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714 

14 CFR Ch. I (1–1–24 Edition) 

§ 33.68 

not be manipulated while the fuel con-
trol device is mounted on the engine 
must be secured by a locking device 
and sealed, or otherwise be inacces-
sible. All other fuel control adjusting 
means must be accessible and marked 
to indicate the function of the adjust-
ment unless the function is obvious. 

(b) There must be a fuel strainer or 

filter between the engine fuel inlet 
opening and the inlet of either the fuel 
metering device or the engine-driven 
positive displacement pump whichever 
is nearer the engine fuel inlet. In addi-
tion, the following provisions apply to 
each strainer or filter required by this 
paragraph (b): 

(1) It must be accessible for draining 

and cleaning and must incorporate a 
screen or element that is easily remov-
able. 

(2) It must have a sediment trap and 

drain except that it need not have a 
drain if the strainer or filter is easily 
removable for drain purposes. 

(3) It must be mounted so that its 

weight is not supported by the con-
necting lines or by the inlet or outlet 
connections of the strainer or filter, 
unless adequate strength margins 
under all loading conditions are pro-
vided in the lines and connections. 

(4) It must have the type and degree 

of fuel filtering specified as necessary 
for protection of the engine fuel system 
against foreign particles in the fuel. 
The applicant must show: 

(i) That foreign particles passing 

through the specified filtering means 
do not impair the engine fuel system 
functioning; and 

(ii) That the fuel system is capable of 

sustained operation throughout its 
flow and pressure range with the fuel 
initially saturated with water at 80 

°

(27 

°

C) and having 0.025 fluid ounces per 

gallon (0.20 milliliters per liter) of free 
water added and cooled to the most 
critical condition for icing likely to be 
encountered in operation. However, 
this requirement may be met by dem-
onstrating the effectiveness of speci-
fied approved fuel anti-icing additives, 
or that the fuel system incorporates a 
fuel heater which maintains the fuel 
temperature at the fuel strainer or fuel 
inlet above 32 

°

F (0 

°

C) under the most 

critical conditions. 

(5) The applicant must demonstrate 

that the filtering means has the capac-
ity (with respect to engine operating 
limitations) to ensure that the engine 
will continue to operate within ap-
proved limits, with fuel contaminated 
to the maximum degree of particle size 
and density likely to be encountered in 
service. Operation under these condi-
tions must be demonstrated for a pe-
riod acceptable to the Administrator, 
beginning when indication of impend-
ing filter blockage is first given by ei-
ther: 

(i) Existing engine instrumentation; 

or 

(ii) Additional means incorporated 

into the engine fuel system. 

(6) Any strainer or filter bypass must 

be designed and constructed so that the 
release of collected contaminants is 
minimized by appropriate location of 
the bypass to ensure that collected 
contaminants are not in the bypass 
flow path. 

(c) 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. 

[Amdt. 33–6, 39 FR 35466, Oct. 1, 1974, as 
amended by Amdt. 33–10, 49 FR 6851, Feb. 23, 
1984; Amdt. 33–18, 61 FR 31328, June 19, 1996; 
Amdt. 33–25, 73 FR 48123, Aug. 18, 2008; Amdt. 
33–26, 73 FR 48285, Aug. 19, 2008] 

§ 33.68

Induction system icing. 

Each engine, with all icing protec-

tion systems operating, must: 

(a) Operate throughout its flight 

power range, including the minimum 
descent idle rotor speeds achievable in 
flight, in the icing conditions defined 
for turbojet, turbofan, and turboprop 
engines in Appendices C and O of part 
25 of this chapter, and Appendix D of 
this part, and for turboshaft engines in 
Appendix C of part 29 of this chapter, 
without the accumulation of ice on the 
engine components that: 

(1) Adversely affects engine operation 

or that causes an unacceptable perma-
nent loss of power or thrust or unac-
ceptable increase in engine operating 
temperature; or 

(2) Results in unacceptable tem-

porary power loss or engine damage; or 

(3) Causes a stall, surge, or flameout 

or loss of engine controllability. The 

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715 

Federal Aviation Administration, DOT 

§ 33.68 

applicant must account for in-flight 
ram effects in any critical point anal-
ysis or test demonstration of these 
flight conditions. 

