generic examples of the calculations used to show compliance to the maximum and timeweighted average ozone concentration limits.
(a) For this example, extracted from report no. FAA-EQ-78-03 (reference (b) of applicant's letter), the maximum certified flight level of the airplane will be FL430. The highest ambient ozone concentrations at flight level 430 are at 80° N latitude, Eastern North America during the month of January (atmospheric ozone concentration, confidence level of 84%). The minimum ozone converter efficiency required for southern and northern hemisphere flights are the same; however, while the worst seasons for high ambient ozone concentrations in the southern hemisphere are the summer and fall, the worst seasons for the northern hemisphere are the winter and spring.
(b) In lieu of an experimentally based retention factor, a retention factor of 0.7 will be used in the following analysis extracted from Docket No. 22438, Notice No. 81-15 (reference (c) of applicant's letter). The FAA will accept a retention factor of 0.7 when computing cabin ozone concentrations if a retention factor has not been measured for the airplane being analyzed.
(c) To comply with § 25.832, an airplane must be designed so that the cabin ozone concentration does not exceed a maximum ozone concentration level of 0.25 parts per million by volume (ppmv), sea level equivalent (SLE), at any time above flight level 320; and time weighted average (TWA) limit 0.1 ppmv, SLE, during any three hour time interval above flight level 270. Compliance will be shown by statistical analysis as described in Advisory Circular 120-38 (reference (d) of applicant's letter).
(d) The following is the calculation of the minimum ozone converter efficiency required to meet the maximum cabin ozone concentration limit (OZMAX) of § 25.832 during a 77
worst case flight. Using equation 1 from Appendix 1 of AC 120-38, the calculation of the resultant cabin ozone concentration is as follows:
1 Given:
Month: January Latitud e: 80° N Flight Level: 430 Flight Time: 3 Hours Cruise; 4 Hours Total
() 16OZ*E1*PP*ROZMAX o −=
where:
OZMAX = Maximum Cabin Ozone Concentration Limit = 0.25 ppmv (§ 25.832) R = Airplane Ozone Retention Factor = 0.7
oP P = Ratio of Cabin Pressure to Sea Level Atmospheric Pressure = 0.74
E = Efficiency of Ozone Converter OZ16 = Ambient Ozone Concentration = 1.6 PPMv (From report no. FAA-EQ-78-03)
Solving:
16OZ*PP*R OZMAX1E o −=
70.06.1*74.0*7.0 25.01E =−= E = 70 % CONVERTER EFFICIENCY; minimum converter efficiency required to comply with § 25.832 OZMAX limit.
(e) The following is the calculation of the minimum ozone converter efficiency required to meet the time-weighted average cabin ozone concentration limit (OZTWA) of § 25.832 during a worst case flight. Using equation 6 from Appendix 1 of AC 120-38, the calculation of the resultant cabin ozone concentration for is as follows:
1 Given:
Month: January Latitud e: 80° N Flight Level: 430 Flight Time: 3 Hours Cruise; 4 Hours Total
∑ = −= N 1i FS i i o T T*)16OZ(*PP*R*)E1(OZTWA where:
OZTWA = Time-Weighted Average Cabin Ozone Concentration Limit = 0.10 ppmv (§ 25.832) E = Efficiency of Ozone Converter R = Airplane Ozone Retention Factor = 0.7
oP P = Ratio of Cabin Pressure to Sea Level Atmospheric Pressure =0.74
N = Number of Individual Flight Segments = 3 OZ16 = Ambient Ozone Concentration = 1.6 ppmv (from report no. FAA-EQ-78-03) Ti = Time of Individual Flight Segment = 1 Hour TFS = Total Time of Flight = 4 Hours
Solving:
i N 1i i o FS T*)16OZ(*PP*R T*OZTWA1E ∑ = −= ( ) () ()[] 84.01*6.11*6.11*6.1*74.0*70.0 4*10.01E =++−= E = 84 % CONVERTER EFFICIENCY; minimum converter efficiency required to comply with § 25.832 OZTWA limit.
