Not just the Royal Navy. HMAS SYDNEY Persian Gulf. I remember 15 January 1991 particularly well as that was the day our Task Group commander sent out a signal "Hostilities are imminent. Shave off -R- Shave off." There went my beard of 12 years. Bloody painful taking it off too......I suspect a gas mask though requires a perfect seal whereas a supplemental O2 mask...maybe not so much. Are the pilots masks more of a "demand" type" or are they "Positive Pressure"?