The history of US Military Aviation oxygen systems (part 2 of 2) continues to investigate the technical development of aircrew life support equipment designed to provide oxygen to pilots of aircraft flying to high altitude in the Earth's atmosphere. [The image accompanying this segment shows the US Army Air Forces' first hard protective helmet (the P-1) in use with the A-14 Diluter Demand Oxygen Mask, circa 1947]
A History of US Military Aviation Oxygen Systems Development to 1945
(Part Two of Two)
In 1940 and 1941, with the RAF already engaged in meeting Germany’s challenge in the air, a team of American aeromedical observers led by Dr. Armstrong were able to visit England to take notes on the rapidly expanding conflict, as it concerned the strenuous physiological demands of altitude and combat. Chief among concerns explored by Armstrong and his colleagues were the unsuitability of high pressure oxygen storage cylinders when exposed to projectile penetrations and servicing difficulties that the old style systems required. Initial experimental tests suggested that the high-pressure system then in use be replaced by a less hazardous low-pressure system, since high pressure oxygen flasks penetrated by incendiary projectiles tended to burn hotly or explode, with extremely damaging results to the aircraft. These findings prompted a change over to the newer low-pressure system in USAAC aircraft—a move that required a new approach to construction of containment flasks carried on board aircraft. This move, while seemingly beneficial to combat aircraft survival, would be later be shown to hold hidden liabilities and pose technical servicing problems in the field. Meanwhile, the US Navy as well as the RAF, continued to use reinforced high pressure oxygen containment flasks—complicating existing compatibility issues between US and English support & maintenance logistics, as the war expanded and progressed. Of interest is the fact that Germany maintained a high-pressure oxygen system throughout the war, although it had earlier gone to the new Demand Type low-pressure oxygen delivery equipment (mask) that required use of an automatic high-to-low pressure conversion regulator.
On US aircraft, the new low pressure oxygen system required use of a new Type A-9 manual continuous flow regulator, which while similar to the A-8 type, allowed use of a new lower-pressure 500 PSIG storage flask; this regulator was standardised in 1940. A slightly improved A-9A manual continuous flow regulator came into standard use in 1941 it was virtually identical to the A-9, but incorporated internal flow and needle adjustment valve settings. An automatic continuous flow Type A-10 regulator was introduced in ‘service test’ status in 1940, but was never standardised, owing to introduction of the new Demand Type low-pressure automatic regulator in 1941. One other regulator introduced at this time was the Type A-11 continuous flow automatic regulator, that was introduced in 1941 and declared limited standard in 1944. It and its successor (the AN-R-15 automatic continuous flow regulator) were intended for use in cargo or transport type aircraft and supplied a number of users wearing A-7 and A-8 type rebreathing masks.
The New Diluter-Demand Type Oxygen System: America Enters the War
As America’s entry into the new European War appeared more likely, second thoughts emerged over the wisdom of the Army’s conversion to the new ‘low-pressure’ oxygen delivery system. This became more evident as statistics gathered during the early phases of the war showed that more aircraft had been lost to ground based FLAK than to actual dogfight ‘kills’. However, it was far too late to take any action on this somewhat already mooted point, as America’s entry into the war was almost a given (as seen by many in the War Department’s inner circles) and a low-pressure oxygen standard had been adopted (flasks filled to about 450 to 500 PSIG only, as compared to 1800 to 3000 PSIG in the earlier high-pressure systems).
Armstrong and his team of observers had noted early on that the rates of oxygen consumption required on missions carried out on the newer extended range aircraft (especially on bomber type aircraft, operating necessarily at higher altitudes due to FLAK threats) were excessively high using older continuous flow oxygen systems. By 1941, the US observers had had ample opportunity to recover functioning examples of the new German Demand Type (the Auer Company first developed this system in 1936) oxygen breathing regulators and masks from downed Luftwaffe aircraft. The apparent advantages the German system offered over the older continuous flow systems were immediately evident and the captured technology was quickly removed to the Wright Aeromedical and Engineering Labs for analysis in June of 1941.
The German system used high pressure compressed oxygen storage flasks, which fed a supply of oxygen (still under high pressure) to a novel ‘high-to-low’ pressure regulator at the individual crew station. The aircrewman was provided with a molded latex rubber facemask lined with soft chamois leather; this facepiece was fitted with a corrugated rubber, large bore and low-pressure hose that featured a quick connect/disconnect fitting on its distal (regulator) end. The German regulator, after reducing the higher pressure, automatically cycled low-pressure breathing oxygen to the wearer only on demand; this was accomplished through use of valving in the regulator and resulted in a remarkable reduction in overall oxygen use (the system satisfied user consumption requirements perfectly without incurring needless wastage). The heart of the German regulator lay in use of a special demand valve engineered by the Draeger Company, a company long experience in both chemical warfare respiratory protection technology and mining rescue apparatus development.
