6-9 tension force should be approximately as listed in Table 6-2, Approximate Main Ski Tension Forces.
Table 6-2. Approximate Main Ski Tension Forces Ski Limit Load Rating Downward Force (Pounds) 1500–3000 20–40 3000–5000 40–60 5000–7000 60–120 7000–9000 120–200 Note: Do not rely upon these tension force values for main ski installations on airplanes with spring steel or other flexible landing gear. Shock cord tensions great enough to require the downward forces listed in Table 6-2 to overcome them may produce excessive toe-in of the main skis on such airplanes. Variations in gear leg flexibility make it difficult to establish a generic table of tension forces appropriate for all airplanes with flexible landing gear.
6.6.3.2 The shock cord tension must also be sufficient to return the skis to the normal flight attitude from their maximum negative incidence at all airspeeds up to the airplane’s never-exceed speed (VNE) with skis installed. In the absence of more precise data, each shock cord must be able to produce a nose-up moment about the ski pedestal bearing centerline of M = (0.0000036)(W)(VNE)2 ft/lbs, when the ski is at its maximum negative incidence, where W is the maximum certificated gross weight of the airplane, and VNE is its never-exceed speed with skis installed.
6.6.4 Springs in Place of Shock Cords. If springs are used in place of shock cords to provide rigging tension, they must be able to withstand extreme cold and slight external scratching without premature fatigue failure and must not cause skis, rigging, or landing gear to experience flutter or objectionable vibration during airplane flight and dive tests.
6.6.5 Nose Ski Installation. Install the nose ski on an airplane with tricycle landing gear in the same manner as the main skis (see Figure 6-7, Typical Nose Ski Installation), except:
6.6.5.1 Adjust the length of the safety cable to provide ±5- to ±15-degree ski incidence.
6.6.5.2 Where it is possible for the nose ski rigging to contact the propeller tips due to vibration, install a 1/4-inch shock cord to hold the rigging out of the propeller arc.
6-10 Figure 6-7. Typical Nose Ski Installation
6.6.6 Tail Ski Installation.
6.6.6.1 When installing a tail ski on an airplane with conventional landing gear, use a tail ski that has been approved on an airplane of approximately the same weight and whose tail wheel bears approximately the same fraction of that weight when the airplane is in the three-point attitude (within 10 percent) or select the tail ski as outlined in paragraphs 6.2 and 6.4. Some types of tail ski require that the tail wheel be removed to install the rest on its upper surface ski.
6.6.6.2 Adjust the length of the limiting cable (see Figure 6-8, Typical Tail Ski Installation) to allow the ski to turn approximately 35 degrees to either side of the straight-forward position with the weight of the airplane resting on the ski.
6.6.6.3 The shock cord (see Figure 6-8) must be of a length that will hold the ski in the straight-forward position during flight.
6-11 Figure 6-8. Typical Tail Ski Installation
6.7 Documentation.
6.7.1 Airplane or Ski Manufacturer’s Data. Comply with the requirements for placards, markings, and manuals required to operate the airplane as a skiplane, as listed in the approved or accepted documents discussed in paragraph 6.4.1.
6.7.2 Performance Information. The following paragraphs contain the minimum additional performance data required for airplanes equipped with new or altered ski installations.
Refer to AC 43-210, Paragraph 4, Flight Manual Supplements, for additional guidance regarding approved Aircraft Flight Manual Supplements (AFMS).
6.7.2.1 For an airplane that requires an approved AFM, obtain FAA approval for an Aircraft Flight Manual Supplement (AFMS) that adds the following or similar information to the “Performance” section of the manual.
6.7.2.1.1 Takeoff. Under the most favorable conditions of smoothly packed snow at temperatures approximating 32 °F, the skiplane takeoff distance is approximately 10 percent greater than that shown for the landplane.
Note: In estimating takeoff distance for other conditions, caution should be exercised as lower temperatures or other snow conditions will usually increase these distances.