(f) When calculating the minimum ozone converter efficiency required to satisfy the 14 CFR time-weighted average limit of 0.1 ppmv, a four-hour flight was assumed, taking a half-hour each for takeoff and landing and three hours for cruise.
(g) Based on the above calculations, the minimum required ozone converter efficiency for the airplane to comply with § 25.832 of 14 CFR is 84%.
e. References. The addresses for ordering the latest revision of the advisory circulars and other referenced documents listed below can be found in the Appendix to this AC.
AC 120-38, Transport Category Airplanes Cabin Ozone Concentrations.
FAA Report FAA-AM-79-20, Effects of Ozone on Exercising and Sedentary Adult Men and Women Representative of the Flight Attendant Population.
FAA Report FAA-AEQ-77-13, Ozone Concentration by Latitude, Altitude, and Month, Near 80o West.
FAA Report FAA-AM-80-9, Effects of Ozone (0.30 Parts Per Million, ~ 600µg/m3) on Sedentary Men Representative of Airline Passengers and Cockpit Crewmembers.
FAA Report FAA-AM-80-16, Effects of Long-Term Exposure to Low Levels of Ozone: A Review.
32. SECTION 25.833 - COMBUSTION HEATING SYSTEMS.
a. Rule Text.
Combustion heaters must be approved.
[Amdt. 25-72, 55 FR 297783, Jul 20, 1990]
b. Intent of Rule. The intent of this rule is that combustion heaters installed on transport category airplanes must be approved. Technical Standard Order (TSO) C-20 provides standards for approval of these devices.
c. Background. Combustion heaters were common in airplanes certificated to parts 3 and 4b of the Civil Air Regulations (CAR). These airplanes, usually powered by reciprocating engines, supplemented various engine exhaust heating systems by separate combustion heaters.
There are few combustion heaters in use in modern part 25 airplanes, but their use is not prohibited. The certification requirements for combustion heaters appeared in § 4b.372 of the CAR, entitled “Heating Systems,” which stated: "Combustion heaters shall be of an approved type and shall comply with the fire protection requirements of § 4b.386. Engine exhaust heaters shall comply with the provisions of § 4b.467(c) and (d)." Section 4b.386 contained extensive standards for combustion heater fire zones, ventilating air ducts, combustion air ducts, heater controls, air intakes, and exhaust systems.
d. Policy/Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of this rule and the following information.
(1) Standards for Certification of Combustion Heaters. The following policy material is extracted from an FAA memorandum dated March 14, 1948, in response to questions regarding standards for certification of combustion heaters to § 4b.372 of the CAR.
(a) Heater Approval. The approval of heater units is considered essential to safety in all cases. In this regard two separate classes of approval are evidently in order to cover the various applications that can be foreseen.
1 The first class of approval will cover units which will be considered eligible for use only as space heaters. In the case of such units, the criteria for acceptance will involve the general airworthiness of the unit under vibration and operating temperatures and considerations of safety from the standpoint of fire and carbon monoxide contamination only.
Heater performance, i.e., the ability of the heater to deliver a given output between normal overhaul periods, ability to start under all expected conditions of altitude and temperature, etc., will not enter into such approvals as this does not normally constitute a safety item in the case of space heaters.
2 The second class of approval will involve heater performance in addition to the criteria for the first class of approval. This type of approval will cover units that are to be used for de-icing purposes since heater performance obviously becomes a safety consideration when the heater is to be used for windshield, surface, or propeller de-icing.
3 Technical Standard Order TSO-C-20 provides standards for combustion heaters and should be used for new installations.
(b) Heater Isolation. Heaters must be isolated from the remainder of the airplane.
