Following inspection of the fittings, a thin coat of antiseize
compound, Federal Specification TT-A-580, must be applied to the
threads of the fittings, except for hydraulic and oxygen fittings.
Hydraulic fluid must be used to lubricate fittings of hydraulic
Antiseize compound MIL-T-5542 is used to lubricate
the fittings of oxygen systems.
Aircraft plain checknuts are used to secure the tubing and fitting
assembly together and to connect the entire tube assembly to
components of the plumbing system. Only special-fitting nut wrenches
of the torque-indicating type should be used for installing tube
assemblies. If not available, open-end wrenches can be used.
Tightening the fitting nuts to the proper torque during installation
is very important.
Overtorquing these nuts can severely damage the
tube flare, the sleeve, and the nut.
Undertorquing is equally
serious; it can allow the line to blow out of the fitting or to leak
under pressure. When fittings are properly torqued, a tube assembly
can be removed and installed many times before reflareing becomes
When installing a fitting, through a bulkhead.
Take care to ensure
that the nuts are tight enough to prevent any movement between the
bulkhead and the fitting.
If any movement takes place, vibrations
can cause the fittings to enlarge the hole through the bulkhead
beyond tolerance and damage the fitting.
A FITTING NUT MUST NEVER BE TIGHTENED WHEN
THERE IS PRESSURE IN THE SYSTEM, AS THIS
RESULTS IN AN UNDERTORQUES CONDITION AND
TENDS TO CUT THE FLARE.
INSTALLATION OF TUBING ASSEMBLIES
Before the tubing assembly is installed in the aircraft, it must be
carefully inspected, and all dents and nicks must be removed.
Sleeves must be snug-fitting with 1/16 to 1/8 inch of the tube
protruding above the top sleeve. The line assembly must be clean and
free from all foreign matter as described in an earlier paragraph.
During installation, the fitting nuts must be screwed down by hand
until they are seated and then properly torqued.
assemblies must not have to be pulled into place with the nut, but
must be aligned before tightening.