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It’s Re-Bladder Time

It’s Re-Bladder Time

Contributing editor and A&P Steve Ells recently installed an FFC fuel bladder in the left tank of his 1960 Comanche.

I open the cabin door of my airplane to start my preflight inspection I’ve been catching a whiff of Avgas. 

I checked around the shaft of the fuel selector thinking that fuel was wicking up past the selector shaft seal, but didn’t see the telltale blue stain. I pretzel-ed myself into position to double-check, and risked (more?) brain damage by sucking in a big whiff. 

Nope, that wasn’t the source of the leak.

I looked in the engine compartment—maybe there was a leak there? But a thorough visual inspection didn’t reveal any blue staining. 

Uff da, I thought to myself. This lack of evidence narrows the source of the smell to one of the fuel bladders—one of the “rubber bags” snugly hidden away inside the fuel tank bay of the wing. 

I removed the wing root fairing and saw it: a small blue stain. 

It wasn’t very big, yet it was there.

Bladder backstory

According to one source, fuel bladders were installed in Piper airplanes for two reasons. First, the military used bladders because it thought that the bladder—being flexible—was much less likely than an aluminum or integral tank to burst during a crash. 

And second, installing a bladder—even the semi-stiff bladders manufactured 70 years ago—took less time than manufacturing and welding up a rigid tank or sealing the wing structure to create an integral tank. 

 

Causes of fuel bladder failure 

The most common cause of bladder failure is porosity of the upper surface. 

It’s pretty well known that fuel bladder life is maximized by keeping the tank as full as possible. This lessens plasticizer loss by keeping temperatures moderated. 

But the breakdown of the upper surface is inevitable and when it gets to a certain point, fuel starts to evaporate out of the tank. 

I had noticed that the fuel level in the left tank had dropped by an inch or so between flights; another sure sign the fuel was evaporating out the porous top surface of the tank. 

I needed a bladder and after checking with a couple of trusted techs for suggestions, ordered one from Floats & Fuel Cells (FFC) in Memphis, Tenn. 

FFC technology

A few days after ordering, a big box arrived. I was surprised at how light it was, but after opening the box and inspecting the new bladder, I understood why. 

New fuel bladders from Floats & Fuel Cells are very flexible and are visibly smoother than the old, semi-rigid bladder. Sort of like the difference between a 1960 Cadillac El Dorado and a 2015 Tesla. 

Each corner of my new bladder tank is rounded and each potential wear point and corner is smoothly reinforced. According to Brewer, this is because bladders from FFC are cured in an autoclave that melds all the parts together by utilizing temperature and pressure to create a one-piece unit.

And it’s light. The weight of the 30-gallon bladder is 4.9 pounds.

Modern bladder-style tanks from Floats & Fuel Cells are constructed of a proprietary P2393 nitrile rubber. Nitrile combines “excellent resistance to petroleum-based oils and fuels, silicone greases, hydraulic fluids, water and alcohols with a good balance of such desirable working properties as low compression set, high tensile strength and high abrasion resistance,” according to one reference book. The only drawback is a low resistance to attacks by ozone. 

FFC’s bladders consist of a four-layer construction. The layers (from inner to outer) are a nitrile layer that retards permeability; a transparent fuel vapor barrier that’s bonded to a nylon fabric layer; and an outer layer of nitrile rubber formulated to resist ozone attacks. 

Remove and replace

I flew the fuel level in the left bladder down to six gallons before siphoning the remaining fuel into grounded fuel cans. (Important: Always establish a ground between the aircraft and the fuel cans to prevent static electricity from spontaneously igniting the fuel vapors.)

I next removed the large access (top) plate and the fuel quantity sender from the top of the wing and removed the screws and bolts connected to the reinforcing/mounting ring molded into the top of the bladder. 

I tried to wrestle the old semi-rigid bladder out and up through the access hole, but soon realized there was a vast difference between the new, flexible, lightweight FFC bladder that I had just unpacked and inspected and the semi-rigid heavy bladder that was leaking. 

I needed enough room to shove most of my right arm down inside the old bladder to free the feed nipple and pull loose the clips holding it in position. 

I took the easy way—I cut the old bladder into pieces. It was a good decision; a box knife and scissors was all it took. That transformed the process of working in the tank bay from an arm-bruising struggle into an easy job. 

The rate-determining step in the removal of older bladder tanks is getting the fuel feed nipples that are molded into each bladder to release from the rigid airframe fuel feed lines. The bladder in my Piper Comanche has a single feed nipple located in the aft inboard corner of the tank.