However, this need not necessarily mean a complete shield around the entire heater unit (although this would be satisfactory) since in many heater designs, the air jacket largely surrounds the flame chamber. Thus, the heater design itself practically provides a steel shield between the combustion unit and the remainder of the airplane. In such cases, it is acceptable to provide isolation for the fuel system components mounted on the heater, and for the heater exhaust system and combustion chamber drains. The following schematic sketch shows an example of an installation which should be satisfactory:
1 The shut-off valve shown in the sketch should be provided if there are fuel system components within the ventilating air shroud which may be subject to leakage or failure.
In such cases, that portion of the ventilating air duct up to the valve, as well as the valve itself, should be of fire resistant construction and the valve should provide as flame tight a seal as possible. If the fuel system is so arranged that there are no fittings or connections within the 81
ventilating air shroud, the downstream air shut-off valve and fire resistant duct between the heater and the valve may be dispensed with.
2 For shrouds for the combustion chamber drain lines, the necessity for these will generally depend upon the location of the drain in the heater. If the drain outlet from the combustion chamber is so located that products of combustion can issue through the drain line, it will no doubt become hot and require isolation. However, drains are sometimes connected in such a manner that they do not carry exhaust gases and remain relatively cool. In such cases, shrouds are not necessary.
(c) Fire Detector and Extinguisher Equipment.
1 For transport category airplanes, extinguishers and detectors will normally be required, in addition to isolation provisions.
2 Detectors and extinguishers should be provided wherever potential sources of fuel leakage and sources of ignition are in close proximity. In the foregoing sketch, the space within the shield would require such protection. In addition, detectors and extinguisher nozzles should be installed in the ventilating air passages of the heater if this chamber contains fuel system fittings or connections that may be subject to leakage. Hand fire extinguishers should be considered equivalent to a fixed fire extinguisher installation only when the heater is located in such a manner that it is readily accessible to the crew and when all fire zones in the installation can easily be reached with a hand extinguisher. All extinguishers may also be dispensed with when the heater is so shielded and located that a fire could be permitted to burn itself out without danger of damage to any important structural members or otherwise endangering the safety of the airplane.
3 Detectors may be dispensed with only when the heater is so located that the occurrence of fire would immediately be noted by the crew.
(d) Heater Fuel System. The heater fuel system should comply with airworthiness standards for the engine fuel system as regards fuel lines, fittings and accessories. While in flight, valves should be provided for shutting off the flow of fuel at its source, unless equivalent provisions in the form of a separate heater fuel pump are available. In the case of transport category airplanes, fuel lines located in fire zones should, in addition, be of fire resistant construction. In the foregoing sketch, this would apply to that portion of the fuel line within the shield and in the heater ventilating air compartment. All pressure lines should comply with the provisions of §§ 03.4211 and 04.4211 of the CAR, regarding pressure cross feed arrangements.
e. Reference. The address for ordering the latest revision of the referenced document listed below can be found in the Appendix to this AC.
Technical Standard Order No. TSO-C-20, Combustion Heaters.
33. SECTION 25.841 - PRESSURIZED CABINS.
a. Rule Text.
(a) Pressurized cabins and compartments to be occupied must be equipped to provide a cabin pressure altitude of not more than 8,000 feet at the maximum operating altitude of the airplane under normal operating conditions.
(1) If certification for operation above 25,000 feet is requested, the airplane must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of 15,000 feet after any probable failure condition in the pressurization system.
(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 automatically 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 appreciable 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 pressure differential that would damage the structure. One valve is enough, however, if it is of a design that reasonably 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 altitude, and the rate of change of the cabin pressure altitude.
(6) Warning indication at the pilot or flight engineer station to indicate when the safe or preset pressure differential and cabin pressure altitude limits are exceeded. Appropriate warning markings on the cabin pressure differential indicator meet the warning requirement for pressure differential limits and an aural or visual signal (in addition to cabin altitude indicating means) meets the warning requirement for cabin pressure altitude limits if it warns the flight crew when the cabin pressure altitude exceeds 10,000 feet.
