45,000 or 51,000 feet). The amendment added the requirement that each occupant be provided with 0.55 pounds (equivalent to 10 cubic feet) per minute of fresh air, reference § 25.831(a).
The amendment also specifies limits for interior temperatures after specific failures, reference § 25.831(g). Compliance to Amendment 25-87 is required for only those airplanes whose certification basis includes this amendment. Most of the airplanes that are currently produced do not fall into this category. An accompanying advisory circular, AC 25-20, Pressurization, Ventilation, and Oxygen Systems Assessment for Subsonic Flight including High Altitude Operation, was also issued to provide one means, but not the only means, of compliance to the regulations affected by Amendment 25-87.
(3) Harmonization. This regulation is the subject of a Federal Aviation Regulations/Joint Aviation Requirements (FAR/JAR) harmonization effort under the Aviation Rulemaking Advisory Committee (ARAC). The ARAC working group will be tasked to recommend revisions to §§ 25.831, 25.841, and related regulations and to recommend revisions to AC 25.20.
(4) Amendment 25-89 (December 2, 1996) revised § 25.831(b)(2) to change the carbon dioxide requirements from three percent by volume to 0.5 percent by volume (sea level equivalent). The following information is extracted from an FAA letter dated May 18, 1999, to the Society of Automotive Engineers (SAE) Committee AC-9, Aircraft Environmental Systems, to acknowledge an error in the preamble to Amendment 25-89, page P-610, 3rd paragraph. It states that ". . . sea level equivalent (SLE) for a cabin altitude of 8,000 feet would be 0.5 percent multiplied by 0.74 . . ." The word "multiplied" should have read "divided." The FAA does not feel that an increase in carbon dioxide concentration from 0.5 percent SLE to 0.68 percent at altitude will introduce an appreciable discomfort or hazard to passengers and maintains a concentration level well below industry recommendations for carbon dioxide exposure.
d. Policy/Compliance Method. For guidance on compliance with this requirement, refer to the preambles of this rule and the following information.
(1) Ventilation rates are determined by tests and/or analyses. For compliance with § 25.831(b) and (c), carbon monoxide and carbon dioxide levels may be measured with hand held samplers or gas measurement devices for the various flight conditions, such as takeoff, cruise, and landing. Measurements are made at "head" level in the cockpit, other compartments occupied by crewmembers, and in the passenger cabin.
(2) Smoke Evacuation. For compliance with § 25.831(d), cockpit smoke evacuation test methods are addressed in AC 25-9A, Smoke Detection, Penetration, and Evacuation Tests and Related Flight Manual Emergency Procedures. The AC 25-9A also provides guidance regarding lavatories, galleys, crew rest areas, equipment cooling bays, and cargo compartments smoke detection/penetration/evacuation.
(3) Ventilation System Design. The ventilation system design should be reviewed and tests or analyses conducted to ensure compliance with § 25.831(e) and (f), which address control of temperatures and ventilating air quantity in compartments occupied by crewmembers.
(4) Airplane Operation with Airconditioning Packs Off. The following policy is extracted from an FAA memorandum dated September 3, 1999, in response to questions regarding compliance with § 25.831 while operating with the airconditioning packs off.
(a) Background. The following summarizes some key elements that are necessary to understand in determining the certification policy regarding cabin ventilation, smoke detection and evacuation, and equipment cooling.
(b) The provisions of § 25.831(a), as amended by Amendment 25-41, state: “Each passenger and crew compartment must be ventilated, and each crew compartment must have enough fresh air (but not less than 10 cu. ft. per minute per crewmember) to enable crewmembers to perform their duties without undue discomfort or fatigue”.
(c) A 1983 memo addressing airconditioning packs-off operation provides additional guidance to the regulatory provisions of § 25.831(a), as amended by Amendment
25-41.
1 The memo states: “The proposed environmental control system (ECS) takeoff procedure for the Model ABC should be processed for certification by an exemption to
25.831(a).”
2 The memo also states: “Analysis and tests have been used to substantiate that operating without the required 10 cubic feet per minute per pilot (§ 25.831(a)) for short duration will not impair pilot performance or significantly affect equipment reliability. This is not to say that the pilots need be comfortable. These ECS ‘off’ approvals were made in accordance with adequate criteria, but not all were made in accordance with correct certification procedures (i.e., equivalency or exemption).”
