PRESSURIZATION
23.841 Pressurized cabins Original Issue and Subsequent Paragraph (c) in § 23.841 requires there be a means to rapidly equalize the pressure differential. Assuming isothermal conditions, the time for the pressures to equalize depends on the cabin volume, the effective area of the safety-dump valves, the cabin inflow, and the pressures inside and outside the cabin. If the size of the effective area of the valve is small in comparison to the cabin volume, the rate of pressure change may be too slow to equalize the pressures before an adverse event could occur. The time period to rapidly equalize the pressures should consider maximum certificated cabin pressure differential, operation of the pressurization system, and either operation of the emergency exits or the cabin entrance doors, or both. When landing the airplane under emergency conditions, the safety-dump valve should have sufficient flow capacity to rapidly equalize the cabin pressure within a time period so that the cabin doors and emergency exits can be opened and evacuation is not impaired. Time to equalize the ambient and cabin pressures should be demonstrated.
Paragraph (f) of § 23.841 requires a warning device for safe or preset pressure differential and absolute cabin pressure. A warning is interpreted to convey the need for an immediate corrective action, so it may not operate unless there is a failure, and the visual indication should be red per § 23.1322. Red lines on altimeters or pressure indicators are used to indicate operating limits, but they are not acceptable warning means.
Inflatable door seals, if installed, are subject to the requirements of this rule.
Amendment 23-14 and Subsequent This amendment requires that cabin pressure altitude not exceed 15,000 feet in any probable failure for airplanes certificated to operate over 31,000 feet. It is not appropriate to use an emergency descent procedure to demonstrate compliance to this rule when compliance can be achieved through design. The Airplane Flight Manual, Emergency Operations Section, should include an emergency descent procedure for use in a rapid decompression from any failure not withstanding the probability of its occurrence.
Amendment 23-17 and Subsequent This amendment established 10,000 feet as the maximum absolute cabin pressure for operation of the pressure altitude warning. Therefore, the pressure sensors used in the warning system cannot have an operating set point and tolerance that would prevent the warning from being given at or before 10,000 feet. A feature that automatically changes the
122 warning altitude to 15,000 feet for operations at field elevations above 10,000 feet is acceptable to prevent nuisance warnings.
The following material is a means of compliance to § 23.841(b)(3) that requires a means by which the pressure differential can be rapidly equalized. Section 23.841(b)(6) offers a provision for a warning indication at the pilot station to indicate when a cabin pressure altitude of 10,000 feet is exceeded.
1. RELATED 14 CFR PART 23 SECTIONS. These acceptable means of compliance refer to certain provisions of part 23 and the corresponding provisions of part 3 of Civil Air Regulations (CAR) in the case of airplanes for which those regulations are applicable.
Listed below are the applicable and the related part 23 sections with the corresponding CAR sections shown in parenthesis:
a. § 23.365 (3.197)
b. § 23.775(c) (3.383)
c. § 23.841 (3.395)
d. § 23.843 (3.396)
2. DISCUSSION OF REQUIREMENTS. In discussing these requirements, a brief history on the development of the applicable airworthiness regulations is first presented.
The purpose of the airworthiness requirements for small airplanes is then explained.
a. Rapidly Equalizing the Pressure
(1) History. The requirement for a means by which the pressure differential can be rapidly equalized was introduced in the airworthiness regulations for pressurized cabins for transport category airplanes when part 04 of the CAR became effective on November 9, 1945. Due to the trend to develop pressurized cabins for small airplanes, the 1956 Annual Airworthiness Review established similar requirements for pressurized cabins for small airplanes. The criteria were developed by using the principles that were applicable to pressurized cabins on transport category airplanes since most of the cabin pressure control system design for small airplanes drew heavily upon the equipment designed and developed for transport category airplanes. As a result, many of the provisions added to part 3 of the CAR by Amendment 3-2, effective August 12, 1957, were substantially the same as those which applied to transport category airplanes. Under the recodification program in 1965, part 23 replaced part 3 of the CAR and these requirements are now in § 23.841(b)(3).
(2) The purpose of this requirement is to provide the crew with a means to rapidly equalize the differential pressure to permit quick opening of the emergency exits and entry door(s) in the event of a gear up landing under emergency conditions.
This means may be used for other events such as over pressurization and reducing cabin contamination. These functions are described in further detail as follows:
(i) Due to a malfunction in the pressurization system or abnormal operational conditions, the cabin pressure is above normal conditions during the airplane landing phase. In this case, the cabin pressure may be vented by the safetydump valve operated through a manual controller or triggered by the landing gear safety switch so the emergency exits and the cabin entrance doors could be opened.
(ii) If a failure such as a cracked window or windshield occurs, the cabin pressure should be capable of being rapidly reduced so the loads due to cabin pressure differential can be reduced accordingly.
(iii) When a threatening cabin overpressure condition exists due to cabin pressurization system malfunction, the cabin pressure can be reduced by the safety-dump valve to prevent a structural failure of the pressure vessel.
