FUEL TANKS AND TANK
VENTING SYSTEM
TIP TANKS
The tip tanks are positioned at 2° nose up rel-
ative to the wings. Two attach points secure
the tank to the wing. Access covers on the top
of the tank provide entry for inspection and
maintenance. Baffles are installed to mini-
mize fuel shift and prevent adverse effects of
shifting fuel on the airplane center of gravity
during extreme flight attitudes. Each tip tank
holds approximately 1,235 pounds of fuel.
An electric boost pump (23 models without jet
pumps) or a jet pump (all other) in each tip tank
transfers fuel into the associated wing tank.
Fuel gravity flows through flapper check
valves into the wing tank, but fuel in the lower
half must be transferred by the pump. A vac-
uum-relief valve is installed in the left tip tank
on the airplanes SNs 24-190 and 25-030 and
subsequent. This is available on earlier 20 se-
ries Learjets as a retrofit. This valve prevents
a vacuum in the fuel vent system.
The tip tanks contain a fuel quantity probe
and two sump drains. The tank is vented
through two vent float valves located in the
forward and aft ends of the tank. A filler cap
located on each tip tank is used to service the
entire airplane fuel system.
WING TANKS
Each wing tank extends from the airplane cen-
terline to the tip tank and holds 1,092 pounds
of usable fuel (23 model) or 1,160 pounds (all
other models). Areas which are not part of the
wing fuel cell are the main landing gear wheel
well, the leading edge forward of spar one (wing
heat area), and the trailing edge between spars
seven and eight (flap, spoiler, and aileron areas).
The 2.5° wing dihedral makes the inboard por-
tions of the wing tanks the lowest areas. Two
fuel pumps are located within each of these
areas and will remain submerged in fuel until
the tanks are nearly empty.
NOTE
The 23 models without jet pumps
have two electric fuel pumps in each
wing tank. All other Learjets have
one electric standby pump and one jet
pump in each wing tank.
Wing ribs and spars act as baffles to minimize
fuel shifting. Flapper valves located in the wing
ribs allow unrestricted inboard flow of fuel and
limit outboard flow. Two pressure-relief valves
at the centerline rib equal equalize internal
pressures between the two wing tanks (except
23 models). The wing tanks begin to fill through
the two tip tank flapper valves as tip tank fuel
increases beyond one-half full.
Two fuel probes in each wing tank provide in-
formation to the fuel quantity indicating system.
FUSELAGE TANK
The fuselage tank consists of two bladder-
type cells located in the cavity between frames
22 and 24 on 23, 24, and 24B models and be-
tween frames 22 and 25 on the 25 model. The
tank contains one electric fuel transfer pump,
a float switch, a fuel probe, and a drain valve.
The crossover interconnects the two cells to
form one fuel storage tank. Fuselage tank fuel
must be transferred to the wing tanks for use.
Fuel capacity on models 23, 24, and 24B is ap-
proximately 840 pounds, and on model 25,
approximately 1,306 pounds.
RAM-AIR VENT SYSTEM
The fuel vent system supplies ram-air pressure
to the fuel tanks through two ram-air masts.
One mast is located on each side of the aft fuse-
lage. Air from the left mast flows through a
check valve to the forward wing tank sump
(Figure 5A-4). Air from the right mast flows
to the forward wing tank sump and to the fuse-
lage tank. From the forward sump, ram air
flows through vent lines to the tip tanks and
to the wing tanks through the wingtip inter-
connects. Relief valves limit pressure to .8 psi.
The vent system prevents fuel tank overpres-
surization or vacuum formation.
5A-5
FOR TRAINING PURPOSES ONLY
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