(d) Subsequently, Amendment 25-87, effective April 30, 1998, amended several of the airworthiness provisions concerning cabin ventilation. Section 25.831(a) was changed to require that “. . . the ventilation system must be designed to provide a sufficient amount of uncontaminated air to enable the crewmembers 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 containing at least
0.55 pounds of fresh air per minute.”
1 The preamble to Amendment 25-87 adds the following additional information. “One commenter recommends allowing the fresh air requirements proposed to be required under § 25.831(a) to remain a crewmember requirement only. The FAA does not concur with this recommendation. It has been determined that this level of airflow is required for several reasons. Members of the flightcrew performing their functions in the passenger cabin are not sedentary and must perform their duties without undue discomfort or fatigue. In addition, fresh airflow has been determined to be necessary to provide adequate smoke clearance in the event of smoke accumulation due to a system failure or fire. However, it is clear that the 66
additional airflow is not required at all times and under all operating conditions. Therefore, the wording in the final rule has been changed to state that the ventilation system must be designed to provide the fresh airflow. This also addresses concerns regarding the low fresh airflow capability that occurs during descent at low power levels.”
(e) Advisory Circular 25-20 was developed concurrent with Amendment 25-87.
Paragraph 5d states: “Takeoff with airconditioning or bleed air system ‘off’ may be an acceptable procedure provided the ventilation system continues to provide an acceptable environment in the passenger cabin and cockpit for the brief period when the ventilation system is not operating normally.”
(f) The Draft Version of this AC (dated November 30, 1998) was issued for public comment as well as coordinated for directorate comments. This AC includes a statement that recommends that the exemption process be used to approve the cabin ventilation system for operation with airconditioning packs-off operation. However, the Aerospace Industries Association (AIA) has questioned the need for an exemption and stated that the no packs takeoff procedures already exist; that as the transient for this condition is short, this is an accepted practice; and that it does not cause the cabin environment to be unsafe. In addition, the Los Angeles Aircraft Certification Office (ACO) commented that an equivalent safety finding was more appropriate than an exemption.
(g) Analysis. The Transport Standards Staff’s (TSS) interpretation of the provisions of § 25.831(a), as amended by Amendment 25-41, is that the prescribed airflow for the flightcrew is required to be provided during normal operation. However, the certification records do not show that any such equivalent safety finding or exemption for airconditioning packs-off operation (i.e., no fresh air for crewmembers) was issued for the model ABC or for any other transport category airplane. Therefore, the TSS assumes that any transport category airplane for which the AFM allows packs-off operation, does not strictly comply with the provision of § 25.831(a), as amended by Amendment 25-41.
1 Recognizing that the cabin ventilation provisions of Amendment 25-41 were overly restrictive, the FAA, in Amendment 25-87, changed the requirement of § 25.831 so that the regulatory provision now reads: “ . . . the ventilation system must be designed to provide a sufficient amount of uncontaminated air to enable the crewmembers to perform their duties without undue discomfort or fatigue . . .”
2 The preamble discussion quoted above states that the reason for this change is that “. . . it is clear that the additional airflow is not required at all times and under all operating conditions.”
3 Therefore, provisions of § 25.831, as amended by Amendment 25-87, specify a design requirement and allow for limited interruptions of the specified design airflow.
However, this interruption may not result in “undue” discomfort or fatigue to the crewmembers.
Advisory Circular (AC) 25-20, which was developed concurrently with Amendment 25-87, supports this interpretation (see excerpt above).
4 Furthermore, an airplane accident heightened FAA awareness to the need to address smoke penetration and include cargo compartment fire protection (detection and suppression) throughout the flight, including taxi, takeoff and climb. When determining compliance with the sections of part 25 relating to flight crew compartment air quality, ventilation, smoke penetration/evacuation, cargo compartment fire protection (detection and suppression) and equipment cooling, all the impacts related to operating with the airconditioning packs off for a limited time must be determined. The TSS considers the failure to document any consideration of these issues in previous compliance findings an oversight by the FAA.
5 While direct compliance to § 25.831(a), as amended by Amendment 25-87, for airconditioning packs-off operation is possible without using an equivalent safety finding; an equivalent safety finding (§ 21.21(b)(1)) may be used for showing compliance with the provisions of § 25.831(a), as amended by Amendment 25-41, for airconditioning packs-off operation.
