December 1, 2006
Steven W. Ells
Fuel screens usually don't require maintenance between 100-hour or annual inspections, so there's scant likelihood that pilots will ever need to know how to inspect and clean these important components of their airplane's fuel systems. With that said, however, it's always a good idea for pilots to participate in the maintenance of their airplanes. Helping and watching your maintenance technician perform an annual inspection on your airplane — including inspecting the fuel screens — will add immeasurably to a better understanding of your airplane's systems. This in turn will further help you to describe discrepancy symptoms when talking with A&P technicians. You may never need to inspect a fuel screen, but it never hurts to know how to if the day comes.
Suggestions that aircraft owners install oil filters, as well as general directions for changing spin-on-type filters, were discussed in AOPA Pilot in May 2005 ("Airframe & Powerplant: The Unfiltered Truth"). Cleaning fuel filters is a new topic so let's take a look at the basics of fuel-system filtration in a typical single- or multiengine piston-powered light airplane.
FAR Part 23, the current airworthiness standards rule for certified aircraft, says basically that fuel strainers must be capable of capturing contaminants so that clean, dry fuel is always supplied to fuel-metering devices such as carburetors and fuel-injection systems. Some airplanes' strainers contain a great deal of contaminant-catching screen area, and other older aircraft have very small screens.
Fuel-borne contaminants include dirt that enters during refueling; lint; fuel tank sealants that break down because of age; rubberlike material from fuel bladders that deteriorate because of abrasion or age; and water. The last three items are the most common. Water is removed via quick drains—the screens must be removed to get at the solid contaminants.
The safeguards installed in the fuel system of a 1966 Cessna 182 are typical for single-engine general aviation airplanes.
Cessna installed flexible baglike fuel bladders in 182s until 1978, when integral "wet wing" tanks were adopted. With very few exceptions every fuel tank in a general aviation airplane is equipped with a forward and a rear fuel-tank outlet. A coarse brass screen — eight to 16 meshes per inch — projects 2 or 3 inches into the bladder tank. Fuel must pass through the screen before entering the feed line. Imagine that your little finger was covered with a coarse-mesh screen. That's about the size of it. This screen keeps out chunks of contamination. It is not required to be inspected or cleaned during routine maintenance.
The next contaminant catcher is the primary one. In most Cessnas the main fuel screen is incorporated into the fuel-strainer assembly. When the term assembly is used in aircraft maintenance it simply means all the parts that make up the main component. For instance, the fuel-strainer assembly consists of the top casting, screen gasket, screen, bowl, bowl gasket, plunger and assorted seals, standpipe, retainer washer, and retainer nut. For convenience, the assembly is just referred to as the "fuel strainer."
The Cessna main fuel filter — that's what Cessna calls it; in reality it's a screen — consists of a pair of oval-shape brass caps that form the top and bottom of the screen assembly. The sides are formed by fine brass screen — 200-mesh per inch — that is soldered to the edges of the caps. Fuel flowing in from the fuel selector must pass through the screen assembly before flowing on to the fuel-metering device.
The Cessna fuel filter/screen has quite a bit of area compared with the "fuel filters" of aircraft made by some other manufacturers. Some fuel-system filters are nothing more than a single layer of screen no larger than the top of a small juice can. Newer designs such as the Columbia line of high-performance singles have much higher-quality fuel-strainer/screen assemblies with a great deal more screen area.
Piper, Mooney, and Beechcraft fuel systems have the same basic clean-fuel safeguards as Cessna systems, with one addition. Carbureted low-wing airplanes such as Pipers and Mooneys must pump fuel up from wing-located fuel tanks to the engine compartment. These aircraft are equipped with electric low-pressure clicker-type (3 to 5 pounds per square inch) fuel pumps to provide redundancy for the engine-driven fuel pump. These boost pumps also have a small internal filter that is accessible by removing the bottom twist-on cap.
Carburetors and fuel-injection systems all have internal screens as the last bulwark of defense to keep contaminants from gumming up or fouling the internal close-tolerance parts. In spite of all these safeguards, clogged fuel-injection nozzles are evidence that the system isn't foolproof.
These internal screens are typically called "finger screens" because of their appearance. These screens are the finest in the system at about 200-mesh per inch. Screens must be cleaned at least every 100 hours under normal conditions. During dusty conditions or off-airport operations this interval should be shortened as necessary.
As with many of the preventive maintenance tasks listed in Part 43, Appendix A, there are vast differences of opinion as to how much latitude is granted an owner under the "clean and inspect fuel or oil system strainers or filter elements" sentence in the FARs.
