Twin Otter Wing Life Extension

July 04, 2014

Twin Otter Wing Life Extension FAA STC #SA00364LA
Transport Canada STC #SA97-100


Structural Integrity Engineering (SIE) has received an STC from both the FAA and Transport Canada pursuant to the above subject. The technical approach that SIE used to develop and obtain the subject STC consisted of employing a fracture mechanics based damage tolerance analysis to develop a safe and prudent repetitive inspection program and a minor spar splice modification. We hold a very similar STC & STA for the Beechcraft 99 series aircraft. The technical approach used in both programs is the same as that used by Boeing, Airbus, and the U.S. Air force in generating their Instruction for Continued Airworthiness programs.


In order to proceed with the subject STC requirements, an initial inspection “Candidacy Inspection Program” must be accomplished. This “Candidacy Inspection Program” which is outlined in our report #SIE-23-702 is required to assure that before any aircraft utilizes our program,it is sufficiently free from corrosion and other damage so as not to diminish the safety of that aircraft. SIE will send our representative to your facility, or whatever facility you choose to have the aircraft made available to SIE, to perform the “Candidacy Inspection Program”. We require the aircraft be made available to us for one eight hour period. SIE personnel will perform the inspection and no manpower requirements other than a liaison to assist our representative gain access to your aircraft is required.

SIE will visually inspect in detail the wing interior through available access and inspection holes. This inspection focuses on the primary structure, (i.e., main spar web and caps, ribs, stringers, skins) and their respective fasteners for signs of cracking, corrosion, exfoliation, pitting and distress. Exterior inspection includes all exterior surfaces, notably any line corrosion along the drag angles, predominately found on the upper skin exhaust track regions, as well as similar corrosion along the leading edge of the upper trailing edge shroud. Note, although the specific inspections are primarily concerned with signs of cracking and corrosion, any abnormality such as paint discontinuity, surface scratches, dubious repairs, missing fasteners, etc. are noteworthy of reporting.

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