ADVANCED MATERIALS & PROCESSES | APRIL 2025 1 7 an approved one from a different manufacturer. The geometry of the housing leaves a thin area of material that must withstand the internal hydraulic pressure, with the corners of the snap ring groove concentrating stress. Further, the housing and barrel are made from a wrought aluminum alloy, tempered to maximum strength and hardness, which can leave the material more susceptible to specific corrosion mechanisms[11]. Corrosion pitting, observed on interior surfaces of these parts, further reduces fatigue life[12]. To date, no significant injuries have been reported due to the failure of these actuators. However, loss of hydraulic fluid, which impairs a pilot’s control of the main landing gear, is a serious safety issue. Belly landings of aircraft, like the one that occurred in the Tacoma, Washington, accident, present a substantial risk to pilots and passengers of vehicles. Several service bulletins from the manufacturer, Cessna, have been issued concerning these hydraulic actuators since 1967. Service Letter 67-16 called for inspections of a specific series of the airplanes to look for and replace particular Electrol actuators with one from another manufacturer. Cessna later released service letters SE69-17 and SE75-21, eventually applying to all 210 airplanes manufactured between 1960 and 1964. In 1976, the Federal Aviation Administration (FAA) issued Airworthiness Directive (AD) 76-04-01 requiring compliance with SE75-21— this AD covers the part numbers listed in Table 1[13]. Cessna then updated the service manual in 2011 with Supplemental Inspection 32-10-01, calling for the actuator to be inspected after 6000 hours or 10 years, and then additionally every 1000 hours or three years. This inspection affects the airplanes in these investigations, as they were older than 10 years. This process can be a fluorescent penetrant inspection, a visual method looking for cracks[14]. A branch of the FAA is compiling information on the affected part numbers, and the NTSB is working in concert with them to ensure this problem can be remediated as soon as possible. For the airplane (N3607Y) that underwent this problem twice, it could not be determined if it had been maintained according to the AD or the supplemental inspections. It is imperative that older aircraft are inspected regularly at their required annual maintenance. The current annual inspection criteria specify operating the MLG repeatedly while checking the hydraulic system for leaks[15,16]. CONCLUSIONS The NTSB investigated multiple airplane accidents involving emergency landings where the pilots could not fully extend the main landing gear to a fully locked position. The agency determined that the probable cause was a loss of hydraulic fluid due to a fatigue failure of the main landing gear hydraulic actuator, which resulted in the pilot’s inability to fully extend the landing gear and inability to attain directional control during the landing roll. The fatigue cracking initiated along the retaining ring groove of the actuator at corrosion pitting present on the snap ring groove. Propagating outward circumferentially, once the fatigue crack grew to its terminal size, the remainder of the cross-section fractured from overstress. The design of the overhauled actuator and material, as well the high stress concentrations and corrosive ambient environments, all led to the premature failure of these actuators. Additionally, current inspection and overhaul procedures may not be robust enough to identify the fatigue in its early stages. ~AM&P Acknowledgments The authors would like to thank all the NTSB field investigators who have worked on these cases over the years, including Shaun Williams, Noreen Price, and John Brannen. The authors would also like to thank Henry Soderlund for his invaluable assistance in accessing historical information and service data. In accordance with Title 5 Code of Federal Regulations §2635.807(b)(2), the views expressed in this publication do not necessarily represent the views of the National Transportation Safety Board or the United States. For more information: Erik Mueller, materials research engineer, National Transportation Safety Board, 490 L’Enfant Plaza, SW, Washington, DC, 20594, erik.m.mueller@gmail.com, ntsb.gov. References 1. ASTM, ASTM E18 – Standard Test Methods for Rockwell Hardness of Metallic Materials, ASTM International, West Conshohocken, PA, 2017. 2. ASTM, ASTM E1004 – Standard Test Method for Determining Electrical Conductivity using the Electromagnetic (Eddy Current) Method, ASTM International, West Conshohocken, PA, 2017. 3. AMS D Nonferrous Alloys Committee, AMS 2658 ‑ Hardness and Conductivity Inspection of Wrought Aluminum Alloy Parts, SAE International, Warrendale, PA, 2016. 4. D.L. DuQuesnay, P.R. Underhill, and H.J. Britt, Fatigue Crack Growth from Corrosion Damage in 7075-T6511 Aluminium Alloy under Aircraft Loading, International Journal of Fatigue, 25(5), p 371-377, 2003, doi.org/10.1016/ S0142-1123(02)00168-8. 5. A.K. Vasudévan and S. Suresh, Influence of Corrosion Deposits on Near-threshold Fatigue Crack Growth Behavior in 2xxx and 7xxx Series Aluminum Alloys, Metallurgical and Materials Transactions A, 13, p 2271- 2280, 1982, doi.org/10.1007/BF02648397. 6. L. Lee, S. Descartes, and R.R. Chromik, Comparison of Fretting Behaviour of Electrodeposited Zn-Ni and Cd Coatings, Tribology International, 120, p 535-546, 2018, doi.org/10.1016/j. triboint.2018.01.021. 7. G. Chen, et al., Pitting Corrosion and Fatigue Crack Nucleation, Effects of the Environment on the Initiation of Crack Growth, Ed. W. Van Der Sluys, R. Piascik, and R. Zawierucha, West Conshohocken, PA, ASTM International, p 18-33, 1997, doi.org/10.1520/STP19951S. 8. P.S. Pao, C.R. Feng, and S.J. Gill, Corrosion Fatigue Crack Initiation in Aluminum Alloys 7075 and 7050,
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