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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