AD NUMBER: 2009-18-03 R1

MANUFACTURER: Pilatus Aircraft Ltd. Models PC-6, PC- 6-H1, PC-6-H2, PC-6/350, PC-6/350-H1, PC-6/350-H2, PC-6/A, PC-6/A-H1, PC-6/A-H2, PC-6/B-H2, PC-6/B1-H2, PC-6/B2-H2, PC-6/B2-H4, PC-6/C-H2, and PC-6/C1-H2
SUBJECT: Airworthiness Directive 2009-18-03 R1

SUMMARY:  This AD results from mandatory continuing airworthiness information (MCAI) issued by an aviation authority of another country to identify and correct an unsafe condition on an aviation product. The MCAI describes the unsafe condition as:

Findings of corrosion, wear and cracks in the upper wing strut fittings on some PC-6 aircraft have been reported in the past. It is possible that the spherical bearing of the wing strut fittings installed in the underwing can be loose in the fitting or cannot rotate because of corrosion. In this condition, the joint cannot function as designed and fatigue cracks may then develop. Undetected cracks, wear and/or corrosion in this area could cause failure of the upper attachment fitting, leading to failure of the wing structure and subsequent loss of control of the aircraft.

To address this problem, FOCA published AD TM-L Nr. 80.627-6/ Index 72-2 and HB-2006-400 and EASA published AD 2007-0114 to require specific inspections and to obtain a fleet status. Since the issuance of AD 2007-0114, the reported data proved that it was necessary to establish and require repetitive inspections.

EASA published Emergency AD 2007-0241-E to extend the applicability and to require repetitive eddy current and visual inspections of the upper wing strut fitting for evidence of cracks, wear and/or corrosion and examination of the spherical bearing and replacement of cracked fittings. Collected data received in response to Emergency AD 2007-0241-E resulted in the issuance of EASA AD 2007-0241R1 that permitted extending the intervals for the repetitive eddy current and visual inspections from 100 Flight Hours (FH) to 300 FH and from 150 Flight Cycles (FC) to 450 FC, respectively. In addition, oversize bolts were introduced by Pilatus PC-6 Service Bulletin (SB) 57-005 R1 and the fitting replacement procedure was adjusted accordingly.

Based on fatigue test results, EASA AD 2007-0241R2 was issued to extend the repetitive inspection interval to 1100 FH or 12 calendar months, whichever occurs first, and to delete the related flight cycle intervals and the requirement for the "Mild Corrosion Severity Zone'. In addition, some editorial changes have been made for reasons of standardization and readability.

Revision 3 of this AD referred to the latest revision of the PC- 6 Aircraft Maintenance Manual (AMM) Chapter 5 limitations which have included the same repetitive inspection intervals and procedures already mandated in the revision 2 of AD 2007-0241. Besides the inspections, in the latest revision of the PC-6 AMM, the replacement procedures for the fittings were included.

Additionally, EASA AD 2007-0241R3 introduced the possibility to replace the wing strut fitting with a new designed wing strut fitting. With this optional part replacement, in the repetitive inspection procedure the 1100 FH interval is deleted so that only calendar defined intervals of inspections remained applicable.

FMI: http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/0/A12FF7F26B22F5428625781600536EAD?OpenDocument