(7) A warning placard at the pilot or flight engineer station if the structure is not designed for pressure differentials 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 (including upper and lower lobe galleys), the warning and automatic presentation devices, required by those provisions, will be actuated without any delay that would significantly increase the hazards resulting from decompression.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-38, 41 FR 55466, Dec. 20, 1976; Amdt. 25-87, 61 FR 28696, Jun. 5, 1996]
b. Intent of Rule. This rule provides standards for pressurized compartments in transport category airplanes. The requirements for various controls and pressure relief valves are addressed. Testing required for demonstrating compliance with many of the requirements of this section are addressed in § 25.843.
c. Background. The rules in this section originated in sections 4b.373 through 4b.376 of the Civil Air Regulations, and were carried over essentially unchanged when part 25 was codified.
(1) Amendment 25-38 (December 20, 1976) added § 25.841(b)(8). This amendment requires pressure sensors and systems designed so the indicators (and automatic oxygen mask presentation required in § 25.1447(c)) will be actuated without any delay that would significantly increase the hazards resulting from decompression.
(2) Amendment 25-87 (June 5, 1996) changed section 25.841(a) based largely on special conditions that had been applied to aircraft operating at high altitudes. The objective of the amended § 25.841(a) (when applied in conjunction with amended § 25.1447(c)) was to provide airworthiness standards that allow subsonic airplanes to operate at their maximum achievable altitudes. This is the highest altitude for which an applicant chooses to demonstrate that, after decompression caused by a single failure or combination of failures that are not shown to be extremely improbable: (1) the flightcrew will remain alert and be able to fly the airplane;
(2) the cabin occupants will be protected from the effects of hypoxia; and (3) in the event that some occupants do not receive supplemental oxygen, they nevertheless will be protected against permanent physiological damage. Compliance to Amendment 25-87 is required for only those airplanes whose certification basis includes this amendment. Most of the currently produced airplanes do not fall into this category.
(3) Harmonization. This regulation is the subject of a Federal Aviation Regulations/Joint Aviation Requirements (FAR/JAR) harmonization effort under Aviation Rulemaking Advisory Committee (ARAC). The ARAC working group will be tasked to recommend revisions to §§ 25.831, 25.841 and related regulations as well as a revised AC
25.20.
d. Policy/Compliance Methods. Flight tests for demonstrating compliance with the requirements of the section are addressed in § 25.843. For guidance on compliance with this requirement, refer to the preamble of this rule and the following information. Advisory Circular (AC) 25-7A, Flight Test Guide for Certification of Transport Category Airplanes, provides additional guidance on tests to verify the 8,000 feet and 15,000 feet cabin altitude requirements as well as tests for high altitude (i.e., greater than 8,000 feet) airports.
(1) Rapid Equalization of Pressure. The following policy is extracted from an FAA memorandum dated June 21, 1982, in response to an inquiry about the intent of, and need for, § 25.841(b)(3).
(a) Section 25.841(b)(3) ensures that a means exists to provide rapid equalization of pressure in the event that (1) a plug type door must be opened for an emergency, and (2) a windshield cracks (§ 25.775(d)). Part 25 manufacturers typically have complied with this regulation by providing a manual position indication and control for the outflow valves. Later certification documents reference, in addition to the manual control, the ability to shut off cabin air inflow as a means of meeting this requirement.
(b) The FAA does not recommend a specific depressurization rate other than the pilot being able to control the rate. In other words, in the manual pressure control mode, he should be able to set the rate as fast or as slow as the pilot judges the emergency requires or as the flight manual specifies. Typical maximum rates for large aircraft will open the outflow valve in 30 seconds, and the depressurization that follows will occur in two minutes or less (with the air conditioning packs on).