Immediately after arrival at Wright Field, a collaborative effort between the Wright Equipment Lab, the Aeromedical Laboratory, the Bendix Corporation, and the Air Reduction Company ensued, with the object being to design and develop a functional American counterpart to the German demand system. The result of this intensive cooperative effort rapidly produced the first American Demand Oxygen breathing system prototypes. These early systems featured an automatic regulator of the diluter design, meaning that the oxygen concentration was automatically diluted proportionate to the specific altitude’s predetermined human oxygen consumption requirement. As such, this reengineered concept was a largely American innovation, although it did share the German use of a venturi dilution control approach and air-mixture valving. The automatic mixture adjustment was accomplished through use of the aneroid capsule, following previous methods. So quickly was the entire project successfully completed, that designs from both Bendix and Air Reduction were standardised by September of 1941—less than 3 months after the original captured German systems had been received at Wright Field. Both designs were type standardised as the new A-12 Diluter-Demand Regulator and with the introduction of the new regulators, the US Army Air Corps at last had an ideally engineered and designed automatic oxygen delivery system—an unrealised goal of aviation engineers and altitude physiologists since the end of World War One!
In operational function, the new diluter-demand system used a suction actuated valve that opened on the initiation of the inspiratory cycle by the user. The mix-selector was usually left in the ‘on’ position, which automatically selected the correct oxygen/ambient air ratio required by the user up to about 30,000 feet. Thereafter it supplied 100% oxygen. With the mix-selector in the ‘off’ position, 100% oxygen was delivered at all times, no matter what the given altitude the user was at. Shortly after introduction, due to some confusion over specific meanings, these two selections were relabeled 'Normal Oxygen' and '100% Oxygen'. The automatic response of the diluter-demand system also accommodated the variable use needs of the user without further inputs from the wearer; it additionally featured an "Emergency" control that would bypass all of the circuitry and deliver a constant flow of 100% oxygen to the wearer, if selected, irregardless of altitude or respiratory needs.
Production of the new Type A-12 automatic Diluter-Demand Regulator was handed over to several manufacturing contractors so as to speed up introduction and supply of the new equipment. In July of 1942 the A-12 regulator was reduced to ‘Limited Standard’ by introduction of the replacement Type AN-R-5. [In February of 1945 an improved Type A-12A Regulator was standardised, resulting in the A-12 and AN-R-5 regulators being relegated to ‘Limited Standard’ status at that time. In addition to the A-12 type regulators, a portable diluter-demand regulator was also designed for use with small ‘walk-around’ bottles (A-13), for use between crew stations.]
The First Diluter-Demand Oxygen Breathing Mask: the A-9
As part of the new Diluter Demand oxygen breathing system, an entirely new breathing mask was required for aircrew use, since only a mask configured for response to discrete respiratory cycling would suffice (although as previously noted, the older A-8B type continuous flow mask could be retrofitted with valves and a hose that would allow it to be used as a ‘demand mask’). This new mask was quickly developed by the combined Aeromedical and Equipment research teams at Wright Field, in cooperation with commercial engineers. The development of this new mask was considerably aided through research done earlier, by various rubber companies in cooperation with the US Army’s research teams at Aberdeen and MIT, on the need to devise a new generation molded rubber facemask to protect soldiers against chemical warfare agents. [It is reasonable to conclude that the German demand-type oxygen mask designs had all been carefully studied, as well, since some of the features of the new American masks would bear a striking similarity to features found on their German counterparts.] The new mask was molded of latex rubber and was designed in such manner that any formation of ice in the critical oxygen channels or orifices could be dislodged by gentle squeezing of the facepiece. Early A-9 Diluter-Demand oxygen masks were molded from grey rubber, had a 12 inch long grey corrugated rubber hose(attached to the lower front chin area), and were equipped with a quick attach & disconnect fitting on the hose’s distal end that connected to the crew station regulator. Frequently, a variable length hose extension was used between the regulator and the mask’s hose, and there was a small pocket in the nose of the mask in which a Type MC-253 or MC-254 microphone could be placed. Construction of the A-9 mask was such that layers of rubber were glued in place to form channels through which exhaled air could be directed. The single inlet valve had a rubber one way flapper valve (‘check valve’), which would close off the inspired airflow upon expiration, thereby directing exhaled air out through the expiratory channels in the mask’s facepiece.
The new A-9 Diluter-Demand Mask featured a soft, facially conforming periphery, which was issued in only two sizes: large and small. The mask featured characteristic upwards extending cheek flaps, as well as an external wire former in the nose section, and was attached to the B-9 flying helmet with hooks. A separate so-called 'Juliette' head strap suspension could also be used with the mask in the absence of a helmet with the attachment hooks. This new mask was standardised in December of 1941, only two days after the Japanese attack on the Pearl Harbor Naval Base, but was changed to Limited Standard on April 20 of 1942 when the improved A-10 Diluter-Demand Oxygen Mask was standardised. Only a very limited number of the A-9 masks was initially procured before the A-10 improved mask was introduced, hence specimens of this early, original model are today in somewhat short supply. The A-9 mask was declared obsolete in August of 1943.
