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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