6.7.2.1.2 Landing. Under the most favorable conditions of smoothly packed snow at temperatures approximately 32 °F, the skiplane landing distance is approximately 20 percent greater than that shown for the landplane.
Note: In estimating landing distances for other conditions, caution should be exercised as other temperatures or other snow conditions may either decrease or increase these distances.
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6.7.2.1.3 Climb Performance. In cases where the landing gear is fixed (both landplane and skiplane), where climb requirements are not critical, and the climb reduction is small (30 to 50 feet per minute), the FAA will accept a statement of the approximate reduction in climb performance placed in the performance information section of the AFMS. For larger variations in climb performance, where the minimum requirements are critical or the landing gear of the landplane is retractable, appropriate climb data should be obtained to determine the changes, and new curves, tables, or a note should be incorporated into the AFMS.
6.7.2.2 For an airplane that does not require an AFM, make the information in paragraph 6.7.2.1 available to the pilot in the form of placards, markings, manuals, or any combination thereof. One type of acceptable manual is an approved supplementary AFM.
6.8 Flight and Handling Operational Checks. Accomplish an operational check in accordance with 14 CFR § 91.407(b) to determine the takeoff, landing, and ground handling characteristics. Ensure that the ski angles during tail high and tail low landings will not cause the skis to dig in or fail from localized stress. Verify that ground control is adequate to satisfactorily complete a landing run with a turnoff at slow speed. In flight, the skis must ride steady with check cables taut and must not produce excessive drag or unsatisfactory flight characteristics. Enter a notation of this check in the airplane records.
6.9 Maintenance (Including Inspection).
6.9.1 Inspection and Repair Data Sources. Contact the airplane and ski manufacturers for any specific inspection and maintenance instructions they may have developed. Refer to AC 43.13-1, Chapter 9, Aircraft Systems and Components, for more information.
6.9.2 Instructions for Continued Airworthiness (ICA). The modifier (developer of the ski installation or alteration) must provide instructions for future inspection, maintenance, and repair of the added or altered parts and is also responsible for assessing the need for any changes to the product-level ICA (changes that affect the airplane as a whole when the skis are installed). For simple airplane/ski combinations where skis of the same model have been approved on similar airplanes with the appropriate ICA, it may only be necessary to reference those ICA in the maintenance records of the newly altered airplane and/or in block 8 of FAA Form 337, Major Repair and Alteration (Airframe, Powerplant, Propeller, or Appliance), documenting the ski installation. For complex ski installations requiring special considerations, the modifier may need to develop new installation-specific ICA. In either case, the modifier must ensure that adequate and appropriate ICA is available to the skiplane owner or operator. Refer to AC 43-210, Paragraph 5, Maintenance Information, and Order 8110.54 for additional guidance regarding ICA.
6.9.3 Interchanging of Skis and Wheels. A person appropriately authorized by 14 CFR § 43.3 must perform a new W&B computation when the skis are initially installed. The FAA recommends that the airplane be weighed for this initial computation. After the initial
6-13 installation, removing the skis and reinstalling the wheels or vice versa is considered a preventive maintenance operation if it does not involve complex assembly operations or a new W&B computation. Refer to 14 CFR part 43 appendix A, paragraph (c)(18).
Note: During subsequent W&B changes to the airplane, be sure to update its W&B records and its equipment list to account for all approved ski, wheel, and float installations.
6.9.4 Periodic Inspection Required. Seasonally removed and installed equipment items, such as skis, should be inspected at installation to comply with 14 CFR §§ 91.407(a) and 91.409(a) and 14 CFR part 43 appendix D, paragraphs (e)(1) and (e)(10) if they were not installed on the airplane at the time of the last inspection. All available data described in this paragraph should be used during the inspection.