Problems associated with the A-9 mask were found to include less than satisfactory mask retention characteristics under high-G combat conditions, an expiratory valve that was slightly too small, inadequate face-sealing (particularly around the nose section), and a tendency to draw outside air in during inhalation (both around the edges of the mask and through the exhalation ducts). In terms of comfort, an all important criteria in use of any close-fitting facemask, the A-9 mask was a considerable improvement over previous masks, but it was still far from perfect. These shortcomings soon resulted in introduction of an improved mask designated the A-10.
Concurrent with the introduction of new complexities associated with the Diluter-Demand oxygen breathing system, a new requirement arose for more intensive training and instruction of aircrew in use of the new systems. This need dovetailed with concerns over the growing number of aircrew personal equipment items that required not just careful instruction in their safe use, but more complex logistical support and maintenance. Initially, this training was undertaken and carried out by personnel of the USAAF Altitude Training Program and special unit officers known as 'Unit Oxygen Officers'. Slightly later, and in order to more adequately rectify this growing need, a new officer position was created in European combat squadrons to be henceforth known as the 'Personal Equipment Officer'. A non-flying officer, his job would be to coordinate inspection, maintenance, and readiness of aircrew personal equipment items and see it to it that all required personnel training, safety orientations, user instruction, and associated support & maintenance work were properly carried out in his unit. This idea worked so well that US Army Air Force Command Headquarters issued uniform Air Force wide orders establishing a Personal Equipment support specialty. providing specific training for officer and enlisted personnel assigned to the new area of work.
As the new area of Personal Equipment support grew during the war, so did the administration & logistics of this area of concern itself, eventually resulting in later dedicated Air Force Specialty Codes (AFSCs) created specifically to carry out this work, after the 1947 National Defense Act reorganisation that allowed for a separate and distinct branch of service known as the 'US Air Force'. ['Personal Equipment Support' of the WWII and Korean eras would later become known as today's modern USAF 'Aircrew Life Support Technology'].
Problems with the older continuous flow oxygen system persisted, due to the fact that the Japanese attack on December 7th of 1941 rather precipitously drew America into the new war before a successful transition had been made from the old continuous flow system to the newer diluter-demand system. This posed a special concern for the crews in early bombers, who carried out the brunt of the European bombing on long high-altitude missions. Instances of oxygen hoses becoming disconnected, masks (A-8 type) freezing, and/or regulators being rendered dysfunctional due to ice and other faults were quite common and posed serious risks to the crews, who once the combat action started had little time to worry about making required adjustments to their oxygen systems. Despite promising to address many of these concerns, the newer A-9 mask was also not without its own problems, although the concurrently used A-12 Diluter-Demand Regulator worked quite well in nearly all situations.
As the war rapidly absorbed American forces, priority was given to these European bomber crews and their fighter escort units to replace their older continuous flow oxygen systems with the newer diluter-demand system, though this was a process that was somewhat handicapped by the rapid and compelling development of combat in the European theatre. By 1943, however, most new American bombers reaching European combat theatres were fitted with the new diluter-demand oxygen system; adequate (but barely) training of aircrews to use the new equipment safely had been included in both flight orientation training and advanced flight training, but the effects of this training was complicated by experiences arising in the field that had not been fully or completely anticipated.
Mask freezing in particular, continued to be a serious problem with the new A-9 mask on the longer, higher altitude flights that the European war necessitated. In answer to the many problems left remaining in the A-9 mask, a cooperative team charged with looking into the possibility of creating an improved mask was formed of the Wright Aeromedical Lab personnel, the Acushnet Rubber Company, and several other distinguished members of the NDRC (National Defense Research Committee). This committee, headed by Dr. C.K. Drinker (a noted American pulmonary specialist), selected a design submitted by Mr. Frank Mauer, which was actually a modification of the existing A-9 mask. The improved mask was initially designated the "L-12 proposal".
Upon review, the new mask was standardised (April 1942) and rushed into production. It was molded from grey rubber and featured the same upward sweeping cheek flaps as the A-9, although it had a slightly larger facial contact area, enlarged exhalation valve, and most particularly an additional strap in the suspension component assembly. This additional strap was integrally molded into the nosepiece of the mask, rising directly upwards over the nose and between the eyes, to attach to a helmet or head-suspension assembly. In this respect it greatly resembled the German Draeger Typ 10-6072 mask and other German masks (Typ 10-69, etc) of the ‘three-point’ suspension type, using a feature that had been in use on some German masks since 1934. [The mask included a small pocket for installation of a T42 (carbon element) or T-44 (magnetic element), but the practical preference of aircrews seems to have been for use of these early diluter demand masks with a standard T-30 throat microphone.] The new mask was placed into immediate production with wartime priority status and was soon being distributed to combat crews within a few months (the first being handed out in late 1942).