(h) Conclusion. The TSS has determined that while direct compliance to § 25.831(a), as amended by Amendment 25-87, for airconditioning packs-off operation is possible without using an equivalent safety finding, an equivalent safety finding (§ 21.21(b)(1)) may be used for showing similar compliance with the provisions of § 25.831(a), as amended by Amendment 25-41. The equivalent safety finding must document that the ventilation system continues to provide an acceptable environment in the passenger cabin and cockpit for the brief period when the ventilation system is not operating normally. The degradation of crewmember air quality must not reach the level that would cause undue discomfort and fatigue to the point that it could affect the performance of their duties.
(i) Advisory Circular (AC) 25.1581-1, Airplane Flight Manual, issued in 1997, states in paragraph 2b(11), Systems and Equipment Limitations, that; “all limitations applicable to systems and equipment installations that are considered necessary for safe operation must be included.” Airconditioning packs-off operation is intended to be a short duration operation.
Therefore, the maximum period of operation in this configuration should be defined by the applicant and specified in the AFM, along with any related operating procedures necessary to maintain compliance with the regulatory issues discussed above. An example of establishing "the maximum period of operation (short duration) for takeoff" would be an operational phase beginning with turning packs off when cleared into position for takeoff, and ending when packs were turned back on after takeoff thrust was reduced to climb thrust or when accomplishing the "after-takeoff" checklist.
(j) Packs-On and Packs-Off. In accordance with §§ 25.855 and 25.857, there must be a means to exclude hazardous quantities of smoke, flames, or extinguishing agent from any compartment occupied by crew or passengers. The FAA has historically found compliance to these requirements for packs-on operation but has not recognized the need to assess compliance with requirements for limited duration packs-off operation. A change in precedent set by historical practice is beyond the scope of this informational memorandum. The Transport Standards Staff (TSS) recognizes that service experience to date has not demonstrated that 68
packs-off operations have resulted in hazardous quantities of smoke. Therefore, at this time, determination that packs-off operation is acceptable can be predicated on an FAA finding that no unsafe condition due to limited duration packs-off operation would result should a cargo compartment fire occur. Criteria that should be considered include: (1) packs-on operation will not allow any smoke from the cargo compartment to penetrate the passenger compartment; and
(2) during limited duration packs-off operations the cargo compartment smoke detection system is effective and the aircondition packs can be turned on and returned to the approved packs-on configuration to exclude hazardous quantities of smoke. The local ACO may make a determination of compliance to the aforementioned criteria based upon available test data, analysis and extrapolation of these data, as has been historically accepted to demonstrate compliance to these and other sections of part 25.
(5) Fresh Air and Normal Operating Conditions. The following policy is extracted from an FAA memorandum dated September 10, 1997, in response to questions regarding the terms “fresh air” and “normal operating conditions” as it applies to compliance with § 25.831.
(a) The term “fresh air” was first documented with the ventilation requirements in § 4b.371 of the CAR. Section 4b.371(a) of the CAR required that all crew compartments be ventilated with a sufficient amount of fresh air. It was specified that “an outside air supply of approximately 10 cubic feet per minute is considered a minimum for each crewmember.” This requirement was later codified into 14 CFR § 25.831(a).
(b) Notice of Proposed Rulemaking (NPRM) 89-31 first proposed that § 25.831(a) be amended to include additional minimum airflow requirements for both crew and passengers.
NPRM 89-31 and Amendment 25-87 proposed that the minimum fresh airflow of 10 cubic feet per minute per crewmember be provided to each occupant during normal operation. Both the “Discussion” section of NPRM 89-31 and the “Background” section of Amendment 25-87 state that some airplanes “incorporate ventilation systems in which fresh air is augmented with conditioned and recirculated air.” It was specified that the rule change would permit a ventilation system that uses a mixture of the minimum amount of fresh air and any desired quantity of recirculated air that is shown to be uncontaminated by odors, particulates, or gases. It is stated that the “minimum amount of fresh air is specified by weight. . . .” Ten cubic feet of standard air at a typical cabin altitude of 8,000 feet weighs approximately 0.55 pounds, which is how the final rule was stated. This provides a clear distinction that the minimum amount of fresh air (outside air) is 0.55 pounds per minute per person, but may be augmented by additional conditioned and recirculated air.
(c) Requests have been made that “normal operating conditions” as used in § 25.831(a) be interpreted to mean normal cruise conditions with this interpretation excluding ground, climb, descent, and holding operations. The FAA does not agree. Ground, climb, descent, and holding operations are considered normal operating conditions. However, it should be noted that the wording in the preamble to the final rule stated that for normal operating conditions, the ventilation system must be designed to provide the fresh airflow. This was a change from the NPRM that which stated that each occupant must be provided the fresh airflow.
