13-8
13.4.3.2 The critical load factors to which the installation is to be substantiated are generally available from the holder of the aircraft’s type certificate (TC).
When the TC holder is no longer active, contact the responsible Aircraft Certification Service office.
13.4.3.3 The maximum loading is a requirement to which the aircraft manufacturer must adhere. Aircraft manufacturer floor loading limitations take precedence over the load rating of the net, fitting, and tiedown devices.
13.4.4 Static Tests.
13.4.4.1 It is recommended that static testing be conducted on a duplicate installation in a jig or mockup that simulates the related aircraft structure. See paragraph 2.6 for static test information.
13.4.4.2 If the actual installation is used for static testing, inspect both the aircraft and the cargo tiedown device installation thoroughly before releasing to service.
Check all members and fittings for cracks, distortion, wrinkles, or elongated holes. Replace all bolts and threaded fittings that are not inspected by magnetic particle or other acceptable Nondestructive Testing (NDT) inspection process. Inspect riveted joints for tipped rivet heads and other indications of partially sheared rivets or elongated holes.
13.4.4.3 All cargo tiedown installations must be tested to the critical ultimate load factor. See Chapter 1, General, for computation and testing procedures.
13.4.4.4 When the cargo compartment is separated from the flight deck by a bulkhead that is capable of withstanding the inertia forces of emergency conditions, a forward load factor of 4.5 g may be used. All other applications require the use of an 18 g forward load factor.
13.4.4.5 Each cargo tiedown fitting installation must be static tested under forward, side, and up-load conditions. Individual fittings may be tested by applying a single pull of 12.6 g forward load at an angle of 18.5° up and 9.5° to the left or right (as applicable) of the aircraft longitudinal axis. For example, assuming a 5,000-pound static pull (rating of a typical tiedown fitting) is applied as described and divided by the g load factor, we find the fitting installation will be capable of restraining a 397-pound load under emergency conditions.
9 g x 1.33 = 12.0
13.4.4.6 When a cargo-restraining net or cargo container with multiple attachments is used, the static load requirements for each tiedown fitting may be divided equally between the fittings. For example, if the maximum cargo load to be carried is 1,800 pounds and 10 tiedown fittings are to be used, the static load requirement for each fitting is approximately 2,155 pounds.
13-9 Example: static load for each tiedown fitting 9 g x 1.33 x 1,800/10 = 2,154.6 Placard individual tiedowns for the maximum weight to be secured.
13.5 Operating Limitations, Loading Instructions, and Placards.
13.5.1 General. Revisions or supplements to the approved portions of the Aircraft Flight Manual (AFM) regarding markings, placards, or other operating limitations require FAA engineering approval. Submit the requested changes and supporting data to the responsible Flight Standards District Office (FSDO) for review and processing.
13.5.2 Operating Limitations and Loading Instructions.
13.5.2.1 Prepare revisions or supplements to the AFM or operating limitations, W&B records, and equipment list changes as necessitated by the installation of the cargo tiedown systems.
13.5.2.2 Provide instructions covering the installation and use of the cargo restraint system. For aircraft that require a flight manual, incorporate these instructions as a supplement. On other aircraft, provide a placard that references the appropriate instruction. In the instructions, cover such items as removal and reinstallation of seats or other equipment exchanged for cargo restraint systems, use of cargo nets, barrier nets, number and positioning of tiedown straps, maximum load for each compartment or tiedown area, permissible load per square foot, number of tiedown points allowable per foot of track, and maximum height of the load’s CG above the floor.
13.5.3 Placards: Cargo Area Placards. Install placards or other permanent markings to indicate the maximum allowable cargo load and weight per square foot limitation for each cargo area. A placard seat tracks the number of tiedown points permissible per foot of track.
Attach a permanent label or other marking on each cargo net, barrier net, and at cargo tiedowns to indicate the maximum cargo weight that the net or attachment will restrain when installed according to the loading instructions.
13.6 Airworthiness Compliance Check Sheet: Cargo Tiedown Device Installations.
13.6.1 General. Cargo tiedown device installations that are the same as those made by the manufacturer, or other installations which are already approved, may be accepted without further investigation. On other installations, the following points should be checked to determine that the installation is satisfactory.
13.6.2 Applicable Civil Air Regulations (CAR).
• CAR 3.390, Seats and Berths.
• CAR 6.355, Seats and Berths.
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13.6.3 Applicable 14 CFR References.
• Section 21.9―Replacement and Modification Articles.