7-1 CHAPTER 7. OXYGEN SYSTEM INSTALLATIONS IN NONPRESSURIZED
AIRCRAFT Section 1. General
7.1 Purpose. This chapter provides data for acceptable means of gaseous oxygen system installations in nonpressurized aircraft. For other oxygen system installations (e.g., liquid oxygen), installers should contact the responsible Flight Standards District Office (FSDO) for assistance in applying for a Supplemental Type Certificate (STC).
7.2 Hazards and Warnings to Consider When Installing an Oxygen System.
7.2.1 Oxygen itself does not burn, but materials that burn in air will burn much hotter and more vigorously in an oxygen-rich environment.
7.2.2 Oil and grease burn with explosive violence in the presence of oxygen.
7.2.3 Rapid release of high-pressure oxygen in the presence of foreign particles can cause temperatures sufficient to ignite combustible materials and materials that would not normally burn in air.
7.2.4 Pressurized oxygen cylinder failures, particularly aluminum-lined composite cylinders, have the potential of producing violent explosions.
7.3 Additional References (current editions).
• Title 14 CFR parts 23, 43, and 91.
• Civil Air Regulation (CAR) 3, Airplane Airworthiness—Normal, Utility, Acrobatic, and Restricted Purpose Categories.
• CAR 6, Rotorcraft Airworthiness; Normal Category.
• AC 27-1, Certification of Normal Category Rotorcraft.
• AC 43.13-1, Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair.
• FAA Order 8300.16, Major Repair and Alteration Data Approval.
• Society of Automotive Engineers (SAE) (now SAE International) Aerospace Information Report (AIR) 825, Oxygen Equipment for Aircraft.
• SAE AIR 822, Oxygen Systems for General Aviation.
7-2 Section 2. Installation of the Oxygen System
7.4 System Requirements. Gaseous oxygen systems may be a higher-pressure system with oxygen stored at 1,850 pounds per square inch (psi) or a low-pressure system with the oxygen stored at 425 psi. All oxygen systems contain a storage tank, a regulation system, and a distribution system. The main difference in system type is in the regulation of the oxygen to the user (see paragraph 7.4.6 below).
7.4.1 Cylinders. Install oxygen cylinders conforming to Interstate Commerce Commission (ICC) requirements for gas cylinders that carry the ICC or Department of Transportation (DOT) 3A, 3AA, or 3HT designation followed by the service pressure metal-stamp on the cylinder.
7.4.2 Tubing/Lines.
7.4.2.1 In systems with low pressure, use seamless aluminum alloy or equivalent, with having an outside diameter of 5/16 inch and a wall thickness of
0.035 inch. Double flare the ends to attach to fittings.
7.4.2.2 In high-pressure systems (1,800 psi), use 3/16-inch outside diameter (O.D.),
0.035-inch wall thickness, seamless copper alloy tubing meeting Specification WWT-779 (as revised) type N, or stainless steel between the filler valve and the pressure-reducing valve. Depending on the process used, silver-solder cone nipples to the ends of the tubing in accordance with American Welding Society Specification C3.4, C3.5, C3.6, or C3.7.
7.4.2.3 Use 5/16-inch O.D. aluminum alloy tubing after the pressure-reducer (low-pressure side).
Note 1: Any lines that pass through potential fire zones should be stainless steel.
Note 2: If lines are located behind upholstery or are not 100 percent visible during normal operations, they should be solid metal lines or high-pressure flexible lines.
7.4.3 Fittings. All fittings must be manufactured from materials that are compatible for use with oxygen systems. Fittings should not be made of mild steel or materials that are prone to corrosion when in contact with another material.
7.4.3.1 High Pressure. Intercylinder connections are made with regular flared or flareless tube fittings made of stainless steel. Usually, fittings are of the same material as the lines. Mild steel or aluminum alloy fittings with stainless steel lines are discouraged. Titanium fittings should never be used because of a possible chemical reaction and resulting fire.
