Fuel Tank Repairs – How We Fix Them

Every week I receive several phone calls and e-mails regarding fuel tank leaks, with questions such as:

“I smell fuel when I first get in the plane and then the smell goes away”;

“I have a screw on the panel at the cabin door that leaks when the tank is full”;

“I have fuel stains in the wheel well”;

“ I have a panel under the wing that leaks”; and

“ How much will it cost to have my tanks resealed?”

In most cases, the leak can be repaired and a total “strip and reseal” is not needed. We repair at least one fuel tank per week.

Fuel Smells in the Cabin

Your plane has been closed up for a period of time. You open the cabin door and smell fuel, and within a few minutes the smell is gone.

The source of the fuel smell can be a fuel tank leak, a leaking fuel selector valve, or leaking fuel gauge sending unit. The leading edge of our Mooney wing is hollow. A fuel leak in the forward or outboard tanks will leak into the leading edge of the wing. The leading edge of the wing is open to the cabin at the forward edge of the seat track on each side of the cabin. Fuel fumes or smells enter the cabin at this point. Holes are drilled in the lower leading edge of the wing at each rib and at the fuselage. These holes are to prevent fuel from accumulating at the ribs and from entering the cabin. Fig. 1 is an example of the weep hole at the fuselage showing fuel leakage stains.

Fuel Selector

Fuel selector valve leaks are evident by stains around the fuel selector stem. In most cases these leaks are repaired by disassembling the selector and replacing the O-rings.

Sending Units

Mooney M20 B thru E models have fuel gauge sending units located inside the cabin, just in front of the rear seats on each side of the cabin. Models F thru S have sending units in the same location, as well as, having an outboard sending unit in each wing. These sending units have cork or neoprene gaskets. Leaks in these areas will be detected by blue stains on the tank walls below the sending units. In most cases, tightening the screws that attach the sending unit will stop these leaks.

Panel Screws

A leak found at a screw in an access panel is usually caused by a cracked nut plate. Fig. 2. The nut plate in the fuel tank has a plastic cap to keep the fuel from leaking around the screw threads. These caps, over time, become cracked due to age or possibly someone has replaced the screw with one that is too long. To repair the leak, remove the screw. You do not have to drain the tank. Apply a fuel resistant sealant, such as Permatex #3, to the screw threads and replace the screw in the panel.

Fuel Tank Construction

A Mooney fuel tank is a metal box constructed out of several individual pieces. Once assembled, each seam is sealed with layers of a fuel resistant sealant. If this sealant is damaged by a hard landing, or by age, fuel can leak between the sealant and the tank walls, migrating to a point where it can exit. This point of exit may be a rivet or seam between wing skins.

I have come across logbook entries where the same rivet has been replaced several times trying to stop a leak. Using our method, we have located the source of these leaks in other parts of the tank.

Finding Leaks

I did not invent this procedure, nor do I remember who told me; however, this is how our service center finds leaks in Mooney fuel tanks.

1. The first rule to remember in chasing a fuel leak is: The source of the leak is never where the leak appears on the out side of the tank.

2. Remove the fuel from the tank.

3. Turn the fuel selector off.

4. Remove the top fuel tank access panels on the wing that has the leak. The panels are removed by first removing the screws.

5. Once the screws are removed, we use masking tape to tape around the access panel and the middle of the panel for protection of the paint. The panel is sealed with a sealant. Do not beat on the panels. An elephant could stand on the panel and it would not come loose.

6. Next, we use a thin putty knife, that has been sharpened on one side, to slide in between the seam of the access panel and the wing. Using a nonmetallic hammer, gently work the putty knife between the skin and panel. It may take several times around the panel with the blade and hammer, each round a little deeper under the skin, until the panel releases.

7. Once the panels are removed we place mirrors in the bottom of the tank so that we will be able to see the upper seams of the tank. Do not cover the stringers in the bottom of the tank. Many leaks are in this area.

8. Next we apply liquid hand soap, thicker the better, to all the seams in the tank.

9. We now cover all the removed access holes in the wing with Plexiglas. Fig. 3. The Plexiglas covers the entire hole including screw holes. The Plexiglas is held in place by masking or duct tape around the out side border forming a seal. The Plexiglas can be cut from an old windshield or purchased at most hardware stores. New glass is preferred as you will be looking thru the glass to find our leak source.

10. Next we take a standard shop vacuum cleaner. We attach the hose from the vacuum cleaner to the vent on the fuel tank. And no, this will not collapse your tank. Fig. 4

11. Turn on the vacuum and, using a flashlight, look thru the Plexiglas panel and look for bubbles in the soap. The mirrors should be positioned so that you can see the entire interior of the tank.

12. Bubbles indicate the source of the leak as air is drawn into the tank thru the leak.

13. Once the leak, or leaks, are found, Fig .5, the area will need to be thoroughly rinsed with water, dried, cleaned and repairs made using procedures in the Mooney Service Manual..

14. Two types of sealant are recommended in the manual. PRC and Flame Master brands. We use Flame Master CS3204B-2 for repairs in the tank and topped with CS-3600 for a protective coating and CS3330B-2 for access panel sealant. Fig 6. We use sealants in Semkits that contain both sealant & activators. They are mixed together in a small caulking tube. Once mixed together, we transfer the sealant to a cup and apply with small acid brushes. The sealant will apply easily if thinned with a small amount of MEK.

15. Replace the access panels using CS3330B-2. I recommend at least 48 hours before fuel is added to allow for proper curing of the sealant. Temperature is key to a successful repair. Avoid cold weather & high humidity if possible.

Most leaks are easily found and repaired using this method. I share this procedure with you so that you and your mechanic can locate and repair your Mooney’s fuel leak. We welcome you to visit our service center at any time.

Please visit our Web Site at www.donmaxwell.com for this and other articles published in the Mooney Log.

Don Maxwell
903-643-9902 or e-mail: dmaxwell@donmaxwell.com

MAPA Log (February 2005)