rigid or flexible hydraulic lines that have become unserviceable. There is an FAA Technical Standard Order (TSO) which provides minimum airworthiness requirements for hydraulic hose assemblies intended for use on transport category airplanes. Technical Standard Order TSO- C75, Hydraulic Hose Assemblies, and the accompanying Federal Aviation Standard should be considered. It should be noted that while the TSO provides an authorization for manufacturing a part that meets certain standards, it does not automatically approve the part for a specific installation.
(b) Section 43.13(a) requires that repairs must be accomplished in accordance with the manufacturers' maintenance manuals. However, air carriers are supposed to follow their Air Carrier Manual approved under part 121.
(c) The only mechanism available for certification of these repair systems by a manufacturer other than the type certificate holder would be a Supplemental Type Certificate (STC). An STC, issued for each installation on every model of airplane on which the applicant desired approval, would be a daunting task. Major airframe manufacturers, because they have all the pertinent data regarding design parameters for the hydraulic systems on their airplanes, can issue appropriate instructions in their maintenance manuals for installation of temporary repairs.
Airlines should use the manufacturer's manuals per § 43.13(a).
(d) One of the ways a manufacturer, other than the type certificate holder who wishes to manufacture parts for a certificated airplane, can apply for a Parts Manufacturing Authorization (PMA) is to show that the parts it makes are identical to those produced by the original manufacturer. Unless the applicant has original drawings complete with all necessary standards and specifications, it is difficult to show identicality. The cognizant FAA Manufacturing Inspection District Office can provide insight into the appropriate PMA application procedures, reference Order 8110.42A.
(3) Use of Hydraulic Lines for Temporary Repairs. The following was extracted from an FAA memorandum dated January 25, 1988, which addresses the use of flexible hydraulic hoses as temporary repairs for unserviceable hydraulic tubing.
(a) The use of flexible hydraulic hoses as temporary repairs for unserviceable hydraulic tubing has been commonplace on transport category airplanes for a number of years.
The lack of adverse service history indicates that this practice can be a viable method of repair, when used with the proper cautions. Strictly speaking, such substitution of hoses for rigid tubing would cause the airplane to be not in compliance with its type design; however, the provisions of Part 43 allow repairs and alterations to be made, provided the work is done in accordance with the methods, techniques, and practices in the current manufacturer's maintenance manual. With respect to multiple repairs, the FAA has concluded that they would be acceptable, provided that each one is individually performed in accordance with an approved procedure, and, collectively, the repairs will not interfere with each other or with any structure, or with the operation or performance of any system on the airplane.
(b) The key issue is the length of time that these temporary repairs may be allowed to exist on the airplane. Only by limiting the exposure can the repair be considered to be a safe alternative to the original design, much in the same manner that the Minimum Equipment List allows operation of the airplane for a short time with certain items of equipment inoperative.
Since a temporary repair would not have been evaluated and certificated in the same manner as the original system, it would not be proper to allow the repair to become, in any way, a permanent part of the type design. It has been suggested by one manufacturer that these repairs be replaced at the next "C" check, which in some cases may be a year in the future. The FAA has concluded that this much time is unwarranted, and may seriously compromise the safety of the repair. An interval of 300 to 350 hours would, in most cases, allow for the ordering and delivery of any needed parts and the scheduling of the airplane for restoration of the system to the original configuration. Also, limiting the time in this manner will reduce the problem of multiple repairs on the system. Any time limit would be largely arbitrary, but this figure, which represents approximately a month of operations, appears to be a good compromise between the economic interests of the operators, and the need for a short exposure to the temporary repair.
(4) Hydraulic System Certification Philosophy. The following was extracted from an FAA letter dated March 5, 1982, which addresses guidance concerning compliance with § 25.1435(b)(2), 14 CFR.
(a) The FAA has not been requested to accept endurance and reliability data based solely on flight test. This is because it is impractical, within a given certification time frame, to conduct the number of test cycles required to comply with § 25.1435(b)(2). The FAA would have no objection to using flight test data in addition to simulated functional endurance and reliability data, provided the manufacturer establishes that the flights are representative of expected service cycles.