Unfortunately, the A-10 mask, while offering some improvement over the earlier A-9 mask, still suffered from certain characteristic faults. Although reports of its freezing were few, it had a tendency to ride too high up on the face to allow a clear and fully unobstructed forward view by the wearer, and yet still tended to slip down over the face in severe combat-induced high-G maneuvering. A wire nose clip was still used to provide proper fitting in this critical area of face-contact, but the wire was uncomfortable, posed a small visual distraction in the field of vision, and did not provide the desired fit expected. Difficulties were also experienced donning the mask, due to its somewhat more complicated suspension system and the need for fighter crews in particular to be in the air at a moment’s notice.
The A-10 mask was not declared obsolete until 1946, but was made limited standard with the introduction of several modified versions (A-10 Revised, A-10 Corrected, and the final A-10A mask). Using feedback from field users, an improved A-10 mask was produced in limited numbers known as the A-10 Revised (also known as the Type A-10R) that was standardised in ‘late 1942’. Further, a limited number of the existing A-10 masks were slightly modified and remained in use; these were known as the A-10 Converted mask. The Type A-10 Revised mask eliminated the molded rubber third strap between the eyes of the wearer and featured a slightly reduced face-contact conformation. USAAF TO 03-50B-1, dated February of 1943, states that the Type A-10 Revised mask could be readily identified by the presence of 4 characteristic molded-in rubber ridges on each side of the nose section (where the third strap originally had been located). Supposedly also a molded –in "R" was to be found near the manufacturer’s trade-mark in the chin-area. The original wire nose shaping component was still present in the original manner as the A-10, although of slightly different shape, and as with previous examples of the new diluter-demand mask, a T-44 (magnetic) or ANB-M-C1 microphone could be fitted.
One important modification was the elimination of the upward sweeping extended flaps on the mask’s facepiece and the substitution of a new two point strap suspension assembly that had to be used only with a standard fabric or leather flight helmet (such as the A-10 or A-11 helmet); this was the same strap suspension used by the A-14 mask, soon to be introduced. The A-10 Revised Mask was made in 4 discrete sizes and gave fairly adequate protection to the face, assured satisfactory oxygen delivery, and enhanced face-fit adjustment needs. In terms of appearance, the new A-10 Revised Mask had an appearance that is much closer to later appearing masks. Operational use reports showed that despite all the improvements, the A-10 Revised Mask still demonstrated a few areas of serious concern (such as face-fit, sealing, icing, etc.).
Due to the demands made upon Allied aircrew in the massive bomber formations over Europe, the new diluter-demand oxygen masks were still in relatively short supply (despite priority production orders), therefore use of even the slightly less than perfect masks were still required. Consequently, efforts continued towards the goal of coming up with an even more improved version of the A-10 mask. This eventually resulted in the final A-10A Mask, even though the newer A-14 mask was about to be widely introduced (the Ohio Chemical & Manufacturing A-14 mask production was beset with problems dealing with its molds that considerably delayed initial introduction of the new style mask).
The new A-10A mask was adopted as a ‘Substitute Standard’ on October 15 1943, although used less and less as the war progressed and the new style A-14 mask started to come into wider distribution. The A-10A mask benefited from A-10 user experiences that resulted in improved construction and which gave it a slightly different appearance from the A-10 Revised Mask. TMost importantly, the A-10A mask featured a more structurally built-up nose section that molded better to the nose without need of the previous and problematic wire reinforcing component. It featured an integrally molded-in microphone pocket, was made in three sizes, and featured the same two-point strap suspension as had the A-10 Revised and A-14 masks.
Despite these additional improvements, problems with mask freezing continued, although these seemed to come mostly from the European Eight Air Force bomber units, which were much more subject to cold and freezing effects at altitude than wearers of the mask in fully enclosed fighter cockpits. In response to this, the Wright team at the Aeromedical Lab was able to devise rubber baffle flaps that helped prevent blockage of the A-10A oxygen inlet ports, although this did not do entirely away with some frost build-up. It should be noted that all of the A-10 masks were made from soft rubber that was difficult to achieve a suitable face-seal with, given the fact that sizing options were limited and not entirely satisfactory to meet the wide ranging facial contours of American servicemen; fortunately, this was about to change with introduction of the new A-14 diluter demand mask.
