HGU-33/P and related helmets based on the PRK-37/P helmet shell
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HGU-33/P, HGU-34/P, HGU-37/P, HGU-43/P, HGU-44/P, HGU-45/P, HGU-46/P, HGU-47/P, HGU-49/P, HGU-50/P, HGU-52/P

In parallel with these studies the US Navy decided to cover the immediate need for lightweight helmets to pilots in high performance aircraft by ordering 500 commercial lightweight helmets from each of the two major military aviation helmet manufacturers of the day, Gentex Corp through their subsidiary Protection Inc. (PI), and Sierra Engineering. Although the sources do not mention this specifically, circumstantial evidence strongly suggests that the Gentex/PI helmets were based on the DH-151 helmet shell and Sierra's helmets were the 570-1302 custom fit helmets with swing-box receivers. For Gentex/PI' helmets it was necessary to make a wax mould of the pilot's head and for Sierra's helmets plaster-of-paris impregnated bandages were used to make a mould. These moulds were then shipped to the manufacturer for fabrication of the form-fit liner. The logistic problems were great but so was the need.

US Navy helmet design was re-evaluated through a study undertaken by the Naval Aerospace Medical Institute (NAMI). The APH-6 had been designed based primarily on the requirement to protect the wearer's head which meant that the helmet became comparatively heavy, making the helmet less acceptable to the users. The NAMI study showed that a heavy helmet would give increased protection in some types of accidents but decreased protection in others. This prepared the way to accept lighter helmets, especially as such helmets with form-fit liners would improve fit and stability under "g".

NADC finally decided against modifying the APH-6 helmet and went for a lightweight helmet based on features from the DH-151 and the 570-1302 helmets. The new helmet type was planned in two versions, one with a form-fit liner and one with styro-liner and fitting pads. The two helmets were designated HGU-33/P and HGU-34/P respectively on 12 June 1973 by request of the Naval Air Engineering Center. Further development studies were undertaken, leading to operational test and evaluation in 1976. HGU-33/P and HGU-34/P helmet assemblies were issued to operational aircrews for testing. The HGU-33/P was preferred to the HGU-34/P but both helmets were found to be operationally effective, mission compatible and acceptable to the users. The OT&E results were reported on 9 February 1977, and the helmets were recommended for service use. As one of the first steps in preparing for production, military specification MIL-H-85047 was issued on 31 July 1978. By 1980 the procurement of HGU-33/P and HGU-34/P helmets was well underway from both Sierra and Gentex but initially they had to be used together with the old A-13A oxygen mask as its successor, the MBU-14/P, only became available from early 1981 onwards. The HGU-33/P and HGU-34/P and sub-types (see below) were US Navy standard for fixed-wing aircraft for more than 15 years until gradually replaced by newer types like HGU-55/P as a stop-gap measure and then the HGU-68/P. The HGU-33/P was still authorised for use by 2002 but no spares were carried any longer. Any deficiency would therefore automatically initiate an upgrade to a new helmet.

Helmet components
The HGU-33/P and HGU-34/P helmets formed the basis for a number of different helmet configurations with varying liners, communications, and visors. In line with US Navy practice these configurations each received their own HGU-designation. The details of each are listed below.

Helmet Shell
The HGU-33/P and related helmets are based on the PRK-37/P shell assembly. The PRK-37/P shell is constructed of fibreglass cloth moulded with epoxy resin. The helmet is painted a non-yellowing white, conforming to MIL-C-83286B. A foam edgeroll around the periphery is covered in black leather. The helmet shell comes in three sizes: Medium, Large and X-Large. Medium and Large were standard issue from th US Navy supply chain wheras the X-Large was only available through direct procurement from Gentex. A chin strap is used, but the nape strap was not part of the original configuration. It was added later in order to improve retention. Originally the standard HGU-33/P used a foamed in-place custom fit liner, designated PRU-39/P, but later helmets were equipped with the VTEC form fit liner, designated PRU-39A/P. Gentex manufactures both a Thermal Plastic Liner (TPL) and a custom form fit liner that meet the penetration requirements of MIL-H-84047. But these are not standard procurement for this helmet. The HGU-34/P uses the PRK-40/P liner comprising an energy absorbing styrene liner combined with leather covered fitting pads.

Visor Assembly 
The standard EEK-4 A/P single visor assembly is used for many configurations. A clear and neutral gray lens are interchangeable without removal of the visor housing. Optional lenses for the EEK-4A/P include gradient, amber, vermillion, and for laser protection. Some configurations use the PRU-36/P side-actuated dual visor often associated with the USAF HGU-26/P helmet. When fitted with a neodymium laser protective lens as an inner lens the PRU-36/P visor assembly is designated EEK-3/P.