7.4.3.2 Low Pressure. Fittings for metallic low-pressure lines are flared or flareless, similar to high-pressure lines. Line assemblies should be terminated with
7-3 “B” nuts in a similar manner to a manufactured terminating connection.
Universal adapters (AN-807) or friction nipples used in conjunction with hose clamps are not acceptable for use in pressurized oxygen systems.
7.4.4 Valves. Each system must contain a slow opening or closing shutoff valve that is accessible to a flight crewmember to turn on and shut off the oxygen supply at the highpressure source.
7.4.5 Regulators. The cylinder or system pressure is reduced to the individual cabin outlets by means of a pressure-reducing regulator that can be manually or automatically controlled.
The regulator should be mounted as close as possible to the cylinders and certificated for aviation environment.
7.4.6 Types of Regulators. The four basic types of oxygen systems, classified according to the type of regulator employed, are:
7.4.6.1 Continuous-Flow Type. The constant flow type provides the same output pressure or flow regardless of altitude. Continuous-flow oxygen systems provide protection for passengers up to 25,000 feet mean sea level (MSL) and a continuous flow of 100 percent oxygen to the user. They may be automatic or manual in their operation.
7.4.6.2 Diluter-Demand Type. This type dilutes pure oxygen with ambient air and maintains the proper portion of oxygen in the breathing gas depending on altitude. Oxygen concentration is automatically diluted proportionate to the specific altitude’s predetermined human oxygen consumption requirement.
Such systems only supply oxygen, mixed with cabin air, during inhalation.
7.4.6.3 Demand Type. This type uses high-pressure compressed oxygen that feeds a supply of oxygen to a “high-to-low” pressure regulator at the individual crew station. The regulator, after reducing the higher pressure, automatically cycles low-pressure breathing oxygen to the wearer only on demand. Title 14 CFR § 23.1441 requires aircraft that are certified to operate above 25,000 feet to be equipped with a demand system to supply required crewmembers.
7.4.6.4 Pressure-Demand Type. This type is not likely to be included on small general-aviation models. Aircraft certified to exceed 40,000 feet must have a pressure-demand system, which delivers pressurized oxygen to pilots, such as a high-altitude oxygen breathing system (above 40,000 feet) using anthropomorphic facial measurements of aircrew to produce a mask that would satisfactorily contain the pressure required to allow breathing under pressure while maintaining an airtight face-seal and also remaining relatively comfortable to the wearer.
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7.4.7 Flow Indicators.
7.4.7.1 A pith-ball flow indicator, vane, wheel anemometer, or lateral pressure indicator that fluctuates with changes in flow, or any other satisfactory flow indicator may be used in a continuous-flow type system.
7.4.7.2 An Air Force/Navy (AN) flow indicator or equivalent may be used in a diluter-demand type system. Each flow indicator should give a positive indication when oxygen flow is occurring.
7.4.8 Relief Valve.
7.4.8.1 A relief valve is installed in low-pressure oxygen systems to safely relieve excessive pressure, such as that caused by overcharging.
7.4.8.2 A relief valve is installed in high-pressure oxygen systems to safely relieve excessive pressure, such as that caused by heating.
7.4.9 Gauge. Provide a pressure gauge to show the amount of oxygen in the cylinder during flight.
7.4.10 Masks. Only approved masks designed for the particular system should be used.
7.5 Installation and Design Considerations. Oxygen systems present a hazard. Therefore, follow the precautions and practices listed below.
7.5.1 Before working with oxygen equipment, remove oil, grease (including lip salves, hair oil, etc.), and dirt from hands, clothing, and tools.
7.5.2 Prior to cutting the upholstery, check the intended route of the system.
CAUTION: Ensure all system components are kept completely free of oil or grease during installation and locate components so they will not contact or become contaminated by oil or oil lines.
7.5.3 Keep open ends of cleaned and dried tubing capped or plugged at all times, except during attachment or detachment of parts. Do not use tape, rags, or paper.