Due to my prior experience in struggling to pull an old bladder free of the feed line, I spent at least an hour using my box knife to cut away sections of the nipple from the feed line. It wasn’t difficult because the nipple was old and brittle. 

Not content with that, I then spent time spraying Kroil between the nipple and the feed line after I had shoved a small pointed punch between the two surfaces. (This penetrating oil spray is designed to free up frozen hardware, but I figured that since it penetrates so well it would work to break the seal between the nipple and the rigid feed line.)

I worked the punch—and sprayed the Kroil—between the feed line and the nipple from the wing root and from inside the tank. Then I went home for the day.

The next day I applied straight pulling pressure and to my surprise the bladder nipple slid easily off the fuel feed line and over the coarse screen at the end of the line. Removal complete!

 

They don’t last long? 

There’s a tale that has been passed along at the preflight planning table and in the pilot’s lounge that bladders don’t last long and that they’re very hard to change. Hogwash. 

Six years ago, I changed the right bladder in my airplane. This year, I changed the left bladder. 

The left bladder was manufactured in 1957 and installed in 1959 in my 1960 vintage airplane. There’s a maintenance logbook entry citing, “Replaced fuel tanks” in late 1988. 

This led me to believe that both bladders had been removed, sent out, repaired and reinstalled, since both bladders I removed had the original U.S. Rubber part numbers, serial numbers and acceptance dates. Thirty years before the first repair is not short-lived. (For more information, take a look at “Refurbish, Repair or Replace: What do to when your fuel bladder fails” in the December 2013 issue of Piper Flyer. —Ed.)

Some are repairable (and some aren’t)

There were at least four different bladder tank manufacturers in the past. These companies included industry powerhouses such as U.S. Rubber, BF Goodrich, Firestone and Goodyear Aerospace. 

There were good construction methods and some that weren’t so good—some bladders are very repairable and some are not. Companies in the refurbishment business know which bladders are good candidates for overhaul. (For a link to table on the FFC website with specifics on which brands/types can be repaired, see Resources at the end of this article. —Ed.)

My Comanche’s original tanks had a cotton-based construction. According to Kevin Brewer at FFC, those U.S. Rubber 584 bladders would not be repaired today. The right one lasted 22 years after being refurbished, the left one 28 years. 

There are at least two other companies that produce and sell bladder-type fuel cells. 

 

Clips and hangers

Bladder-type tanks are fabricated to fit as perfectly as possible within the fuel tank bay inside the wing. The Floats & Fuel Cells bladder arrived with a new gasket, a roll of industrial-strength, fabric-backed tape and nine button-type spring clips. 

The tape is used to prevent tank abrasion. It’s laid over all rivet and screw heads and all seams within the fuel tank bay prior to installing the bladder. The tank bay of my Piper was very clean, so all I had to do was remove the old tape and install new tape.

The clips slip under a reinforced ring attached to the outside surface of the bladder. There’s a ring/clip in each bottom corner of the tank that’s there to keep the bottom of the bladder wrinkle-free. 

The bladder in my airplane holds 30 gallons. It’s shaped like a rectangle except for the forward corner of the inboard section which extends forward; it’s like a triangular piece was grafted onto the rest of the rectangle. 

There are five clips that need to be pushed into receptacles to hold the upper surface of the bladders in position. All of the removal, installation, flow nipple and clip installation work has to be done through the access hole in the top of the wing. 

It’s important to use the new clips and to get them snapped into the receptacles without bending them.

AD 68-13-03 applies to the bladder tanks in my PA-24 Comanche. It requires a visual inspection every 100 hours to check the condition of the clips. So installing new clips—and installing them correctly—is important.

Since the bladder in my Piper is small, I didn’t have any problem with arm length/finger strength issues during clip installation. 

Finishing up

One part of the installation is difficult. Two half-inch (inside diameter) flexible tubes need to be slid over tube ends attached to the metal top plate and tube ends mounted in the wing. These tubes connect the top plate fuel spill drain-off tube and the fuel tank vent line tube. 

Once these tubes are slid into position, the reinforcing/mounting ring of the bladder has to be pulled up into position so the screws connecting the two can be started and torqued. 

My solution to pulling the bladder reinforcing/mounting ring up into position—since the top plate completely fills the access hole, thereby cutting off access to the bladder—is to fabricate two long aligning pins out of bolts. 

These are screwed into nut plates in the bladder mounting ring prior to sliding the flexible tubes onto the wing mounted tubes. 

Once the top plate is in position I pull up on the aligning pins—which are nothing more than long bolts with the heads cut off—and start screwing in the screws that hold the top plate and the reinforcing/mounting ring together. 

