INTRODUCTION OF AIRCRAFT STRUCTURAL REPAIR TECHNIQUES FOR STRUCTURAL AIRCRAFT MECHANICS. This Blog’s target audience is A&P sheet metal mechanics and structural engineers. Feel free to comment or ask questions.
E-mail us at email@example.com and visit our website at http://www.Callahan.aero for any questions or special interest.
What are you paying for when you hire a DER?
I can’t speak for others but for me you are paying for my time and what you want done by me. My time includes my education, experience, skill, liability, and certification. And what are you hiring me for is the question I have for my clients.
My clients’ requests range from pure data certification to “we need help. Tell us what to do.” I have hired the IA and mechanics, rented the compressor, and provided transportation, lodging, and meals along with the repair plan, design, analysis, and FAA data approval. The more a DER does the more he costs. We ain’t cheap.
Requesting FAA approval of major repair data may seem straight forward but we need data. Data starting with the TCDS so we can determine the required FARs including their Amendments followed by US, the DER deciding which FARs are applicable making sure we have the proper delegation. We need the make, model, and serial number of the aircraft. Did you send it or did we have to keep asking for it. Cha Ching – time is money.
Asking us only to approve the data means you generated the complete data package. A complete professional data package approval may only take the time to read and comprehend it plus generate the FAA Form 8110-3, sign, scan, and send it. Yes, it can be done quickly, right Bill? Mike?
But alas and thank goodness this is not the norm.
Usually there are numerous iterations until the package is ready or turned over to me to finish. I love drafting repairs at my DER rate but what I’ve noticed is it may be less expensive overall to the client. A few hours of my time may be less expensive and quicker than a week at a drafter’s rate.
Regardless, DERs need reliable data about the aircraft and that may be from OEM drawings, manuals, an email from the manufacturer, or other acceptable sources. What is reliable data is determined by me, the DER. Showing another DERs drawing is not enough. For that matter, I know some SRM repairs that are wrong. My approval stands on its own as it should be.
DERs report to the FAA ACO (engineers) and our relationship with them is paramount. I do not accept data the FAA Engineers wouldn’t accept because I represent them!
By the way, FSDO has no say in what we engineers do or accept. They are all yours but can ask the ACO to assist too.
Back to collecting data. I love getting a cropped page from the SRM all marked up, NOT. If possible send the SRM or at least the whole ATA section. If we go back and forth collecting data it is more time wasted but easy money for me. If you have the time do a rubbing and send it overnight that would be best. If sending it by email lay a ruler on it so The scale can be determine. I do love rubbings. Pictures are quicker and good too but second to rubbings in my opinion.
Again place a ruler in the photo so I can determine the scale. Take many, many pictures from many angles and sides. Don’t take pictures too close because we can zoom in but not out. I need an overview to understand the load path. Remember digital photos are free!
A medical analogy is a skin rash photo sent to the doctor. All he sees is a rash. How big? Where on the body? Yep, I have seen corrosion but not yours so show me where it is. Show me how that part connects to adjacent parts.
I do accept rubbings and photos as good data. Keep in mind I need to know both what the part is suppose to look like and what it looks like damaged.
Once I have sufficient information and review it, a repair plan is visualized. If you suggest a repair I start there. Oh, the time to do all this may have taken six minutes or hours. Having the data in hand a diagnosis may take minutes.
I kind of fibbed up front, I do the TCDS near the end because I usually know what FARs are needed. And while I’m at it, there are more deviations to the rules than there are rules.
“There more of a guideline.”.
It pays to have a sense of humor and a good working relationship.
Whew, that was just getting the data.
Next step: A purchase order would be nice right about now.
I do a stick diagram penciled on scratch paper and sizing takes place. (I just said I do a simple sketch and crunch some numbers.) Once I am satisfied with sizing, we coordinate the repair, materials, skill, schedule, etc. This can be a few minutes or longer depending on several factors. Your experience, your availability, your attitude, your professionalism, etc. are what you bring to the table and personally control. I am an ally in this endeavor. But sometimes a more formal sketch (time) is required to coordinate the damage / repair.
Once we have a repair plan agreed upon you and I have options. You can procure materials, prepare the area, fabricate parts, etc. based upon our “verbal” repair plan or not. You may wait on my approved drawing. Either way I generate a drafted drawing so we can close the communication loop on our repair plan and my formal report begins.
What is the report? It is the structural substantiation of what the drawing specifies. The drawing is designed and drafted communicating and documenting the repair. A general misconception is that the drawings is the engineering. So not true, not that the design is not important but using a medical analogy the drafter is the nurse, the designer is the general practitioner, and the analyst is the surgeon and what we have is a major operation. The analyst is the one whom generates the report. The DER is the Chief of Medicine, a good looking one that is.
The drawing can be quick or very time consuming depending on the geometry, size, complexity, number of views and details and available data. Repair drawings should be fairly quick but it is very depended on skill, confidence, and ability. Experience may actually hurt because not all experience is good experience.
The layout that the drawing is extracted from is critical. A good designer’s layout is one key to success.
The analysis is crucial too but more importantly is the teamwork between the designer and analyst. That is where it pays off to have a good design engineer. His design is readily approved and that translates into lower costs and quicker turn around.
A good stress analyst is important too. He “can show s**t good” and that may mean less work for the mechanic.
THE DEFINITION OF A GOOD REPAIR DEPENDS ON WHO IS DEFINING IT.
The B747 engine strut plastic side fairings circa 1993 had a cracking problem. Airlines needed a repair, a permanent repair. One airline stop drilled the cracks and applied epoxy trying to retard the crack growth. It seemed to work but the OEM wasn’t buying into it. I was actually analyzing that fairing’s new design and on a visit to that airline another OEM engineer joined me. I was from the Lazy B Ranch and he came from Everett. He was a pompous arrogant stress analyst but so was I. He was new to that group so I used this opportunity to push for that temporary stop drill repair. He replied he already had a repair for that cracking panel. I got excited but first let me explain these side fairings are replaced by back drilling from inside the strut, a narrow box with several small access panels. Basically a real pain and not to mention the cost and availability of a replacement panel. Hmmm, maybe that’s why we were replacing them with a new design.
Well that stress analyst from Everett proudly and with a straight face like he had the cure for the common cold said, “just replace the panel.” No joke. You see he repaired the aircraft and actually believed no one else thought of replacing the damaged part. A real genius.
My airline friend asked my opinion and I suggested he just keep repairing it by doing his “temporary” repair for ever.
Let me ask you MRO engineers what would you do? It is secondary structure.
Me? I would approve the stop drill as a permanent repair after trying to get the A&P accept it on her authority. But once I approved one crack repair, A&Ps could use it as acceptable data from then on.
James W. Callahan
General Manager / FAA DER
P.O. BOX 533
Albertville, AL 35950