Air transportation safety advisory A19C0026-D1-A1

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31 July 2019

Letter addressed to:
Director General, Civil Aviation
Transport Canada

Re :

Air transportation safety advisory A19C0026-D1-A1
Federal Aviation Administration Airworthiness Directive 2015-08-04 – Reliability of Main Spar Wing Lift Strut Assembly Inspection – Punch Test Method

Dear sir,

On 30 March 2019, a privately registered, ski-equipped Piper J3C-65 aircraft (registration C-FLDQ,  serial number 16839) was en route from Pistol Lake, Ontario, to Snowshoe Lake, Ontario (approximately 53 nautical miles northwest of Kenora, Ontario), with the pilot and 1 passenger on board. The intent of the flight was to transport the passenger to a hunting and fishing outpost lodge to complete some renovations. The passenger was an employee of the pilot, who was also the owner of the aircraft and the lodge.

On arrival at Snowshoe Lake, the pilot conducted a low pass from a north- northwest direction, adjacent to  the outpost lodge, to advise lodge guests of their arrival. During the pass, control of the aircraft was lost and the aircraft struck the frozen lake surface. People at the lodge responded immediately and called for emergency services. The pilot was fatally injured and the passenger sustained serious injury. The passenger succumbed to his injuries 6 days later. The aircraft was destroyed.

Inspection of the airframe at the site revealed that the left main spar wing lift strut assembly (strut assembly) had separated near the lower fork end attachment (Figure 1). Visual examination of the strut assembly revealed excessive corrosion in the area of the separation.

The failed strut assembly was sent to the TSB Engineering Laboratory in Ottawa, Ontario, for analysis. The analysis revealed that failure of the strut assembly was initiated by excessive corrosion and thinning of the load-bearing wall inside the strut, followed by fatigue and eventual overload failure.

Figure 1. Left main spar wing lift strut assembly upper side (Source: TSB)
Left main spar wing lift strut assembly upper side (Source: TSB)

The U.S. Federal Aviation Administration issued an Airworthiness Directive (AD 2015-08-04) on 03 June 2015, requiring the inspection or replacement of potentially corroded strut assemblies on several different models of Piper Aircraft. The occurrence aircraft was required to have complied with the AD, but a review of the maintenance records did not identify any record of the AD being conducted.

The AD requires that the strut assembly be inspected, and that this inspection reoccur at intervals no greater than every 24 months. The AD details two permissible methods to inspect the strut assemblies; a punch test method outlined in Piper Mandatory Service Bulletin (MSB) 528D, or an ultrasonic method described in the AD itself. If either of these tests identify significant corrosion, the AD requires that the strut assembly be replaced. The AD also allows for the replacement of the strut assembly instead of conducting one of the two permissible inspection methods.

The TSB laboratory in Ottawa conducted further examination of the failed strut assembly and completed the punch test inspection prescribed in MSB 528D, which states that if the bulletin’s punch test procedure creates a perceptible dentFootnote 1 using a punch testerFootnote 2, then the strut assembly metal is corroded beyond specified limits and the strut assembly is to be replaced before further flight. If no perceptible dent is evident then the strut assembly can remain in service. Representatives of Piper Aircraft Inc. and Transport Canada (TC) observed the tests. Several punch tests were completed in areas immediately surrounding, and further from, the fractured area on both the lower and upper sides of the failed strut assembly.

The alternative inspection method described by the AD is the use of ultrasonic inspection. Where the typical thickness of an exemplar strut assembly wall is between 0.034 and 0.041 inch, the ultrasonic inspection procedure specifies that wall thickness measurements of 0.024 inch or less require replacement of the strut assembly prior to any further flight. Although an ultrasonic inspection was not completed on the failed strut assembly, an examination using a scanning electron microscope was accomplished to take accurate measurements of wall thickness. This examination determined the following:

  • the heavily corroded area of the lower side of the failed strut assembly had a remaining wall thickness between 0.002 and 0.019 inch, well below the required minimum. Punch tests applied to this area revealed one perceptible dent.
  • the corroded area of the upper side of the failed strut assembly had a remaining wall thickness between 0.021 and 0.031 inch; therefore, some areas were below the required minimum. Punch tests applied to these areas did not produce any perceptible dents.

The contrast between the results from these two separate tests demonstrate that the use of the punch test inspection method described as an option in AD 2015-08-04 can be inconclusive and as a result is an unreliable method to determine the amount of corrosion within the strut assembly. TC is therefore advised that current inspection methods describe in the AD may need to be reviewed to ensure the methods are a reliable way to identify the unsafe condition. 

The TSB would appreciate being advised of TC’s position on this issue, and what action, if any, will be taken in this regard. Upon completion of the investigation into occurrence A19C0026, the Board will release its investigation report.

Original signed by

Natacha Van Themsche
Director of Investigations — Air
Transportation Safety Board of Canada

cc:

A/Director, Policy and Regulatory Services, Transport Canada
Director, Office of Aviation Safety, National Transportation Safety Board
Director, Office of Accident Investigation and Prevention, Federal Aviation Administration
Senior Director of Certification and Technical Affairs, Piper Aircraft Inc.

Background information

Occurrence No.: A19C0026

This Safety Communication No.: A19C0026-D1-A1

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