• Section 23.785―Seats, Berths, Litters, Safety Belts, and Shoulder Harnesses.
• Section 23.787―Baggage and Cargo Compartments.
• Section 27.785―Seats, Berths, Litters, Safety Belts, and Harnesses.
• Section 27.787―Cargo and Baggage Compartments.
• Section 29.785―Seats, Berths, Litters, Safety Belts, and Harnesses.
• Section 29.787―Cargo and Baggage Compartments.
13.6.4 Structural Requirements.
13.6.4.1 If the aircraft structure is changed or altered, determine if the original strength and integrity of the structure is retained. (See Chapter 2, Structural Data, and 14 CFR § 23.561.)
13.6.4.2 Determine if the extent to which the modification affects the aircraft CG has been evaluated. Refer to 14 CFR § 23.1589.
13.6.4.3 If the equipment is mounted either on the existing structure or on a bracket attached to the existing structure, confirm that all of the structure (including the bracket, if used) is adequate to support the required loads. Refer to 14 CFR §§ 23.307, 23.561, 23.613, 25.307, 25.561, 25.613, 27.307, 27.561, 27.613,
29.307, 29.561, and 29.613.
13.6.4.4 Verify that the equipment is installed so that it does not adversely affect other structures (either primary or secondary).
13.6.4.5 Confirm that means are provided to permit proper inspections of the installation and related adjacent parts as components. Refer to 14 CFR § 23.611.
13.6.5 Hazards to the Aircraft or Its Occupants. Determine whether the:
13.6.5.1 Modification creates any projections that may cause injury by human impact.
13.6.5.2 Fabric used in the modification complies with flame-resistant requirements.
13.6.5.3 Modifications adversely affect the accessibility of the exits and doors.
13.6.6 Detail Design Standards. Verify the following:
13.6.6.1 If suitable materials are used in the construction, including standard fasteners, and if the method of fabrication will result in a consistently sound structure.
13-11 Refer to 14 CFR §§ 23.305, 23.603, 23.605, 23.607, 23.613, 27.305, 27.603,
27.605, 27.607, and 27.613.
13.6.6.2 That cargo tiedown devices conform to an acceptable standard.
13.6.6.3 That there are written procedures or diagrams concerning cargo equipment installation, removal, and any loading restrictions (see Figure 13-6, Example of Allowable Cargo Loading Diagram).
13.6.6.4 That written equipment serviceability requirements or instructions for continued airworthiness (ICA) are available to ensure the aircraft structure, tiedown devices, nets, fittings, and required placards remain in serviceable condition.
13.6.7 Recordkeeping. Verify whether:
13.6.7.1 A maintenance record entry has been made. Refer to 14 CFR § 43.9.
13.6.7.2 The equipment list and W&B has been revised. Refer to AC 43.13-1, Chapter 10, Weight and Balance.
13.6.7.3 FAA Form 337, Major Repair and Alteration (Airplane, Powerplant, Propeller, or Appliance), and ICA have been completed and accepted by the FAA.
Figure 13-6. Example of Allowable Cargo Loading Diagram 100 Ibs.
NO LOAD (45 kg.)
(heat duct) 50 1bs. each 183 Ibs. (23 kg.)
(83 kg.)
(total) Seat belt fittings 170 Ibs. each (77 kg.)
544 Ibs. STA (247 kg.) 187.834 193 Ibs.
(87 kg.)
(total) STA
168.780 183 Ibs.
(total) STA
128.737 STA
106.628 STA
73.040 STA
53.986 13-12
14-1 CHAPTER 14. LARGER TIRE INSTALLATIONS
14.1 Purpose. Many aircraft operating in harsh and/or rough terrain environments have obtained FAA approval for installation of wheel/tire combinations larger than originally certificated on small single-engine airplanes of less than 6,000 pounds gross weight.
These larger wheel/tire combinations are commonly referred to as tundra tire modifications and better adapt airplanes to a particular operating flying environment. The information provided in this chapter can generally be applied to fixed gear airplanes less than 6,000 pounds gross takeoff weight when installing tires and wheel assemblies that do not exceed 130 percent of the standard tire and wheel combination that the aircraft was originally designed with.
14.2 Hazards and Warnings. Persons operating aircraft with larger size (tundra) tire installations should be aware of the safety risks involved and take appropriate measures to reduce those risks. With a wide range of operating parameters, it is not always possible to predict the effects of vibration, harmonics, drag, loading, or other characteristics induced by aircraft operating with these configurations. Therefore, it is incumbent on the operator to understand the potential risks and take appropriate measures for safe operation of the aircraft.