Calibrating a dipstick

After the bladder is in and is deemed ready, I like to fill the tank in five-gallon steps—with the airplane on level ground and the landing gear struts and tires at normal inflation—for two reasons: this is the perfect time to make up a fuel tank quantity dipstick (I know I sound like a dipstick salesman, but they work and they are a simple safety tool) and it allows you to make sure you know exactly how much fuel your new bladder holds. 

A new bladder should provide good service for at least 20 years, and likely much longer if steps are taken keep the top surface of the bladder cool. 

Do this by shading the top surface of the wings whenever possible and by striving to top off the fuel tanks after each flight since the fuel will moderate bladder surface temperatures by acting as a heat sink. Hangaring an aircraft is the most effective method for preserving its fuel bladders. 

Know your FAR/AIM and check with your mechanic before starting any work.

 

Steve Ells has been an A&P/IA for 44 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He’s a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and the proud owner of a 1960 Piper Comanche. He lives in Templeton, Calif. with his wife Audrey. Send questions and comments to

RESOURCES >>>>>

New PMA fuel cells and fuel cell repair
– PFA supporter
Floats & Fuel Cells, Inc.
Repair and overhaul chart
Floats & Fuel Cells, Inc.
Other fuel cell suppliers
Eagle Fuel Cells, Inc.
Hartwig Fuel Cell Repair
Read more...
Airplane Maintenance for the DIYer: Changing a Tire

Airplane Maintenance for the DIYer: Changing a Tire

In this article—the first in a series describing various preventive maintenance actions—A&P Jacqueline Shipe goes through the entire process of removing and reinstalling an aircraft tire. 

All aircraft owners will periodically have to replace a tire. Even planes that don’t get used much eventually require tire replacement due to dry rot and sidewall cracking of the rubber. 

Tire replacement is one of the items the FAA considers preventive maintenance that owners may legally perform on their aircraft. Changing a tire isn’t mechanically complex, but it does require the owner to use some caution. 

Proper aircraft jacking procedures have to be used, and if the plane is jacked outside, first check the weather conditions to ensure that the wind is not going to be too high. Major damage can be done to the airframe structure whenever a plane falls off a jack. 

Once the plane is properly jacked, the removal of the wheel assembly can begin. (For information about aircraft jacks and proper procedures, take a look at Shipe’s “Airplane Maintenance for the DIYer: First Steps” in the May 2016 issue. —Ed.)

If the plane is equipped with wheel pants, the wheel pant has to be removed first. This is usually a pretty straightforward process, just be sure to keep the removed screws and bolts identified as to which receptacle they came out of, because sometimes several different diameters and lengths are installed. 

Removing a main wheel

The next step is to remove the outboard brake backing plate (if the wheel is a main wheel) so that the brake disc will be free to slide off the axle with the wheel assembly. 

There are two or more bolts that connect the backing plates to the caliper that have to be removed. They may or may not have safety wire on them, depending on the design. 

Once the outboard brake is off, the next step is to deflate the tire by removing the valve core. It is important to do this before removing the big axle nut, because if any bolts holding the wheel halves together are loose or damaged, the wheel assembly could blow apart as the axle nut is loosened. 

Most axle nuts have either a clevis pin with a small cotter key or a single large cotter key to keep them from backing off. 

When the axle nut is removed, the wheel assembly will slide off. Some manufacturers employ a spacer that may or may not slide off with the wheel; care needs to be taken to ensure it doesn’t get misplaced.

Removing a nosewheel

Nosewheels usually have a removable axle that slides through the wheel and nose fork. This is generally held in place with a single long bolt and lock nut that secures two cup-shaped retainers. 

Nosewheels are fairly easy to remove unless the steel axle is corroded and frozen in the aluminum fork, in which case the axle has to be driven out. A wooden dowel should be used to drive out the axle because it won’t gouge or mar the aluminum fork.

 

Breaking the tire bead

Once the wheel is removed, the tire bead needs to be broken from the rim of the wheel halves. Get a piece of plywood to lay the wheel on (so it won’t get marred), then forcefully push the tire down all the way around. It will eventually pop down off the rim. This process has to be repeated on the reverse side as well. 

Some folks use flat blade screwdrivers or pry bars to pry the tire away from the wheel, but this can result in major scarring of the relatively soft cast aluminum that the wheels are made of. 

If the bead is really stubborn and just won’t break loose, you may have to enlist the help of a mechanic and a bead-breaking tool made specifically for the task.

Splitting the wheel and removing the tube 

Most wheel assemblies are two-piece, and the wheel halves are split by removing the through bolts and nuts holding them together. After separating the wheel halves, the tube can then be removed from the tire. 

