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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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530 

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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531 

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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