(iv) When the cabin air becomes contaminated by smoke, fumes, etc., the cabin safety-dump valve may be used, depending on the conditions, to assist the pressurization or ventilation system, or both, in evacuation of the cabin air to reduce the contaminants.
b. Cabin Pressure Altitude Warning
(1) History
(i) The cabin altitude warning and many of the provisions for pressurized cabins for small airplanes were added to part 3 of the CAR by Amendment 3-2, effective August 12, 1957. Section 3.395(f) of part 3 of the CAR required, in pertinent part, that the pilot be provided a warning when safe or preset limits on pressure differential and on absolute cabin pressure were exceeded.
(ii) In May 1958, a quantitative requirement was introduced in the airworthiness regulations when FAA established policy for altitude warning on the sport category airplanes. This policy, which was set forth in § 4b.375-l of CAR part 3, required that the warning for cabin pressure would meet the applicable requirements if it occurred when cabin absolute
124 pressure was reduced below that equivalent to 10,000 feet. Under the recodification program in 1965, part 25 replaced part 4b of the CAR and the 10,000 feet warning policy was carried over as an appropriate means of meeting the warning requirements in § 25.841.
(iii) As part of the First Biennial Airworthiness Review Program in 1975, Amendments 23-17 and 25-28, which changed parts 23 and 25 respectively, were issued and became effective February 1, 1977.
Amendment 25-28 transmitted a minor change to § 25.841 as follows: It changed "cabin absolute pressure is below that equivalent to 10,000 feet" to "cabin pressure altitude exceeds 10,000 feet." Amendment 23-17 brought into § 23.841 of part 23 a warning indication when the cabin pressure altitude of 10,000 feet mean sea level (MSL) is exceeded. The preamble for this change indicated this proposal was adopted because a large number of small airplanes had such a warning and many pilots had begun to rely on this warning.
(2) The purpose of the cabin pressure altitude warning requirement is to indicate a warning at the pilot station when the cabin pressure altitude is greater than 10,000 feet MSL. A possible hazardous condition could be when the airplane reaches the operating altitude, which is greater than 10,000 feet MSL, and a malfunction in the cabin pressurization system occurs. If there was no warning for cabin pressure altitude, the cabin pressure altitude could slowly increase undetected to the airplane altitude, and the crew and passengers could become unconscious due to hypoxia. The effects of hypoxia are usually encountered when the flight crew is exposed to altitudes above 10,000 feet during extended flights.
3. ACCEPTABLE MEANS OF COMPLIANCE Warnings and Cautions. Section 23.1322 provides specific requirements for the assignment of red and amber for visual indications. Specifically, for abnormal operational or airplane systems conditions, a "caution" should be generated for crew awareness and subsequent crew action may be required; the associated color is amber. Under emergency operational or airplane systems conditions, a “warning” should be generated for immediate crew recognition and when corrective or compensatory action may be required; the associated color is red. If the cabin pressure altitude warning is a visual indicator, it should be red to indicate a hazard.
23.843 Pressurization tests Original Issue and Subsequent This rule applies to all doors. This includes doors that open outward, doors that open inward, and emergency exits.
The 1.5 safety factor in § 23.303 does not apply when executing the pressurization tests in this section. Paragraph (a) of the rule specifies the pressure differential of § 23.365(d), which is 1.33 times the maximum relief valve setting. Paragraph (b) requires functional testing to verify operation so there cannot be any gross plastic deformation from the 1.33 factor.
FIRE PROTECTION
23.851 Fire extinguishers Amendment 23-34 and Subsequent See AC 20-42C, Hand Fire Extinguishers for Use in Aircraft, for guidance.
23.853 Passenger and crew compartment interiors Original Issue and Subsequent See AC 23-2, Flammability Tests.
The purpose of this policy is for standardization in the approval of flammability testing of materials used in small airplanes per 14 CFR part 23, §§ 23.853, 23.855 and 23.1359.
The Small Airplane Directorate policy for all certification projects, including engineering assistance for major alterations on FAA Form 337 Field Approvals, is that:
a. The only materials for which flammability testing is not required are those that would not contribute significantly to the propagation of a fire per 14 CFR part 23, Section
23.853. These materials/parts are typically small parts. Material certification by an applicant or their supplier can be used in the determination that the material will not significantly contribute to the propagation of the fire. Company/Supplier material certifications cannot be used in lieu of official FAA flammability testing.
b. All other proposed materials must be tested to the flammability level required by the certification basis and category of airplane. Advisory Circular (AC) 23-2, Flammability Tests, gives guidance on conducting flash-resistant, flame-resistant, fire-resistant, fireproof and self-extinguishing tests. The AC specifies that Appendix F of part 23 is to be used for proposed self-extinguishing materials. These are official FAA certification tests, which require witnessing by either an FAA certification engineer or an FAA Flammability Designated Engineering Representative (DER) who has authorization to witness a test.
