through tubing to an overboard discharge
indicator. In the event of excessive pressure
developing within the low-pressure section of the
pressure reducer, the excess pressure will flow
through the relief valve and out the overboard
discharge line. This flow will rupture the green
disc in the discharge indicator, giving a visual
indication of a malfunctioning pressure reducer.
A line from the high-pressure side of the
pressure reducer connects to a gauge in the
cockpit. This gauge gives the pilot an indication
of pressure in the three storage cylinders.
The oxygen flows from the low-pressure side
of the reducer to the three regulators. A flexible
hose attached to each regulator is for attachment
of the oxygen mask.
Portable Oxygen Systems
Portable oxygen systems include walkaround
cylinders, survival kits, and bailout units. These
systems are used primarily to maintain crew
functions in the event of failure of the fixed
oxygen systems. The survival kit oxygen system
also performs the same function during descent
after bailout. All of these are small, lightweight,
high-pressure, self-contained gaseous systems,
which are readily removed from the aircraft.
Walkaround cylinders are standard equipment
on many transport, patrol, and early warning
aircraft, and are used separately or in addition
to a permanently installed oxygen system. Each
system consists of a reducer and regulator
assembly mounted directly on a small oxygen
Figure 4-6 illustrates a high-pressure walk-
around oxygen system. A 295 or 514 cubic inch-
capacity, 1,800 psi cylinder is equipped with a
regulator, which is connected to the cylinder with
a short coiled length of copper tubing. A short
flexible breathing tube, clamped to the outlet of
the regulator at one end and fitted with a
connector at the other end, provides the necessary
assembly for the attachment of the demand mask
tube. Straps fastened to the cylinder bracket
provide the means for securing the unit to the
users seat or part of the aircrafts structure. The
cylinder bracket may be placed horizontally or
stood on end while in use. The straps can be
used as a handle to carry it from place to place.
Because of its weight, the walkaround unit should
not be carried by its breathing tube, regulator, or
The maintenance procedures discussed in this
section are general in nature. Consult the
applicable MIM prior to performing any
maintenance on each specific type of aircraft.
Routine maintenance includes servicing of
cylinders, checking the system and regulators for
Figure 4-6.Walkaround oxygen bottle cylinder.