CS 25.831  Ventilation

ED Decision 2019/013/R

(See AMC 25.831)

(a) Under normal operating conditions and in the event of any probable failure conditions of any system that would adversely affect the ventilating air, the ventilation system must be designed to provide a sufficient amount of uncontaminated air to enable the crew members to perform their duties without undue discomfort or fatigue, and to provide reasonable passenger comfort. For normal operating conditions, the ventilation system must be designed to provide each occupant with an airflow that contains at least 0.25 Kg (0.55 lb) of fresh air per minute. (See AMC 25.831(a).)

(b) Crew and passenger compartment air must be free from harmful or hazardous concentrations of gases or vapours. In meeting this requirement, the following apply:

(1) Carbon monoxide concentrations in excess of one part in 20 000 parts of air are considered hazardous. For test purposes, any acceptable carbon monoxide detection method may be used.

(2) Carbon dioxide concentration during flight must be shown not to exceed 0·5% by volume (sea level equivalent) in compartments normally occupied by passengers or crewmembers. For the purpose of this subparagraph, “sea level equivalent” refers to conditions of 25° C (77° F) and 1 013·2 hPa (760 millimetres of mercury) pressure.

(c) There must be provisions made to ensure that the conditions prescribed in sub-paragraph (b) of this paragraph are met after reasonably probable failures or malfunctioning of the ventilating, heating, pressurisation or other systems and equipment. (See AMC 25.831(c).)

(d) If accumulation of hazardous quantities of smoke in the cockpit area is reasonably probable, smoke evacuation must be readily accomplished, starting with full pressurisation and without depressurising beyond safe limits.

(e) Except as provided in sub-paragraph (f) of this paragraph, means must be provided to enable the occupants of the following compartments and areas to control the temperature and quantity of ventilating air supplied to their compartment or area independently of the temperature and quantity of air supplied to other compartments and areas:

(1) The flight-crew compartment.

(2) Crew-member compartments and areas other than the flight-crew compartment unless the crewmember compartment or area is ventilated by air interchange with other compartments or areas under all operating conditions.

(f) Means to enable the flight crew to control the temperature and quantity of ventilating air supplied to the flight-crew compartment independently of the temperature and quantity of ventilating air supplied to other compartments are not required if all of the following conditions are met:

(1) The total volume of the flight-crew and passenger compartments is 22.65m3 (800 cubic ft) or less.

(2) The air inlets and passages for air to flow between flight-crew and passenger compartments are arrange to provide compartment temperatures within 2.8°C (5°F) of each other and adequate ventilation to occupants in both compartments.

(3) The temperature and ventilation controls are accessible to the flight crew.

[Amdt No: 25/18]

[Amdt No: 25/23]

AMC 25.831(a) Ventilation

ED Decision 2020/024/R

1. General

CS 25.831(a) specifies that the ventilation system must be designed to provide a minimum of 0.25 kg (0.55 lb) of fresh air per minute per person (i.e. 10 cubic feet per minute of air at 8 000 feet pressure altitude and at a cabin temperature of 24°C (75°F)) for normal operations.

The applicant may demonstrate compliance with this specification by analysis, ground tests, and/or flight tests.

Because it is not practicable to measure the airflow at each occupant’s location, the fresh air supplied per minute per occupant may be determined by averaging the total cabin fresh air supply and cockpit fresh air supply for the number of occupants that each area can accommodate, assuming a uniform ventilation distribution in each area.

2. Low airflow capability during some flight phases

If an applicant proposes not to provide the minimum required fresh airflow during the phases of flight that use low power levels, the applicant must show that the cabin air quality is not compromised during those flight phases.

3. Operations with the air conditioning system ‘off’

The following provisions should be considered for the limited time periods, such as during take-off, during which the air conditioning system is ‘off’:

a. There should be a means to annunciate to the flight crew that the air conditioning system is selected to ‘off’. When, in flight, after the end of the maximum allowed time period (e.g. typically after the take-off), the air conditioning system is still in the ‘off’ position, an alert should be triggered to inform the flight crew of the status of the air conditioning system.

b. It should be demonstrated that the ventilation system continues to provide an acceptable environment in the passenger cabin and the cockpit for the brief period when the air conditioning system is not operating.

c. Furthermore, the equipment environment should be evaluated during those periods to ensure that the reliability and performance of the equipment are not impaired. This evaluation should cover the extremes of ambient hot and cold air temperatures in which the aeroplane is expected to operate.

d. In addition, it should be demonstrated that no unsafe condition will result from operation for a limited time with the air conditioning system ‘off’, if a fire occurs. When demonstrating compliance with CS 25.831(d) (cockpit smoke removal), CS 25.857 (occupied areas smoke penetration), and CS 25.858 (smoke detection), the following should be considered:

i. During the operation of the aeroplane for any limited period of time with the air conditioning system ‘off’, the smoke detection systems should be effective.

ii. It should be possible for the air conditioning system to be turned ‘on’ and returned to the approved air conditioning system ‘on’ configuration to extract any hazardous quantities of smoke.

e. Finally, the period during which the aeroplane is operated with the air conditioning system ‘off’ is intended to be of short duration. Therefore, the maximum time period allowed for the operation of an aeroplane in this configuration should be defined by the applicant and specified in the appropriate operating manuals, along with any related operating procedures that are necessary to ensure that the above items are addressed.

4. Probable failure conditions

For probable failure conditions, the ventilation system should be designed to provide enough fresh air to prevent the accumulation of odours and pollutants such as carbon dioxide. Under these conditions, the supply of fresh air should not be less than 0.18 kg/min (0·4 lb/min) per person for any period exceeding five minutes. However, temporary reductions below this flow rate may be accepted provided that the compartment environment can be maintained at a level which is not hazardous to the occupant; for this purpose, the applicant may refer to international cabin air quality standards.

[Amdt 25/23]

[Amdt 25/26]

AMC 25.831(c) Ventilation

ED Decision 2003/2/RM

1 To avoid contamination the fresh air supply should be suitably ducted where it passes through any compartment inaccessible in flight.

2 Where the air supply is supplemented by a recirculating system, it should be possible to stop the recirculating system and –

a. Still maintain the fresh air supply prescribed, and

b. Still achieve 1.

CS 25.832 Cabin ozone concentration

ED Decision 2003/2/RM

(a) The aeroplane cabin ozone concentration during flight must be shown not to exceed –

(1) 0·25 parts per million by volume, sea level equivalent, at any time above flight level 320; and

(2) 0·1 parts per million by volume, sea level equivalent, time-weighted average during any 3-hour interval above flight level 270.

(b) For the purpose of this paragraph, “sea level equivalent” refers to conditions of 25° C (77° F) and 1 013·2 hPa (760 millimetres of mercury) pressure.

(c) Compliance with this paragraph must be shown by analysis or tests based on aeroplane operational procedures and performance limitations, that demonstrated that either –

(1) The aeroplane cannot be operated at an altitude which would result in cabin ozone concentrations exceeding the limits prescribed by sub-paragraph (a) of this paragraph; or

(2) The aeroplane ventilation system, including any ozone control equipment, will maintain cabin ozone concentrations at or below the limits prescribed by sub-paragraph (a) of this paragraph.

CS 25.833 Combustion heating systems

ED Decision 2003/2/RM

Combustion heaters must be approved.