Also of note is the fact that as the special difficulties posed by the need to more adequately fit aircrew faces became widely acknowledged, the decision was made to transfer responsibility for engineering requirements from the Equipment Branch at Wright to the Aeromedical Laboratory’s newly established ‘Oxygen Branch’ in April of 1943, headed by Captain Loren D. Carlson. This change in administrative and logistical support for oxygen equipment design and development was to greatly further the success of later research efforts. As part of the new activity undertaken by the Oxygen Branch of the Aeromedical Lab, one of the first purpose-specific anthropomorphic studies was completed (with the cooperation of Harvard University’s Department of Anthropology), based upon a precisely measured study of the facial configuration specifications of over 1800 young airmen. This ground-breaking study produced 7 generalised ‘standard’ facial configurations that were molded into dummy heads and used as a basis for production of more adequate face-sealing facepiece master molds for production of oxygen masks. This allowed optimal production of facepieces that minimised leakage, which was to be best demonstrated in the performance of the A-14 diluter-demand mask, considered one of the best (if not THE best) American oxygen mask produced during the energetic researches of the World War Two period.
Several other areas of work that the Wright Aeromedical Lab engaged in, concurrent with their central concentration of design and development of adequate diluter-demand oxygen masks, included investigations into the possible use of experimental integrated mask/helmet assemblies. Intended as complete protection for the entire human head and respiratory system, several prototype models were produced (these included the plastic Type A-11 oxygen-helmet-mask assembly, and a later improved version designated the Type A-12). A definitive prototype proposal oxygen-mask-helmet assembly was finally produced, designated the Type C-1, but none of these ideas were ever taken off experimental status before the war ended.
The Type A-14 Diluter-Demand Oxygen Mask: Benchmark
Sharing awareness with Army and commercial design engineers charged with the daunting task of helping develop a chemical warfare protective respirator facepiece molded from latex rubber, the US Army Air Corps’s Wright Aeromedical Lab teams realised that given the state-of-the-art, their goal of producing a completely functional and fully satisfactory oxygen facemask was an extremely challenging and difficult one. The challenge, both from a medical and engineering standpoint, had shown itself to be immensely complex from the earliest days of such investigations. Experience with the succeeding generations of rubber facepieces had demonstrated that an air-tight seal between face and mask was essential for proper function. This contrasted considerably with the need for such a facepiece to also be comfortable to wear for extended periods of time, since conventional rubber compounds could not meet all requirements for fit, seal, and comfort equally. [This would be a frustrating and recurrent problem that the US Army would face much later again, in attempting to develop a new generation chemical defense facemasks for soldiers in the 1970s.]
Hence, as work on the A-9 and A-10 masks had continued, a separate developmental investigation into perfecting a more suitable diluter-demand mask was carried out in cooperation with the Army, the Mayo Clinic researchers, and the Ohio Chemical and Manufacturing Company. Based upon designs submitted by Dr. Arthur Bulbulian, Ohio Chemical and Manufacturing quickly devised a new design that would eventually be standardised as the A-14 Diluter-Demand Oxygen Mask.
The pre-production prototype model of the A-14 mask was completed by October 1941, but much subsequent work was required to perfect this new mask until it was ready for standardisation on 1 July 1943; the molding processes required by the new facepiece were considerably complex and demanding, requiring an inordinately high level of precision on the part of the mold makers and production teams. Initial assessments of the new mask showed that it had great promise, however. So great did its promise appear that it was actually rushed into production before it had officially become standardised. The A-14 facepiece marked a new level of developmental expertise in both engineering design and manufacturing process, as it permitted an entirely new standard in face-seal to be achieved. Molded from latex rubber in a green color, the mask was of sturdy, though flexible construction (much sturdier and less flexible that the A-9 and A-10 series masks). It was molded with a microphone pocket in front of the nose to hold a standard T-44 or ANB-M-1C mic, had a nose shaping wire embedded into the rubber of the facepiece, and used a two-point rubber strap assembly for attachment to A-10 type helmets; it featured a quick release buckle on the right side of the facepiece to allow easy on/easy off donning capability, had a standard corrugated rubber hose with quick release fitting distally, sealed well around the wearer’s face, and best of all, was far more comfortable than any previous mask had been found to be by a wide range of wearers.
In initial operational tests conducted in 1943, the mask had been first issued to fighter pilots in Europe, who found the mask quite suitable in virtually every respect. Only when the mask was issued to bomber crews were there found to be problems with freezing, since fighter pilots in their fully enclosed and protected cockpits did not have extremes of wind and cold to contend with. These problems were addressed to a satisfactory extent by the insertion of a rubber baffle flap, which protected the oxygen inlets from fully obstructing the inlets; this idea came from a member of the RAF’s Medical Branch and worked fairly well until an electrically heated mask-warming cover could be devised to fully resolve the problem.
When the new A-14 mask was standardised in 1943, it quickly became the principal standard diluter-demand oxygen breathing mask of the late World War Two period. So satisfactory was the A-14 mask found to be that improved models remained in regular use well into the 1980s.
