529
Federal Aviation Administration, DOT
§ 27.923
(3) Following the in-flight shutdown
of all engines, in-flight engine restart
capability must be provided.
[Doc. No. 5074, 29 FR 15695, Nov. 24, 1964, as
amended by Amdt. 27–11, 41 FR 55469, Dec. 20,
1976; Amdt. 27–23, 53 FR 34211, Sept. 2, 1988;
Amdt. 27–44, 73 FR 11000, Feb. 29, 2008; Amdt.
27–51, 88 FR 8737, Feb. 10, 2023]
§ 27.907
Engine vibration.
(a) Each engine must be installed to
prevent the harmful vibration of any
part of the engine or rotorcraft.
(b) The addition of the rotor and the
rotor drive system to the engine may
not subject the principal rotating parts
of the engine to excessive vibration
stresses. This must be shown by a vi-
bration investigation.
(c) No part of the rotor drive system
may be subjected to excessive vibra-
tion stresses.
R
OTOR
D
RIVE
S
YSTEM
§ 27.917
Design.
(a) Each rotor drive system must in-
corporate a unit for each engine to
automatically disengage that engine
from the main and auxiliary rotors if
that engine fails.
(b) Each rotor drive system must be
arranged so that each rotor necessary
for control in autorotation will con-
tinue to be driven by the main rotors
after disengagement of the engine from
the main and auxiliary rotors.
(c) If a torque limiting device is used
in the rotor drive system, it must be
located so as to allow continued con-
trol of the rotorcraft when the device
is operating.
(d) The rotor drive system includes
any part necessary to transmit power
from the engines to the rotor hubs.
This includes gear boxes, shafting, uni-
versal joints, couplings, rotor brake as-
semblies, clutches, supporting bearings
for shafting, any attendant accessory
pads or drives, and any cooling fans
that are a part of, attached to, or
mounted on the rotor drive system.
[Doc. No. 5074, 29 FR 15695, Nov. 24, 1964, as
amended by Amdt. 27–11, 41 FR 55469, Dec. 20,
1976]
§ 27.921
Rotor brake.
If there is a means to control the ro-
tation of the rotor drive system inde-
pendently of the engine, any limita-
tions on the use of that means must be
specified, and the control for that
means must be guarded to prevent in-
advertent operation.
§ 27.923
Rotor drive system and con-
trol mechanism tests.
(a) Each part tested as prescribed in
this section must be in a serviceable
condition at the end of the tests. No in-
tervening disassembly which might af-
fect test results may be conducted.
(b) Each rotor drive system and con-
trol mechanism must be tested for not
less than 100 hours. The test must be
conducted on the rotorcraft, 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 condi-
tions of support and vibration closely
simulate the conditions that would
exist during a test on the rotorcraft.
(c) A 60-hour part of the test pre-
scribed in paragraph (b) of this section
must be run at not less than maximum
continuous torque and the maximum
speed for use with maximum contin-
uous torque. In this test, the main
rotor controls must be set in the posi-
tion that will give maximum longitu-
dinal cyclic pitch change to simulate
forward flight. The auxiliary rotor con-
trols must be in the position for nor-
mal operation under the conditions of
the test.
(d) A 30-hour or, for rotorcraft for
which the use of either 30-minute OEI
power or continuous OEI power is re-
quested, a 25-hour part of the test pre-
scribed in paragraph (b) of this section
must be run at not less than 75 percent
of maximum continuous torque and the
minimum speed for use with 75 percent
of maximum continuous torque. The
main and auxiliary rotor controls must
be in the position for normal operation
under the conditions of the test.
(e) A 10-hour part of the test pre-
scribed in paragraph (b) of this section
must be run at not less than takeoff
torque and the maximum speed for use
with takeoff torque. The main and aux-
iliary rotor controls must be in the
normal position for vertical ascent.
(1) For multiengine rotorcraft for
which the use of 2
1
⁄
2
minute OEI power
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14 CFR Ch. I (1–1–24 Edition)
§ 27.923
is requested, 12 runs during the 10-hour
test must be conducted as follows:
(i) Each run must consist of at least
one period of 2
1
⁄
2
minutes with takeoff
torque and the maximum speed for use
with takeoff torque on all engines.
(ii) Each run must consist of at least
one period for each engine in sequence,
during which that engine simulates a
power failure and the remaining en-
gines are run at 2
1
⁄
2
minute OEI torque
and the maximum speed for use with
2
1
⁄
2
minute OEI torque for 2
1
⁄
2
minutes.
(2) For multiengine turbine-powered
rotorcraft for which the use of 30-sec-
ond and 2-minute OEI power is re-
quested, 10 runs must be conducted as
follows:
(i) Immediately following a takeoff
run of at least 5 minutes, each power
source must simulate a failure, in turn,
and apply the maximum torque and the
maximum speed for use with 30-second
OEI power to the remaining affected
drive system power inputs for not less
than 30 seconds, followed by applica-
tion of the maximum torque and the
maximum speed for use with 2-minute
OEI power for not less than 2 minutes.
