RAM-AIR VENT SYSTEM
A NASA ram-air scoop located on the under-
side of each wing (Figure 5-2) supplies posi-
tive air pressure in flight to a manifold which
directly vents the fuselage tank and both tip
tanks. Each wing tank is indirectly vented to
its own tip tank through a length of tubing, the
ends of which extend to the uppermost area of
each tank (Figure 5-3). The NASA scoops, by
design, do not require heating to remain ice-
free. Two vent float valves are located in each
tip tank and are in the fuselage tank except in
model 25C. The float valves close when the
fuel level reaches the vent ports, preventing
fuel from entering the vent lines. A vacuum-
relief valve in each tip tank and the fuselage
tank opens to allow air to enter the tanks should
vacuum conditions occur during fuel trans-
fer. Each tip tank has two pressure-relief valves
which protect the tanks from excessive pres-
sure due to thermal expansion of fuel in the
tank when the vent float valves are closed.The
pressure-relief valves are set at 1.0 and 1.5 psi,
the second valve providing a backup in case
the first valve fails.
Thermal expansion of fuselage fuel is accounted
for by a continuation of the fuselage vent line
bypassing the fuselage vent float valve, thereby
relieving pressure overboard through the NASA
scoops. A sump, installed in the vent manifold,
located at the bottom center fuselage just aft of
the main landing gear, collects any fuel which
might enter the vent lines during refueling. A
vent drain valve permits draining of the sump,
which must be accomplished prior to every
flight to ensure that the vent line to the fuse-
lage tank is unobstructed.
FUEL INDICATING
SYSTEMS
FUEL QUANTITY INDICATING
SYSTEM/LOW FUEL WARNING
The fuel quantity indicating system includes an
indicator and tank selector switch located on
the fuel control panel (Figure 5-4). A red LOW
FUEL warning light (“Annunciator Panel” sec-
tion) illuminates if either wing tank fuel level
is less than approximately 450 pounds.
The fuel quantity indicating system uses DC
power from the right and left essential buses
through the FUEL QTY circuit breakers. The
six-position rotary selector switch enables the
pilot to check the fuel quantity in each of the
five tanks and the airplane total fuel quantity.
The fuel quantity for the position selected is
read on the fuel quantity indicator. The quan-
tities printed beside each selector switch po-
sition indicate usable fuel capacities in pounds.
There are seven capacitance fuel probes. One
fuel probe is located in each tip tank and in the
fuselage tank. Each wing tank has two probes
wired in parallel. The inboard probe in the left
wing contains a temperature compensator which
adjusts quantity readings for all switch selec-
tions for fuel density change due to temperature.
5-4
FOR TRAINING PURPOSES ONLY
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
Figure 5-2. Ram-Air Vent and Vent Sump
Drain (Typical)