At the time of its introduction, the A-14 mask was believed to be the very best mask of its type ever produced in the United States and appeared to be the ultimate oxygen breath mask that had been so long sought after by American aeromedical researchers since investigations into use of a facemask began, back in the 1920s. Suffice it to observe that one of the major impediments to the successful development of this level of efficient oxygen delivery in a facepiece was the fact that the materials and fabrication technologies of those early decades were simply not adequate to the challenge presented them. It remained for developments in chemical and polymer formulations technology to catch up with the visions focused upon earlier, before the high level of effectiveness embodied in the A-14 mask’s design could finally be realised. [Despite the excellent performance of the A-14 mask design, it should be reiterated that there is still today no such thing as 'the perfect oxygen facemask', since the human face is different in each distinctive individual.]
A slightly modified version of the A-14 mask would be shortly produced in January 1945; almost too late for use in the war, it featured refinements to the internal baffles and was also manufactured in 4 sizes instead of the A-14’s basic 3. This slightly improved model would be standardised as the Type A-14A mask 01/45), examples of which would remain in use in the US Army Air Force in the late 40s, even later be used by the US Air Force in the Korean War, and remain in use in actual fact until the final silicone rubber based A-14B mask was introduced in the 1960s.
The Pressure-Demand Oxygen System:
High altitude flight has always been an attractive goal for the aviation minded adventurer. However, in the first part of the new European war (WWII), it rather quickly became apparent not only that flight to higher altitudes was possible (thanks to newly emerging technology), but that due to the increasing deadliness of defensive weaponry, flight to higher (and therefore paradoxically ‘safer’) altitudes would soon be mandatory. Mindful of this, and well aware of the basic laws of human physiology that dictate the uppermost limits of human respiratory function, an urgent requirement soon developed at the Wright aeromedical Laboratory for a means by which high altitude flight could be safely reached without protection such as was provided by pressurised cabins or pressure suits.
Standard Demand Type oxygen systems were simply not adequate for extended flights above 30,000 feet. Even on full 100% oxygen at 30,000 feet, the law of partial pressures mandates a severe decline in alveolar oxygen saturation levels of aircrew. Further and even more severe declines were encountered at up to about 40,000+ feet, at which point alveolar oxygen partial pressure produced by ambient pressure at that height was barely capable of sustaining life for a short period of time—despite inspiration of 100% oxygen at ambient pressure.
Some body of medical research had already been accumulated, however, with particular application to treatment of pulmonary edema, by the start of the 1930s; especially of note was that research done by Dr. Alvin Barach at Columbia University. In 1940, Dr. Barach was approached by the Wright Aeromedical Lab to see if his research with higher inspiratory pressures ('positive pressure') might not provide a reasonable theoretical basis for the development of a similar pressure-breathing system that would support human life at higher altitudes. Since the partial pressure of oxygen fell away in direct proportion to the ambient pressure in reference, it stood to reason that breathing oxygen under pressure might help artificially boost the partial pressure of oxygen in the lungs above and beyond the normal limits of flight without it.
Captain A. Pharo Gagge of the Wright Aeromedical Lab determined to apply Dr. Barach’s work towards this end and using himself as a test subject 'flew’ the facility’s hypobaric chamber to a simulated altitude of 50,000 feet on 12 December 1941, using an experimental breathing circuit of his own design. The NRDC paid careful attention to this fascinating proposal and as a result, quickly initiated a complete research project whose purpose was to investigate the design and development of a practical military pressure breathing system for use at high altitudes. By June 1942 the J.H. Emmerson Company, specialists in medical respiratory equipment, had come up with a specially modified A-12 Demand Regulator that used a spring weighted valve modification. Other effort was directed towards devising a suitable facemask with which to use the experimental pressure-demand regulator. In 1942 development of a pressure-demand face mask was initiated by Captain Francis Randall, again using anthropomorphic facial measurements of aircrew to produce a mask that would satisfactorily contain the pressure required to allow breathing under pressure, while maintaining an airtight face-seal and also remaining relatively comfortable to the wearer. The prototype mask Randall had devised was initially designated the Type XA-13 mask, and was turned over the Mine Safety Appliances Company (MSA), a company with a long history of research work on mining respiratory rescue and breathing systems, for possible production development. Further, by January of 1943 the Bendix Corporation had brought out an improved (experimental) pressure demand oxygen regulator that was later designated the Type A-17 Pressure Demand Oxygen Regulator.
One of the key components of the A-13 pressure demand breathing mask was the critical exhalation valve design, the initial example of which had been designed by Captain Gagge and his colleague, Captain Randall. Proper function of this valve was responsible for allowing the pressure breathing cycle to be carried out properly. The first functioning engineering specimen of the Gagge/Randall valve was produced by the Linde Air Products Company in the short span of a week. According to the story, an unidentified engineer took the Gagge/Randall exhalation valve drawings and built the first working valve, inserting a small counterpressure spring between the main diaphragm and the compensating diaphragm, despite the fact that this small spring had been lacking in the original Gagge/Randall blueprints. This simple and anonymous modification of the original design is credited with ensuring the fact that the new valve passed every single functional test imposed upon it. Consequently, it was quickly put into production.