The preamble to Amendment 25-87 states that it is clear that the additional airflow is not 69
required at all times and under all operating conditions. The change in wording reflects the need to express certification rules in terms of design requirements rather than operating restrictions and addresses concerns regarding potential low airflow capability that occurs during descent at low power levels. If an applicant proposes not to provide the minimum required fresh airflow during phases of flight using low power levels, the applicant must show that the cabin air quality is not compromised for that period of time.
(6) Ventilation During Takeoff with the Environmental Control System Turned Off.
The following policy is extracted from an FAA memorandum dated May 20, 1983, in response to questions regarding compliance with § 25.831 while conducting takeoff with the environmental control system (ECS) off.
(a) Proposals for takeoff procedures with the ECS off should be processed for certification by an exemption to § 25.831(a). [See paragraph (h) Conclusion, above for a 1999 update] Transient ventilation conditions have been required to be analyzed for current transport aircraft in the past. Analysis and tests should be used to substantiate that operating without the required 10 cubic feet per minute per occupant (§ 25.831(a)) for short durations will not impair pilot performance, passenger safety, or significantly affect equipment reliability.
(b) Both equipment and the pilot environment should be evaluated during taxi and takeoff to ensure pilot performance and equipment reliability are not impaired. This evaluation should cover the extremes of ambient hot air temperatures in which the airplane is expected to operate. The temperatures evaluated should be no less than the maximum ambient hot air temperature limits of engine or equipment, whichever is more limiting. Analysis may be used to extrapolate test temperatures.
e. References. The address for ordering the latest revision of the advisory circulars listed below can be found in the Appendix to this AC.
AC 25-9A, Smoke Detection, Penetration, and Evacuation Tests and Related Flight Manual Procedures.
AC 25-20, Pressurization, Ventilation, and Oxygen Systems Assessment for Subsonic Flight Including High Altitude Operation.
AC 20-32B, Carbon Monoxide (CO) Contamination in Aircraft-Detection and Prevention.
AC 25-7A, Flight Test Guide for Certification of Transport Category Airplanes.
31. SECTION 25.832 - CABIN OZONE CONCENTRATION.
a. Rule Text.
(a) The airplane cabin ozone concentration during flight must be shown not to exceed- 70
(1) 0.25 parts per million by volume, sea level equivalent, at any time above flight level 320; and
(2) 0.10 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 section, ''sea level equivalent'' refers to conditions of 25o C and 760 millimeters of mercury pressure.
(c) Compliance with this section must be shown by analysis or tests based on airplane operational procedures and performance limitations, that demonstrate that either-
(1) The airplane cannot be operated at an altitude which would result in cabin ozone concentrations exceeding the limits prescribed by paragraph (a) of this section; or
(2) The airplane ventilation system, including any ozone control equipment, will maintain cabin ozone concentrations 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]
b. Intent of Rule. Ozone is an odorless, colorless gas that can be irritating to the respiratory tract and eyes when present in high enough concentrations. This rule was formulated to limit the ozone concentrations in the occupied areas of transport category airplanes. Because the level of discomfort is proportional to the level of activity of the parties exposed, cabin attendants are more likely to be adversely affected. The ozone limits embodied in this section will prevent crew members and passengers from being exposed to concentrations high enough to be hazardous. There is a parallel requirement in the operating rules (§ 121.578).
c. Background. Following complaints from crewmembers regarding various adverse health effects associated with ozone in the airplane cabins, and a petition for rulemaking, parts 25 and 121 were amended at Amendments 25-50 (January 1, 1980) and 121-154 (January 1, 1980), to add standards for maximum ozone concentration in transport category airplanes.
(1) Amendment 25-56 (December 30, 1982) raised the flight levels at which compliance must be demonstrated, and excluded certain airplanes from the requirements of the part 121 rule.
(2) The Operating Rule, § 121.578, allows route and altitude adjustments to be used, in conjunction with global ozone concentration data and airplane ozone retention factors, to avoid exceeding safe ozone levels. Part 25 requirements were intended to be addressed through design features, e.g., ozone filters or catalytic converters, to ensure that ozone limits would not be exceeded regardless of altitude, latitude, and time of year for the operation. However, some manufacturers have requested that they be allowed to comply with § 25.832 by placing limitations in the airplane flight manual that would stipulate altitude limits for operations at specific latitudes during certain times of the year.
d. Policy/Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of this rule and the following information.
