6-5 separate means of attaching cables and shock cords at the forward and aft ends of the skis.
6.5.3.3 Fabrication. Approved skis are normally supplied with cables, shock cords, and fittings. However, the specifications in Table 6-1, paragraphs 6.5.3.3.1 through 6.5.3.3.3, and Figure 6-3 may be used for fabricating and installing cables, shock cord assemblies, and fuselage fittings.
Note: Field experience indicates that accelerated wear and damage can occur to a 1/8-inch safety or crust-cutting cable and its attachment hardware in normal service on skis having a limit load rating of 1,500 pounds or more. The FAA recommends a minimum cable size of 5/32 inch for use in fabricating safety or crust-cutting cables for use with skis of a 1,500 pounds or greater limit load rating. 1/8-inch cables may be suitable for use on airplanes with light-weight skis and maximum certificated weights less than 1,500 pounds, such as those meeting the definition of light sport aircraft.
Table 6-1. Recommended Minimum Cable and Shock Cord Sizes Ski Limit Load Rating (Pounds) Single Safety Cable Double Safety Cable Single Crust-Cutting Cable Double Crust-Cutting Cable Single Shock Cord Double Shock Cord Less than 1,500 1/8 inch 1/8 inch 1/8 inch 1/8 inch 1/2 inch 1/2 inch 1,500–3,000 5/32 inch 5/32 inch 5/32 inch 5/32 inch 1/2 inch 1/2 inch 3,000–5,000 Do Not Use 5/32 inch 5/32 inch 5/32 inch Do Not Use 1/2 inch 5,000–7,000 Do Not Use 5/32 inch 5/32 inch 5/32 inch 3/4 inch 3/4 inch 7,000–9,000 Do Not Use 3/16 inch Do Not Use 5/32 inch Do Not Use 3/4 inch
6.5.3.3.1 Make check cable, safety cable, and crust-cutting cable ends by the splice method or swaging, such as the Nicopress process. Cable clamps may be used if adjustable lengths are desired, but they are not recommended. Use standard airplane hardware only. (Hardware used to attach cables must be compatible with cable size.) Refer to AC 43.13-1, Chapter 7, Aircraft Hardware, Control Cables, and Turnbuckles, for more information on cable fabrication.
6.5.3.3.2 Shock cord ends may be fabricated by any of the following methods:
1. Make a wrapped splice using a properly sized rope thimble and No. 9 cotton cord, 0.041 inch (minimum) safety wire (refer to Aerospace Industries Association (AIA) National Aerospace Standard (NAS) NASM20995, Wire, Safety or Lock), 1025 steel, or its equivalent
6-6 (AISI 4130). A ttach with a clevis or spring steel snap fastener. (Do not use cast iron snaps.)
2. Us e approved spring-type shock cord end fasteners, 1025 steel, or its equivalent (AISI 4130).
6.5.3.3.3 Fitting (Figure 6-3) specifications and installation:
1. Fittings fabricated for 1/8-inch cable or 1/2-inch shock cord must be at least 0.065-inch 1025 steel or its equivalent.
2. Fittings fabricated for 5/32-inch cable or 3/4-inch shock cord must be at least 0.080-inch 1025 steel or its equivalent.
3. An improperly installed fitting may impose excessive eccentric loads on the fitting and attach bolts and result in failure of the fitting or bolts.
4. If attaching cables directly to holes in fittings, radius the hole edges to reduce stress concentration and accelerated wear of the thimble. Stainless steel thimbles are recommended for increased wear resistance.
5. If attaching cables to fittings using clevises, clevis bolt castellated nuts should be used and then properly torqued and safe tied with cotter pins.
Field experience has shown that diaper-pin-style quick-releasing safety devices are more prone to failure during operation and are not recommended.
Figure 6-3. Typical Fuselage Fittings
6.5.4 Provisions for Inspection. An airplane using fabric-covered landing gear should have at least the lower 4 inches of fabric removed to facilitate inspection of the axle attachment area and to prevent the entrapment of snow and ice, which can lead to damage and corrosion of the landing gear (see item 6 in Figure 6-2).
6-7
6.6 Rigging of Skis.
6.6.1 Location of Attach Fittings on Fuselage or Landing Gear. Locate fittings so the shock cord and cable angles are not less than 20 degrees when measured in the vertical plane with the shock absorber in the fully extended position (see angle B, Figures 6-4, Main Ski at Maximum Positive Incidence (Check Cable Taut), and 6-5, Main Ski at Maximum Negative Incidence (Safety Cable Taut)).
Note: Do not attach fittings to wing-brace struts, except by special approval, either manufacturer or FAA.
6.6.2 Main Ski Incidence Angles.
6.6.2.1 Set cable lengths with the airplane level and no weight on the landing gear.
6.6.2.2 Adjust the length of the check cable to provide a 0 (zero) to 5-degree ski incidence angle (see Figures 6-4 and 6-6, Main Ski Incidence Angles).
6.6.2.3 Adjust the length of the safety cable to provide a -20 to -35-degree ski incidence angle (see Figures 6-5 and 6-6).
Figure 6-4. Main Ski at Maximum Positive Incidence (Check Cable Taut)
6-8 Figure 6-5. Main Ski at Maximum Negative Incidence (Safety Cable Taut) Fig ure 6-6. Main Ski Incidence Angles
6.6.3 T ension Required in Main Ski Shock Cords.
6.6.3.1 Apply sufficient shock cord tension to the forward ends of the skis to prevent flutter and “dumping” throughout the range of airspeeds and attitudes at which the airplane will operate on skis. Because of the various angles used in attaching the shock cord to the skis, shock cord tension cannot be specified. It is possible to specify the downward force that must be applied to the forward end of the ski in order to overcome the shock cord tension and cause the check cable to slacken when the ski is in the normal flight attitude. That downward force is commonly referred to as the shock cord tension force or simply the tension force. In most installations on rigid, truss type landing gear, the
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.
6-12
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.