We will also accept from a DER submittal of an 8110-3 that attests to the validity of the data being approved. This can occur when the people running the test are well known to the DER and judged by the DER to be technically competent and reliable. Test data may only be approved by the FAA or by a DER with that approval authority.
Note: DOT/FAA/CT-89/15” Aircraft Material Fire Test Handbook” is good reference material.
c. A DER should not use FAA Form 8110-3 for flammability test results for a material when the testing is for quality assurance purposes for either a manufacturer or a repair station. Testing done for these purposes should be documented in a quality assurance report. When the material supports a certification project or an alteration or repair, FAA form 8110-3 is the DER’s only means of approving the technical data. The DER should determine if the testing documented in the test report adequately addresses the applicable airworthiness standards for the intended use of the material. If found
128 acceptable, the DER may generate an 8110-3 that references the test report. For the purposes of flammability testing, this technical data includes records of preconditioning of the test specimen. The flammability testing required by the certification basis is as follows:
Passenger and Crew Compartment Interiors, § 23.853 Certification Basis Category Material Flammability Testing CAR 3 NormalFlame resistant if smoking allowed (1945-1946) Restricted Effective Experimental November 13, 1945 CAR 3 Normal, Flash resistant (1946-1949) Utility & Effective Acrobatic December 15, 1946 CAR 3 Normal, Flash resistant or flame resistant if (1949-1965) Utility & smoking is allowed in a specific Effective Acrobatic compartment November 1, 1949 Part 23 Normal, Flame resistant Through Amdt. 23-22Utility & (1965-1978) Acrobatic Effective December 5, 1978 Part 23 (78-present) Normal, Flame resistant or self-extinguishing Amdt. 23-23 & Subs Utility & per Appendix F for materials located & Subsequent Acrobatic on the cabin side of the firewall (1978-present) Effective December 1, 1978 Part 23 Commuter Self-extinguishing per Appendix F Amdt. 23-34 except for small parts in & Subsequent § 23.853(d)(3)(v) (1987-present) Effective February 17, 1987
129 Cargo and Baggage Compartment Fire Protection, § 23.855 Part 23 Normal, Meet provisions of § 23.853(d)(3) Amdt. 23-49 Utility & (1996-present) Acrobatic Effective March 11, 1996 Part 23 Commuter Meet provisions of § 23.853(d)(3) Amdt. 23-49 (1996-present) Effective March 11, 1996 Electrical System Fire Protection, § 23.1359 Part 23 (96-present) Normal, Meet provisions of § 23.863 and Amdt. 23-49 Utility, § 23.1182 and wire insulation self Effective Acrobatic & extinguishing per Appendix F, March 11, 1996 Commuter 60 degree angle test
1. Per AC 23-2, cloth, wire and sheet specimens may be taken from a sample segment (batch/roll/sheet). In this case, FAA conformed test specimens/parts per 14 CFR part 21, § 21.33, are not required. Instead, conformity can be established for these types of materials on the basis of bill of materials, roll identification, etc. The 8110-3 should state it applies to the specific batch/roll/sheet for which the test was conducted.
Note: Wire specified in AC 43-13-1B, Section 7, has been determined to be acceptable for use in certified airplanes and may be used without flammability testing.
2. In other cases, test specimens must be fabricated to accurately represent the production assembly or must be cut from actual parts. These parts should be conformed per 14 CFR part 21, § 21.33, prior to testing.
3. In both 1 and 2 above, a DER must comply with Order 8110.37C, Designated Engineering Representative Guidance Handbook. While we understand that in the past some DER’s have not always submitted 8110-3 forms with Make and Model information, 8110-3 forms must always be complete, including the Make and Model information.
d. Flammability requirements have not been applied to conventional aircraft structure.
However, the use of composite structure can result in a need to test a representative
130 build-up panel with an interior material, adhesive and composite structure, unless it is demonstrated the interior material does not permit an ignition source to penetrate it.
e. Interior flammability tests may be required with build-up samples. Experience has shown that the thin exposed layer can burn away and expose the adhesive layer, which in many cases is extremely flammable and would contribute significantly to the propagation of a fire. Testing only the exposed layer without the adhesive backing would not be representative. Adhesives with a flame-retardant additive should be encouraged and listed; known flammable adhesives should not be used.
f. We have reviewed the test criteria of part 25, Appendix F, part I and have determined that parts/materials tested to the part 25 test criteria are acceptable data to show compliance with the flame-resistant material requirement of § 23.853(a).
23.855 Cargo and baggage compartment fire protection See guidance for § 23.853.
All-Cargo or Combination Passenger/Cargo Operations Special conditions will be required for airplanes approved for all-cargo or combination passenger/cargo operations because 14 CFR part 23 does not have applicable airworthiness standards for these operations.
23.859 Combustion heater fire protection No policy available as of June 30, 2001.