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Resurrecting a 170: Tearing Down The Fuselage

How do you repair a fuselage that's been through a crash-landing - and shows the signs of it? It involves lots of sheet metal work; a task Kyle Fosso perfected while restoring his 1954 Cessna 170B.


Rebuilding the Fuselage

The Cessna 170, like most Cessna aircraft, uses a monocoque, or "stressed-skin," fuselage. The thin aluminum skin carries twisting and bending forces, and internal bulkheads hold the skin in-shape. The design is extremely light - but if you over stress the skin to the point where it stretches or wrinkles, it can fail.

As Kyle worked his way down the fuselage, he examined each piece for large dents and signs of stress. If the skin passed inspection, he cleaned it. If it didn't, he replaced it.

Rigging A Jig - Making Life Easier

Working on a fuselage while you're standing is much easier than crawling around the plane on your hands and knees. Kyle's first step in the teardown process was to rig a couple of jigs to elevate and support the fuselage. In his case, a 55 gallon drum fit the tail perfectly, and a stepladder combined with a wooden frame held up the midsection.

Cleaning - The Easy Part

Cleaning is the task you want to do. First, you scrub the dirt and grime off the inside of the metal skin and bulkheads. Then, you inspect it for corrosion.

Aluminum corrosion looks like white dust, almost like powdered sugar. If it's topical, you can scrub it off with a Scotch-Brite pad. Once it's removed, the metal underneath can't show any signs of damage, like pitting or nicking. If it's pitted or nicked, you'll need to replace it.

Kyle was lucky - when they towed the wreck out of the inlet in 1972, they hosed out all of the water. Kyle had a small amount of surface corrosion on the skin and some bulkheads, but no pitting or nicking.

Once each skin panel was cleaned, Kyle painted the interior side with a corrosion inhibitor and moved on to the next piece.

Replacement - The Hard Part

Unfortunately for Kyle, the aircraft went through a catastrophic water crash landing. The floats were torn and impacted the aft fuselage. Most of the pieces of skin weren't corroded, but they were deformed. They needed to be replaced.

This was a tedious and seemingly never-ending task. On the 170, hundreds of rivets join each skin panel to a bulkhead or neighboring skin. To replace a piece, you first drill out each rivet, then form new aluminum panel to match the old panel, and finally rivet the new panel in place.

Drilling Out Rivets

Rivets join two or more sheets of metal between a head and a base, which are connected by a shaft. To remove the rivet, you drill into the shaft with a drill bit slightly smaller than the shaft's diameter. Once you've drilled halfway down the shaft, you use a punch and hammer to remove the rivet's head, and then pop out the rivet's base. Then you repeat that process hundreds of times. It's a great task for a rainy afternoon.

Forming A Panel

Once you've removed the old panel, you cut sheet aluminum to fit using shears and a Dremel tool or grinding wheel. Each edge must be straight and perfectly smooth, and shears leave a nick after every cut. If left in place, vibration and stress can cause those nicks to tear - and your fuselage will fail in flight.

To dress the edge and remove the nicks, you run a long Vixen file along the edge until it's perfectly smooth.

The Cessna 170's rear fuselage uses two flat pieces of skin along the sides, and then curved pieces of skin along the top and bottom. If you're replacing a curved segment, you'll need to form the panel by carefully bending it over a piece of wood, adding pressure until it takes the original skin's shape. Patience is the key - you don't want to crease the metal.

With the skin formed, you're ready to pre-drill your rivet holes. By overlaying the old panel on the new, you can see where your rivets go - this is called "back-drilling." You then drill each rivet hole to match the rivet's diameter. Once drilled, you deburr each hole. Any rough edges or burrs can damage the rivet.

Mounting The Panel

With the metal formed, it's ready to install. You fit the skin to the fuselage, using temporary fasteners - called Clecos - to hold the skin in place.

You've probably seen Clecos before, but not known what they were. Clecos look like copper or silver spines sticking up from an under-construction wing or fuselage. A skin section fully Cleco'ed looks like an aluminum porcupine. They're color-coded by size - the copper ones are commonly seen on aircraft projects.

