Document ID: chunk:federal_register_of_legislation:F2012L00161:body:0:p2
Version: federal_register_of_legislation:F2012L00161
Segment Type: other
Provision Reference: 
Character Range: 8935–12082

2:  The available fatigue data covers wing main spars and pressure cabins only.  The durability of other parts of the airframe has not been evaluated, and is therefore unknown.

                                 Based on experience, the Authority is prepared to credit unevaluated structure with a retirement life equal to double the life for the wing spar (Requirement para 2).  An extension beyond this may be available, but any application must be supported by a fatigue substantiation covering all the remaining primary structure.  This should include empennage, wing centre section, wing rear spar, control surfaces, unpressurised fuselages, etc.  The substantiation may take the form of a sufficiently rigorous inspection program to ensure that any fatigue cracking which could occur in service will be detected before the aircraft is endangered.

                                 Revised airframe lives approved against earlier issues of this Directive still apply.

                                 Note 3.  The model designations in the Retirement Schedule are as listed in FAA Type Certificate Data Sheets A2050 and A8EA.  The variation between the popular and the official designation for some models has caused confusion in the past; AAC Articles 133-5 (August 1982) and 135-5 (October 1982) refer.  All PA-31 series aircraft have life-limited wings.  If your model is not listed in the Retirement Schedule, or if you are uncertain of the official model designation for your aircraft, please contact your CASA District Office.

             Note 4.  The requirement for tracking weights and average flight times has been removed as a result of a sensitivity analysis that was conducted.  To operate beyond the retirement life refer to requirements 1.b. & 2.b.

             Note 5.  In developing an inspection program consideration must be given to the principles of damage tolerance.  A Damage Tolerant analysis of critical parts is required such that inspections include probable locations of damage due to fatigue, corrosion or accidental damage per FAR 23.573(b).  For guidance on this aspect, refer to FAA Advisory Circular 23-13A "Fatigue, Fail-Safe, and Damage Tolerance Evaluation of Metallic Structure for Normal, Utility, Acrobatic, and Commuter Category Airplanes".
Compliance:  Forthwith.
             This Amendment becomes effective on 2 February 2012.
Background:  The retirement lives promulgated in this Directive are based on fatigue tests and analyses conducted by the manufacturer.  According to these evaluations, the risk of airframe cracking increases to an unacceptable level beyond the specified life limitations.

             Amendment 11 provides alleviation to the retirement requirement of paragraphs 1.a. and 2.a. by allowing a CASA Approved Damage Tolerance inspection program as an alternative.

             Amendment 10 became effective on 25 February 1999 and removed the requirement to record average weight and flight times.  This was a result of a sensitivity analysis carried out by CASA on the relative effects of different sortie profiles and their relative effect on true fatigue life.