(b) It is normal practice, for certification purposes, to comply with the requirements of § 25.1435(b)(2) by the use of simulators such as an "iron bird." The simulators are intended to be representative of the aircraft systems. Mounting brackets, hydraulic and electrical system flight controls, and control surfaces are all the same as the actual airplane. For practical purposes, simulation may be broken into functional subsystems, i.e., wing, rudder, gear, etc. Deviations from actual simulation have been allowed where the deviation will not interfere with the intent of establishing endurance and reliability. Deviation cases have to be ruled on in a case by case basis.
(c) The FAA would not accept compliance with § 25.1435(b)(2) based on only vendor component tests and flight tests. Functional tests and endurance tests, either using an airplane or mock-up (iron bird, simulator) at the system or subsystem level, are required.
Functional tests are required prior to first flight test.
(d) The manufacturer will develop load data through analysis, wind tunnel data, flight test experience, and/or any combination of these (spoilers and slots require flight test data, § 25.459). This load data is submitted to the FAA in the form of a loads document. The loads document is usually approved by FAA designated engineering representatives (DER) and 142
submitted to the FAA for a spot check review when and where it is considered necessary. Flight test load verification is not required when the methods used in determining those loading conditions are shown to be reliable (§ 25.301(b)). Most manufacturers go beyond FAA requirements and check the loads with strain gauges during flight tests.
(e) The airframe manufacturer (type design applicant) identifies the loading spectrum (amplitude and cycles) for the hydraulic systems based on the loads document and expected service. The FAA will be involved only indirectly in most cases, through the loads DER’s.
(f) The manufacturer subjects components and systems or the aircraft to the selected load/cycle spectrum in demonstrating compliance with the endurance requirements of § 25.1435(b)(2). Normally, vendor testing will be used for verification of the components for endurance as well as other requirements, such as environmental qualifications, fatigue, etc. The airframe manufacturer's simulator (iron bird) or simulators will be used for endurance tests and functional tests. The qualification reports documenting these tests are reviewed by FAA DER’s and approved or submitted to the FAA with a recommendation for approval. This option is designated by the FAA.
(5) Interpretation of § 25.1435(a)(4) of 14 CFR. The following was extracted from an FAA memorandum dated September 14, 1978, which addresses interpretation of § 25.1435(a)(4), Amendment 25-41.
(a) Section 25.1435(a)(4)(i) states: "There must be a means to keep hydraulic system pressures within +/- 10 percent of the pressure at the discharge of the pump outlet or the transient pressure dampening device, if provided."
(b) This rule was originally established to cover pump ripple pressure variations.
It can be seen that, under extreme system loading (high flow) conditions, the pump average discharge pressure can decrease as much as 25 percent. Apparently, the system pressure tolerance of +/- 10 percent mentioned above would then apply to this reduced pump average discharge pressure, decreased due to high flow conditions.
(c) Section 25.1435(a)(4)(ii) states: "Except as provided in Paragraph (a)(7) of this section, system pressures will not exceed 125 percent of the design operating pressure, excluding pressure at the pump outlet or dampening device. Design operating pressure is defined as the maximum steady operating pressure."
(d) The FAA has determined that Paragraph (4)(i) covers pressure variations due to pump ripple and high flow conditions while Paragraph (4)(ii) covers variations due to pressure transients. Note the use of "pump average discharge pressure" in (4)(i) and "design operating pressure" in (4)(ii).
e. References. The addresses for ordering the latest revision of advisory circulars, technical standard orders, and other referenced documents listed below can be found in the Appendix to this AC.
AC 25.1309-1A - System Design and Analysis.
AC 120-42A - Extended Range Operation with Two Engine Airplanes.
AC 20-128 - Design Considerations for Minimizing Hazards Caused by Uncontained Turbine Engine and Auxiliary Power Unit Rotor and Fan Blade Failures.
TSO-C47 - Pressure Instruments-Fuel, Oil, and Hydraulic.
TSO-C75 - Hydraulic Hose Assemblies.
SAE AS-595B - Civil Type Aircraft and Variable Delivery Hydraulic Pump.
SAE ARP-763 - Accumulators, Ground, Hydropneumatic Pressure.