The actual first flight test of the new pressure-demand oxygen breathing system was made by Dr. (Colonel) Lovelace in a specially modified B-17E, during which an altitude of over 42,000 was reached. A subsequent test was flown in a specially modified two-seat P-38 Lightning to an altitude of nearly 45,000 feet without incident. The prototype pressure-demand system was then operationally flight tested in the 28th Photo-Recon Sqdn in October of 1943 and subsequently adopted for continued photo recce work by the Army Air Force in November of that year. Initial orders were quickly placed with MSA for 4,000 sets of equipment consisting of the new MSA produced A-13 mask (fitted with the Linde-modified pressure-compensated exhalation valve) and a new Aro A-14 pressure demand regulator that was still under development.
The first operational combat mission flown using the new pressure-demand oxygen equipment was carried out in February of 1944 by the RAF Spitfire flying 14th Photo Recon Sqdn, and in April of 1944 the new system was used in bombing missions over the German capitol. By late 1944, all F-5 and F-13 aircraft were fitted with the new Type A-14A pressure demand oxygen regulators and pilot training was carried out in several US ZI (Zone of the Interior) training bases. Several squadrons of Photo Recon aircraft in the Pacific Theatre were also equipped with the initial pressure-demand breathing system, as well.
According to source documentation, the original experimental A-13 mask was made from very hard green rubber and was referred to as the 'XA-13' mask. It was used with the 'XA-16' portable pressure-demand oxygen breathing regulator. However, the original rubber compound was found to be too hard and did not sufficiently conform to facial contours, so the production model manufactured by MSA was produced in a medium-green natural rubber compound that was softer and therefore more capable of providing a firm, air-tight face seal on the user’s face. The A-13 type mask was distinctive for featuring two black rubber one-way check valves in the inlet circuit that were covered by transparent plastic protective covers, opening downward and marked with an arrow to show proper directional orientation. The exhalation valve, which has been previously described in some detail, was manufactured by MSA and was located in the bottom frontal part of the inner mask area. Directly in front of the nose a circular cavity was molded in which a standard ANB-M-1C type microphone could be inserted, for communication.
One of the main and visually distinctive features of the new A-13 type pressure demand mask was the mask’s reversed peripheral face-seal lip. This was separated into upper and lower orifices by a horizontal ‘bar’ of soft rubber that crossed directly over the mouth, just under the nose. This bar served to keep the lateral sides of the facepiece from distending sideways under pressure and removal of the bar (as was occasionally done by wearers for comfort, since the bar could be somewhat irritating to some wearers) could dangerously degrade the mask’s ability to contain inlet pressure of the oxygen. The reversed peripheral face-seal served to help press the pressurised mask to the face, when properly secured. [This ‘reversed peripheral seal’ feature would later be revived and successfully used by the US Army and many foreign nations in development of new 1970s/80s generation chemical defense respirators.].
As with previous oxygen masks produced in the USA and in Germany, the A-13 mask featured full lateral face flaps to help protect the wearer against flash effects and extreme cold. As originally designed, the A-13 incorporated a two point suspension system, using doubled snap tabs on the left side to attach to a flight helmet, while the right side strap assembly featured a quick release buckle so that the mask could be quickly attached and detached, as suited to needs. The rubber facepiece had two upper protrusions (frequently described as 'lugs') through which a nose strap was fitted over a shaped black or OD colored plastic nosepiece to help retain the mask; on each lower lateral margin of the mask, two other molded-in protrusions were situated (covering fabric internal reinforcements), where small metal strap buckles were inserted to secure attachment straps. To provide for attachment of the mask to an H-2 type 10-minute emergency bailout oxygen bottle, a 'T piece' connector was installed between the end of the mask’s chin inlet and the proximal end of the corrugated rubber oxygen supply hose. The H-2 bailout bottle was initially kept in a flightsuit leg pocket, but was later supplied with a tie-on fabric container to secure in place on the leg; a three foot length of small-bore rubber tubing connected the bail-out bottle valve to the mask’s "T-piece" connector.
Designed to provide higher inspiratory oxygen pressure than ambient pressure, no effort was required to inspire (other than the end of a regular expiration), as the oxygen was forced into the lungs automatically. Some effort was required to exhale, however, and it was this work that over a prolonged period of time made pressure-breathing at altitude somewhat tiring. Aircrew using the new mask soon also found that communication using the mask ANB-M-1C microphone while pressure breathing was somewhat unclear. Due to the 'reverse breathing' process pressure breathing required (effortless automatic inflation of the lungs, followed by forced effort expiration), training requirements for proper use of the pressure breathing mask were considerably more detailed.
