is installed in the converter system to relieve
The supply sequence of the liquid oxygen
system consists of two modes of operationthe
economy mode, in which gaseous oxygen is fed
from the converter, and the demand mode, in
which oxygen flows from the converter as a
liquid and vaporizes to a gas in the feed line.
In the economy mode of operation (fig. 4-15,
supply sequence, view C), limited demand upon
the system allows the converter to supply gaseous
oxygen directly as a result of drawing off the
gaseous oxygen stored within the top of the
converter. At approximately 82 psi, the pressure
opening valve unseats and allows gaseous oxygen
to flow from the converter to the supply system.
Oxygen then flows from the upper (gas) portion
of the converter, rather than the liquid side. When
the amount of oxygen demanded by the crew
exceeds the supply capabilities of the economy
mode, the pressure opening valve closes. As the
crew continues to draw upon the oxygen supply,
the supply system pressure becomes lower than
that of the converter. When a pressure differential
of 5 psi occurs, the differential check valve opens
(fig. 4-15, supply sequence, views C and D) and
allows liquid oxygen to flow into the supply line,
thus creating the demand mode. Converter
pressure will build up while the system is operating
in the demand mode. As the pressure again
approaches 82 psi, the pressure opening valve will
again unseat, switching the supply sequence back
into the economy mode. The converter
automatically switches itself back and forth
between the economy and demand modes while
supplying oxygen to the crew.
Extreme care must be exercised when installing
units in an oxygen system. The life of the pilot
and crew depends on the thoroughness with which
the AME does this job.
All maintenance of LOX systems must be
done in accordance with the instructions contained
in the applicable MIM. The AME assigned to do
LOX system maintenance should also be familiar
with the various instructions pertaining to
handling LOX and maintenance of the related
The actual removal and installation procedures
used in maintaining LOX systems will vary from
one aircraft to another; however, the following
precautions will apply to almost any aircraft
1. Use only tubing assemblies that have been
tested, cleaned, capped, and properly identified
as oxygen lines.
If lines are fabricated locally, ensure that
only clean, oil-free tubing and fittings are
used. Also ensure that no hydraulic fluid is
used in the fabrication procedure.
2. Use only the type of fittings specified for
the particular oxygen system. Never use fittings
with pitted or otherwise disfigured cones or
3. It is MANDATORY that EXTREME
CAUTION be exercised with regard to cleanliness
of hands, clothing, and tools. It must be
emphasized that all items that come into contact
with the oxygen system must be free of dirt, oil,
4. Use thread antiseize tape that is approved
under specification MIL-T-27730A.
5. When installing tubing assemblies between
fixed units, the tube assembly should align without
the use of undue force.
6. The torque values specified for the
particular oxygen system should be strictly
adhered to when tightening the fittings.
7. If a section of line is left open or
disconnected during an installation, the open
fittings must be covered with suitable caps or
plugs. When making connections, be certain that
no lint, dust, chips, or other foreign material is
allowed to enter the oxygen system.
8. Upon completion of the installation of a
tube assembly or component, a pressure check of
the system should be conducted. The system
should be pressurized and the connections checked
with a leak-test solution conforming to specifica-
tion MIL-L-25567B. After the connections have
been checked, the leak-test solution should be
washed off with clean water.
9. The aircraft liquid oxygen system should
be purged after the replacement of any compo-
nent or tubing assembly.
10. The type of clothing and footwear that is
worn when maintaining and servicing a liquid
oxygen system is an extremely important factor.
Do not wear anything that will produce sparks
or static electricity, such as nylon clothing or shoes