SAE AIR-4150 - Inspection of In-Service Airborne Accumulators.
SAE AIR-1047C - A Guide for Selection of Quick Disconnect Couplings for Aerospace Fluid Systems.
SAE ARP-1709 - Coupling Assembly, Hydraulic Self Sealing, Quick Disconnect.
SAE AIR-786A - Elastomer Compatibility Considerations Relative to O-Ring and Sealant Selection.
SAE AS-1241B - Fire Resistant Phosphate Ester Hydraulic Fluid for Aircraft.
SAE AIR-1116 - Fluid Properties.
SAE ARP-1084 - Hydraulic External Leakage for In-service Components.
SAE AIR-1362 - Physical Properties of Hydraulic Fluids.
SAE ARP-1832 - Color Identification for O-Ring Seals.
SAE AIR-737E - Hydraulic and Pneumatic Specifications and Standards.
SAE ARP-994 - Design of Tubing Installations for Aerospace Fluid Power Systems.
SAE AIR-4003 - Turbine Engine Containment.
SAE AIR-1083B - Airborne Hydraulic and Control System Survivability for Military Aircraft.
SAE AIR-1899 - Aircraft Hydraulic System Characteristics.
SAE AIR-1918 - Comparison of Hydraulic System Cleanliness Procedures and Requirements for Ten Aerospace Companies.
SAE ARP-24B - Determination of Hydraulic Pressure Drop.
SAE ARP-490E - Electro-Hydraulic Servovalves.
SAE ARP-1280A - Aerospace Application Guide for Hydraulic Power Transfer Units.
SAE ARP-1281B - Actuators: Aircraft Flight Controls, Power Operated, Hydraulic, General Specification.
SAE ARP-4379 - Accumulator, Hydraulic, Cylindrical Aircraft.
SAE ARP-4752 - Aerospace - Design and Installation of Commercial Transport Aircraft Hydraulic Systems.
FAA Order 8000.40D - Maintenance of Pressure Cylinders in Use as Aircraft Equipment.
61. SECTION 25.1438 PRESSURIZATION AND PNEUMATIC SYSTEMS.
a. Rule Text.
(a) Pressurization system elements must be burst pressure tested to 2.0 times, and proof pressure tested to 1.5 times, the maximum normal operating pressure.
(b) Pneumatic system elements must be burst pressure tested to 3.0 times, and proof pressure tested to 1.5 times, the maximum normal operating pressure.
(c) An analysis, or a combination of analysis and test, may be substituted for any test required by paragraph (a) or (b) of this section if the Administrator finds it equivalent to the required test.
[Amdt. 25-41, 42 FR 36971, Jul. 18, 1977]
b. Intent of Rule. This rule provides standards for the tests that must be conducted to demonstrate that pneumatic and pressurization ducting and components will not fail in normal operation. The rule was originated to address the pneumatic system, from the engine bleed port(s) to the pressure regulating and/or shut-off valve, and the pressurization system, which comprises all ducting and components of the air distribution system downstream of the abovementioned valve.
c. Background. This subject was not addressed in either part 4b of the Civil Air Regulations (CAR) or part 25 of Title 14, Code of Federal Regulations (14 CFR) when it was originally codified. The reason for proposing the new standards was: "Components (such as ducts and couplings) of pressurization and pneumatic systems have failed at an unacceptable rate in service. The proposed standards for these components have been effective in preventing design deficiencies in the past."
(1) 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 may recommend a revised § 25.1438/JAR 25.1438 and JAR 25X1436, and may develop an accompanying new advisory circular.
d. Policy/Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of this rule and the following information.
(1) Pneumatic Versus Pressurization System Identification: The following policy was extracted from an FAA letter dated October 19, 1993, in response to questions regarding § 24.1438 and the difference between pressurization system elements and pneumatic system elements.
(a) Section 25.1438(a) states: "Pressurization system elements must be burst pressure tested to 2.0 times, and proof pressure tested to 1.5 times, the maximum normal operating pressure," and § 25.1438(b) states: "Pneumatic system elements must be burst pressure tested to 3.0 times, and proof pressure tested to 1.5 times, the maximum normal operating pressure."