Only a limited number of the original A-13 pressure demand masks were ordered on January 24th 1944, due to the urgent requirements of units scheduled to operationally use the new system, and in August of the same year, the original A-13 pressure demand oxygen mask was designated as ‘Substitute Standard’ on the same date that the improved A-13A mask was standardised. The improvements to the A-13 design that resulted in the new A-13A standard were a result of feedback from aircrew who had used the new mask operationally, as was the usual custom. The improved A-13A mask was very similar to the A-13 mask in overall appearance, but had a few slight changes incorporated into it. The improved A-13A mask could be used either as a pressure-demand mask, or as a regular diluter demand mask by substitution of a straight rubber flappered exhalation valve for the pressure-compensated exhalation valve and removal of the one-way check inlet check valves. Other small changes incorporated included sturdier OD colored cotton fabric straps for the original black cotton straps, an improved quick-release right side mask attachment buckle, and the mask was issued in three basic sizes (small, large, and medium), just as was the A-13 mask.
Somewhat later, a few additional changes were incorporated into the US Air Force A-13A mask, which included addition of a snap to the nose strap (the snap was added to the left side of the strap cross-piece, for quick attachment adjustments), the substitution of OD or sage green colored nylon straps for the original black or OD colored cotton straps, substitution of sturdier (redesigned) upper and lower buckles for mask strap attachment, and perhaps most importantly, the use of the new A-2 (later to be known as the MC-3) bailout quick-released connector at the distal end of the oxygen supply hose in place of the original "T-piece" bailout bottle connector (between mask body and hose). These changes were incorporated into the mask shortly before the improved A-13A mask was redesignated the MS22001 mask (early 50s).
[The US Navy continued to use the improved A-13A mask well into the 60s, although changes in the suspension were incorporated (this involved use of a Hardman Tool Company mask suspension shell and quick attach bayonets in place of the original snap attached suspension) in the mid to late 50s. Other changes in the US Navy version included use of a single snap at the helmet end of a stitched two-point 'Y' fabric strap on each side of the mask (no right-side quick release buckle, as on the Air Force version) up until use of the the Hardman kit became standard Navy configuration. The Air Force version of the mask, soon known universally as the MS22001 Mask, remained in use well into the late 50s and early 60s, at which time it became increasingly supplanted by the newer MBU-5/P pressure-demand mask. A variant version of the MS22001 mask was also designated the MBU-3/P, late in the 50s].
One other type of pressure-demand breathing mask was developed during the later part of the war. This was the Type A-15 pressure-demand oxygen breathing mask. In October of 1943, the Ohio Chemical and Manufacturing Company also undertook to develop a version of the original pressure-demand mask proposal. This Ohio built mask became known as the A-15 mask; it resembled the A-13 mask in many ways, as both had been alternate versions of the original design proposal, and had many of the same features as the A-13. However, instead of a hard plastic nose piece and two rubber lugs protruding through it (as on the A-13), the A-15 mask used a simple rubber nose strap, attached to the facepiece by the same sort of plastic rivet system used on the A-14 mask (which Ohio had developed and produced). Otherwise, its functional features were virtually identical to those of the A-13 and after service tests conducted by the Army Air Force in November 1944, it was not accepted as having any essential additional benefits to offer over the already accepted MSA produced A-13 design. An improved mask, designated the A-15A, entered service tests in May of 1945, just as the European War was ending. It too had inlet check valves and a pressure-compensated exhalation valve that operated similarly to that used in the A-13, and could also be used as a straight demand mask by installation of a simple flapper exhalation valve and removal of the inlet check valves.
Further testing by both the Army Air Force and the US Navy revealed that the Ohio built A-15A mask also offered little improvement over the existing A-13, and was noted to perhaps have a few deficiencies in certain respects. Thus the A-15A mask was never adopted for standard service use. As a result, it is today considered a very rare mask and examples are far and few between, owing to its never having reached a regular standard, mass-procurement stage of production.
Pressure-demand regulator development continued with the two versions of the A-14B pressure demand regulator, one by the Aro Company and then other by Bendix. Both were found to have certain defects and as a result of the comparisons, only the Aro regulator was recommended as being kept in use. The revised A-14 pressure-demand regulator became fully standardised on 1 November 1944 as a modification of the Aro Type A-12 regulator (or AN-R-5 regulator). It differed from the A-12 only in having a manual dial added to its control face, to regulate delivery of positive pressure up to about 30.2 TORR over ambient pressure.
Other pressure-demand regulators under development included the Type A-15, and the Type A-16 (which was a low pressure regulator for use at extremely high altitudes). The latter was standardised in February of 1944 but subsequently proved to have a number of problems, so that by January of 1945 it was declared obsolete. Eventual standardisation of the Aro Type A-14 regulator made further development of the A-16 redundant. The very last pressure demand regulator developed in the war years was the experimental Type A-17, previously mentioned.
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