Correcting  “Jet Age Flight Helmets” and the helmet description of the United Kingdom Mk.V on page 197, the correct name for the helmet described is the Mk.5 High Speed Anti-Blast helmet manufactured by M L Aviation Company Limited of Maidenhead, Berkshire and not by Helmets Limited. It was never used by Vulcan crews but was used for Tornado development trials. Also I might add that the British helmet numbering system has never used the Roman numeral system. The M L HSAB (High Speed Anti Blast) Mk.5 was developed during the late 1960’s and was first introduced around 1971. The following was taken from the Provisional Issue of the “Descriptive, Instruction and Servicing Manual” dated July 1971.

Introduction
The M L Aircrew Protective Helmet Assembly is of the enclosed helmet type, having a fully integrated mask oxygen system, as opposed to the conventional open helmet with separate oxygen mask. It is designed to form a component of an Aircrew Equipment Assembly compatible with the flight envelope of modern high-performance aircraft. In addition to affording air-blast protection at high ejection speeds, by means of an automatically closing visor, the helmet assembly also provides a high degree of impact protection and survival at high altitude – in both the routine and emergency case. The helmet assembly is of low weight construction to improve comfort and reduce wearer fatigue. An anti-glare visor is fitted, and a communication system, coupled with efficient sound attenuation, provides signal/noise ratio. 

Description  
The helmet assembly consists of a main shell of double-skinned sandwich reinforced plastic construction, with integrally bonded taped cradle harnesses for the crown, back and sides of the head. A frontal chin-cup – hinged on the right and latched on the left – carries the oxygen mask and suspension, breathing valves and ductings, and provides support for the lower edge of the blast visor. All facilities and personal service systems are contained within or on this structure.

The side-pivot mounted visor – dyed to dark and light tint density over upper and lower areas respectively – can be raised and lowered, or set to any intermediate position, and also locked into the aperture seal formed by the main shell and chin-cup. The left side pivot incorporates a gas-operated mechanism linked to the visor and actuated by cartridge initiator for automatic operation. At the commencement of the ejection sequence the initiator is fired electrically, by means of a seat/aircraft mounted accumulator pack or pulse generator – the current passing through the helmet intercom loom – to close and lock the visor. 

The oxygen mask, having a single point adjustable front suspension for the exo-skeleton, is mounted at the centre of the chin-cup. Breathing oxygen, or air/oxygen mixture, is drawn into the mask cavity from the supply hose and inspiratory valve, located at the right side of the chin-cup, via a short length of ducting and a T-piece mounted at the front of the mask. Exhaled air passes back through the T-piece and another similar  ducting and exits to atmosphere through an expiratory valve – compensated to inlet pressure – located at the left side of the chin-cup. Depending upon the type of oxygen regulator in use, an anti-suffocation valve can be fitted into the wall of the mask and a sound attenuating supply hose fitted in place of the standard hose. 

The communication system comprises: two hard-wall plastic ear pads containing telephone receivers, a microphone fitted in the wall of the mask, a microphone switch mounted at left centre in the chin-cup and a supply cable, terminating in a jack-in type plug, depending from the lower right hand edge of the main shell.  The ear pads are faced with replaceable foam filled – or alternatively, liquid filled – circumaural seals, and suspended by side cradle harnesses incorporating a tensioning toggle mechanism. 

Soft leatherfaced pads, located at the brow and back of the head, in conjunction with chin-strap incorporating a quick-release buckle, the ear pads and the supporting cradle harnesses, provide location and stability for the helmet assembly.A two-piece lined fabric neck skirt, overlapping and joined at the hinge/latch interface, is attached to the lower edge of the main shell and chin-cup for additional air-blast protection and comfort.  

The two examples in my collection consist of a Prototype Mk.5 and a conventional Mk.5/6. There is a variation which was for use where operational conditions relating to ejection air-blast requirements were not so stringent and the high speed capability was not warranted. This helmet consisted of the conventional open type retaining many features of the basic type HSAB, including an automatically closing visor, but was fitted with attachments for the type MBU-5/P (or similar) oxygen mask assembly for general purpose application. It was maybe this latter type that was used in the Vulcan aircraft although I do not know of any recorded  instances of its use. 

prototype of the HSAB

Leftside view of the HSAB

visor and facepiece open

Closeup of the mask

Closeup of the facepiece

The conventional HSAB

Leftside view