Leaf-Spring Landing Gear Failures Have Increased, FAA Says

The FAA has published a Safety Alert For Operators (SAFO) for airplanes with leaf-spring landing gear which have been modified with oversized "tundra" tires, skis, or a combination of the two.

The FAA notes that many aircraft operating in harsh and/or rough terrain environments have obtained approval for installation of wheel/tire combinations larger than originally certificated. These wheel/tire combinations are commonly referred to as tundra tire modifications. Modifications also include the installation of ski and wheel/ski modifications to better adapt to a particular flying environment.

There has been an increase of structural failures on aircraft operating with these modifications, according to the SAFO. To date there are at least 32 documented accidents caused by failure of the landing gear leg near the attachment holes of the axle. Based on statistical data the expectation is that under current conditions approximately 15 additional accidents will occur on aircraft with leaf spring gear having tundra tire, ski, and wheel/ski modifications producing a potential risk for serious or fatal injuries to occur.

The SAFO notes that with a wide range of operating parameters, it is not always possible to predict the affects 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.

The FAA published guidance in Advisory Circular (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 oversized tundra tires. This information is useful for other aircraft installations even though the test aircraft for the data collected was primarily a Piper PA-18.

Legend Cub Equipped With Skis

The SAFO urges that persons interested in operating aircraft with oversize tundra tire, ski, or wheel/ski installations should be aware of the safety risks involved and take appropriate measures to reduce those risks.

The FAA recommends the following actions for aircraft so equipped:

• Inspect the area of the axle attaching holes 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 penetrant method of nondestructive testing (NDT) with the aide of magnification, or other methods techniques and practices acceptable to the FAA.
• Corrosion, fatigue, and/or hardware substitution 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 noncompatible materials. The effects of time in service on hardware should be considered and replacement intervals established.
• Aircraft in service with these modifications should have an increased inspection frequency with records maintained to indicate the method and status since last inspection.
• Instructions for Continued Airworthiness (ICA) are required for continuous operation of the aircraft. Apply careful consideration when developing maintenance and inspection procedures that define the methods, techniques, and practices used in determining safe operation of the aircraft.
• When 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.

Aircraft operators who obtained approval for these installations prior to the data package requirement for field approval or ICA requirement for supplemental type certificate (STC) are encouraged to develop an ICA for their aircraft.

FMI: www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo/all_safos/media/2010/SAFO10007.pdf