(1) Sea Level Equivalent. The following guidance is extracted from an FAA letter dated May 18, 1999. It was developed in response to inquiries regarding a definition of the term “sea level equivalent” and to acknowledge an error in the preamble to Amendment 25-89 with reference to AC 120-38. The following guidance will demonstrate that for cabin air concentration the FAA intended to maintain constant parital pressures; however, the concentrations may change with altitude.
(a) Confusion still exists pertaining to the term “sea-level equivalent.” This term has been used as far back as 1953 in the Civil Air Regulations (CAR) part 4b, § 4b.371, Ventilation, to address carbon dioxide concentrations. This later became Title 14, Code of Federal Regulations (14 CFR), part 25, § 25.831(b)(2). Amendments 121-154 (Operations) and 25-50 (Certification), effective February 20, 1980, introduced §§ 121.578 and 25.832, Cabin Ozone Concentration. These regulations defined the term sea level equivalent in §§
121.578(a)(2) and 25.832(b) as follows: “Sea level equivalent (SLE) refers to conditions of 250 C and 760 millimeters of mercury pressure." The preamble to these amendments further states that ozone concentration is stated in parts per million by volume (ppmv) and expressed as a sea level equivalent, i.e., the ratio of ozone to air that would exist at 760 millimeters of mercury (mm Hg) pressure and 250 C.
(b) The NPRM that originally proposed ozone limitations, (NPRM No. 78-15), introduced an ozone limit of 0.3 ppmv at any given time. During the comment period, confusion was expressed as to what was meant by the proposed cabin ozone concentrations. The ratios proposed were those that would be expected at the air pressure which is normally maintained in the passenger cabins of the affected aircraft (it was considered to be 6,000 feet). This original proposal was modified, however, since concentration limits are typically expressed at standard sea level pressure (760 mm Hg) and temperature (250 C), including those adopted by the Occupational Safety and Health Administration (OSHA).
(c) The FAA adjusted the final rule, Amendment. Nos. 121-154 and 25-50, to be expressed based on the sea level criteria. The preamble states, “Since there is more air in a given volume at sea level, the proposed 0.3 ppmv limit converts to 0.25 as a ppmv, sea level equivalent.” The intent was not to allow a higher ozone concentration at altitude, but to take the proposed cabin ozone concentration and express it in standardized terms based on published OSHA limits.
(d) Advisory Circular (AC) 120-38, provides illustrations for determining a sea level equivalent concentration. Per the AC, the terms OZSLE and O3 are concentrations in ppmv. OZSLE is the sea level equivalent absolute ozone concentration and O3 is the absolute concentration at a given altitude (assuming a cabin temperature of 250 C). The AC provides the following equation to relate O3 and OZSLE based on a pressure ratio (P/Po), which will always be less than zero for altitudes higher than sea level.
1 OZSLE = (P/Po)(O3)
P/P o is the ratio of the total cabin pressure (P) to total sea level pressure (Po). This equation is nothing more than a conversion of a concentration value at altitude back to sea level.
(e) Dalton’s Law states that the total pressure of a gas mixture is the sum of the individual pressure (or partial pressures) that each gas would exert if it alone occupied the whole volume. Each kind of gas exerts its own pressure depending on the percentage of that gas in the mixture; thus, even though the percentage or concentration of oxygen in the atmosphere is nearly constant (around 20.9%), its partial pressure will decrease proportionately as atmospheric pressure decreases. If c is the concentration of a gas, P is the absolute pressure, and Pp is the partial pressure, they are related by the following equation.
2 c = (Pp/P)
Advisory Circular (AC) 120-38 uses SLE to essentially provide a constant partial pressure requirement independent from the cabin pressure. Substituting the concentrations of ozone at sea level (OZSLE) and of ozone under cabin pressure (O3) into equation (1) you get:
3 P p,OZSLE/P = (P/Po)(Pp,O3/P)
At sea level, P = Po, therefore, equation 3 becomes
4 P p,OZSLE/ Po = (P/Po)(Pp,O3/P)
which reduces to Pp,OZSLE = Pp,O3.