14.2.1 The FAA published guidance in AC 23-17, Systems and Equipment Guide for Certification of Part 23 Airplanes and Airships, summarizing flight test results, possible safety hazards, and general information about certification of aircraft with larger tires.
This information is useful for other installations, even though the test aircraft for the data collected was primarily a Piper PA-18.
14.2.2 Operation of airplanes with larger tires may expose the occupants to additional risks because there is evidence of increased structural failure of the landing gear or components on aircraft operating with these modifications. The additional weight and surface area of tires can impose additional ground and air loads on the airplane. Ground handling, taxiing, takeoff, and landing can place significant side loads and twisting moments to the landing gear and its attachment structure, which can cause hidden damage or cumulative damage, or both. In-service failure of main landing gear struts, wheels and attachment hardware have resulted in aircraft accidents and incidents with injury to the occupants of the airplane. Installation, rigging, periodic inspection, and maintenance of larger tires, wheels, landing gear, and their attaching parts are of utmost importance to safety. Consultation with maintenance technicians and operators experienced in installing and operating aircraft with larger tires is strongly recommended when considering any new installation or alteration of an existing aircraft with larger tires.
14.3 Additional Reference (current editions).
• Airframe, tire, wheel, and brake manufacturers’ data, if available.
• AC 43.9-1, Instructions for Completion of FAA Form 337.
14-2
• AC
43.13-1 , Acceptable Methods, Techniques, and Practices—Airplane Inspection and Repair.
• AC 43-210, Standardized Procedures for Obtaining Approval of Data Used in the Performance of Major Repairs and Major Alterations.
• Original certification standards of the aircraft being modified (i.e., 14 CFR part 23, Civil Air Regulation (CAR) 3, etc.).
• National Transportation Safety Board (NTSB) Safety Recommendation A-95-13, dated February 7, 1995.
• Technical Standard Order (TSO)-C62e, Aircraft Tires.
14.4 Installation Considerations.
14.4.1 Determining Eligibility of an Airplane. Many aircraft have been approved to use larger tires through the original manufacturer, field approval, or Supplemental Type Certificate (STC) process. It is advisable to research the aircraft Type Certificate Data Sheet (TCDS), specification sheet, or FAA STC database available through the FAA website at https://www.faa.gov website to see if the aircraft in question already has approval for the size tire. The FAA should not field approve a wheel/tire/brake combination that is already approved via another method such as STC. If larger tires are not previously approved for your particular airplane, the guidance in this chapter can be used to approve tires and wheels that are not categorized a major design change. The information provided in this chapter is limited to fixed gear airplanes with a gross weight of 6,000 pounds or less.
Prior to an aircraft modification, a conformity evaluation of the aircraft and review of the aircraft records for previous alterations and repairs should be accomplished to ensure there is no effect on the proposed alteration. This evaluation should include inspection of the aircraft wings, tail, and control surfaces for improper repair or alterations and proper rigging.
14.4.2 Identification of Approved Tires, Wheels, and Brakes. Only tires, wheels, and brakes of an FAA-approved type and load rating for aircraft may be utilized. Approved wheels, brakes, and tires will be marked using plates, placards, or other methods identifying the TSO number (TSO-C26d, Aircraft Wheels, Brakes and Wheel/Brake Assemblies for Parts 23, 27 and 29 Aircraft, wheels and brakes) (TSO-C62e tires), an aircraft component, accessory, equipment type certificate (TC) number, an airplane part number (for type certificated airplanes), or a Parts Manufacturer Approval (PMA).
14.4.3 Maximum Limit Load Rating.
14.4.3.1 Known limit landing load factor. Before installation, determine that the maximum limit load rating (L) of the wheels specified on its identification plate or placard is at least equal to the maximum static load on it (S) times the limit landing load factor (fl) previously determined from drop tests of the
14-3 airplane by its manufacturer. This requirement can be expressed by the following equation:
L = S x fl
14.4.3.2 Unknown limit landing load factor (fl). In lieu of a value fl determined from such drop tests, a value of fl determined from the following formula may be used:
9,000 fl = 2.80 + (W + 4000) where “W” is the certificated gross weight of the airplane
14.4.4 Landing Gear Moment Reactions: Landing Gear Bending Moments. In order to avoid excessive bending moments on the landing gear and attachment structure, the axle height
(y) measured from the bottom surface of the tire to the axle centerline, as indicated in the figure below, must not exceed 130 percent of the static rolling radius of the standard tire/wheel approved for the airplane when the tire is inflated to the maximum rated inflation pressure. The maximum rated tire pressure can be found stamped on the side of the tire or will be included in the servicing information for the aircraft. It is not appropriate to use oversize or “tundra” tires to determine the static rolling radius. If lower-than-maximum rated tire pressure is used on the proposed larger tire to meet this requirement then that becomes a limitation on the installation. For example, if the larger tire is rated at 30 pounds per square inch (psi) maximum and can only be inflated to 26 psi to meet the axle height requirement, 26 psi becomes the new maximum rated inflation limitation and needs to be placarded as a limitation.