(b) Operate throughout its flight 

power range, including minimum de-
scent idle rotor speeds achievable in 
flight, in the icing conditions defined 
for turbojet, turbofan, and turboprop 
engines in Appendices C and O of part 
25 of this chapter, and for turboshaft 
engines in Appendix C of part 29 of this 
chapter. In addition: 

(1) It must be shown through Critical 

Point Analysis (CPA) that the com-
plete ice envelope has been analyzed, 
and that the most critical points must 
be demonstrated by engine test, anal-
ysis, or a combination of the two to op-
erate acceptably. Extended flight in 
critical flight conditions such as hold, 
descent, approach, climb, and cruise, 
must be addressed, for the ice condi-
tions defined in these appendices. 

(2) It must be shown by engine test, 

analysis, or a combination of the two 
that the engine can operate acceptably 
for the following durations: 

(i) At engine powers that can sustain 

level flight: A duration that achieves 
repetitive, stabilized operation for tur-
bojet, turbofan, and turboprop engines 
in the icing conditions defined in Ap-
pendices C and O of part 25 of this 
chapter, and for turboshaft engines in 
the icing conditions defined in Appen-
dix C of part 29 of this chapter. 

(ii) At engine power below that which 

can sustain level flight: 

(A) Demonstration in altitude flight 

simulation test facility: A duration of 
10 minutes consistent with a simulated 
flight descent of 10,000 ft (3 km) in alti-
tude while operating in Continuous 
Maximum icing conditions defined in 
Appendix C of part 25 of this chapter 
for turbojet, turbofan, and turboprop 
engines, and for turboshaft engines in 
the icing conditions defined in Appen-
dix C of part 29 of this chapter, plus 40 
percent liquid water content margin, 
at the critical level of airspeed and air 
temperature; or 

(B) Demonstration in ground test fa-

cility: A duration of 3 cycles of alter-
nating icing exposure corresponding to 
the liquid water content levels and 
standard cloud lengths starting in 
Intermittent Maximum and then in 
Continuous Maximum icing conditions 
defined in Appendix C of part 25 of this 
chapter for turbojet, turbofan, and tur-
boprop engines, and for turboshaft en-
gines in the icing conditions defined in 
Appendix C of part 29 of this chapter, 
at the critical level of air temperature. 

(c) In addition to complying with 

paragraph (b) of this section, the fol-
lowing conditions shown in Table 1 of 
this section unless replaced by similar 
CPA test conditions that are more crit-
ical or produce an equivalent level of 
severity, must be demonstrated by an 
engine test: 

T

ABLE

1—C

ONDITIONS

T

HAT

M

UST

B

E

D

EMONSTRATED BY AN

E

NGINE

T

EST

 

Condition 

Total air temperature 

Supercooled water 

concentrations 

(minimum) 

Median volume drop diameter 

Duration 

1. Glaze ice condi-

tions.

21 to 25 

°

F (

¥

6 to 

¥

°

C).

2 g/m

3

........................

25 to 35 microns ...................

(a) 10-minutes for 

power below sus-
tainable level flight 
(idle descent). 

(b) Must show repet-

itive, stabilized op-
eration for higher 
powers (50%, 75%, 
100%MC). 

2. Rime ice conditions 

¥

10 to 0 

°

F (

¥

23 to 

¥

18 

°

C).

1 g/m

3

........................

15 to 25 microns ...................

(a) 10-minutes for 

power below sus-
tainable level flight 
(idle descent). 

(b) Must show repet-

itive, stabilized op-
eration for higher 
powers (50%, 75%, 
100%MC). 

