The Empennage Kit
Towards the end of 2017, after I had decided to for sure build an RV-7, I decided it was time to purchase the kit. My workplace essentially shuts down for two weeks during the holidays, so I’d have all that time to start building. My plan was to order the kit from Van’s at the beginning of December to have the kit in time, however I also kept an eye on Van’s Airforce for the possibility of getting a ‘used’ kit. For me to purchase a used kit it had to meet two requirements: be local(ish) and almost completely untouched. I didn’t want to inherit someone else’s bad workmanship.
At some point in November I got lucky and a nearly untouched empennage kit came up in Houston. I believe only 6 rivets were installed. Houston is about a 4 hour drive from where I live, but this is the enormous state of Texas; 4 hours of driving is a reasonable day trip in these parts. Even better, I’d be headed to Houston to visit family for Thanksgiving, so that would be a perfect time to take a look. So I got in touch with the seller and arranged a meeting.
On the way to look at the kit I got a disheartening text. The seller was pulling together all of the kit and found some corrosion on a handful of the parts. I gave him a call to discuss it and it sounded relatively severe. I decided to take a look anyway, after all I spend about half of my time at work fixing corroded aircraft parts.
Upon arriving, I took a good look at all the parts. The parts had been stored with the blue film still on which is notorious for causing corrosion. Additionally, they were stored in a non-climate controlled garage, however the seller did assure me that they were not flooded during the recent hurricane. Taking a good look eased most of my concerns regarding the corrosion. A majority of the corrosion was very superficial, less than a thousandth of an inch deep. It was mostly powdery white surface corrosion, with some pitting and filiform corrosion. I’d estimate that 15% of the parts had corrosion, although most of it was only along the edges in localized areas. Around half of the skins had corrosion, although it was slight.
Taking a closer look revealed two parts that I wouldn’t feel comfortable installing: one of the HS609PP Horizontal Stabilizer Spar Reinforcements and the WD605R1 Elevator Horn Weldment. Being corrosion prone extrusion, the HS609PP had visible intergranular and pitting corrosion. Perhaps it could have been removed and saved, but this is highly critical structure and I’m building a new airplane; it just wasn’t worth the risk. This part was nearly $100, however. The WD605R1 had some of the powdercoat bubbling from rust underneath. After ordering a new one, I removed the corroded area from the old one and discovered that it wasn’t as bad as it looked. I’m positive the original one would have been fine, but the replacement was fairly cheap. With the replacement parts I also ordered the practice toolbox and control surface kits.
To give an idea of the scale of the corrosion, here is the absolute worst spot on the skins. As mentioned it is almost entirely along the edge.
Here it is after rework (I may call it ‘blending’, but it just involves using Scotchbrite or fine sandpaper to remove the corrosion and give 125 RHR or better surface finish):
Here you can see I’ve drawn a ¼” x ¼” grid on the blended areas and measured the remaining thicknesses. I measured with a properly calibrated micrometer. Even that nasty looking corrosion only required .002 material removal to completely remove the corrosion. Since the original sheet thickness was .001 over nominal, I only ended removing .001 from the designed thickness (.019 remaining thickness on a .020 skin).
How can I be sure that this is acceptable? There are many methods that are used in the industry, however the easiest and safest is simply to look at the allowable tolerances that the aluminum sheet had leaving the factory. These are contained in a specific ANSI document. For this sheet thickness I believe the tolerances were +/- .002 (although I’m saying that from memory and it has been awhile, it may have been .0015, it varies depending on the thickness of the sheet). That means a brand new sheet could be as thin a .018 leaving the factory and still be perfectly acceptable. Therefore my blend of .001 is acceptable.
The sheet thickness tolerance isn’t extremely large; often it is necessary to justify more than a couple thousandths material removal. For parts that aren’t extremely critical, I would hesitate to remove 10% of the material thickness (which is only .002 in this case, but increases for thicker sheets) without doing any actual numbers-on-paper analysis (especially if it is only locally removed). 10% is sort of standard in the industry for sheet metal parts, although it can vary depending on the criticality of the structure. Often times more than 10% is allowed by the OEM manuals for certain structure. Any greater than 10% or if the structure is extremely critical, I’ll want to develop the worst case loads on the part and ensure that the area I’ve reworked will not be the first thing to fail. If it is the first thing to fail, I’ll have to show that it still meets the 150% ultimate load requirement. There are many methods to do this, but I won’t bore you. If I ever have to get this in depth on my RV build, I’ll probably just reorder the part. Van’s is great about replacement parts. You oftentimes can’t find all the parts for the big airplanes.
What about the cladding, you ask? I’ve worked in the industry for a while. I’ve seen hundreds of corroded parts, maybe thousands. I have to find out what the part is made of to remake or repair it, so I’ll know if it is bare or clad. A majority of the corroded parts I deal with are extrusion (which is never clad), but for sheet metal parts the bare/clad doesn’t seem to make a difference. I see just as many clad parts that have corroded as bare parts. I can also find no rhyme or reason as to why the OEMs pick clad or bare for certain parts. Perhaps there are more clad parts on aircraft and that explains it, but my opinion is this: if the conditions are right to form corrosion, the cladding may help for a while, but the corrosion will win. I believe the best thing is to apply the best line of defense you can: a corrosion inhibiting primer. If someone has some actual testing data to prove me wrong, please provide it.
I should also note that after rework I apply Alodine (from a pen) to the areas where I have removed the cladding. Cladding is very good for preventing ‘flash’ corrosion, which can happen surprisingly quickly on certain aluminum alloys. So cladding is certainly useful when parts are sitting around unprimed, I’m saying that once the parts are primed it becomes nearly irrelevant. Don’t get me wrong, there are plenty of 50 year old aircraft flying with unprimed, unclad aluminum. And there is no process to end corrosion once and for all. I like having as much insurance as I can get. If I can have cladding, conversion coating, and primer then great, but I’m not going to shed a tear if the cladding gets removed (and I’m also not planning to Alodine, other than touch up, due to the harshness of chromates and the environmental concerns).
Take everything said here with a grain of salt. This isn’t a complete guide to reworking corroded aircraft parts, I’ve left out about 99% of what I know of it and there is so much that I don’t know. Use your own sound judgement and call Van’s if you don’t feel 110% comfortable with the structural integrity of your parts. I’m not advocating anyone else do anything written here and am not responsible if they do.
In the end, removing corrosion is a lot of work. After accounting for the cost of the parts I replaced and all the extra work, I probably paid too much for the kit, although it was certainly cheaper than new and I did get it cheaper than originally advertised due to the corrosion. I suppose I did also save shipping and all the hardware was very neatly organized in nice containers. So who know, maybe it wasn’t that bad of a deal after all.
Whew, I can get to the actual building now.