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Federal Aviation Administration, DOT 

§ 25.979 

(1) Each vent must be arranged to 

avoid stoppage by dirt or ice forma-

(2) The vent arrangement must pre-

vent siphoning of fuel during normal 

(3) The venting capacity and vent 

pressure levels must maintain accept-
able differences of pressure between 
the interior and exterior of the tank, 

(i) Normal flight operation; 
(ii) Maximum rate of ascent and de-

scent; and 

(iii) Refueling and defueling (where 


(4) Airspaces of tanks with inter-

connected outlets must be inter-

(5) There may be no point in any vent 

line where moisture can accumulate 
with the airplane in the ground atti-
tude or the level flight attitude, unless 
drainage is provided; 

(6) No vent or drainage provision may 

end at any point— 

(i) Where the discharge of fuel from 

the vent outlet would constitute a fire 
hazard; or 

(ii) From which fumes could enter 

personnel compartments; and 

(7) Each fuel tank vent system must 

prevent explosions, for a minimum of 2 
minutes and 30 seconds, caused by 
propagation of flames from outside the 
tank through the fuel tank vents into 
fuel tank vapor spaces when any fuel 
tank vent is continuously exposed to 


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 

(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 

[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; 

(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 

(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, 

§ 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 

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