(f) This exercise demonstrates that the FAA intended to maintain constant partial pressures; however, the concentrations may change with altitude. It was not the intent of the FAA to allow higher partial pressures of a gas at low cabin pressures. In fact, the partial pressures are intended to be independent of the cabin altitude. With this in mind, the sea level equivalent of a gas can be considered the gas concentration (volume %) at sea level for which the partial pressure of the gas matches the ambient partial pressure of the gas at a given cabin altitude.
(g) Based on this interpretation of the preamble to Amendment. Nos. 121-154 and 25-50, and AC 120-38 material, the preamble to Amendment. No. 25-89 should be corrected to read as shown in paragraph 36(c)(4)(a) above.
(2) Ground test. Applicants have demonstrated compliance with this rule by conducting a ground test to establish the ozone retention of the aircraft. This should be accomplished by inducing a known ozone concentration into the engine inlet/bleed air system, representative of the worst case concentration predicted for the atmospheric environment, and measuring the concentration in the passenger cabin and cockpit. Alternatively, ground tests could be conducted utilizing ambient ozone (vs. machine generated ozone) in regions with high 73
levels of ozone provided the cognizant ACO finds them acceptable. The airplane engines and ventilation systems by themselves reduce the concentration without any additional filtering.
Using the ratio of the ozone introduced to the amount of ozone retained in the cabin environment (the retention ratio), the airplane operating characteristics in terms of the ozone concentration to be expected in the occupied areas can be determined. If an ozone filter or catalytic converter is installed, the manufacturer may perform a similar ground test (or an equivalent laboratory test acceptable to the cognizant ACO) to demonstrate that the ozone concentration in the airplane will never exceed the ozone limits in service, regardless of the atmospheric concentration.
Typically, a maintenance manual limit is established for removing and cleaning the units. The maintenance interval ensures that the converters will be removed when the efficiency of the converter degrades to the point that the airplane would no longer be in compliance with § 25.832.
(3) Use of Default Retention Ratio. The following guidance is extracted from an FAA letter dated February 18, 1999. It was developed in response to an inquiry regarding certification of cabin ozone limits to satisfy § 25.832:
(a) One letter requested FAA concurrence that for future certification programs a value of 0.7 be accepted as a default value for the ozone retention ratio when demonstrating compliance to 14 CFR § 25.832.
(b) An applicant for a type certificate under part 25, or a part 121 operator, must demonstrate compliance with the maximum ozone concentration requirements of § 25.832 or § 121.578, respectively. These two regulations were introduced per Amendments 25-50 and 121-154. Amendments No. 25-56 and 121-181 made revisions stipulating these ozone concentration limits be applicable at revised flight levels. Section 25.832 stipulates a limit for a time weighted average during any three hour interval above flight level 270. Section 121.578 stipulates a limit for a time weighted average for each flight segment that exceeds four hours and includes flight above flight level 270 during that segment. The time difference is referenced in the preamble material in Notice No. 78-15 and ensuing Amendments No. 25-50 and 121-154.
As explained in the notice, approximately one hour of the scheduled flight segment would not be at higher altitudes. This was based on conservative times for start, taxi, takeoff, climb, descent, approach, and landing. The operating rule considers the entire flight segment involving time at lower altitudes.
(c) Certification for ozone concentration limits under both parts 25 and 121 entails demonstrating that the cabin and cockpit ozone concentration will not exceed the limits mandated by the regulations. In either case, the testing and/or analysis for showing compliance utilizes a retention ratio. This ratio is the fractional value of the cabin interior ozone concentration compared with the ambient concentration.
(d) The FAA published a table of previously measured transport airplane retention ratios in Notice No. 81-15. The notice also stipulated that the FAA would accept a retention ratio of 0.7 to facilitate compliance to § 121.578 when computing cabin ozone concentrations, even when a retention ratio has not been measured for that particular airplane. The final rule 74
preamble (Amendment Nos. 25-56 and 121-181) incorporating the proposed changes to both part 25 and part 121 states that the FAA considers it reasonable to accept the value of 0.7.
(e) Since that notice was published, retention ratios have been determined for a number of airplanes. The retention values that have been demonstrated on turbine-powered aircraft indicate that 0.7 continues to be a reasonable value. The FAA accepts a default value of
0.7 for the ozone retention ratio when demonstrating compliance to both §§ 25.832 and 121.578.
If you propose to use a retention value of less than 0.7, the FAA will require certification testing to validate the proposed retention level.
