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.
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• 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.
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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.
14-8 Figure 14-2. Typical Main Landing Gear Spring Strut and Axle Assembly
14.7.2.2 Corrosion, fatigue, or hardware substitution, or all, can be detrimental to other areas of the strut as well, and the manufacturer’s recommendations should be thoroughly reviewed and understood. Compatibility of materials should be considered, particularly when aircraft have been modified with components of different base metals than originally certified, such as titanium/cadmium combinations or other non-compatible materials. The effects of time in service on hardware should be considered and replacement intervals established.
14.7.2.3 Aircraft in service with these modifications should have an increased inspection frequency with records maintained to indicate the method and status since last inspection.
14.7.2.4 Under current guidance, ICA are required to be addressed prior to returning the aircraft to service. Apply careful consideration when developing field approval data packages that define the methods, techniques, and practices used in determining the continuous airworthiness of the aircraft. AC 43-210, Figure 5-1, Major Repair or Major Alteration Maintenance Information Checklist, contains the minimum requirements that should be addressed. The ICA should be referenced in block 8 of FAA Form 337.
14.7.2.5 In establishing the frequency of these repetitive inspections, special consideration should be given to the number of hours flown, type of flying, operational environment, age, and previous operating history of the aircraft.
14.7.2.6 Operators of aircraft having obtained approval for these installations, prior to the data package requirement for field approval, may wish to develop ICA for their aircraft.
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14.8 Interchanging of Skis and Wheels.
14.8.1 W&B Computation. A person appropriately authorized by 14 CFR § 43.3 must perform a new W&B computation when the larger tires are initially installed. The FAA recommends that the airplane be weighed for this initial computation. After the initial installation, removing the larger tires and/or brakes and returning the aircraft to its original configuration 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 the W&B records and equipment list to ensure proper configuration of all approved ski, wheel, and float installations.
14.8.2 Periodic Inspection Required. Seasonally removed and installed equipment items such as tires, skis and floats should be inspected at installation to comply with 14 CFR §§ 91.407(a) and 91.409(a), and 14 CFR part 43 appendix D, 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.
14.9 Documentation.
14.9.1 Alterations. Modifications not listed in the aircraft specifications, incorporating larger tires, wheels, or brakes are considered a major alteration. Therefore, it is recommended that the procedures contained in ACs 43.9-1 and 43-210 be utilized to meet the recording requirements contained in 14 CFR part 43.
14.9.2 Performance Information. The following paragraphs contain the minimum additional performance data required for airplanes equipped with larger tire installations. Refer to AC 43-210, Paragraph 4, Flight Manual Supplements, for additional guidance regarding approved AFMS.
Note: In estimating landing distances for other conditions, caution should be exercised as temperatures or other environmental conditions may either decrease or increase these distances.
14.10 Summary of Considerations for Approval of Larger Tire Installation (may be used as a job aid).
14.10.1 If an STC exists, it must be used.
14.10.2 All tires must be either TSO or PMA approved.
14.10.3 Tire size must not exceed 35 x 15 inches.
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14.10.4 Procedures:
1. Review ACs 23-17 and 43-210.
2. Review the aircraft records for previous alterations and repairs to ensure there is no effect on the proposed alteration.
3. Conduct a conformity evaluation to ensure that the proposed alteration will not impact the airworthiness of the aircraft.
4. Submit FAA Form 337 prepared in accordance with 14 CFR part 43 and ACs 43-210 and 43.9-1, including the certification basis used for the field approval, a description of the proposed alteration, and ICA.
5. The alteration must be performed by a certificated airframe mechanic, and accomplished using acceptable methods, techniques, and practices. Refer to 14 CFR §§ 43.7 and 43.13.
6. Aircraft must be weighed with tires installed, and a new W&B generated. Refer to 14 CFR §§ 43.13, 91.9, and 91.103.
7. Aircraft CG must be determined for the most forward and aft conditions, at both empty and gross weights. Refer to AC 43.13-1, Chapter 10, Weight and Balance.
8. The holder of an Inspection Authorization (IA) should inspect the aircraft for:
• Previous alterations or repairs that may not have been recorded.
• Compatibility of previous alterations or repairs.
• Airworthiness of the aircraft.
14.10.4.1 Ensure that the aircraft wing, tail, and control surfaces have not been improperly altered and the aircraft is properly rigged (log book entry by a certified mechanic).
14.10.4.2 Perform a series of maintenance operational check flights as follows:
• No passengers permitted.
• All flights conducted in accordance with the aircraft’s flight manual or limitations.
• Turning and braking tests for tire slippage and interference. Recommend conducting at minimum and maximum tire pressures.
• Aircraft minimum weight: Flight(s) conducted at the most forward and at the most aft CG.
• Aircraft at gross weight: Flight(s) conducted at the most forward and at the most aft CG.
14.10.4.3 After completion of check flights, 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. Refer to 14 CFR § 91.407(b).
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