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Federal Aviation Administration, DOT 

§ 121.243 

§ 121.225 Propeller deicing fluid. 

If combustible fluid is used for pro-

peller deicing, the certificate holder 
must comply with § 121.255. 

§ 121.227 Pressure cross-feed arrange-

ments. 

(a) Pressure cross-feed lines may not 

pass through parts of the airplane used 
for carrying persons or cargo unless— 

(1) There is a means to allow crew-

members to shut off the supply of fuel 
to these lines; or 

(2) The lines are enclosed in a fuel 

and fume-proof enclosure that is venti-
lated and drained to the exterior of the 
airplane. 

However, such an enclosure need not be 
used if those lines incorporate no fit-
tings on or within the personnel or 
cargo areas and are suitably routed or 
protected to prevent accidental dam-
age. 

(b) Lines that can be isolated from 

the rest of the fuel system by valves at 
each end must incorporate provisions 
for relieving excessive pressures that 
may result from exposure of the iso-
lated line to high temperatures. 

§ 121.229 Location of fuel tanks. 

(a) Fuel tanks must be located in ac-

cordance with § 121.255. 

(b) No part of the engine nacelle skin 

that lies immediately behind a major 
air outlet from the engine compart-
ment may be used as the wall of an in-
tegral tank. 

(c) Fuel tanks must be isolated from 

personnel compartments by means of 
fume- and fuel-proof enclosures. 

§ 121.231 Fuel system lines and fit-

tings. 

(a) Fuel lines must be installed and 

supported so as to prevent excessive vi-
bration and so as to be adequate to 
withstand loads due to fuel pressure 
and accelerated flight conditions. 

(b) Lines connected to components of 

the airplanes between which there may 
be relative motion must incorporate 
provisions for flexibility. 

(c) Flexible connections in lines that 

may be under pressure and subject to 
axial loading must use flexible hose as-
semblies rather than hose clamp con-
nections. 

(d) Flexible hose must be of an ac-

ceptable type or proven suitable for the 
particular application. 

§ 121.233 Fuel lines and fittings in des-

ignated fire zones. 

Fuel lines and fittings in each des-

ignated fire zone must comply with 
§ 121.259. 

§ 121.235 Fuel valves. 

Each fuel valve must— 
(a) Comply with § 121.257; 
(b) Have positive stops or suitable 

index provisions in the ‘‘on’’ and ‘‘off’’ 
positions; and 

(c) Be supported so that loads result-

ing from its operation or from acceler-
ated flight conditions are not trans-
mitted to the lines connected to the 
valve. 

§ 121.237 Oil lines and fittings in des-

ignated fire zones. 

Oil line and fittings in each des-

ignated fire zone must comply with 
§ 121.259. 

§ 121.239 Oil valves. 

(a) Each oil valve must— 
(1) Comply with § 121.257; 
(2) Have positive stops or suitable 

index provisions in the ‘‘on’’ and ‘‘off’’ 
positions; and 

(3) Be supported so that loads result-

ing from its operation or from acceler-
ated flight conditions are not trans-
mitted to the lines attached to the 
valve. 

(b) The closing of an oil shutoff 

means must not prevent feathering the 
propeller, unless equivalent safety pro-
visions are incorporated. 

§ 121.241 Oil system drains. 

Accessible drains incorporating ei-

ther a manual or automatic means for 
positive locking in the closed position, 
must be provided to allow safe drainage 
of the entire oil system. 

§ 121.243 Engine breather lines. 

(a) Engine breather lines must be so 

arranged that condensed water vapor 
that may freeze and obstruct the line 
cannot accumulate at any point. 

(b) Engine breathers must discharge 

in a location that does not constitute a 
fire hazard in case foaming occurs and