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14 CFR Ch. I (1–1–24 Edition) 

§ 33.37 

permit the use of a means for ice pre-
vention. 

(c) The type and degree of fuel fil-

tering necessary for protection of the 
engine fuel system against foreign par-
ticles in the fuel must be specified. The 
applicant must show that foreign par-
ticles passing through the prescribed 
filtering means will not critically im-
pair engine fuel system functioning. 

(d) Each passage in the induction sys-

tem that conducts a mixture of fuel 
and air must be self-draining, to pre-
vent a liquid lock in the cylinders, in 
all attitudes that the applicant estab-
lishes as those the engine can have 
when the aircraft in which it is in-
stalled is in the static ground attitude. 

(e) If provided as part of the engine, 

the applicant must show for each fluid 
injection (other than fuel) system and 
its controls that the flow of the in-
jected fluid is adequately controlled. 

[Doc. No. 3025, 29 FR 7453, June 10, 1964, as 
amended by Amdt. 33–10, 49 FR 6851, Feb. 23, 
1984] 

§ 33.37

Ignition system. 

Each spark ignition engine must 

have a dual ignition system with at 
least two spark plugs for each cylinder 
and two separate electric circuits with 
separate sources of electrical energy, 
or have an ignition system of equiva-
lent in-flight reliability. 

§ 33.39

Lubrication system. 

(a) The lubrication system of the en-

gine must be designed and constructed 
so that it will function properly in all 
flight attitudes and atmospheric condi-
tions in which the airplane is expected 
to operate. In wet sump engines, this 
requirement must be met when only 
one-half of the maximum lubricant 
supply is in the engine. 

(b) The lubrication system of the en-

gine must be designed and constructed 
to allow installing a means of cooling 
the lubricant. 

(c) The crankcase must be vented to 

the atmosphere to preclude leakage of 
oil from excessive pressure in the 
crankcase. 

Subpart D—Block Tests; 

Reciprocating Aircraft Engines 

§ 33.41

Applicability. 

This subpart prescribes the block 

tests and inspections for reciprocating 
aircraft engines. 

§ 33.42

General. 

Before each endurance test required 

by this subpart, the adjustment setting 
and functioning characteristic of each 
component having an adjustment set-
ting and a functioning characteristic 
that can be established independent of 
installation on the engine must be es-
tablished and recorded. 

[Amdt. 33–6, 39 FR 35465, Oct. 1, 1974] 

§ 33.43

Vibration test. 

(a) Each engine must undergo a vi-

bration survey to establish the tor-
sional and bending vibration character-
istics of the crankshaft and the pro-
peller shaft or other output shaft, over 
the range of crankshaft speed and en-
gine power, under steady state and 
transient conditions, from idling speed 
to either 110 percent of the desired 
maximum continuous speed rating or 
103 percent of the maximum desired 
takeoff speed rating, whichever is high-
er. The survey must be conducted 
using, for airplane engines, the same 
configuration of the propeller type 
which is used for the endurance test, 
and using, for other engines, the same 
configuration of the loading device 
type which is used for the endurance 
test. 

(b) The torsional and bending vibra-

tion stresses of the crankshaft and the 
propeller shaft or other output shaft 
may not exceed the endurance limit 
stress of the material from which the 
shaft is made. If the maximum stress 
in the shaft cannot be shown to be 
below the endurance limit by measure-
ment, the vibration frequency and am-
plitude must be measured. The peak 
amplitude must be shown to produce a 
stress below the endurance limit; if 
not, the engine must be run at the con-
dition producing the peak amplitude 
until, for steel shafts, 10 million stress 
reversals have been sustained without 
fatigue failure and, for other shafts, 
until it is shown that fatigue will not 

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