title-14-part-27-sec27-953

533

Federal Aviation Administration, DOT

§ 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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534

14 CFR Ch. I (1–1–24 Edition)

§ 27.954

valve required by § 27.995 if the line be-
tween the valve and the engine com-
partment does not contain a hazardous
amount of fuel that can drain into the
engine compartment.

(2) At least two vents arranged to

minimize the probability of both vents
becoming obstructed simultaneously.

(3) Filler caps designed to minimize

the probability of incorrect installa-
tion or inflight loss.

(4) A fuel system in which those parts

of the system from each tank outlet to
any engine are independent of each
part of each system supplying fuel to
other engines.

§ 27.954

Fuel system lightning protec-

tion.

The fuel system must be designed

and arranged to prevent the ignition of
fuel vapor within the system by—

(a) Direct lightning strikes to areas

having a high probability of stroke at-
tachment;

(b) Swept lightning strokes to areas

where swept strokes are highly prob-
able; or

vent outlets.

[Amdt. 27–23, 53 FR 34212, Sept. 2, 1988]

§ 27.955

Fuel flow.

(a)

General. The fuel system for each

engine must be shown to provide the
engine with at least 100 percent of the
fuel required under each operating and
maneuvering condition to be approved
for the rotorcraft including, as applica-
ble, the fuel required to operate the en-
gine(s) under the test conditions re-
quired by § 27.927. Unless equivalent
methods are used, compliance must be
shown by test during which the fol-
lowing provisions are met except that
combinations of conditions which are
shown to be improbable need not be
considered.

(1) The fuel pressure, corrected for

critical accelerations, must be within
the limits specified by the engine type
certificate data sheet.

(2) The fuel level in the tank may not

exceed that established as the unusable
fuel supply for that tank under § 27.959,
plus the minimum additional fuel nec-
essary to conduct the test.

(3) The fuel head between the tank

outlet and the engine inlet must be

critical with respect to rotorcraft
flight attitudes.

(4) The critical fuel pump (for pump-

fed systems) is installed to produce (by
actual or simulated failure) the critical
restriction to fuel flow to be expected
from pump failure.

(5) Critical values of engine rotation

speed, electrical power, or other
sources of fuel pump motive power
must be applied.

(6) Critical values of fuel properties

which adversely affect fuel flow must
be applied.

(7) The fuel filter required by § 27.997

must be blocked to the degree nec-
essary to simulate the accumulation of
fuel contamination required to acti-
vate the indicator required by
§ 27.1305(q).

(b)

Fuel transfer systems. If normal op-

eration of the fuel system requires fuel
to be transferred to an engine feed
tank, the transfer must occur auto-
matically via a system which has been
shown to maintain the fuel level in the
engine feed tank within acceptable
limits during flight or surface oper-
ation of the rotorcraft.

(c)

Multiple fuel tanks. If an engine

can be supplied with fuel from more
than one tank, the fuel systems must,
in addition to having appropriate man-
ual switching capability, be designed to
prevent interruption of fuel flow to
that engine, without attention by the
flightcrew, when any tank supplying
fuel to that engine is depleted of usable
fuel during normal operation, and any
other tank that normally supplies fuel
to the engine alone contains usable
fuel.

[Amdt. 27–23, 53 FR 34212, Sept. 2, 1988]

§ 27.959

Unusable fuel supply.

The unusable fuel supply for each

tank must be established as not less
than the quantity at which the first
evidence of malfunction occurs under
the most adverse fuel feed condition
occurring under any intended oper-
ations and flight maneuvers involving
that tank.

§ 27.961

Fuel system hot weather oper-

ation.

Each suction lift fuel system and

other fuel systems with features condu-
cive to vapor formation must be shown