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716 

14 CFR Ch. I (1–1–24 Edition) 

§ 33.69 

T

ABLE

1—C

ONDITIONS

T

HAT

M

UST

B

E

D

EMONSTRATED BY AN

E

NGINE

T

EST

—Continued 

Condition 

Total air temperature 

Supercooled water 

concentrations 

(minimum) 

Median volume drop diameter 

Duration 

3. Glaze ice holding 

conditions.

(Turbojet, turbofan, 

and turboprop only).

Turbojet and Tur-

bofan, only: 10 to 
18 

°

F (

¥

12 to 

¥

°

C).

Alternating cycle: First 

1.7 g/m

3

(1 minute), 

Then 0.3 g/m

3

(6 

minute).

20 to 30 microns ...................

Must show repetitive, 

stabilized operation 
(or 45 minutes 
max). 

Turboprop, only: 2 to 

10 

°

F (

¥

17 to 

¥

12 

°

C).

.

4. Rime ice holding 

conditions.

(Turbojet, turbofan, 

and turboprop only).

Turbojet and Tur-

bofan, only: 

¥

10 to 

°

F (

¥

23 to 

¥

18 

°

C).

0.25 g/m

3

...................

20 to 30 microns ...................

Must show repetitive, 

stabilized operation 
(or 45 minutes 
max). 

Turboprop, only: 2 to 

10 

°

F (

¥

17 to 

¥

12 

°

C).

....................................

................................................

(d) Operate at ground idle speed for a 

minimum of 30 minutes at each of the 
following icing conditions shown in 
Table 2 of this section with the avail-
able air bleed for icing protection at its 
critical condition, without adverse ef-
fect, followed by acceleration to take-
off power or thrust. During the idle op-
eration, the engine may be run up peri-
odically to a moderate power or thrust 
setting in a manner acceptable to the 

Administrator. Analysis may be used 
to show ambient temperatures below 
the tested temperature are less crit-
ical. The applicant must document any 
demonstrated run ups and minimum 
ambient temperature capability in the 
engine operating manual as mandatory 
in icing conditions. The applicant must 
demonstrate, with consideration of ex-
pected airport elevations, the fol-
lowing: 

T

ABLE

2—D

EMONSTRATION

M

ETHODS FOR

S

PECIFIC

I

CING

C

ONDITIONS

 

Condition 

Total air temperature 

Supercooled water 

concentrations 

(minimum) 

Mean effective particle di-

ameter 

Demonstration 

1. Rime ice condition

0 to 15 

°

F (

¥

18 to 

¥

°

C).

Liquid—0.3 g/m

3

........

15–25 microns .......................

By engine test. 

2. Glaze ice condition 

20 to 30 

°

F (

¥

7 to 

¥

°

C).

Liquid—0.3 g/m

3

........

15–25 microns .......................

By engine test. 

3. Snow ice condition 

26 to 32 

°

F (

¥

3 to 0 

°

C).

Ice—0.9 g/m

3

............

100 microns ...........................
(minimum) ..............................

By test, analysis or 

combination of the 
two. 

4. Large drop glaze 

ice condition (Tur-
bojet, turbofan, and 
turboprop only).

15 to 30 

°

F (-9 to -1 

°

C).

Liquid—0.3 g/m

3

........

100 microns (minimum) .........

By test, analysis or 

combination of the 
two. 

(e) Demonstrate by test, analysis, or 

combination of the two, acceptable op-
eration for turbojet, turbofan, and tur-
boprop engines in mixed phase and ice 
crystal icing conditions throughout 
Appendix D of this part, icing envelope 
throughout its flight power range, in-
cluding minimum descent idling 
speeds. 

[Amdt. 33–34, 79 FR 66536, Nov. 4, 2014] 

§ 33.69

Ignitions system. 

Each engine must be equipped with 

an ignition system for starting the en-
gine on the ground and in flight. An 
electric ignition system must have at 
least two igniters and two separate sec-
ondary electric circuits, except that 
only one igniter is required for fuel 
burning augmentation systems. 

[Amdt. 33–6, 39 FR 35466, Oct. 1, 1974] 

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