Some mechanics replace the tube with every tire change, and some re-use the old tube as long as it looks good. Tube manufacturers recommend always replacing tubes when replacing a tire because they stretch while in use. 

Once an old tube is removed, it can be barely inflated—just enough to expand it a little—so the entire exterior can be inspected. Tubes with deep wrinkles or that have signs of damage or age, such as dry rot cracks around the valve stem, should always be replaced. 

When ordering a replacement tube, pilots may want to get the type that doesn’t lose air, such as a Leakguard or Airstop inner tube. They do reduce the frequency of having to air up the tires by quite a bit, especially if they are serviced with nitrogen instead of compressed air. 

Reinstalling the tube

The outside of the tube and inner part of the tire need to be coated in talcum powder before installing the tube. The powder keeps the tube from sticking to the sides of the tire and helps prevent chafing. 

The tube should have a balance mark on it. This needs to be aligned with the balance mark on the tire, which is generally a red dot. 

In the absence of a balance mark on the tube, align the valve stem with the red dot. This matches the heaviest part of the tube with the lightest part of the tire and makes it much easier to balance.

 

Reassembling the wheel

Once the tube is installed in the tire, the wheel halves (and the brake disc, if it is a main wheel) can be assembled together. Slightly inflating the tube a tiny amount helps to ensure it won’t be pinched between the wheel halves. 

Place the wheel halves together so the bolt holes align. The bolts can then be slid through and the washers and lock nuts installed. Lock nuts should have enough tension on them so that they cannot be tightened by hand, otherwise they should be replaced. 

The correct torque should be observed when assembling the wheel halves. These are made of cast aluminum and are strong, but over-tightening the nuts and bolts can lead to cracking. 

 

Balancing the tire

A wheel balancer is a fairly expensive tool to buy, and tires aren’t usually replaced on an individual airplane often enough to merit owning one for most folks. Large imbalances in a wheel assembly can be detected by mounting the wheel on the axle and installing the axle nut, but leaving it slightly loose so the wheel rotates freely. 

Once the tire is spun a few times, if the same spot always ends up coming to rest on the bottom, this indicates an imbalance and weights will need to be added to the light side. Using stick-on lead type weights purchased from an aviation parts warehouse or automotive store, add enough weight so that the wheel comes to a stop in random places as it is spun freely on the axle. The balance is more critical on the nosewheel assembly because it will cause a shimmy if there is even a slight imbalance. 

 

Reinstalling the wheel on the aircraft

Once the wheel is balanced, it is ready for installation. The axle should be wiped off and greased, and any corrosion should be removed with an abrasive cleaning pad. 

A general-purpose Scotch-Brite 7447 pad (maroon color) works well and can be purchased anywhere automotive paint products are sold. These pads are abrasive enough to clean off rust, but not so abrasive so as to scratch the metal. Use elbow grease to scrub the axle until it is shiny.  

The nosewheel axle is prone to rusting internally. Any rust should be removed here too, and the internal part of the axle should be either painted with a rusty-metal primer, or coated in LPS 3 or other corrosion-inhibiting compound. 

During the final installation, the axle nut needs to be tightened enough so there is no free play detected as the tire is grasped and pushed inboard on the top while pulling outboard on the bottom, or vice versa. (This checks for side-to-side free play, and there should be none.) 

The wheel should spin somewhat freely, but there needs to be a slight amount of tension on the axle nut. If the nut is too tight, the wheel won’t spin much at all by hand, and the wheel bearings will be more likely to fail from having too much of a pre-load placed on them. 

Once the correct tension is achieved, align the cotter key hole in the axle with the opening in the nut and install the cotter key. Be sure to bend the edges of the cotter key in such a way that they won’t get entangled in the valve stem or rub on the wheel bearing retainer. 

After the wheel is secured in place, the valve stem should be removed and the tube inflated and deflated two or three times to remove any wrinkles. Then the valve stem can be reinstalled and the tire inflated to its proper pressure. (Correct tire pressures are found in the POH.) 

After the outboard brake and/or wheel pant are reinstalled, the tire change will be complete. The aircraft owner will also have the satisfaction of having completed the work himself (or herself)—and will have hopefully have saved a few bucks in the process.

 

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe attended Kentucky Tech to obtain her A&P license. She has worked as an airline mechanic and on a variety of General Aviation planes, and has logged over 5,000 hours of flight instruction time. Send question or comments to .

RESOURCES >>>>>

Leakguard butyl inner tubes by Aero Classic
Aircraft Spruce & Specialty
Airstop® aircraft inner tubes
Michelin North America Inc.
LPS 3 Premier Rust Inhibitor
ITW Pro Brands
Read more...
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