(4) Use of Operational Limitations. The following guidance is extracted from an FAA memorandum dated March 31, 1997. It was developed in response to inquiries regarding certification of cabin ozone limits to satisfy § 25.832:
(a) The cabin ozone limits specified in § 25.832 were first introduced in Amendment 25-50, and were simultaneously placed into part 121. When the part 25 rule went into effect, it was expected that many of the existing airplanes in the fleet would not be modified with ozone filters; since § 25.832 was not part of their type design, these airplanes did not have to comply with this rule. Most operators of these airplanes chose, instead, to comply with the ozone limits required by § 121.578 by way of operational considerations, i.e., reduced cruise altitudes, amended routes of flight, etc. For the existing airplanes at that time, this solution was deemed the most economical method of compliance, and Advisory Circular (AC) 120-38 was developed to provide guidance on acceptable methods of operational compliance to § 121.578.
(b) Compliance with § 25.832 requires a demonstration (by test or analysis) that the cabin ozone concentration will not exceed the stated limits at the maximum atmospheric ozone concentration expected in service. If an ozone filter or other device is installed, it must be shown that this device, plus the natural ozone dissociation which occurs in the engines and in the airplane air conditioning system, will meet the requirements of the rule at the airplane’s certificated altitude ceiling. If an ozone filter or other device is not used, however, certification testing may reveal that the cabin ozone concentration may exceed the limits at the airplane’s certificated altitude ceiling, and this would cause the certificated ceiling of the airplane to be lowered to whatever altitude would allow compliance with § 25.832. Proposals have been made to comply with § 25.832 by applying operational limitations to the Airplane Flight Manual in lieu of installing ozone control equipment. These limitations would restrict the airplane to certain maximum altitudes based on the latitude and time of year. Such data is currently considered acceptable in finding compliance to § 121.578. This concept could be an option for compliance to § 25.832, but a part 25 approval is not geographically limited. Therefore, charts need to be available for ozone limitations globally for this method of complying with § 25.832.
This type of compliance would have to be approved by the Manager of the responsible Aircraft Certification Office.
(5) Meaning of the Term Time-Weighted Average in § 25.832. The following policy material is extracted from an FAA letter dated October 26, 1987. It was in response to a request 75
for information on the calculation of time-weighted-average ozone exposure for compliance with § 25.832(a)(2).
(a) Time-weighted-average (TWA) ozone exposure, as described in AC 120-38, is calculated for flight segments that occur in different atmospheric ozone concentrations by averaging the individual exposures and the duration of each exposure over the entire flight. This concept can be explained by an example.
(b) Assume a 2 hour flight in zero parts per million (PPM) atmospheric ozone concentration, and then a 4 hour segment in a 1.3 ppm concentration. Assume also a cabin ozone retention ratio of 0.7, a cabin altitude of 7,000 feet, and no ozone filter installed.
(c) The time-weighted-average (TWA) atmospheric ozone concentration is calculated as follows:
(0.0ppm)(2 hrs) + (1.3ppm)(4 hrs) = 0.867 ppm time-weighted-average (TWA) 6 hrs atmospheric ozone concentration
(d) The 7,000-foot cabin altitude results in a ratio of cabin altitude/sea level pressure of 0.77 (the allowable cabin ozone concentration is corrected to sea level conditions).
The ozone retention ratio of 0.7 is a value that the FAA will accept without a test to determine the actual value.:
1 This is equation 6 from Appendix 1 of AC 120-38:
0Z16 = (l-E)(R)(P/PO) ( ) /OZ Ti Ti F Si N 16 1= ∑⎡⎣⎢ ⎤⎦⎥
2 Using equation 6 from Appendix 1 of AC 120-38, the calculation of the resultant cabin ozone concentration for this sample flight is as follows:
TWA = 0.77 x 0.7 x 0.867 = 0.467 ppm cabin ozone concentration.
(e) In this example, the cabin ozone concentration exceeds the 0.1 ppm limit in § 121.578, and the flight could not be made under these conditions. Either the altitude, route of flight, or duration of exposure would have to be changed to make the flight acceptable. This procedure is explained, with several examples, in AC 120-38.
(f) The calculation of the TWA ozone exposure for compliance with § 25.832(a)(2) is done in the same way except that the TWA value would be based on the 4-hour segment, not the 6-hour segment used in the example.
(6) Example of Compliance with § 25.832. The following is extracted from an applicant's letter in response to the FAA’s request for an example of the calculation methods used to certify previous models under § 25.832, Cabin Ozone Concentration. Included below are 76