Figure 14-1. Diagram of Axle Height Measurement
14.4.5 Field Experience. This chapter contains feedback from maintenance personnel and pilots with extensive operational experience in these types of installations. The information is for informational purposes to aid in understanding some aspects concerning installation of larger wheels, tires, and brakes. Larger tires usually require more effective braking to meet certification requirements. Typical installations involve replacing drum wheels and brake assemblies with more effective hydraulic wheel and brake assemblies or in the case
14-4 of the latter, changing from a single puck to a double puck style caliper to increase brake effectiveness. While additional brake effectiveness is needed to meet certain certification standards, it has been found that too much brake can be detrimental. As tire size and brake effectiveness increase, so do the effects on the aircraft. For example, the larger footprint and dynamic force of the tire combined with more effective braking will result in the increased tendency for the aircraft to nose over. On conventional gear aircraft, it results in an increased chance of propeller strike and ground loop. On tricycle gear aircraft, it increases the stress on the forward landing gear structure to include the firewall and engine mount. Moving the forward center of gravity (CG) aft will reduce the stresses on the nose landing gear.
14.4.5.1 When increasing tire size, it is recommended that the axle strength be increased to reduce in-service failures. For example, Piper model J-3 through PA-20 aircraft equipped with 1¼ inch axles are known to bend at the inner bearing stop. This has been prevented by pressing a Chrome-Moly sleeve inside the axle to reinforce it or by changing to the larger 1½ inch axles. On Cessna aircraft, it is recommended that the higher strength axles designed for ski installations should be installed.
14.4.5.2 In addition, several other modifications are available to adapt aircraft to a harsher operating environment such as landing gear attach reinforcement kits, check cables, horizontal stabilizer leading edge abrasion boots, and gravel guards that have been born out of necessity to reduce damage and increase the longevity of aircraft while operating off airports.
14.4.6 Performing the Modification. The modification must be performed by appropriately certificated persons using aircraft quality parts in accordance with acceptable methods, techniques, and practices. ACs 23-17, 43-210, and 43.13-1, and manufactures’ information are good sources of information to evaluate, perform, and document the larger tire installations. After the installation is complete, the equipment list and Weight and Balance (W&B) data need to be updated to reflect the new installation. The W&B can be accomplished by actual weighing or computation. A summary of considerations, not all inclusive, for approval of larger tire installations is provided and may be useful as a job aid.
14.5 Flight and Ground Handling Operational Checks.
14.5.1 Characteristics. Accomplish an operational check in accordance with 14 CFR § 91.407(b) to determine the takeoff, landing, and ground handling characteristics. Verify that ground control is adequate to satisfactorily complete a landing run with a turnoff at slow speed.
In flight, the larger tires must not produce excessive drag or undesirable flight characteristics. Where climb requirements are not critical, and the climb reduction is small (not exceeding 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 Aircraft Flight Manual Supplement (AFMS). For larger variations in climb performance, where the minimum requirements are critical, appropriate climb data should
14-5 be obtained to determine the changes; new curves, tables, or a note should be incorporated into the AFMS.
14.5.2 Airplane Without an AFM. For an airplane that does not require an AFM, make the revised performance information available to the pilot in the form of placards, markings, manuals, or any combination thereof. One type of acceptable manual is an approved AFMS. Enter a notation of this check-in the airplane records.
14.5.3 Potential Adverse Effects of Larger Tire Installations. Larger tire installations on airplanes may produce one or more of the following effects on handling characteristics:
1. Reduced ability of brakes to hold against takeoff power.
2. Reduced brake effectiveness during rejected takeoff and balked landing.
3. Reduced stability and controllability during rejected or balked landing and go around.
4. Change in either trim range or trim authority, or both.
5. Reduced directional stability and control during takeoff and landing ground rolls, with increased tendency to ground loop.