7.5.4 Clean all lines and fittings that have not been cleaned and sealed by one of the following methods:
7.5.4.1 A vapor-degreasing method with stabilized trichloroethylene conforming to Specification MIL-T-7003 or carbon tetrachloride. Blow tubing clean and dry with a stream of clean, dried, water-pumped air or dry nitrogen (water-vapor content of less than 0.005 milligrams per liter of gas at 700 ºF and 760 millimeters of mercury pressure).
7-5
7.5.4.2 Flush with naphtha conforming to Specification TT-N-95B; blow clean and dry off all solvent with water-pumped air; flush with anti-icing fluid conforming to Specification TT-1-735A or anhydrous ethyl alcohol; rinse thoroughly with fresh water; and dry thoroughly with a stream of clean, dried, water-pumped air or by heating at a temperature of 250 to 300 ºF for a half hour.
7.5.4.3 Flush with hot inhibited alkaline cleaner until free from oil and grease. Rinse thoroughly with fresh water, and dry thoroughly with a stream of clean, dried, water-pumped air or by heating at a temperature of 250 to 300 ºF for a half an hour.
7.5.5 Install lines, fittings, and equipment above and at least 6 inches away from fuel, oil, and hydraulic systems. Use deflector plates where necessary to keep hydraulic fluids away from the lines, fittings, and equipment.
7.5.6 Allow at least a 2-inch clearance between the plumbing and any flexible control cable or other flexible moving parts of the aircraft. Provide at least a 1/2-inch clearance between the plumbing and any rigid control tubes or other rigid moving parts of the aircraft.
7.5.7 Allow a 6-inch separation between the plumbing and the flight and engine control cables and electrical lines. When electrical conduit is used, this separation between the plumbing and conduit may be reduced to 2 inches.
7.5.8 Route the oxygen system tubing, fittings, and equipment away from hot air ducts and equipment. Insulate or provide space between these items to prevent heating the oxygen system.
7.5.9 Mount all plumbing in a manner that prevents vibration and chafing. Support a 3/16-inch O.D. copper line each 24 inches and a 3/16-inch O.D. aluminum each 36 inches with cushioned loop-type line support clamps (AN-742) or equivalent.
7.5.10 Locate the oxygen supply valve (control valve) to allow its operation by the pilot during flight. The cylinder shutoff valve may be used as the supply control valve if it is operable from the pilot’s seat. Manifold plug-in type outlets, which are incorporated in automatic systems, may be considered oxygen supply valves since the pilot can control the flow of oxygen by engaging and disengaging the plug-in type of oxygen mask.
7.5.11 Filler connections, if provided, are recommended to be located outside the fuselage skin or isolated in a manner that would prevent leaking oxygen from entering the aircraft. Any nearby source of fuel, oil, or hydraulic fuel should be carefully evaluated under normal or malfunction conditions. Each filler connection should be placarded. Additionally, any valve (aircraft or ground servicing equipment) associated with high pressure should be slow acting.
Note: Locate the oxygen shutoff valve on or as close as practicable to the cylinder to prevent loss of oxygen due to leakage in the system.
7-6
7.6 Equipment Location and Mounting. Prior to commencing the installation, determine the weight/load factors and center of gravity (CG) limits.
7.6.1 Mount the cylinder in the baggage compartment or other suitable location in such a position that the shutoff valve is readily accessible. Provide access to this valve from inside the cabin so that it may be turned on or off in flight.
7.6.2 Fasten the cylinder brackets securely to the aircraft, preferably to a frame member or floorboard using AN bolts with fiber or similar locking nuts. Add sufficient plates, gussets, stringers, cross-bracing, or other reinforcements, where necessary, to provide a mounting that will withstand the inertia forces, stipulated in Chapter 2, Structural Data.
7.6.3 When cylinders are located where baggage or stored materials may damage them, protect them with a suitable guard or covering.
