Piper PA-25 main landing gear hydrasorb shock absorber assemblies - Civil Aviation Safety Alerts (CASA) No. 2011-02


Owners and maintainers of Piper PA-25 series aeroplanes

Issuing Office: National Aircraft Certification Branch
Document No.: CASA 2011-02
Issue No.: 01
RDIMS No.: 6561866
Effective Date: 2011-05-31


This CASA is issued to inform owners and maintainers of PA-25 series aeroplanes of a potential unsafe condition regarding the fatigue failure of end fittings on Piper PA-25-235 Main Landing Gear Hydrasorb shock absorber assemblies.


Following the investigation of a main landing gear collapse on a Piper PA-25 aeroplane, the Transportation Safety Board of Canada (TSB) has issued to Transport Canada Civil Aviation (TCCA), TSB Advisory Letter A10W0092-D1-A1.

It has been determined that there have been at least six similar occurrences involving both foreign and Canadian registered aeroplanes.

TCCA believes this information is of value to all Piper PA-25 series aeroplane owners and maintainers as it may assist in preventing further incidents.

The following information is derived from the TSB Advisory Letter.

Following a normal three-point touchdown, the right main gear of a Piper PA-25-235 collapsed. The aeroplane ground looped and came to rest with the right wing on the ground. The propeller struck the ground under low power and the right wing sustained minor damage. There were no injuries.

Initial examination determined that the right main landing gear Hydrasorb shock absorber had failed at the bolt hole in the lower fitting, resulting in separation of the upper end of the right main landing gear vee assembly. Subsequent dye penetrant inspection (DPI) determined that the lower fitting on the left Hydrasorb shock absorber was also cracked through the bolt hole. The Hydrasorb shock absorbers, in combination with shock chords, absorb and dissipate landing loads within the shock strut assemblies.


Shock Strut Assembly.

Both main landing gear shock strut assemblies were submitted to the TSB Laboratory for metallurgical examination. It was determined that the right Hydrasorb shock absorber had failed due to the instantaneous overstress extension of a high-cycle fatigue crack through the bolt hole in the bottom end fitting (see Photo 2).


Photo 2. Fatigue crack in end fitting

A similar fatigue crack was found on the bottom end fitting of the left Hydrasorb shock absorber (see Photo 2). In both cases the fatigue cracking had initiated from multiple origins located at the sharp edge of the bolt hole and at residual tool marks on the bolt hole surface which had acted as stress risers. Examination showed that the lack of a chamfer on the bottom end bolt holes and the use of non heat-treated steel were contributory to the fatigue failures of the bottom fittings.

The part number (P/N) of the shock strut assemblies was U64052-003. These shock assemblies are fitted to Piper PA-25-235 and PA-25-260 aeroplanes. Each shock strut assembly consists of a top fitting (P/N U64026-006), a bottom fitting (P/N U64029-000), a Hydrasorb shock absorber (P/N U486-604-1) and three shock cord rings (P/N U31322-005). 

The Hydrasorb shock absorbers had been manufactured by Univair Aircraft Corporation in 2003 under a Federal Aviation Administration Parts Manufacturer Approval (FAA-PMA). The Univair label on each Hydrasorb shock absorber identified the production work order as 104040. The Hydrasorb shock absorbers had been installed on the aeroplane as components of two shock strut assemblies on 5 April 2005. The shock strut assemblies had accumulated 352.1 flight hours and an estimated 1400 take-off/landing cycles since installation.

Univair Aircraft Corporation received FAA-PMA approval to manufacture the U486-604-1 Hydrasorb shock absorber on 15 September 2003. A total of 45 shock absorbers were produced under Univair work order 104040.  These shock absorbers were sold individually or as part of the U64052-003 assembly. Neither the Piper Aircraft Corporation original equipment manufacturer (OEM) specifications nor the Univair specifications required chamfering of the bolt hole in the top or bottom fittings or heat treatment of the fittings.

Univair Revision A of the OEM specification, dated 30 September 2003, required the sharp edges on the top and bottom end fitting bolt holes to be chamfered. Univair Revision B of the OEM specification dated 14 October 2009, required heat treatment of the top and bottom fittings. The top and bottom fittings on Hydrasorb shock absorbers manufactured to Univair Revision A and Revision B specifications are expected to be more resistant to fatigue cracking.

Piper PA-25-235 and PA-25-260 main landing gear Hydrasorb shock absorber assemblies manufactured in accordance with OEM specifications under Univair Aircraft Corporation work order 104040, without chamfering of the end bolt holes and heat treatment of the top and bottom end fittings, are at a risk of developing fatigue cracks through the end bolt holes. Piper PA-25 main landing gear Hydrasorb shock absorbers manufactured by other vendors with no chamfering of the end bolt holes and no heat treatment of the end fittings, may have the same metallurgical properties, and therefore the same vulnerability to fatigue cracking, as the pre-revision A and B Hydrasorb shock absorbers manufactured by Univair.

Initiation and growth of fatigue cracks in the end fittings of the Hydrasorb shock absorbers can lead to failure of the end fittings and collapse of the main landing gear.

TCCA Comments:
There are approximately 92 Piper PA25 series aeroplanes presently registered in Canada.

The Design approval holder, Laviasa of Argentina, is aware of these shock absorber end fitting failures and has developed Service Bulletin (SB) that introduces a safety cable to prevent spread or lateral movement of the main landing gear and also helps prevent wing damage should the landing gear collapse.

Recommeded action:

TCCA strongly recommends that all affected owners and operators pay close attention to the main landing gear shock absorber assembly, both during scheduled maintenance activity and during post and pre-flight inspection. To prevent unnecessary damage in the event of a failed Hydrasorb shock assembly, it is recommended to incorporate the Laviasa SB 25-32-04, or at a minimum, incorporate the cable installation as per the Service Bulletin.

Defects, malfunctions and failures occurring on aeronautical products are to be reported to Transport Canada, Continuing Airworthiness in accordance with CAR 521 mandatory Service Difficulty Reporting requirements.

Contact office:

For related service information, contact :
Avenida Colón 412, P.B. Dpto B C.P. 5500,
Mendoza República Argentina

For further information, contact a Transport Canada Center, or Paul Jones, Continuing Airworthiness, Ottawa at 613-952-4357, facsimile 613-996-9178, or email CAWWEBFeedback@tc.gc.ca

For the electronic version of this document, please consult the following Web address:

For Director, National Aircraft Certification

Derek Ferguson
Chief, Continuing Airworthiness

The Transport Canada Civil Aviation Safety Alert (CASA) is used to convey important safety information and contains recommended action items. The CASA strives to assist the aviation industry's efforts to provide a service with the highest possible degree of safety. The information contained herein is often critical and must be conveyed to the appropriate office in a timely manner. The CASA may be changed or amended should new information become available.