Clecos use a retractable pair of pins to hold two pieces of metal together, temporarily doing the job of a rivet. You extend the pins with a Cleco pliers, pushing down on a shaft on top of the Cleco. Then, you insert the pins through the rivet hole and release the pliers. The pins retract and expand, gripping and compressing the two sheets together.

Riveting Everything Together

Once the skin's in place and held together by Clecos, It's time to rivet. You remove every other Cleco, replacing it with a rivet. By following an alternating pattern, you make sure the skin fits evenly over the fuselage, avoiding any wrinkling or warping.

Riveting itself is a long task. The 170 uses "solid rivets," which look like a long "T," or a bolt without any threads. Rivets commonly have a round head or a "countersunk," flush head.

Round-head rivets are found all over the 170. On modern, laminar-flow wings, you'll find flush mounted rivets near the leading edge. Kyle's 170B uses flush rivets near the leading edge, because it has a Sportsman STOL kit installed. However, the original 170B uses round-head rivets throughout the wing. The 170 also uses flush rivets when another sheet of metal will overlay a rivet, and where parts of the aircraft's structure don't provide enought clearance for a raised, rounded head.

There are a number of ways to install solid rivets. On a 170, rivet gun and a heavy metal block, called a "bucking bar," do the trick. The process is relatively simple - you'll beat the rivet's base into shape using the gun and the bucking bar. The denser and heavier the bucking bar, the better.

First, remove the Cleco and insert the rivet into the hole, with the rivet head on the exterior side of the aircraft. Place the rivet gun's nosepiece, called a "rivet set," on the head of the rivet and a bucking bar against the base. The gun, rivet and bar must be perfectly straight and perpendicular to the aircraft's skin.

Once you're perfectly aligned, gently pull the gun's trigger to deliver a few light bursts. Make sure that the alignment remains straight, and then drive the rivet with a short, heavy burst.

The rivet gun uses air pulses to slam pressure against the rivet's head, forcing the rivet against the bucking bar. The bar hammers the base flat, compressing the two pieces of skin together. The base deforms into a buck-tail, creating a squat block of evenly thick metal. This is a great reason why you never arm-wrestle with your A&P.

It sounds like a job of sheer brute force, but it takes quite a bit of finesse. A correctly driven rivet evenly joins the metal without damaging the holes or the skin. If you've done the job correctly, you can drill out the rivet and the hole won't show any damage - it's ready for another rivet.

The aircraft's skin isn't the best place to perfect riveting technique. In Kyle's case, he used rivet cards, each holding nearly 300 rivets. After the card was full, Mac - Kyle's mentor - inspected the work. Then, Kyle drilled out each rivet to make sure the holes were undamaged - and then re-riveted the card. In total, Kyle drove a little over 2000 practice rivets before touching an aircraft.

A Time Consuming Process

Working quickly, Kyle could replace a piece of skin in three to four days. You're forearm strength may be the limiting factor in this operation.

In total, Kyle replaced 13 skin panels on the fuselage and aircraft floor - roughly 75% of the aircraft's skin. The process changed the way he looks at aircraft damage.

"The work's the same whether a piece of skin has one dent or a hundred - so paying more for a fuselage that has only a few obvious signs of damage doesn't make sense. A fuselage with heavily damaged skin may price lower, but require the same amount of repair work. When it comes to aircraft skin, looks can be deceiving."

Preparing For The Control Surfaces

Rebuilding the fuselage helped Kyle perfect his sheet metal and riveting skill. And it prepared him for his next task, rebuilding the control surfaces, which we'll cover in our next article. In the meantime, check out Kyle's Facebook Page, 2771C Progress. If you want to help him out, you can contribute through his GoFundMe site.

Check Out This Story's Previous Parts

Part 1: Resurrecting a Cessna 170B

Part 2: 11 Facts About The Cessna 170

Aleks Udris

Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at aleks@boldmethod.com.

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