7.6.4 Provide at least 1/2-inch of clear airspace between any cylinder and a firewall or shroud isolating a designated fire zone.
7.6.5 Mount the regulator close to the cylinder to avoid long high-pressure lines.
7.6.6 Store the masks so that there will be a minimum delay in removing and putting them into use.
7.7 Thread Compound. Use anti-seize or thread-sealing compound conforming to Specification MIL-PRF-27617F or equivalent.
7.7.1 Do not use compound on aluminum alloy flared tube fittings having straight threads.
Proper flaring and tightening should be sufficient to make a flared tube connection leakproof.
7.7.2 Treat all male-tapered pipe threads with antiseize and sealing compound (MIL-PRF-27617F, or polytetrafluorethylene tape A-A-58092) or equivalent.
7.7.3 Apply the compound in accordance with the manufacturer’s recommendation. Make sure that the compounds are carefully and sparingly applied only to male threads, coating the first three threads from the end of the fitting. Do not use compound on the coupling sleeves or on the outside of the tube flares.
7.8 Functional Test. Before inspection plates, cover plates, or upholstering are replaced, make a system check that includes at least the following:
7.8.1 Open the cylinder valve slowly and observe the pressure gauge.
7.8.2 Open the supply valve and remove one of the mask tubes and bayonet fittings from one of the masks in the kit. Plug the bayonet into each of the oxygen outlets. A small flow should be noted from each of the outlets. This can be detected by holding the tube to the lips while the bayonet is plugged into an outlet.
7-7
7.8.3 Check the complete system for leaks. This can be done with a soap solution made only from mild (castile) soap or by leak-detector solution supplied by the oxygen equipment manufacturer.
7.8.4 If leaks are found, close the cylinder shutoff valve and reduce the pressure in the system by plugging a mask tube into one of the outlets or by carefully loosening one of the connections in the system. When the pressure has been reduced to zero, make the necessary repairs. Repeat the procedure until no leaks are found in the system.
WARNING: Never tighten oxygen system fittings with oxygen pressure applied.
7.8.5 Test each outlet for leaks at the point where the mask tube plugs in. This can be done by using a soapy solution over each of the outlets. Use the solution sparingly to prevent clogging the outlet with soap. Remove all residue to prevent the accumulation of dirt.
7.8.6 Examine the system to determine if the flow of oxygen through each outlet is at least equal to the minimum required by the pertinent requirements at all altitudes at which the aircraft is operated. This can be accomplished by one of the following methods:
7.8.6.1 In a continuous-flow system, when the calibration (inlet pressure versus flow) of the orifices used at the plug-in outlets is known, the pressure in the low-pressure distribution line can be measured at the point subject to the greatest pressure drop. Do this with oxygen flowing from all outlets. The pressure thus measured should indicate a flow equal to or greater than the minimum flow required.
7.8.6.2 In lieu of the above procedure, the flow of oxygen through the outlet that is subject to the greatest pressure drop may be measured with all other outlets open. Gas meters, rotameters, or other suitable means may be used to measure flows.
7.8.6.3 The measurement of oxygen flow in a continuous-flow system that uses a manually adjusted regulator can be accomplished at sea level. However, in a continuous-flow system that uses an automatic-type regulator, it may be necessary to check the flow at the maximum altitude that will be encountered during the aircraft’s normal operation. The manufacturer of the particular continuous-flow regulator used should be able to furnish data on the operating characteristics of the regulator from which it can be determined if a flight check is necessary.
7.8.6.4 Checking the amount of flow through the various outlets in a diluter-demand or straight-demand system is not necessary since the flow characteristics of the particular regulator being used may be obtained from the manufacturer of the regulator. However, in such systems, the availability of oxygen to each regulator should be checked by turning the lever of the diluter-demand regulator to the “100 percent oxygen” position and inhaling through the tube via the mask to determine whether the regulator valve and the flow indicator are operating.