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