532 

14 CFR Ch. I (1–1–19 Edition) 

§ 27.953 

incorporate the following design fea-
tures: 

(i) The load necessary to separate a 

breakaway coupling must be between 
25 to 50 percent of the minimum ulti-
mate failure load (ultimate strength) 
of the weakest component in the fluid- 
carrying line. The separation load 
must in no case be less than 300 pounds, 
regardless of the size of the fluid line. 

(ii) A breakaway coupling must sepa-

rate whenever its ultimate load (as de-
fined in paragraph (c)(1)(i) of this sec-
tion) is applied in the failure modes 
most likely to occur. 

(iii) All breakaway couplings must 

incorporate design provisions to vis-
ually ascertain that the coupling is 
locked together (leak-free) and is open 
during normal installation and service. 

(iv) All breakaway couplings must in-

corporate design provisions to prevent 
uncoupling or unintended closing due 
to operational shocks, vibrations, or 
accelerations. 

(v) No breakaway coupling design 

may allow the release of fuel once the 
coupling has performed its intended 
function. 

(2) All individual breakaway cou-

plings, coupling fuel feed systems, or 
equivalent means must be designed, 
tested, installed, and maintained so 
that inadvertent fuel shutoff in flight 
is improbable in accordance with 
§ 27.955(a) and must comply with the fa-
tigue evaluation requirements of 
§ 27.571 without leaking. 

(3) Alternate, equivalent means to 

the use of breakaway couplings must 
not create a survivable impact-induced 
load on the fuel line to which it is in-
stalled greater than 25 to 50 percent of 
the ultimate load (strength) of the 
weakest component in the line and 
must comply with the fatigue require-
ments of § 27.571 without leaking. 

(d) 

Frangible or deformable structural 

attachments. 

Unless hazardous relative 

motion of fuel tanks and fuel system 
components to local rotorcraft struc-
ture is demonstrated to be extremely 
improbable in an otherwise survivable 
impact, frangible or locally deformable 
attachments of fuel tanks and fuel sys-
tem components to local rotorcraft 
structure must be used. The attach-
ment of fuel tanks and fuel system 
components to local rotorcraft struc-

ture, whether frangible or locally de-
formable, must be designed such that 
its separation or relative local defor-
mation will occur without rupture or 
local tear-out of the fuel tank or fuel 
system components that will cause fuel 
leakage. The ultimate strength of fran-
gible or deformable attachments must 
be as follows: 

(1) The load required to separate a 

frangible attachment from its support 
structure, or deform a locally deform-
able attachment relative to its support 
structure, must be between 25 and 50 
percent of the minimum ultimate load 
(ultimate strength) of the weakest 
component in the attached system. In 
no case may the load be less than 300 
pounds. 

(2) A frangible or locally deformable 

attachment must separate or locally 
deform as intended whenever its ulti-
mate load (as defined in paragraph 
(d)(1) of this section) is applied in the 
modes most likely to occur. 

(3) All frangible or locally deformable 

attachments must comply with the fa-
tigue requirements of § 27.571. 

(e) 

Separation of fuel and ignition 

sources. 

To provide maximum crash re-

sistance, fuel must be located as far as 
practicable from all occupiable areas 
and from all potential ignition sources. 

(f) 

Other basic mechanical design cri-

teria. 

Fuel tanks, fuel lines, electrical 

wires, and electrical devices must be 
designed, constructed, and installed, as 
far as practicable, to be crash resist-
ant. 

(g) 

Rigid or semirigid fuel tanks. 

Rigid 

or semirigid fuel tank or bladder walls 
must be impact and tear resistant. 

[Doc. No. 26352, 59 FR 50386, Oct. 3, 1994] 

§ 27.953

Fuel system independence. 

(a) Each fuel system for multiengine 

rotorcraft must allow fuel to be sup-
plied to each engine through a system 
independent of those parts of each sys-
tem supplying fuel to other engines. 
However, separate fuel tanks need not 
be provided for each engine. 

(b) If a single fuel tank is used on a 

multiengine rotorcraft, the following 
must be provided: 

(1) Independent tank outlets for each 

engine, each incorporating a shutoff 
valve at the tank. This shutoff valve 
may also serve as the firewall shutoff 

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