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
necessary to show that it is extremely improbable. As noted in the earlier advisory material regarding System Design Analysis (AC 25.1309-1), "If a quantitative analysis is used to help show compliance with Federal Aviation Regulations for equipment which is installed and required only for a specific operating condition for which the airplane is thereby approved, credit may not be taken for the fact that the operating condition does not always exist." While this note does not appear in AC 25.1309-1A, the basic philosophy has not changed.
(2) Guidance, Availability of Flightcrew Oxygen. The following policy was extracted from an FAA memorandum dated May 11, 1992, in response to an FAA Safety Recommendation regarding annunciation that oxygen is available for use by the flightcrew when in actuality there is no oxygen available.
(a) On a recent twin-engine transport airplane flight, the crew elected to divert after the first officer donned his oxygen mask and discovered he had no oxygen available. After landing, the crew oxygen bottle valve was found turned to the closed position. Prior to the flight, maintenance had started to replace the bottle due to low pressure, and the valve located at the bottle was turned off. When the bottle pressure was found to be acceptable, the bottle was reinstalled but the valve was left turned off and safety wired in that position. The maintenance crew checked the bottle pressure on the airplane Engine Indication and Crew Alerting System (EICAS) status page and because the EICAS displays the pressure in the line downstream of the valve, the residual pressure in the line resulted in a displayed pressure that was within limits.
(b) The existing system on this airplane measures the oxygen pressure in the line to the flightcrew regulator. If the shutoff valve at the pressure bottle is shut off, there is sufficient pressure remaining in the line to indicate a pressure high enough for dispatch. Even when the crew checks the masks, only a small amount of oxygen is allowed to escape, and there are currently no means to determine that the valve is off. Further, the oxygen pressure, and therefore the quantity, is observable on EICAS only when the crew selects the status page.
(c) The FAA determined that this is unacceptable for the following reasons:
1 . Section 25.1441(c) requires that the crew be able to determine the quantity of oxygen available. When the valve is in the "OFF" position, the oxygen is not available, but the indication on EICAS (the residual pressure in the line) is that oxygen is available if needed.
The pressure measurement gives information about quantity only when the valve is open. If the valve is inadvertently left closed, the information provided is misleading.
2 . 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.” It is clear that the existing design gives misleading information to the flightcrew, which could lead them into an unsafe operation. If depressurization takes place at a significant altitude, and oxygen is not available to the crew, an unsafe condition exists.
3 . Further, § 121.333(c)(4) states: "Before the takeoff of a flight, each flight crewmember shall personally preflight his oxygen equipment to insure that the oxygen mask is functioning, fitted properly, and connected to appropriate supply terminals, and that the oxygen supply and pressure are adequate for use." In theory, adherence to this rule should ensure that the oxygen supply is available.
(d) In order for the scenario discussed above to be a hazard, four separate events would have to occur:
1 There would have to be maintenance performed on the airplane oxygen system that required the valve, located on the oxygen bottle, to be turned off for some reason.
The airplane maintenance manual contains both visual and operational checks to ensure that the oxygen valve is open and the pressure is adequate. These procedures were not followed in the case of interest because the bottle was never removed.
2 The flightcrew check of their oxygen equipment would have to fail to indicate that oxygen is not available. This is possible with the present preflight procedure, but the procedure is being changed.
3 A decompression would have to occur that required the use of oxygen by the flightcrew.
4 Event 3 would have to occur before the flightcrew had attempted to use supplemental oxygen (which would demonstrate no oxygen available) or checked the EICAS Status Page, which would indicate low oxygen pressure. In either case, airline procedures and Federal Aviation Regulations require that the crew divert to fix the problem.
(e) The manufacturer of the airplane involved in the incident which prompted this memorandum is changing their operational procedures, which are used by the airlines for their Operations Manuals, to describe an acceptable procedure to verify that adequate oxygen is available at the pilots mask. This procedure involves two separate flow tests (Normal and 100 percent), followed by a pressure check on EICAS. As all flightcrew masks must be tested (at least two on any transport category airplane), there will be ample tests and pressure checks to identify a problem prior to dispatch. Based on the above considerations, the FAA has determined an adequate level of safety is achieved through the existing and proposed procedures with the certificated design. However, this mechanization is not considered to meet the requirements of § 25.1309(c).
(f) Flightcrew oxygen systems should be reviewed to ensure that on future certification programs, a system design that can lead to misinformation is not allowed.
(3) Oxygen Installation for Medical Use. The following policy was extracted from an FAA facsimile message dated January 3, 1991, that was written in response to a request for policy relative to the installation of shutoff valves, pressure relief devices and overboard vent lines installed on oxygen bottles for medical use.
(a) There is no current written FAA policy relative to the installation of manifolded oxygen systems other than Civil Aeronautics Manual (CAM) 4b.651-1. CAM 4b.651-1 recommends that low pressure oxygen systems have a pressure relief device to prevent over pressurization during the filling operation.
(b) Department of Transportation (DOT) regulation (49 CFR, § 173.34(d)) requires that all pressure cylinders be provided with a safety device (a rupture disc) to prevent explosion of the bottle if the bottle is subjected to over-pressure or excessive heat. The pressure relief device may be installed on the bottle or on the valve of the bottle (on the pressure side of the valve).
(c) To meet the requirements of § 25.1441(b) and § 1451(c) (note: Amendment 25-72 deleted § 25.1451 and moved the requirements to § 25.869(c)), aircraft manufacturers have been installing oxygen systems that have a pressure relief that vents through a vent line, normally stainless steel, to the outside of the aircraft. A colored blowout plug is installed at the end of the vent line on the surface of the aircraft to indicate if the rupture disc has blown.
(d) The inquiring FAA office indicated that they may have approved single high pressure bottle (air ambulance) installations that do not have the overboard drain line. Further, these single bottle installations are filled off the airplane. There may be other installations without a vent line because there doesn't appear to be a written policy on this subject.
(e) To determine if a vent line is needed to comply with §§ 25.1441(b) and § 25.869(c)(3) (formerly 25.1451(c)), the consequences of the pressure relief device venting the total contents of the bottle or bottles (if manifolded together) into the compartment should be considered, i.e., overpressurizing the compartment; exposure to grease, flammable fluids and ignition sources.
(f) All the installations, which the FAA is aware of, have a high pressure shutoff valve located on the bottle to isolate the bottle from the oxygen system. On a manifolded system, these valves normally are lockwired in the open position. However, the bottle shutoff valve is downstream of the pressure relief device in order that the pressure relief device will protect the bottle from exploding if the bottle is isolated (valve shut) from the system and is exposed to heat or over- pressurization.
(g) The FAA is not aware of any objection to having a common high pressure manifold without the bottle shutoff valves as long as there is a manifold system shutoff valve and a pressure relief device between the bottles and the shutoff valve.
(h) One objection might be exposing the airplane and maintenance personnel to the hazards of a fully pressurized vented bottle or bottles if someone tries to remove a bottle from the system. The total system has to be depressurized before the bottle can be removed as opposed to the manifold system where each bottle shutoff valve can be closed, and an individual bottle with its valve can be removed.