An examination of two emergency breathing aids for use during helicopter underwater escape.

BACKGROUND: There is a paucity of published work in which the performance of Emergency Underwater Breathing Aids (EUBA) has been examined in the wide range of scenarios in which helicopter underwater escape may be necessary. In the present investigation two EUBA were examined: the Air Pocket (AP) rebreather and the Short Term Air Supply System (STASS) mini SCUBA set. METHOD: Young, healthy male subjects undertook simple simulated helicopter underwater escapes in water at 15 degrees C and/or 5 degrees C. During the immersions the subjects attempted to remain submerged for 60 s while traversing back and forth along a ladder secured at a depth of 1.25 m. At each temperature the subjects used AP and STASS twice. The subjects were dressed in the Royal Navy winter sea helicopter aircrew equipment assembly and an aircrew helmet. RESULTS: Both AP and STASS significantly extended the underwater survival time of individuals when compared to their maximum breath-hold time (BHT). It is clear from the measurements made of gas concentrations in AP; the volume of air used from STASS; and subjective responses, that the 60-s submersions were achieved more easily with STASS than AP. CONCLUSION: It is concluded that in conditions similar to those of the present experiment STASS will give a longer underwater duration than AP, but this benefit must be offset against the possible risk of pulmonary barotrauma associated with the use of STASS, as well as increased training and maintenance costs. Irrespective of the EUBA which is provided, in-water training, preferably including exposure to cold water, will significantly improve the ability of an individual to use it.

The effect of leakage on the insulation provided by immersion-protection clothing.

The effect of controlled, incremental water leakage on the thermal insulation provided by three immersion-protection assemblies has been measured using a thermal manikin. The results show an average loss of 30% of the initial insulation for a leak of 500 g, 40% for a leak of 1000 g, and nearly 60% for a leak of 3000 g. The assemblies differed only in the thermal insulation layer which consisted of: A. A single thickness of wool (initial immersed insulation 0.63 clo) B. A double thickness of wool (initial immersed insulation 0.79 clo) and C. A layer of polypropylene batting between nylon covers (initial immersed insulation 0.76 clo). Differences between the assemblies in loss of insulation with leakage were small and no evidence was found to support claims that the insulating properties of polypropylene are more resistant to wetting than those of wool. It is considered that the substantial loss of insulation even with small leaks makes it essential that tests of the water-excluding performance of immersion suits are undertaken in realistic conditions rather than in calm water.

The effect of the UK aircrew chemical defence assembly on thermal strain.

The thermal strain imposed on helicopter aircrew by chemical protective (NBC) clothing in summer in Germany has been assessed in a laboratory simulation. The environmental conditions used were dry bulb temperature 35 degrees C, wet bulb temperature 19 degrees C and a wind speed of 2.0 m X s-1. The NBC equipment imposed a significant thermal strain on the crewman when compared with standard summer flying clothing, but not on the pilot whose tasks involve lower energy expenditures. Deep body temperature exceeded 37.6 degrees C and a significant degree of dehydration (1% of body weight) also occurred, despite the availability of a drinking facility in the respirator. It is recommended that the only practical way of preventing thermal strain in helicopter crewmen under NBC conditions is by providing personal conditioning.

Prediction of oesophageal temperatures from core temperatures measured at other sites in man.

Transient changes in core temperature were induced in seven subjects by immersion in a hot, then a cold, bath and by light intermittent exercise. Measurements of core temperature were made at four sites: the external auditory canal, the oesophagus, the rectum and the gastrointestinal tract. A mathematical model was derived to enable prediction of oesophageal temperature from measurements made at any one of the other sites. The equations for auditory canal and rectal temperatures were very similar to those previously derived in the Institute of Aviation Medicine; use of an equation to predict oesophageal temperatures from gastrointestinal tract temperatures measured in the field using a radio-pill would therefore appear to be reasonable.

Effect of direction and rate of change of deep body and skin temperatures on performance of a rotary pursuit task.

Performance at a pursuit rotor task has been studied during the overshoot of core temperature caused by sudden cooling after heating, and the undershoot caused by sudden heating after cooling. Conditions were chosen so that effects of the absolute levels of core and skin temperature could be discounted. The results showed that the direction of change of core and skin temperature, rates of change of core temperature between -0.07 and +0.06 degrees C/min, and rates of change of skin temperature between -1.0 and +1.0 C/min did not affect performance; particular circumstances of this experiment, expecially the short duration of the changes in direction studied, make this conclusion tentative. Comparison with earlier studies indicates that the major determinants of performance at elevated body temperatures are absolute levels of mean skin temperature, with the absolute level of core temperature having a less significant role.