(b) The FAA considers the pneumatic system to include all air supply elements from the bleed port on the engine to the pressure regulating and shutoff valve. Pressurization system elements are all other elements in the air distribution system.
(c) The burst pressure test requirement for pneumatic and pressure system elements has been used to demonstrate that high pressure air will be contained within the system or element. The element is not required to function during or after the burst pressure test, but pressurized air must be contained within the element or system. The proof pressure test is used to demonstrate that pneumatic and pressure system elements can function after a higher than normal pressure (1.5 times the maximum normal operating pressure) is introduced into the element or system.
(d) Section 25.1438(c) allows the applicant to use an analysis as a method of showing compliance to the burst and proof pressure test requirement in §§ 25.1438(a) & (b).
e. References. None.
62. SECTION 25.1439 PROTECTIVE BREATHING EQUIPMENT.
a. Rule Text.
(a) If there is a class A, B, or E cargo compartment, protective breathing equipment must be installed for the use of appropriate crewmembers. In addition, protective breathing equipment must be installed in each isolated separate compartment in the airplane, including upper and lower lobe galleys, in which crewmember occupancy is permitted during flight for the maximum number of crewmembers expected to be in the area during any operation.
(b) For protective breathing equipment required by paragraph (a) of this section or by any operating rule of this chapter, the following apply:
(1) The equipment must be designed to protect the flight crew from smoke, carbon dioxide, and other harmful gases while on flight deck duty and while combating fires in cargo compartments.
(2) The equipment must include-
(i) Masks covering the eyes, nose, and mouth; or
(ii) Masks covering the nose and mouth, plus accessory equipment to cover the eyes.
(3) The equipment, while in use, must allow the flight crew to use the radio equipment and to communicate with each other, while at their assigned duty stations.
(4) The part of the equipment protecting the eyes may not cause any appreciable adverse effect on vision and must allow corrective glasses to be worn.
(5) The equipment must supply protective oxygen of 15 minutes duration per crewmember at a pressure altitude of 8,000 feet with a respiratory minute volume of 30 liters per minute BTPD. If a demand oxygen system is used, a supply of 300 146
liters of free oxygen at 70 o F. and 760 mm. Hg. pressure is considered to be of 15-minute duration at the prescribed altitude and minute volume. If a continuous flow protective breathing system is used (including a mask with a standard rebreather bag) a flow rate of 60 liters per minute at 8,000 feet (45 liters per minute at sea level) and a supply of 600 liters of free oxygen at 70 o F. and 760 mm. Hg. pressure is considered to be of 15-minute duration at the prescribed altitude and minute volume. BTPD refers to body temperature conditions (that is, 37o C., at ambient pressure, dry).
(6) The equipment must meet the requirements of paragraphs (b) and (c) of § 25.1441.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-38, 41 FR 55468, Dec. 20, 1976]
b. Intent of Rule. The intent of this rule is for the protective breathing equipment (PBE) provisions to protect crewmembers from the effects of hazardous gasses and smoke, either while on flight deck duty or combating a fire, if there is a class A, B, or E cargo compartment or an isolated compartment in which crewmembers are permitted access during flight. This rule does not address passenger protective breathing equipment. Section 25.1439(b) contains wording that requires PBE if required "by any operating rule of this chapter. . . " There are two technical standard orders (TSO) providing standards for approval of PBE. Technical Standard Order, TSO-C99, provides standards for use by flight crewmembers while on flight deck duty. Per the TSO, the PBE must provide eye protection in addition to preventing inspiration of smoke/fumes.
The flightcrew supplemental oxygen system can provide this via a full face mask with a setting that provides a positive pressure differential between the mask and ambient pressure in the cockpit to prevent smoke/fumes from entering the mask. If the crew supplemental oxygen system is equipped with an oro-nasal type mask (covers nose and mouth only) rather than a full mask, the eye protection can be achieved via a pair of goggles located within easy reach. This type of equipment is intended for use by flight crewmembers at their stations for a prolonged length of time. It would not generally be acceptable for firefighting since the crewmember's ability to reach the fire would be limited by the amount of extra oxygen tubing provided by the stationary system. TSO-C116 provides standards for PBE to be used by crewmembers while locating and fighting a fire. Per the TSO, it also provides head and shoulder protection from "drippings," and typically provides protective oxygen for 15 minutes. This type of PBE would not be suitable for use by the flightcrew at their stations.
c. Background. Effective February 1, 1965, part 25 was added to Title 14, Code of Federal Regulations (14 CFR) to replace part 4b of the Civil Air Regulations (CAR). Many of the requirements in section 4b.651 of the CAR were carried over essentially unchanged to § 25.1439 of 14 CFR. Part 25 standards state that PBE must be installed “if there is a class A, B, or E cargo compartment.”
