Partial Pressure Helmet, Type 12P, Mk. 4G. Size 2. Manufactured by the ML Aviation Company Ltd., White Waltham Aerodrome, Maidenhead, Berkshire. Dating from the early to mid 1960s, the servicing manual states that 'The helmet is designed for use with a partial pressure suit or jerkin effecting survival at high altitude, by means of an automatically closing visor activated by barometric pressure. The helmet also provides crash and blast protection, a separate anti-glare visor, an intercommunication system, a mouth access flap and a ventilation system, if required.' The helmet is manufactured with a tough reinforced polyester fibreglass laminate shell. This MK.4 is also fitted with a rear shell that is hinged from the top of the front shell and provides additional protection. Some internal insulation is provided and the helmet has a transparent moveable visor that seals against the shell when closed. Partial pressurisation is achieved by means of a balaclava-like bag of soft rubber which covers the head from the cheeks to the nape of the neck. A fabric liner, detachable for laundering, is fitted to the inside of the pressure bag. The airman's face fills the aperture in the bag and completes the seal. The rear of the helmet is closed by flexible fabric flaps joined with a heavy duty zip fastener, holding the complete helmet and pressure bag in position on the head. Lacing provides adjustment for differing head sizes and an emergeny release knob is also incorporated. An integral oxygen mask is fitted for use in normal conditions when the visor is open. In an emergency, due to loss of ambient pressure, the visor closes automatically by action of a barometic release.
The shell has minor wear and scratching but is in very good overall condition bar a crack in the hinge that joins the front and rear (see photographs). The fabric lining is also very good to excellent and the zip fastener runs freely. The fine rubber pressure bag has some damage and tears but this doesn't detract as it is not particularly visible. The helmet retains the wiring loom/plug and oxygen hose/connectors. The rubber shroud over the breathing outlet valve is perishing and there is a hole in the outer layer of the main clear visor. Some deterioration of the rubber of the oxygen mask inlet pipe. Both mechanisms for outer and inner visors function correctly. Supplied with a photocopy of the Instruction & Servicing Manual. The helmet is a real rarity and even with a few faults makes a superb display piece. Numerous further photographs available on request. Stock code H22029.