Figure 12–40.—Bleeder bomb.
bleeder valve, and the free end of the hose is placed in
a container that has enough hydraulic fluid to cover the
end of the hose. The air-laden fluid is then forced from
the system by applying the brakes. If the brake system
is a part of the main hydraulic system, a portable
hydraulic test stand may be used to supply the pressure.
If the system is an independent master cylinder system,
the master cylinder will supply the necessary pressure.
In either case, each time the brake pedal is released, the
bleeder valve must be closed or the bleeder hose pinched
off; otherwise, more air will be drawn back into the
system. Bleeding should continue until no more air
bubbles come through the bleeder hose into the bleeder
container.
BOTTOM-UP METHOD.—In the bottom-up
method, the air is expelled through the brake system
reservoir or other specially provided location. Some
aircraft have a bleeder valve located in the upper brake
line. In this method of bleeding, pressure is supplied by
a bleeder bomb. A bleeder bomb (fig. 12-40) is a
portable tank in which hydraulic fluid is placed, and then
put under pressure with compressed air. The bleeder
bomb is equipped with an air valve, air gauge, and a
connector hose. The connector hose, which attaches to
the bleeder valve on the brake assembly, is provided
with a shutoff valve. Normally, the hose is connected to
the lowest bleed fitting on the brake assembly. With the
brake bleed fitting opened, opening the bleeder bomb
shutoff valve allows pressurized fluid to flow from the
bleeder bomb through the brake system until all the
trapped air is expelled. The brake bleeder valve is then
secured, and the bleeder bomb hose is disconnected.
This method of bleeding should be performed
strictly in accordance with specific instructions for the
aircraft concerned. Although the bleeding of individual
systems presents individual problems, the following
precautions should be observed in all bleeding
operations:
1. Ensure that the bleeding equipment is absolutely
clean and filled with the proper type of hydraulic fluid.
2. Maintain an adequate supply of fluid during the
entire operation. A low fluid supply will allow more air
to be drawn into the system.
3. Continue bleeding until no more air bubbles are
expelled from the system and a firm brake pedal is
obtained.
4. Check the reservoir fluid level after the bleeding
operation is completed. With brake pressure on, check
the entire system for leaks.
Overheated Wheel Brakes
In the event an aircraft has been subjected to
excessive braking, the wheels may be heated to the point
where there is danger of a blowout or fire.
NOTE: Excessive brake heating weakens tire
and wheel structures, increases tire pressure,
and creates the possibility of fire in the
magnesium wheels. When the brakes on an
aircraft have been used excessively, the fire
department should be notified immediately, and
all unnecessary personnel should be advised to
leave the immediate area.
If blowout screens, such as the one shown in figure
12-41, are available, they should be placed around both
main wheels. These screens help to eliminate the
possibility of damage or injury in the event of a blowout.
Sudden cooling may cause an overheated wheel to
fracture or fly apart, which could hurl bolts or fragments
through the air with sufficient speed to injure personnel.
Required personnel should approach overheated wheels
with extreme caution in the fore or aft directions-never
in line with the axle.
NOTE: The area on both sides of the tire and
wheel, in line with the axle, is where the
fragments would be hurled if the tire were to
explode; therefore, it is called the danger area.
See figure 12-41.
12-44
