16 meshes per inch; 
and 

(2) For turbine engine powered air-

planes, prevent the passage of any ob-
ject that could restrict fuel flow or 
damage any fuel system component. 

(b) [Reserved] 
(c) The clear area of each fuel tank 

outlet strainer must be at least five 
times the area of the outlet line. 

(d) The diameter of each strainer 

must be at least that of the fuel tank 
outlet. 

(e) Each finger strainer must be ac-

cessible for inspection and cleaning. 

[Amdt. 25–11, 32 FR 6913, May 5, 1967, as 
amended by Amdt. 25–36, 39 FR 35460, Oct. 1, 
1974] 

§ 25.979

Pressure fueling system. 

For pressure fueling systems, the fol-

lowing apply: 

(a) Each pressure fueling system fuel 

manifold connection must have means 
to prevent the escape of hazardous 
quantities of fuel from the system if 
the fuel entry valve fails. 

(b) An automatic shutoff means must 

be provided to prevent the quantity of 
fuel in each tank from exceeding the 
maximum quantity approved for that 
tank. This means must— 

(1) Allow checking for proper shutoff 

operation before each fueling of the 
tank; and 

(2) Provide indication at each fueling 

station of failure of the shutoff means 

to stop the fuel flow at the maximum 
quantity approved for that tank. 

(c) A means must be provided to pre-

vent damage to the fuel system in the 
event of failure of the automatic shut-
off means prescribed in paragraph (b) 
of this section. 

(d) The airplane pressure fueling sys-

tem (not including fuel tanks and fuel 
tank vents) must withstand an ulti-
mate load that is 2.0 times the load 
arising from the maximum pressures, 
including surge, that is likely to occur 
during fueling. The maximum surge 
pressure must be established with any 
combination of tank valves being ei-
ther intentionally or inadvertently 
closed. 

(e) The airplane defueling system 

(not including fuel tanks and fuel tank 
vents) must withstand an ultimate 
load that is 2.0 times the load arising 
from the maximum permissible 
defueling pressure (positive or nega-
tive) at the airplane fueling connec-
tion. 

[Amdt. 25–11, 32 FR 6913, May 5, 1967, as 
amended by Amdt. 25–38, 41 FR 55467, Dec. 20, 
1976; Amdt. 25–72, 55 FR 29785, July 20, 1990] 

§ 25.981

Fuel tank ignition prevention. 

(a) No ignition source may be present 

at each point in the fuel tank or fuel 
tank system where catastrophic failure 
could occur due to ignition of fuel or 
vapors. This must be shown by: 

(1) Determining the highest tempera-

ture allowing a safe margin below the 
lowest expected autoignition tempera-
ture of the fuel in the fuel tanks. 

(2) Demonstrating that no tempera-

ture at each place inside each fuel tank 
where fuel ignition is possible will ex-
ceed the temperature determined under 
paragraph (a)(1) of this section. This 
must be verified under all probable op-
erating, failure, and malfunction con-
ditions of each component whose oper-
ation, failure, or malfunction could in-
crease the temperature inside the tank. 

(3) Demonstrating that an ignition 

source could not result from each sin-
gle failure, from each single failure in 
combination with each latent failure 
condition not shown to be extremely 
remote, and from all combinations of 
failures not shown to be extremely im-
probable. The effects of manufacturing 

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