It is my opinion that many inspectors are uncomfortable with the idea of an average pilot performing the task of pulling the finger screen out of a fuel-metering device such as a carburetor or fuel-injection system. I also would bet that every airframe and powerplant (A&P) mechanic I have ever met would want to watch closely during an owner's first or second attempt to clean the main fuel screen, or finger screen, of a carburetor or fuel-injection system. It's not technically difficult, but the margin between saving a few dollars and stripping the threads in the $1,500 aluminum housing by over-torquing the steel fuel line adapter is pretty slim.
With the exception of the coarse fuel-tank inlet finger screens, every other screen is typically removed and cleaned at each annual inspection.
If you, as an airplane owner, want to know how to do this task, ask your A&P technician to show you the ropes when your airplane is next in for an annual. A rough description of the procedure, with a few added caveats gleaned from my experience cleaning fuel screens, follows.
First, turn the fuel selector to the Off position. Since skin contact with hydrocarbons is known to cause health problems, pick up a pack of disposable gloves and wear them while working on your airplane's fuel system.
After locating the fuel-strainer assembly, which is always at the low point in the entire fuel system, take a close look at it. It should be clean and dry. If any blue stain is visible, there's a possibility that the assembly wasn't cleaned at the last annual, but it's more likely the stain indicates a fuel leak.
Leaks are commonly seen where the shaft of the drain plunger enters the body of the top casting, and at the joint where the bowl separates from the top casting. If there's evidence of a leak, find out why and fix it.
The next step is to cut the safety wire and loosen the screw or nut that holds the fuel-strainer cover in place. Take a good look at the parts because they have to be reinstalled in the proper order. A trick that helps is to lay out a shop towel and put the parts on the towel in the order and in the orientation that they are removed. In other words, write down or up on one end of the towel and then put every part on the towel in order of removal and in orientation to the top or bottom as selected.
After removing the bowl, clean it, and take a good look inside. If there has been water standing in the strainer at some time in the past, it will be visible because of discoloring and/or corrosion. If the corroded area is small, you may be able to polish it out with a piece of a Scotch-Brite pad. If you have any doubts about the airworthiness of the bowl, consult your mechanic. This also is the best time to install a new sealing O-ring on the fuel-bowl drain petcock.
Next, check to see that fuel doesn't continue to drip out of the fuel-strainer housing. If you're seeing a steady drip it means that fuel is leaking past the shut-off position of the fuel-selector valve. These valves are not supposed to leak. Consider rebuilding or sending the valve for overhaul if it's leaking. Contact your type club for names of parts suppliers and qualified overhaul shops.
Once the cover is off, the filter screen may fall out of the housing. That's what happens when I remove the cover of the strainer assembly on my 1960 Piper Comanche. Cessna screens are held in place by a spring pin, or by the threaded standpipe. The standpipe must be removed to remove and clean the screen.
There's a right and a wrong way to remove the threaded standpipe. The wrong way is to grab the standpipe with a pair of pliers and apply an unscrewing force. The correct way is to pull up on the fuel-strainer drain knob, and apply an unscrewing force by inserting a pin punch through the holes in the standpipe.
Remove all debris from the screen. Some well-equipped shops use ultrasonic cleaners for this task, but most just blow the debris from the inside out with compressed air. I wouldn't recommend taking your filter screen home to run it through the wife's ultrasonic jewelry cleaner. It may be the same thing to you but it's most assuredly not to her (unless, of course, she's a pilot too).
Some screens have been in service for a long time and may reflect it by being bent, having fraying edges, or just being so tired that the screen material is thread thin. If you have any doubts about the screen, buy and install a new one; it's money well spent.
When fuel strainers are disassembled for inspection and cleaning, all seals and sealing O-rings should be changed before starting the reassembly. There's a temptation to skimp on this, but remember that the fuel strainer is located behind the firewall in almost every GA airplane. That means that any fuel-strainer leakage will result in an ongoing fuel smell in the cabin. If you smell fuel in the cabin when first opening your airplane door but don't smell it during flight, there's a good chance you have a small fuel leak at the fuel-strainer assembly.
If new seals are installed and the assembly is reassembled but the smell and staining remain, then it's time to look for warped sealing services, or cracks. Unfortunately, it's not uncommon for the sealing surfaces of the top casting of older strainers to be bent.
The reason? The seals and O-rings were not changed and in an attempt to make the tired, old, worn-out things seal, too much torque was applied to the securing screw or nut. This often bends the soft metal of the top casting into a slight U-shape. It's possible to save these by bending the flanges of the casting back into position, but this always requires that the assembly be removed from the airplane.
If the flanges can't be straightened or there's a crack in the assembly, then it's time to get a new strainer assembly. This can add up to some big dollars spent for older airplanes. If a new fuel screen or screen assembly is needed, the best source of information will again be the tech staff at your type club.
If, after reassembly, the fuel valve is turned back on and there is no evidence of leaking, install the appropriate safety wire and sign off your work. Remember that owners must have the appropriate service publications and tools, and must perform their work to the same high standards as a certificated airframe and powerplant technician.
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