(c) From a design and certification standpoint, the need to establish a rate of equalization of pressure can be assessed from failure analyses, abnormal procedures, and the need to prevent a failure from becoming a disaster.
(2) Access to a Pressurized Cargo Compartment in the Engine Burst Zone During High Altitude Operation. The following policy is extracted from an FAA memorandum dated June 25, 1986, in response to an inquiry regarding access to a cargo compartment located in the engine burst zone during operation above 10,000 feet.
(a) Section 25.841(a)(2) requires that the cabin pressure altitude not exceed 40,000 feet for any duration, or 25,000 feet for more than two minutes after any failure not shown to be extremely improbable. Section 25.841(a)(3) states that engine failures are to be considered in evaluating the cabin decompression, this includes rotor burst. Prior to Amendment 25-87, these stipulations were made through special conditions on several aircraft. Compliance has been shown for many airplanes by locating the pressure bulkhead forward of the aft mounted engines for aft fuselage mounted engines.
(b) In some cases, the primary pressure bulkhead of the airplane is located aft of the engines; however, an additional pressure bulkhead installed forward of the cargo 85
compartment, forward of the engine, and forward of the aft pressure bulkhead was proposed to meet these requirements. The additional pressure bulkhead is likely to have a door that is used for access to the cargo compartment from the main passenger compartment. Should a depressurization of the cargo compartment occur, the integrity of this pressure bulkhead is dependent on the door being closed. This arrangement is considered satisfactory for compliance with the above requirements, provided there is a means of ensuring that the door is not opened during high altitude flight.
(c) Allowing the door to the aft cargo compartment of the airplane to be opened at high altitudes would not ensure the level of safety established by the high altitude special conditions and subsequent rule change. Preventing access to the cargo compartment to ensure that the door is always closed and latched at the higher altitudes is an inconvenience. Most present day airplane designs provide overhead storage bins or limited underseat storage in the main cabin. With some forethought, the limited baggage needed during flight can be stored in these locations prior to operation at high altitudes without exposing the airplane and occupants to possible catastrophic decompression. For these reasons, the FAA has determined that these conditions do not impose an undue burden on the applicant.
(d) In view of the above, the applicant must provide a positive means of ensuring that: (1) The cargo compartment is inaccessible and unoccupied for high altitude flight; (2) There is an indication in the cockpit to warn the flight crew when the cargo compartment door is not closed and latched; and (3) There is a means to automatically close the door. A flight manual limitation should specify that the cargo compartment door must be closed and latched during any operation above 10,000 feet.
(3) Cabin Altitude Limit; § 25.841(a). The following policy is extracted from an FAA memorandum dated January 12, 1994, in response to the question: Is the 8,000 foot cabin altitude limit in § 25.841(a) an absolute limit?
(a) Background. The pressurization systems on the airplanes of interest allow cabin altitudes to exceed the 8,000 feet maximum allowed by § 25.841(a). The manufacturer states that the systems currently used on these airplanes allow 8,000 feet plus or minus 200 feet.
It is also noted that the cabin altitude display employs a tolerance of an additional plus or minus 250 feet. The applicant states that this design does not result in an unsafe condition, and while the system does not literally comply with the 8,000 feet limit, the intent of § 25.841(a) is met.
(b) The preamble to both the Notice of Proposed Rulemaking (NPRM) and the final rule associated with part 4b of the Civil Air Regulations (CAR) have been reviewed.
Amendment 4b-6, effective August 12, 1957, changed the requirements in section 4b.374 of the CAR to limit cabin altitude to 8,000 feet. Prior to that amendment, the limit was 10,000 feet.
