Cessna Hawk XP R172K Pilot Operating Handbook page 72

SECTION 7
AIRPLANE
&
SYSTEMS DESCRIPTIONS
CESSN/\
MODEL R172h
1
CI';SSNA
MODEL R172K
SECTION 7
AIRPLANE
&
SYSTEMS DESCRIPTIONS
skids, slips, or unusual attitudes.
The fuel selector valve should be in the BOTH position for takeoff.
climb, landing, and maneuvers that involve prolonged slips or skids.
NOTE
When the fuel selector valve handle is in the BOTH posi-
tion in cruising flight, unequal fuel flow from each tank
may occur if the wings are not maintained exactly level.
Resulting wing heaviness can be alleviated gradually by
turning the selector valve handle to the tank in the
"heavy" wing.
NOTE
It
is not practical to measure the time required to con-
sume all of the fuel in one tank, and, after switching to
the opposite tank, expect an equal duration from the re-
maining fuel. The airspace in both fuel tanks is inter-
connected by a vent line and, therefore, some sloshing of
fuel between tanks can be expected when the tanks are
nearly full and the wings are not level.
The auxiliary fuel pump switch is a toggle-type switch labeled
HIGH, OFF, and LOW, and is located on the left side of the instrument
panel. The HIGH position of the switch is spring loaded to the
OFF
position, and is used primarily for engine starting. The HIGH position
is also used in the event of an engine-driven fuel pump failure during
takeoff or high power operations, and for extreme vapor purging.
Holding the switch in the HIGH position will provide maxirnum fuel
flow.
NOTE
If the auxiliary fuel pump switch is accidentally placed
in the HIGH or LOW position with the master switch on,
mixture rich, and the engine stopped, the intake mani-
folds will be flooded.
The LOW position of the switch is used for minor vapor purging
and continued engine operation in the event of an engine-driven fuel
pump failure. When the switch is placed in the LOW position, the
auxiliary fuel pump will operate at one of two flow rates depending on
the position of the throttle. With the throttle in a cruise flight position,
the pump will provide a high enough fuel flow to maintain flight in the
7-24
ovent of an engine-driven fuel pump failure. As the throttle is moved
toward the closed position (during letdown, landing, or taxiing), fuel
tlow
provided by the pump is automatically reduced by a throttle-
uctuated
switch, preventing an excessively rich mixture during periods
of reduced engine speed.
NOTE
If the engine-driven fuel pump is functioning and the
auxiliary fuel pump switch is placed in the LOW posi-
tion, an excessively rich fuel/ air ratio is produced unless
the mixture is leaned. Therefore, this switch should be
turned off during takeoff.
If it is desired to completely exhaust a fuel tank quantity in flight,
the auxiliary fuel pump will be needed to assist in restarting the engine
when fuel exhaustion occurs. Therefore, it is recommended that proper
operation of the auxiliary fuel pump be verified prior to running a fuel
tank dry by placing the auxiliary fuel pump switch in the HIGH posi-
tion momentarily and checking for a slight rise in fuel flow indication.
To ensure a prompt engine restart in flight after running a fuel
tank dry, immediately switch to the tank containing fuel at the first
indication of fuel pressure fluctuation and/ or power loss. Then place
the auxiliary fuel pump switch in the HIGH position momentarily (3 to
5 seconds) with the throttle at least
1/2
open. Excessive use of the HIGH
position at high altitude and full rich mixture can cause flooding of the
engine as indicated by a short
(1
to 2 seconds) period of power followed
by a loss of power. This can later be detected by a fuel flow indication
accompanied by a lack of power. If flooding does occur, turn off the
auxiliary fuel pump switch, and normal propeller windmilling should
start the engine in 1 to 2 seconds.
If the propeller should stop (possible at very low airspeeds) before
the tank containing fuel is selected, place the auxiliary fuel pump
switch in the HIGH position and advance the throttle promptly until
the fuel flow indicator registers approximately
1/2
way into the green
arc for 1 to 2 seconds duration. Then retard the throttle, turn off the
auxiliary fuel pump, and use the starter to turn the engine over until a
start is obtained.
The fuel system is equipped with drain valves to provide a means
for the examination of fuel in the system for contamination and grade.
The system should be examined before the first flight of every day and
after each refueling, by using the sampler cup provided to drain fuel
from the wing tank sumps, and by utilizing the fuel strainer drain
7-25