Aircraft Modifications

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.

This concept was foreign to some of the maintenance industry since with most field approvals, continued airworthiness was considered to be handled just fine by the inspection/maintenance program issued by the manufacturer. Aircraft certification disagrees, so today an FAA Order is available to help us understand what must be included in any ICA created to support a major alteration or repair. That order, “FAA Order 8110.54 - Instructions for Continued Airworthiness Responsibilities, Requirements & Contents” is available at, thousands of modifications were performed well before the FAA began requiring ICAs. This is where the problem lies.

Those aircraft are still airworthy and the approvals are still valid, except in a couple of cases, such as moving aircraft operations from Part 91 to Part 135. When an aircraft undergoes an FAA conformity inspection in preparation for being added to the operations specifications for an air carrier under Part 135, some FAA offices are now requiring that all major modifications be reviewed. They may also require ICAs to be developed for those that do not already have one, even though the modification was approved when it was accomplished.

It seems ridiculous to me that the aircraft is airworthy when operating under Part 91 and suddenly not when being added to the air carrier certificate. I know that many of you have already been up against that, and as I mentioned before, the FAA Order can be an aide in making sure that all of the requirements have been met before providing an ICA to the FAA for approval.

Corrosion control
The second case that I’m referring to is the case in which a corrosion prevention and control program (CPCP) is required. Beginning with large scheduled airline aircraft, and as a result of the 737 that came apart in flight several years ago, the FAA implemented the CPCP concept. To begin with, an Airworthiness Directive is issued requiring that the operators of affected aircraft implement a CPCP. ADs were quickly issued for several of the large airline transport models.

The CPCP model used for the larger operators has been worked and modified over the last few years and seems to fit the major airlines well. Each of these operators generally operates tens and hundreds of these models, so implementing a CPCP into their standard program is relatively easy.

This will not be the case for the business aviation community. Soon business aircraft manufacturers will begin to incorporate CPCPs into their recommended inspection programs — some already have. On the surface, it sounds like the manufacturers are doing the ground work and all we will need to do is complete the inspections. Not quite — the manufacturer will set a starting point for the CPCPs they create and that inspection will cover all of the standard aircraft. The truth is the only standard aircraft is one that has not received a major modification.

When aircraft reach the CPCP implementation deadline defined by the manufacturer in hours, cycles, and/or date, the FAA will require that operators obtain a letter of authorization (LOA) for their aircraft-specific CPCP. This is similar to when an aircraft moves from Part 91 to Part 135. In order to receive that CPCP LOA, the operator will be required to review all major modifications (alterations and repairs) accomplished on that aircraft to determine if an ICA exists and if that ICA includes inspection criteria to ensure corrosion control for the structural elements of the modification.

It is the FAA’s position that any modifications performed after delivery must be reviewed and verified to meet the airworthiness requirements for the aircraft and provide for continued airworthiness ongoing. This won’t be easy.

Unlike the major airline operators, hundreds of operators will need to have help in developing new ICAs for their modifications that will include all of the necessary inspections, or include a statement from the FAA that the modification does not require any special inspections. Neither Flight Standards nor Aircraft Certification will have resources available to handle the effort with any reasonable timeframe. The only other option that we will have is to find industry engineers designated by the FAA to approve data and to help in the evaluation and creation of ICAs. Once created, the ICAs will still need to be reviewed by flight standards and an LOA issued for that aircraft.

In addition to the inspection criteria required within each ICA, the operator will need to provide a corrosion prevention and control program process that explains how discovered corrosion will be evaluated and how recurring inspections will be established and by whom. In many cases, the appropriate corrective action will be a new specific inspection of that area at new intervals. These resultant inspection requirements must also be tracked to ensure corrosion control.

This is something that can really sneak up on an operator. Establishing and getting a CPCP authorized by the FAA will take some time. Get in touch with the manufacturer and find out how soon to expect to need to do this. I suggest planning six months to one year to get prepared to implement your program.

Joe Hertzler is the CEO and co-founder of Avtrak Inc., provider of the industry’s first Internet-based and compliance-focused maintenance tracking service — Avtrak GlobalNet. Avtrak has earned a solid reputation for having the most comprehensive and easy-to-use compliance management system and service in the industry. Avtrak’s GlobalNet technology is the engine behind Gulfstream and Sikorsky HelotracII. GlobalNet is the system of choice for many operators of more than 140 models including Bombardier, Hawker/Beechcraft, and Dassault Falcon aircraft.