(1) Amendment 25-38 (December 20, 1976) added the requirement that PBE be installed in each isolated separate compartment, including upper and lower lobe galleys, in which crewmember occupancy is permitted during flight for the maximum number of crewmembers expected to be in the area.
d. Policy/Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of this rule and the following information.
(1) Protective Breathing Equipment Usage When Approved Under TSO-C99 and TSO-C116. The following policy, extracted from an FAA letter dated May 13, 1993, clarifies compliance with the requirements of § 25.1439 using equipment of the "smoke hood" variety approved under TSO-C116.
(a) The FAA letter clarified whether smoke hood type PBE would be acceptable for meeting the requirements of § 25.1439. The FAA is not aware of any smoke hood type PBE being approved as meeting the requirements of § 25.1439(b)(3), which relates to the use of radio equipment by the flight crew. The FAA would consider a smoke hood type PBE if compliance with § 25.1439(b)(3) is demonstrated.
(b) The FAA letter also clarified that a finding of equivalent safety would be required if an applicant wishes to use a “breathable gas,” in lieu of providing oxygen as specified in § 25.1439(b)(5).
(c) The FAA letter also addressed § 25.1439(b)(6), which specifies that there must be a means provided to allow the crew to readily determine the quantity of oxygen available.
PBE approved in accordance with TSO-C116 may or may not have such features. The FAA does not grant exemptions for the cases where the PBE is used by the flight crewmembers while on flight deck duty. When the PBE is intended for use in locating and combating a fire, the FAA will approve equipment which, when the container is intact, is considered to have a full charge of oxygen or, for chemical oxygen generation, the canister has not been activated. The container is "tamper evident" and the crewmember can readily determine that the equipment is fully charged and ready for use. This PBE would not be suitable for approval in meeting the flight crewmember requirements addressed by § 25.1439.
(d) The letter provided an FAA position on PBE requirements for the first and second observers. The FAA treats the first observer and second observer differently. The first observer must have the same protection, i.e., the same equipment, as the flight crewmembers.
The second observer may be supplied with oxygen equipment suitable for passengers. Again, the first observer must have the same protection (including the ability to communicate on the crewmember interphone) that is provided for the flight crewmembers; the second observer may be provided with passenger-type oxygen equipment.
(2) Protective Breathing Equipment Serviceability. The following policy was extracted from an FAA memorandum dated March 9, 1990, to provide guidance regarding the approval of PBE which met the requirements of § 121.337 and FAA Action Notice 8150.2.
(a) Section 121.337 was revised in 1987 and, in part, incorporated the intent of § 25.1439 and expanded on those requirements. This revision resulted in a new type of PBE for 148
use by any crewmember while combating fires on board an airplane. The approval basis was contained in Action Notice A8150.2, dated September 1, 1987, in advance of TSO-C116.
(b) Section 121.337 clearly intends that any crewmember PBE be immediately ready to perform its intended function during an emergency. In fact, the rule requires that each certificate holder's operations manual designate at least one crewmember to check that each PBE unit is properly stowed and serviceable prior to the first-flight of the day.
(c) The approval basis (TSO-C116) for crewmember PBE requires that the equipment has a means to indicate the serviceability of the unit in its stowed condition. Some manufacturers of the crewmember PBE designs approved by the FAA have elected to use a vacuum sealed envelope as a means of protecting the PBE, and the loss of vacuum has been accepted as an indicator that the PBE is not serviceable.
