Section 27.927

14 CFR Ch. I (1-1-19 Edition)

remaining engine(s) is run for a 1-hour
period.
(Secs. 313(a), 601, and 603, 72 Stat. 752, 775, 49
U.S.C. 1354(a), 1421, and 1423; sec. 6(c), 49
U.S.C. 1655(c))

spaschal on DSK3GDR082PROD with CFR

[Doc. No. 5074, 29 FR 15695, Nov. 24, 1964, as
amended by Amdt. 27-2, 33 FR 963, Jan. 26,
1968; Amdt. 27-12, 42 FR 15044, Mar. 17, 1977;
Amdt. 27-23, 53 FR 34212, Sept. 2, 1988; Amdt.
27-29, 59 FR 47767, Sept. 16, 1994]

Section 27.927 Additional tests.
(a) Any additional dynamic, endurance, and operational tests, and vibratory investigations necessary to determine that the rotor drive mechanism is
safe, must be performed.
(b) If turbine engine torque output to
the transmission can exceed the highest engine or transmission torque rating limit, and that output is not directly controlled by the pilot under
normal operating conditions (such as
where the primary engine power control is accomplished through the flight
control), the following test must be
made:
(1) Under conditions associated with
all engines operating, make 200 applications, for 10 seconds each, or torque
that is at least equal to the lesser of - 
(i) The maximum torque used in
meeting Section 27.923 plus 10 percent; or
(ii) The maximum attainable torque
output of the engines, assuming that
torque limiting devices, if any, function properly.
(2) For multiengine rotorcraft under
conditions associated with each engine,
in turn, becoming inoperative, apply to
the remaining transmission torque inputs the maximum torque attainable
under probable operating conditions,
assuming that torque limiting devices,
if any, function properly. Each transmission input must be tested at this
maximum torque for at least 15 minutes.
(3) The tests prescribed in this paragraph must be conducted on the rotorcraft at the maximum rotational speed
intended for the power condition of the
test and the torque must be absorbed
by the rotors to be installed, except
that other ground or flight test facilities with other appropriate methods of
torque absorption may be used if the
conditions of support and vibration
closely simulate the conditions that

would exist during a test on the rotorcraft.
(c) It must be shown by tests that the
rotor drive system is capable of operating under autorotative conditions for
15 minutes after the loss of pressure in
the rotor drive primary oil system.
(Secs. 313(a), 601, and 603, 72 Stat. 752, 775, 49
U.S.C. 1354(a), 1421, and 1423; sec. 6(c), 49
U.S.C. 1655(c))
[Amdt. 27-2, 33 FR 963, Jan. 26, 1968, as
amended by Amdt. 27-12, 42 FR 15045, Mar. 17,
1977; Amdt. 27-23, 53 FR 34212, Sept. 2, 1988]

Section 27.931 Shafting critical speed.
(a) The critical speeds of any shafting
must be determined by demonstration
except that analytical methods may be
used if reliable methods of analysis are
available for the particular design.
(b) If any critical speed lies within,
or close to, the operating ranges for
idling, power on, and autorotative conditions, the stresses occurring at that
speed must be within safe limits. This
must be shown by tests.
(c) If analytical methods are used and
show that no critical speed lies within
the permissible operating ranges, the
margins between the calculated critical speeds and the limits of the allowable operating ranges must be adequate
to allow for possible variations between the computed and actual values.
Section 27.935 Shafting joints.
Each universal joint, slip joint, and
other shafting joints whose lubrication
is necessary for operation must have
provision for lubrication.
Section 27.939 Turbine
engine
operating
characteristics.
(a) Turbine engine operating characteristics must be investigated in flight
to determine that no adverse characteristics (such as stall, surge, or flameout) are present, to a hazardous degree,
during normal and emergency operation within the range of operating
limitations of the rotorcraft and of the
engine.
(b) The turbine engine air inlet system may not, as a result of airflow distortion during normal operation, cause
vibration harmful to the engine.
(c) For governor-controlled engines,
it must be shown that there exists no
hazardous torsional instability of the

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