320 

14 CFR Ch. I (1–1–19 Edition) 

§ 25.981 

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 explosion preven-

tion. 

(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) Except for ignition sources due to 

lightning addressed by § 25.954, dem-
onstrating that an ignition source 
could not result from each single fail-
ure, from each single failure in com-
bination with each latent failure condi-
tion not shown to be extremely remote, 
and from all combinations of failures 
not shown to be extremely improbable, 
taking into account the effects of man-
ufacturing variability, aging, wear, 
corrosion, and likely damage. 

(b) Except as provided in paragraphs 

(b)(2) and (c) of this section, no fuel 
tank Fleet Average Flammability Ex-
posure on an airplane may exceed three 
percent of the Flammability Exposure 
Evaluation Time (FEET) as defined in 
Appendix N of this part, or that of a 
fuel tank within the wing of the air-
plane model being evaluated, which-
ever is greater. If the wing is not a con-
ventional unheated aluminum wing, 

the analysis must be based on an as-
sumed Equivalent Conventional 
Unheated Aluminum Wing Tank. 

(1) Fleet Average Flammability Ex-

posure is determined in accordance 
with Appendix N of this part. The as-
sessment must be done in accordance 
with the methods and procedures set 
forth in the Fuel Tank Flammability 
Assessment Method User’s Manual, 
dated May 2008, document number 
DOT/FAA/AR–05/8 (incorporated by ref-
erence, see § 25.5). 

(2) Any fuel tank other than a main 

fuel tank on an airplane must meet the 
flammability exposure criteria of Ap-
pendix M to this part if any portion of 
the tank is located within the fuselage 
contour. 

(3) As used in this paragraph, 
(i) 

Equivalent Conventional Unheated 

Aluminum Wing Tank 

is an integral 

tank in an unheated semi-monocoque 
aluminum wing of a subsonic airplane 
that is equivalent in aerodynamic per-
formance, structural capability, fuel 
tank capacity and tank configuration 
to the designed wing. 

(ii) 

Fleet Average Flammability Expo-

sure 

is defined in Appendix N to this 

part and means the percentage of time 
each fuel tank ullage is flammable for 
a fleet of an airplane type operating 
over the range of flight lengths. 

(iii) 

Main Fuel Tank 

means a fuel 

tank that feeds fuel directly into one 
or more engines and holds required fuel 
reserves continually throughout each 
flight. 

(c) Paragraph (b) of this section does 

not apply to a fuel tank if means are 
provided to mitigate the effects of an 
ignition of fuel vapors within that fuel 
tank such that no damage caused by an 
ignition will prevent continued safe 
flight and landing. 

(d) To protect design features that 

prevent catastrophic ignition sources 
within the fuel tank or fuel tank sys-
tem according to paragraph (a) of this 
section, and to prevent increasing the 
flammability exposure of the tanks 
above that permitted in paragraph (b) 
of this section, the type design must 
include critical design configuration 
control limitations (CDCCLs) identi-
fying those features and providing in-
structions on how to protect them. To 
ensure the continued effectiveness of 

VerDate Sep<11>2014 

12:50 Apr 30, 2019

Jkt 247046

PO 00000

Frm 00330

Fmt 8010

Sfmt 8010

Y:\SGML\247046.XXX

247046

spaschal on DSK3GDR082PROD with CFR