At least one run sequence must be con-
ducted from a simulated ‘‘flight idle’’
condition. When conducted on a bench
test, the test sequence must be con-
ducted following stabilization at take-
off power.
(ii) For the purpose of this para-
graph, an affected power input includes
all parts of the rotor drive system
which can be adversely affected by the
application of higher or asymmetric
torque and speed prescribed by the
test.
(iii) This test may be conducted on a
representative bench test facility when
engine limitations either preclude re-
peated use of this power or would re-
sult in premature engine removal dur-
ing the test. The loads, the vibration
frequency, and the methods of applica-
tion to the affected rotor drive system
components must be representative of
rotorcraft conditions. Test components
must be those used to show compliance
with the remainder of this section.
(f) The parts of the test prescribed in
paragraphs (c) and (d) of this section
must be conducted in intervals of not
less than 30 minutes and may be ac-
complished either on the ground or in
flight. The part of the test prescribed
in paragraph (e) of this section must be
conducted in intervals of not less than
five minutes.
(g) At intervals of not more than five
hours during the tests prescribed in
paragraphs (c), (d), and (e) of this sec-
tion, the engine must be stopped rap-
idly enough to allow the engine and
rotor drive to be automatically dis-
engaged from the rotors.
(h) Under the operating conditions
specified in paragraph (c) of this sec-
tion, 500 complete cycles of lateral con-
trol, 500 complete cycles of longitu-
dinal control of the main rotors, and
500 complete cycles of control of each
auxiliary rotor must be accomplished.
A ‘‘complete cycle’’ involves movement
of the controls from the neutral posi-
tion, through both extreme positions,
and back to the neutral position, ex-
cept that control movements need not
produce loads or flapping motions ex-
ceeding the maximum loads or motions
encountered in flight. The cycling may
be accomplished during the testing pre-
scribed in paragraph (c) of this section.
(i) At least 200 start-up clutch en-
gagements must be accomplished—
(1) So that the shaft on the driven
side of the clutch is accelerated; and
(2) Using a speed and method selected
by the applicant.
(j) For multiengine rotorcraft for
which the use of 30-minute OEI power
is requested, five runs must be made at
30-minute OEI torque and the max-
imum speed for use with 30-minute OEI
torque, in which each engine, in se-
quence, is made inoperative and the re-
maining engine(s) is run for a 30-
minute period.
(k) For multiengine rotorcraft for
which the use of continuous OEI power
is requested, five runs must be made at
continuous OEI torque and the max-
imum speed for use with continuous
OEI torque, in which each engine, in
sequence, is made inoperative and the
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Federal Aviation Administration, DOT
§ 27.939
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))
[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]
§ 27.927
Additional tests.
(a) Any additional dynamic, endur-
ance, and operational tests, and vibra-
tory investigations necessary to deter-
mine that the rotor drive mechanism is
safe, must be performed.
(b) If turbine engine torque output to
the transmission can exceed the high-
est engine or transmission torque rat-
ing limit, and that output is not di-
rectly controlled by the pilot under
normal operating conditions (such as
where the primary engine power con-
trol is accomplished through the flight
control), the following test must be
made:
(1) Under conditions associated with
all engines operating, make 200 appli-
cations, for 10 seconds each, or torque
that is at least equal to the lesser of—
(i) The maximum torque used in
meeting § 27.923 plus 10 percent; or
(ii) The maximum attainable torque
output of the engines, assuming that
torque limiting devices, if any, func-
tion properly.
(2) For multiengine rotorcraft under
conditions associated with each engine,
in turn, becoming inoperative, apply to
the remaining transmission torque in-
puts the maximum torque attainable
under probable operating conditions,
assuming that torque limiting devices,
if any, function properly. Each trans-
mission input must be tested at this
maximum torque for at least 15 min-
utes.
(3) The tests prescribed in this para-
graph must be conducted on the rotor-
craft 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 facili-
ties 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 rotor-
craft.
(c) It must be shown by tests that the
rotor drive system is capable of oper-
ating 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]
§ 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 con-
ditions, 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 crit-
ical speeds and the limits of the allow-
able operating ranges must be adequate
to allow for possible variations be-
tween the computed and actual values.
§ 27.935
Shafting joints.
Each universal joint, slip joint, and
other shafting joints whose lubrication
is necessary for operation must have
provision for lubrication.
§ 27.939
Turbine engine operating
characteristics.
(a) Turbine engine operating charac-
teristics must be investigated in flight
to determine that no adverse charac-
teristics (such as stall, surge, or flame-
out) are present, to a hazardous degree,
during normal and emergency oper-
ation within the range of operating
limitations of the rotorcraft and of the
engine.
(b) The turbine engine air inlet sys-
tem may not, as a result of airflow dis-
tortion 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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