(d) One manufacturer recommended that when the vacuum is lost the PBE need not be replaced immediately, since ambient moisture will not deactivate the potassium superoxide in the atmosphere regenerating system for a number of days. This may well be correct, but it is only one consideration. Since the cause of the loss of vacuum will most likely not be known to the crewmember making the inspection, and the extent of damage to the PBE itself may not be evident, the PBE should be considered not serviceable. Another consideration is that a design, which includes a protective envelope material that can readily be damaged, may not perform its intended function.
(e) Based on the above information, ACO's should ensure that the manufacturer submits operating, installation, and maintenance instructions with limitations, warnings, or cautions for FAA approval. The information being provided the end user should be in accord with the safety intent of § 121.337.
(3) Protective Breathing Equipment on the Flight Deck. The following policy was extracted from an FAA memorandum dated December 19, 1988, and provided additional guidance on protective breathing equipment for use on the flight deck.
(a) Section 121.337 specifies provisions for protective breathing equipment with the airplane both pressurized and unpressurized. Section 25.1439 makes no such distinction.
The part 25 rule specifies that protective breathing equipment must be installed as part of the type design if certain types of cargo compartments or isolated, occupied compartments are installed. If these specified compartments are not installed on the airplane, then protective breathing equipment is not required as part of the type design.
(b) For an unpressurized part 25 airplane certified without these specified compartments and operated under Part 121, however, the decision to require protective breathing equipment should be based on an evaluation of the hazard. Section 25.831(d) states that if accumulation of hazardous quantities of smoke in the cockpit area is reasonably probable, smoke evacuation must be readily accomplished, starting with full pressurization and without depressurizing beyond safe limits. This capability must be demonstrated for certification under 149
any normal operating condition, including unpressurized flight; and smoke clearance must be accomplished within three minutes (Advisory Circular 25-9A, or latest revision, explains a test procedure in detail). Section 25.831 also specifies limits for carbon dioxide and carbon monoxide, but does not define unsafe levels of any other gas, vapor, or fume. Since the toxicity of smoke or fumes generated by a fire is unknown, the FAA has required that there be essentially no penetration of smoke into an occupied compartment. Since it is likely that smoke of unknown composition would be generated within the cockpit, and may be present for up to three minutes prior to its removal, installation of protective breathing equipment would be appropriate, unless the applicant can show that it is not necessary.
(4) Definition of Terms Used in § 25.1439. The following policy was extracted from an FAA memorandum dated February 26, 1985, that responded to a request for the meaning of the words "isolated separate compartment," as used in § 25.1439.
(a) The preamble to the Notice 75-10, item 2-91, read: "The proposal would require protective breathing equipment for crewmembers expected in isolated areas." Item 2-91 was based on the First Biennial Airworthiness Review of 1974-1975, proposal No. 812 that read:
"(a) Protective breathing equipment must be installed for each required crewmember in isolated separate compartments, such as upper or lower lobe galleys, in which occupancy is permitted during flight."
(b) Based on the intent of the background quoted above, the FAA concluded that the crew rest area, not being part of the main cabin, is similar in location to a lower lobe galley and, therefore, is an isolated separate compartment. However, as crewmembers are not required in the crew rest area except to fight a fire, only sufficient protective breathing equipment need be provided in the crew rest area for that function. It is also noted that, as long as the lavatory is located next to the flight deck door, there is little chance that a crewmember may be isolated there by a fire. The crew lavatory need not be considered isolated.
e. References. The address for ordering the latest revision of the technical standard orders listed below can be found in the Appendix to this AC.
TSO-C99, Protective Breathing Equipment.
TSO-C116, Crewmember Protective Breathing Equipment.
63. SECTION 25.1441 - OXYGEN EQUIPMENT AND SUPPLY.
a. Rule Text.
(a) If certification with supplemental oxygen equipment is requested, the equipment must meet the requirements of this section and 25.1443 through
25.1453.
(b) The oxygen system must be free from hazards in itself, in its method of operation, and in its effect upon other components.
(c) There must be a means to allow the crew to readily determine, during flight, the quantity of oxygen available in each source of supply.
