Retract units should always be checked for
proper torque on all screws. Removing and
reinstalling them with an application of Loctite
Threadlocker Blue will ensure that they don’t
come loose. Inspect all axles and pivot pins for
Today’s jet fuel systems are complicated and
typically have composite-molded fuel tanks
such as this pair of 100-ounce-capacity tanks.
The tanks should be checked for leaks before
installation to prevent kerosene from getting
inside of the airplane.
Fuel systems and brakes
by Jim Hiller
To pick up where my March 2016 column left off, let’s continue setting up a new turbine model for
a test flight and systems check, starting
with the fuel system.
I use the term system because of the
complexity of a modern jet installation.
My new model has two composite-constructed saddle tanks, each with a
capacity of approximately 100 ounces.
They will be set up to feed a center
24-ounce fuel tank, then the Bob Violett
Models (BVM) Ultimate Air Trap
(UAT). I call this my fuel tank system.
You first need to consider whether the
composite tanks are fiberglass, Kevlar, or
carbon fiber, whether they are fabricated
in a mold, and if they are susceptible to
minor problems that could result in fuel
leaks. I completely assemble each tank,
then pressure test it to ensure there are
no leaks in the tank or in the connecting
fittings. I do this test by submerging the
tank under water, with all but one of the
Through that fitting, I blow into the
tank to pressurize it and look for air
bubbles in the water. It’s an easy way to
find minor air leaks. Be careful, though,
because these tanks are not designed to
be pressure vessels, so they will flex and
can be damaged by excessive pressure.
Use only enough air to ensure that
there are no leaks. I seldom find leaks in
composite tanks because of the quality
of the available products.
Before the final installation, I flush
the tanks. It is amazing what can come
out of a new tank, which includes
fibers, loose trimmings, and release
agents. The fuel tanks are now ready to
install, so go ahead and put them in the
airplane. Plumbing should be done with
appropriate-size tubing, such as Tygon.
The UAT is a small tank that
safeguards the turbine pump so it is
only fed clean kerosene that has no
air bubbles. I have used everything,
including a small 6-ounce plastic tank
with no clunk and a brass kerosene
pump line centered in the tank. This
setup is in my Der Jet Vampire and is
simple, easy, and reliable. I’ve had no
problems throughout many seasons of
The next evaluation of the UAT is
similar, except a paper fuel filter is
attached to the brass feed line, again
centered in the tank. This is a trick I
learned many jets ago that is effective,
easily made, and simple. Many of the
latest UATs on the market still use a
paper fuel filter as a fundamental part of
Replace the paper fuel filter with a
bag filter similar to what is attached to
a fuel-tank-mounted gas pump in cars
today. It’s effective and proven when
used in jet applications.
Conduct a final check of the fuel tank
system before moving to the flightline.
Fill the installed tanks completely until
kerosene comes out of the overflows,
while checking for leaks and any other
problems. When full, remove the fuel
line at the turbine fuel pump and pump
out the kerosene, confirming that the
kerosene is feeding out of the tanks as
Ensure that no air is showing up in
the UAT. If found and remedied now, it
105 Model Aviation MAY 2016