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14 CFR Ch. I (1–1–19 Edition) 

§ 27.659 

(1) There must be means for venting 

the internal pressure of the blade; 

(2) Drainage holes must be provided 

for the blade; and 

(3) The blade must be designed to pre-

vent water from becoming trapped in 

(b) Paragraphs (a)(1) and (2) of this 

section does not apply to sealed rotor 
blades capable of withstanding the 
maximum pressure differentials ex-
pected in service. 

[Amdt. 27–2, 33 FR 963, Jan. 26, 1968] 

§ 27.659

Mass balance. 

(a) The rotors and blades must be 

mass balanced as necessary to— 

(1) Prevent excessive vibration; and 
(2) Prevent flutter at any speed up to 

the maximum forward speed. 

(b) The structural integrity of the 

mass balance installation must be sub-

[Amdt. 27–2, 33 FR 963, Jan. 26, 1968] 

§ 27.661

Rotor blade clearance. 

There must be enough clearance be-

tween the rotor blades and other parts 
of the structure to prevent the blades 
from striking any part of the structure 
during any operating condition. 

[Amdt. 27–2, 33 FR 963, Jan. 26, 1968] 

§ 27.663

Ground resonance prevention 


(a) The reliability of the means for 

preventing ground resonance must be 
shown either by analysis and tests, or 
reliable service experience, or by show-
ing through analysis or tests that mal-
function or failure of a single means 
will not cause ground resonance. 

(b) The probable range of variations, 

during service, of the damping action 
of the ground resonance prevention 
means must be established and must be 
investigated during the test required 
by § 27.241. 

[Amdt. 27–2, 33 FR 963, Jan. 26, 1968, as 
amended by Amdt. 27–26, 55 FR 8000, Mar. 6, 






§ 27.671


(a) Each control and control system 

must operate with the ease, smooth-

ness, and positiveness appropriate to 
its function. 

(b) Each element of each flight con-

trol system must be designed, or dis-
tinctively and permanently marked, to 
minimize the probability of any incor-
rect assembly that could result in the 
malfunction of the system. 

§ 27.672

Stability augmentation, auto-

matic, and power-operated systems. 

If the functioning of stability aug-

mentation or other automatic or 
power-operated systems is necessary to 
show compliance with the flight char-
acteristics requirements of this part, 
such systems must comply with § 27.671 
of this part and the following: 

(a) A warning which is clearly distin-

guishable to the pilot under expected 
flight conditions without requiring the 
pilot’s attention must be provided for 
any failure in the stability augmenta-
tion system or in any other automatic 
or power-operated system which could 
result in an unsafe condition if the 
pilot is unaware of the failure. Warning 
systems must not activate the control 

(b) The design of the stability aug-

mentation system or of any other auto-
matic or power-operated system must 
allow initial counteraction of failures 
without requiring exceptional pilot 
skill or strength by overriding the fail-
ure by movement of the flight controls 
in the normal sense and deactivating 
the failed system. 

(c) It must be shown that after any 

single failure of the stability aug-
mentation system or any other auto-
matic or power-operated system— 

(1) The rotorcraft is safely control-

lable when the failure or malfunction 
occurs at any speed or altitude within 
the approved operating limitations; 

(2) The controllability and maneuver-

ability requirements of this part are 
met within a practical operational 
flight envelope (for example, speed, al-
titude, normal acceleration, and rotor-
craft configurations) which is described 
in the Rotorcraft Flight Manual; and 

(3) The trim and stability character-

istics are not impaired below a level 
needed to permit continued safe flight 
and landing. 

[Amdt. 27–21, 49 FR 44433, Nov. 6, 1984; 49 FR 
47594, Dec. 6, 1984] 

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