(d) The oxygen flow rate and the oxygen equipment for airplanes for which certification for operation above 40,000 feet is requested must be approved.
b. Intent of Rule. This rule is intended to ensure that, if supplemental oxygen equipment is to be included in the type design of an airplane, the oxygen dispensing equipment will protect passengers and crewmembers from the effects of hypoxia. Many of the oxygen system requirements are addressed in §§ 25.1443 through 25.1453, which are incorporated by reference.
c. Background. Effective February 1, 1965, part 25 was added to Title 14, Code of Federal Regulations (14 CFR) to replace part 4b of the Civil Air Regulations (CAR). The requirements in section 4b.651(a) of the CAR for oxygen systems were carried over essentially unchanged to § 25.1441 of 14 CFR.
d. Policy/Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of this rule and the following information.
(1) Compliance with §§ 25.1309(c) and 25.1441(c). The following policy was extracted from an FAA memorandum dated February 23, 1994, and provides an FAA position on the requirements of § 25.1309(c) as related to a Passenger Gaseous Oxygen System that could erroneously indicate oxygen is available.
(a) Section 25.1309(c) states: "Warning information must be provided to alert the crew to unsafe system operating conditions, and to enable them to take appropriate corrective action. Systems, controls, and associated monitoring and warning means must be designed to minimize crew errors which could create additional hazards." Section 25.1441(c) states: "There must be a means to allow the crew to readily determine, during flight, the quantity of oxygen 151
available in each source of supply." An applicant proposed a passenger oxygen system where the means is provided through an indication available to the flight crew of the oxygen pressure in the oxygen system plumbing between the oxygen bottles and the flow control valves. There are several bottles, each with its own shut-off valve, manifolded together. If, during maintenance, the valves on all the bottles are turned off and left in that position, but the tubing between the bottles and the flow control valve is not disturbed, the pressure in the lines will continue to read the same value that would be measured if the bottle valves were open. Therefore, the reading in the cockpit will erroneously indicate that oxygen is available. A concern arises because the flightcrew will dispatch believing that passenger oxygen is available. If a depressurization event occurs, the passengers will not have supplemental oxygen available as required by the regulations. Passengers will be at risk during the emergency descent due to lack of available oxygen.
(b) The non-availability of passenger supplemental oxygen presents a hazard if a depressurization occurs. In most cases of depressurization, the flightcrew executes an emergency descent and the cabin altitude does not exceed safe limits. This is not, of course, always the case; loss of consciousness or more serious injury, and even death, could occur for some passengers if oxygen is not available after depressurization. There is usually no risk to the airplane itself because the flightcrew has a separate oxygen system and, assuming they act properly, the airplane will quickly descend to a safe altitude and proceed to the nearest airport.
For these reasons, passenger supplemental oxygen being unavailable is not an immediate threat to continued safe flight and landing. However, one of the design principles suggested in Advisory Circular (AC) 25.1309-1A for ensuring fail-safe design concepts is error tolerance to allow for possible adverse effects of foreseeable errors during the airplane's design, test, manufacture, operation, and maintenance. This system does not appear to adhere to that principle. In addition, the AC also notes that a warning is required for a failure which makes it necessary for the flightcrew to make an unscheduled landing to reduce exposure to a more hazardous failure condition that would result from subsequent failures or operational conditions, or if the failure must be corrected before a subsequent flight. In conclusion, a warning to the flightcrew for non-availability of passenger oxygen is the type of warning addressed in the first sentence of § 25.1309(c).
(c) The system installed to provide the crew with an indication of the amount of oxygen available does not appear to meet the requirement contained in the second sentence of § 25.1309(c). The false indication of oxygen pressure resulting from the oxygen trapped between the shut off valves and the flow control valves will continue indefinitely, leading to dispatch after dispatch with no means to detect the problem until either a depressurization occurs or some maintenance action leads to the discovery that the oxygen bottle valves are closed. If the crew were aware of the situation, they would not dispatch or, if the problem were discovered in flight, they would immediately divert to an alternate airport. Clearly, the indication system is not designed to minimize crew errors that could create additional hazards.
(d) It is not acceptable to use probability of the event (depressurization) to demonstrate the probability of the unsafe condition (decompression combined with passenger oxygen being unavailable). In this case, the unsafe condition is not catastrophic so it is not 152