302 

14 CFR Ch. I (1–1–24 Edition) 

§ 25.833 

will maintain cabin ozone concentra-
tions at or below the limits prescribed 
by paragraph (a) of this section. 

[Amdt. 25–50, 45 FR 3883, Jan. 1, 1980, as 
amended by Amdt. 25–56, 47 FR 58489, Dec. 30, 
1982; Amdt. 25–94, 63 FR 8848, Feb. 23, 1998] 

§ 25.833

Combustion heating systems. 

Combustion heaters must be ap-

proved. 

[Amdt. 25–72, 55 FR 29783, July 20, 1990] 

P

RESSURIZATION

 

§ 25.841

Pressurized cabins. 

(a) Except as provided in paragraph 

(c) of this section, pressurized cabins 
and compartments to be occupied must 
be equipped to provide a cabin pressure 
altitude of not more than 8,000 feet 
under normal operating conditions. 

(1) If certification for operation 

above 25,000 feet is requested, the air-
plane must be designed so that occu-
pants will not be exposed to cabin pres-
sure altitudes in excess of 15,000 feet 
after any probable failure condition in 
the pressurization system except as 
provided in paragraph (c) of this sec-
tion. 

(2) The airplane must be designed so 

that occupants will not be exposed to a 
cabin pressure altitude that exceeds 
the following after decompression from 
any failure condition not shown to be 
extremely improbable: 

(i) Twenty-five thousand (25,000) feet 

for more than 2 minutes; or 

(ii) Forty thousand (40,000) feet for 

any duration. 

(3) Fuselage structure, engine and 

system failures are to be considered in 
evaluating the cabin decompression. 

(b) Pressurized cabins must have at 

least the following valves, controls, 
and indicators for controlling cabin 
pressure: 

(1) Two pressure relief valves to auto-

matically limit the positive pressure 
differential to a predetermined value 
at the maximum rate of flow delivered 
by the pressure source. The combined 
capacity of the relief valves must be 
large enough so that the failure of any 
one valve would not cause an appre-
ciable rise in the pressure differential. 
The pressure differential is positive 
when the internal pressure is greater 
than the external. 

(2) Two reverse pressure differential 

relief valves (or their equivalents) to 
automatically prevent a negative pres-
sure differential that would damage 
the structure. One valve is enough, 
however, if it is of a design that rea-
sonably precludes its malfunctioning. 

(3) A means by which the pressure 

differential can be rapidly equalized. 

(4) An automatic or manual regulator 

for controlling the intake or exhaust 
airflow, or both, for maintaining the 
required internal pressures and airflow 
rates. 

(5) Instruments at the pilot or flight 

engineer station to show the pressure 
differential, the cabin pressure alti-
tude, and the rate of change of the 
cabin pressure altitude. 

(6) Warning indication to the 

flightcrew when the safe or preset pres-
sure differential or cabin pressure alti-
tude limit is exceeded. Appropriate 
warning markings on the cabin pres-
sure differential indicator meet the 
warning requirement for pressure dif-
ferential limits. An alert meets the 
warning requirement for cabin pressure 
altitude limits if it warns the 
flightcrew when the cabin pressure al-
titude exceeds 10,000 feet, except as 
provided in paragraph (d) of this sec-
tion. 

(7) A warning placard at the pilot or 

flight engineer station if the structure 
is not designed for pressure differen-
tials up to the maximum relief valve 
setting in combination with landing 
loads. 

(8) The pressure sensors necessary to 

meet the requirements of paragraphs 
(b)(5) and (b)(6) of this section and 
§ 25.1447(c), must be located and the 
sensing system designed so that, in the 
event of loss of cabin pressure in any 
passenger or crew compartment (in-
cluding upper and lower lobe galleys), 
the warning and automatic presen-
tation devices, required by those provi-
sions, will be actuated without any 
delay that would significantly increase 
the hazards resulting from decompres-
sion. 

(c) When operating into or out of air-

ports with elevations at or above 8,000 
feet, the cabin pressure altitude in 
pressurized cabins and occupied com-
partments may be up to, or greater 

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