This limit was carried over into § 25.841 of 14 CFR. The NPRM states, ". . . it is proposed to change § 4b.374 to require on airplanes intended for operation over 25,000 feet to maintain under normal conditions a pressure altitude of no more than 8,000 feet. . ." The wording adopted was similar to that found in § 25.841(a). When deciding on the interpretation of a rule, the policy of the FAA has been to determine the intent of the parties preparing the rule. The use of 86
the words "no more than 8,000 feet" indicates that the rule was intended to mandate a cabin altitude limit of 8,000 feet. There is nothing in either preamble to indicate that 8,000 feet plus some equipment tolerance was the goal. Further, the pressurization systems on many other airplanes are designed to limit cabin altitude to 7,950 feet including equipment tolerances.
(c) It is recognized that the difference in the partial pressure of oxygen at 8,000 versus 8,450 feet is not significant. There is, of course, a difference, and if a passenger has a breathing impairment, there could be some effect on their health. There is no adverse effect associated with the cabin altitude indicator tolerance because oxygen partial pressure is a function of actual cabin altitude.
(d) Based on the above review, the FAA has determined that issuing a new or amended Type Certificate (TC) for an airplane which does not meet the criteria as written is not in the public interest. When a maximum (or minimum) value of a parameter is provided in a section of 14 CFR, the airplane should be shown to meet that limit in normal operation, including expected equipment and sensor manufacturing tolerances. This applies to the actual measured value as determined in FAA flight or ground tests, but does not apply to tolerance in the display device, such as the cabin altitude indicator. A displayed value of 50 or 100 feet above the 8,000 feet limit would not be considered unacceptable, as long as the actual pressure altitude in the cabin does not exceed 8,000 feet.
(e) For airplanes that are already certificated, it is not necessary to require a modification to revise the pressurization control system. That action would be required only if an unsafe condition warranting mandatory action is identified.
e. References. The addresses for ordering the latest revision of the advisory circular (AC) listed below can be found in the Appendix to this AC.
AC 25-20, Pressurization, Ventilation and Oxygen Systems Assessment for Subsonic Flight Including High Altitude Operation.
AC 25-7A, Flight Test Guide for Certification of Transport Category Airplanes.
34. SECTION 25.843 - TESTS FOR PRESSURIZED CABINS.
a. Rule Text:
(a) Strength test. The complete pressurized cabin, including doors, windows, and valves, must be tested as a pressure vessel for the pressure differential specified in
25.365(d).
(b) Functional tests. The following functional tests must be performed:
(1) Tests of the functioning and capacity of the positive and negative pressure differential valves, and of the emergency release valve, to simulate the effects of closed regulator valves.
(2) Tests of the pressurization system to show proper functioning under each possible condition of pressure, temperature, and moisture, up to the maximum altitude for which certification is requested.
(3) Flight tests, to show the performance of the pressure supply, pressure and flow regulators, indicators, and warning signals, in steady and stepped climbs and descents at rates corresponding to the maximum attainable within the operating limitations of the airplane, up to the maximum altitude for which certification is requested.
(4) Tests of each door and emergency exit, to show that they operate properly after being subjected to the flight tests prescribed in paragraph (b)(3) of this section.
b. Intent of Rule. This rule provides specific tests that must be conducted in demonstrating compliance with the cabin pressurization requirements contained in § 25.841.
c. Background. The standards in this section originated in section 4b.376 of the Civil Air Regulations (CAR), and were carried over essentially unchanged when part 25 was codified.
(1) Amendment 25-87 (June 5, 1996) added another factor to § 25.365(d), referred to in § 25.843(a). Airplanes operating up to 45,000 feet use a factor of 1.33; airplanes operating over 45,000 feet use a factor of 1.67. Compliance to Amendment 25-87 is required for only those airplanes whose certification basis includes this amendment. Most of the currently produced airplanes do not fall into this category.
(2) Harmonization. This regulation is the subject of a Federal Aviation Regulations/Joint Aviation Requirements (FAR/JAR) harmonization effort under Aviation Rulemaking Advisory Committee (ARAC). The ARAC working group will be tasked to recommend revisions to §§ 25.365, 25.831, 25.841, and related regulations as well as revisions to AC 25.20.