Original Item: Only One Available. This is a United States Helicopter Flight Helmet type SPH-4 made by Gentex. The helmet is in overall excellent complete condition with the liner, headset, visor, microphone, and tint visor. The helmet is marked X-Large This is a rare Late Vietnam War era original rare helicopter helmet in wonderful complete condition!

The Sound Protective Helmet-4 (SPH-4) is a derivative of the US Navy SPH-3 and was used by the US Army since 1970. The SPH-4 is a single-visor lighter-weight version of the SPH-3 and it replaced the two Army aircrew helmet then in use: The Navy-developed Aircrew Protective Helmet no 5 (APH-5) and the Army-developed Anti-fragmentation Helmet No. 1 (AFH-1). Both of these helmets were deficient in noise attenuation and retention capability. The SPH-4, which was specifically designed for sound protection, provided superior sound attenuation but the 1970 version provided no more impact protection than the APH-5A. As the sciences of crashworthiness and head injury prevention developed, it became evident that head injuries could be reduced by modifying the SPH-4.

Two types of head injury that might be prevented continued to occur after the introduction of the SPH-4. One was concussions severe enough to prevent the crewmember from saving himself from the crash site, and the other was skull fractures due to blows from the side (lateral). Furthermore, helmet retention proved to be a problem as well. A helmet can only protect a crewmember if it stays in place and it turned out that one in five crewmembers involved in severe crashes lost their helmet.

The original SPH-4 had a shell made of fibreglass cloth layers bonded by epoxy. The inner polystyrene foam energy absorbing liner was 97 mm (0.38") thick with a density of 5.2 lb/ft3. The helmet was fitted with a sling suspension liner and had a nape strap with a single snap on each side fitting to studs on a retention harness. The chin strap had a design strength of 150 lbs. The headset was mounted in 6 mm thick moulded plastic ear cups with excellent sound attenuation characteristics. A size regular helmet weighed 1.54 kg (3.4 lbs).

In 1974 the SPH-4 was modified with a thicker energy absorbing liner to reduce the risk of concussions. The new liner was 1.27 cm (0.50") thick and with the same density as the original liner. In 1982 the risk of concussions was reduced even further by manufacturing the energy absorbing liner with a lower density 4.5 lb/ft3. All in all the impact protection was improved about 33% over the original SPH-4 from 1970.
Nothing was done to the original SPH-4 design to reduce the risk of skull fractures due to blows from the side. The main culprit was the rigid plastic ear cups that turned out to be too strong in comparison with the skull around the ears. In case of a strong blow from the side the ear cup survived but the skull fractured. This problem was not addressed until the SPH-4B helmet was fielded.

Helmet retention, however, was improved. The original 1970 helmet had a chinstrap with single snap fasteners on each side and was designed to withstand a load of 150 lbs. In 1978 a double-Y chinstrap with two snap fasteners was incorporated to reduce failures. This chinstrap had a failure limit of 250 lbs based on the adjustment buckle strength. In 1980 a third chinstrap was introduced. It was fastened to the ear cup assembly on one side with a small screw and T-nut, and the other side with two snap fasteners. This chinstrap had a failure limit of 300 lbs but some failed at 280 lbs.