Thermo Plastic Liner: This system uses layers of firm
plastic that is best described as hard bubble wrap. The "bubble
layers" are covered with a soft, black "sock."
This sock not only provides comfort to the wearer but also
holds the liner in place with the aid of 4 Velcro tabs that
are glued to the styrene liner. TPLs are custom fit the the
wearer by heating the liner and forming it to the head. A
flight helmet can be fitted by adding or removing layers of
the plastic bubbles. Used in all types of current flight helmets,
including: HGU-55/P, HGU-68/P, SPH-4, HGU-84/P.
Fit V-tec: The V-tec was widely used during the 1980s
by US Navy crewmembers. It is still in use today by those
wanting a truly custom fit helmet. The V-tec is placed on
the head, a helmet shell with a hole cut in the top is put
on and pulled down. A special warm expanding foam is poured
in the holes on top of the V-tec and a mold is made of the
wearer's head. The V-tec is made of a leather pad area and
thin fiberglass top. Most commonly found in HGU-33/P, HGU-45/P,
Styrene Liner with Fitting Pads: This is the most
universal type of liner. Many squadrons keep helmets on hand
with this type of liner for training use and other occasional
users. The PRK-40 liner is made of non-melting styrene. It
comes in 2 pieces to allow installation in the helmet shell.
The leather fitting pads have self-adhesive backing and also
come with several layers of sizing pads so you can customize
the fit to some extent. Used on HGU-34/P series flight helmets.
Can also be modified to fit HGU-55/Ps and other helmets.
The suspension liner, like the PRK-40 with fitting pads,
is a very universal type of liner. It is very adjustable
which is important since SPH helmet shells come in 2
sizes- Regular or Extra Large. The liner is also popular
because it allows air to flow over the wearer's head,
providing comfort. This style of liner is found in SPH-4
and SPH-3B flight helmets. Earlier helmets like the
Korean Era "P" and "H" series also
used suspension liners.