Making the best of a messy situation
By Joe Escobar
Fuel cell maintenance is definitely not on the top of the list of a mechanic's most favorite tasks. Fuel leaks and subsequent fuel cell work often mean you go home at the end of your shift with that unmistakable smell of fuel permeating your clothing and skin. But there are some tips you can use to help make this messy job a little easier.
The first indication that there is a leak is usually fuel dripping on the bottom of the aircraft. When you start investigating for the source of the leak, be sure to keep in mind that fuel often times travels a long way before finding an exit point. This can sometimes make it difficult to locate the source of the leak. Don't assume that the leaking fuel cell is near the visible leak. Be sure to follow the leak path before pulling a suspected leaker.
Hartwig Aircraft Fuel Cell Repair offers the following when inspecting for leaks. "Before removing the cell, check carefully to determine possible areas where the leak or leaks may be originating. Inspect the wing or fuselage cavity and trace any stains or fuel back to their original source. Because of the effect gravity exerts on fluid movement, leaks often do not originate close to where the leak is most visible externally."
Also remember that other components of the fuel system can leak. Connections, hoses, and vent areas should be checked for leaks. Also, just because a fuel cell is leaking doesn't necessarily mean it needs to be replaced. There are some leaks on a fuel cell that can be corrected. These include:
Loose hose clamps
Loose transmitter screws
Loose plate or filler neck bolts
Cracks on the filler neck or tubing
Taking the time to investigate these items can sometimes prevent unnecessarily changing a fuel cell.
If you have inspected the fuel cell and determine that it needs to be removed, here are a few tips from Eagle Fuel Cells on making the removal task a little easier.
- Have a positive attitude. Having a good attitude will make this task go easier.
- Read the technical data. Not following procedures can cause damage to the cell and/or the aircraft.
- Tape the edges of openings. This will prevent damage to the cell as well as protect your flesh from the rough metal edges.
- Loosen clamps and let the rubber relax. Remember that the fuel cell may have been installed on the aircraft for many years, and the rubber takes a set underneath the clamps. If you loosen the clamps and let the cell sit for a while, it will allow the rubber to relax, making removal easier.
- Warmth is your friend. Cold fuel cells are stiff, and make the process that much more difficult. If possible, try to work with warm fuel cells. Even letting the aircraft sit in the sun for a short period can warm the cell up enough to make the process easier. Be careful using heaters and blow dryers as they can be an ignition source. Vapors in a defueled aircraft are more combustible than a fully fueled aircraft.
- Roll the fuel cell into a tube lengthwise before removing. This will make removal easier than just grabbing parts of the cell and trying to pull it out the access panel.
Inspect the fuel cell
After removal, inspect the fuel cell for any obvious damage. An inspection can reveal damage to the fuel cell that can be avoided in the future such as FOD damage or damage from a fueling nozzle.
Avoid the pressure
Eagle Fuel Cells strongly recommends not pressurizing a fuel cell to find a leak. Technicians at Eagle Fuel Cells pressurize the bladder to very low limits when checking for leaks. Pressurizing a cell to just over 1/4 pound pressure can stress seams and damage the tank. In addition, soap solutions that a mechanic would typically use to find a leak can actually mask small leaks that can be present in the cell.
Packing the cell for shipping
When ready to pack the fuel cell for shipment, wipe all residual fuel from the cell. Hartwig Aircraft Fuel Cell Repair suggests cleaning the cell with warm water and liquid dish soap. Once the cell is clean and dry, it can be carefully folded and packed for shipment to the repair facility.
Some mechanics apply an oil film to the cell before shipment thinking it helps in the preservation of the cell. Kurt with Eagle Fuel Cells stresses not to do this. "It is not necessary to coat the cell with oil. It just adds work to the process on our end."
Fuel cell bay area inspection
Perform an inspection of the fuel cell bay area. Clean and treat corrosion. Remove bad tape and residue. Tape residue can be removed fairly easily with MEK. Remove all FOD, especially metal shavings, as these can quickly damage a newly installed fuel cell.
When taping the liner, be sure to use fuel cell tape. Although it may seem acceptable to use duct tape or electrical tape, they are not the same as fuel cell tape. The tape and adhesive used in fuel cell tape remains fairly intact in the event of a fuel leak. Duct tape, electrical tape, and other tapes do not. If you have ever had to clean out duct tape residue from a fuel cell cavity, you know the difference.
Fuel cell tape should be applied over all rivets and edges. It is usually easier to work with short sections of tape, around 6-inches long, than trying to work with long sections. This is especially true when taping blindly in hard to reach areas. A mirror can also aid in helping see what you are doing in difficult areas.
Installing the cell
Now it is time to install the replacement cell. There are a few tips to make this job a little easier.
Have a positive attitude. Just as in the removal step, maintaining a positive attitude is essential.
Read the maintenance manual. Be sure you are familiar with the installation process before beginning the installation. Failing to install the cell properly can result in damage to the fuel cell or aircraft or lead to leaks in the future.
Again, warmth is your friend. Cold fuel cells are stiff. If the hangar is cold, try keeping the cell in a warm storage room or office until ready to install. The warmer the cell is, the easier it will be to handle.
Make sure edges of openings are taped to prevent damage to the fuel cell and yourself.
Do a final FOD check of fuel cell bay and for adequate taping.
Roll the tank into a tube lengthwise. Carefully slide fuel cell into bay. Attach all supports (bayonet snaps, lanyards, etc.) and attachments.
Kurt offers a tip on attaching clamps to fuel cells. "If you are working with a fuel cell that is difficult to get to the snaps from outside the cell, you can tape the bayonet snaps in place and work from inside the fuel cell to snap them into the receptacles." To do this, you can slide a slim flat screwdriver underneath the clip and support loop. Then stick a piece of tape to the screwdriver and pull through the loop with the adhesive facing the clip. Once pulled through underneath the clip, just loop around over the clip to hold it in place. Kurt recommends placing the clip at the most outboard (from the center of the cell) position to account for possible fuel cell shrinkage. Kurt also explains another issue pertaining to support straps. "At our facility we sometimes punch a hole in the support strap next to the metal hole to account for cell shrinkage. If you see a punched hole, use that hole instead of the metal one. We punch them when we find noticeable shrinkage has occurred."
Kurt recommends double clamping fuel cell connections whenever possible. This helps prevent leaks in older cells, but is also beneficial for new ones with large interconnects.
After tightening the clamps, the rubber will settle. Because of this, it is a good practice to re-tighten all the clamps an hour after the initial install.
Once you are done with the fuel cell installation and the aircraft is fueled, a final leak check is necessary. John Stewart of Dallas, Texas-based Southwest Airframe and Tank Services (S.W.A.T.) stresses not to rush this process. "Always complete a leak check after the system has been fully fueled for at least eight hours," he states. This way, you are more likely to find any leaks that exist. If you fuel the aircraft and perform your leak check immediately, you will notice large leaks, but slow leaks may not be visible immediately.
Another thing that Stewart recommends is to rely on the experts if you need to. Companies like S.W.A.T. that perform fuel cell maintenance on a regular basis become experts in their field. If you are unfamiliar with the aircraft or don't perform fuel cell changes often, consider relying on these experts to do the job.
These have been a few tips on fuel cell maintenance. They could help ensure your next fuel cell change won't be such a messy situation.
Eagle Fuel Cells
Hartwig Fuel Cells
Floats & Fuel Cells