Aircraft Modifications

Implications for continued airworthiness and corrosion control

Making modifications to an aircraft is not likely a foreign topic to any of us. However, I would like to discuss it briefly here just to set the stage properly for this month’s topic, continued airworthiness and corrosion control.

Repairs and modifications
For the purpose of this discussion, modifications to the aircraft come in two forms: those that add new equipment or features to the aircraft (referred to as alterations), and those accomplished to re-establish the original strength and integrity of damaged areas of the aircraft (referred to as repairs). Regardless of the purpose for the modification, we are also required to determine whether or not the alteration or repair is major or minor in nature. FAR Part 1 and FAR Part 43 together provide basic guidance to help determine whether or not a modification is “major” by definition.

Additionally, the FAA has provided other guidance resources to assist us in making that distinction. Limited though it may be, the FAA continues to try to develop more and clearer guidance for us to use to distinguish between a major and a minor aircraft modification. Alterations and repairs have been a part of aviation for years under the supervision of the FAA. The criteria used for the major/minor distinction has differed from FSDO to FSDO and changed immensely over time, leaving the industry confused, to say the least. In addition, the FAA’s inconsistency over the years has created another issue.

From the beginning
Use of FAA Form 337 was first introduced to the industry to document major repairs and major alterations. Upon the form’s introduction, the FAA flight standards division was capable and anxious to help industry make and approve the modifications. At that time, the field inspectors were comfortable with the methods and techniques being used and the alterations and repairs were straightforward and easier to assess.

Advisory Circular 43.13 “Acceptable Techniques Methods and Practices for Repairs and Alterations” had been created (and has since been divided into two parts) to ensure that the industry used safe methods to accomplish the modifications and that the field safety inspectors would exercise their authority and provide field approvals (a term rarely used today) to approve the data used.

Field approvals
A field approval is a method for obtaining the required FAA-approved data to support a major modification being accomplished. The field safety inspector can sign block 3 of FAA Form 337 to attest that the data contained on the form that describes how the modification was accomplished is satisfactory and certified as approved. When a major alteration or repair would be needed for several aircraft, the safety inspector would authorize “for duplication” on like model and configuration. (Ah, the good old days.)

When aircraft manufacturers began to introduce improved methods for navigation, improved systems on the aircraft, and aircraft that would fly higher and faster with much greater pressure differentials, design improvements were moving faster than the rate at which the FAA could train its inspectors. FAA field safety inspectors were no longer familiar with the methods and techniques being used and thus were no longer willing to approve the data. The Flight Standards Division of the FAA began to look to the Aircraft Certification Division for help. The engineers were more familiar with the emerging technologies and were already working with industry to modify type certificates and approve supplemental type certificates (STCs). STCs quickly became the preferred method of alteration approval and designated engineers were used to review repair data to major repairs.

Now that our picture is focused, we move on to discuss the term “instructions for continued airworthiness (ICAs).” When the aircraft certification group began to get more involved with the field aircraft modifications, the issue of continued airworthiness got much more complicated. As the engineers who approve aircraft type design, the new logical step for any STC is continued airworthiness, even though in-service aircraft are all part of an inspection and maintenance program that encompasses the whole aircraft, in one way or another.

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