6. Increased tendency for conventional gear airplanes to nose over during landing on paved surfaces more than during landings on gravel, grass, or other surfaces that allow the tires to skid more easily.
7. Reduced stall warning margin, change in either aerodynamic stall warning characteristics (warning buffet) or reduced effectiveness of stall warning system, or both, in both level and turning flight with power either on or off, or both.
8. Changes in stalling and stall recovery behavior in both level and turning flight with power either on or off, or both. Stalls may become more abrupt and altitude loss before recovery may increase.
9. Increased tendency to enter an inadvertent spin and reduced ability to recover from the spin.
10. Reduced longitudinal, lateral, and directional stability.
11. Increased airframe vibration and buffet.
14.5.4 Potential Propulsion Systems Effects.
1. Fuel flow may be affected by changes in normal flying attitude.
2. Unusable fuel may be affected by changes in normal flying attitude.
3. The fuel tank sump may be affected by a change in ground attitude.
4. Fuel drains may be affected by a change in ground attitude.
5. Engine cooling may be affected by performance or flying attitude changes.
14-6
6. Changes in tires may affect the air induction system certification for operation on wet runways.
7. Changes in tires that change ground attitude may affect the induction system icing protection.
14.6 Testing the Modification. After the modification has been installed and all necessary maintenance entries have been made for the approval for return to service, an operational maintenance check is required in accordance with 14 CFR § 91.407 to ensure that the installation functions as intended. The operational check should be performed as follows:
14.6.1 No passengers permitted during operational maintenance checks.
14.6.2 All flights conducted in accordance with the aircraft’s flight manual or limitations.
14.6.3 Prepare the aircraft for testing.
1. The checks should be performed at maximum gross weight.
2. Inflate tire to the minimum allowable inflation pressure for the airplane weight.
3. Using white shoe polish or equivalent, coat the brake (inboard) side of tire sidewall. If larger nose tires are installed coat both sides of sidewall also.
14.6.4 Ground turning and braking tests for tire slippage and interference.
1. Conduct at least two maximum effort non-skidding taxi turns to the right and left.
2. With takeoff power applied, ensure that the brakes hold the aircraft static.
3. Conduct a minimum of two short field landings using maximum braking at max gross operating weight.
14.6.5 Aircraft minimum weight: Flight(s) conducted at the most forward and at the most aft CG.
14.6.6 Aircraft at gross weight: Flight(s) conducted at the most forward and at the most aft CG.
14.6.7 After completion of operational flights, inspect the tire, wheel, and brake installation for evidence of abrasion contact on the sidewall of the tire and any other indication that the clearance between the installed components is insufficient. If all ground handling and performance checks are found satisfactory, the pilot will sign the aircraft’s airframe maintenance (log) book, certifying that the aircraft is safe to operate within the operational limits of the aircraft’s design. This certification must contain the pilot’s name, certificate number, and date the operational check was performed.
Note: In the event operational performance is found to be below that established during certification, corrective action needs to be taken. On fixed landing gear landplanes, 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
14-7 information section of the AFMS. For larger variations in climb performance, where the minimum requirements are critical, appropriate climb data should be obtained to determine the changes and new curves, tables, or a note should be incorporated into the AFMS.
14.7 Maintenance (Including Inspection).
14.7.1 Inspection and Repair Data Sources. Contact the airplane, wheel, brake, or tire manufacturers for any specific inspection and maintenance instructions they may have developed. Check the applicable directory for Airworthiness Directives (AD) for the products being used. Refer to AC 43.13-1, Chapter 9, Aircraft Systems and Components, for more information.
14.7.2 Instructions for Continued Airworthiness (ICA). The installer 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 tires are installed). For simple airplane/tire combinations where tires of the same size have been approved on similar airplanes with 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 the FAA Form 337, Major Repair and Alteration (Airframe, Powerplant, Propeller, or Appliance), documenting the larger tire installation. For complex tire 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 owner or operator of the modified aircraft. Refer to AC 43-210, Paragraph 5, Maintenance Information, for additional guidance regarding ICA. Operators intending to modify aircraft to these configurations should consider the following:
14.7.2.1 Inspect the area of the fuselage attachment clamp area, axle attachment area, and the axle for cracks, corrosion, and any other defect that might induce failure. Performance of these inspections should be in accordance with “best practices” commonly used in industry with, at a minimum, the use of a dye penetrate method of Nondestructive Testing (NDT) with the aid of magnification or other methods, techniques, and practices acceptable to the FAA.