Professional Pilot, May 2018

Quick donning oxygen masks can be lifesavers This EROS mask worn by Chris Battaglia formerly a pilot writer for Pro Pilot and now an airline pilot was developed by Intertechnique and is now produced by Zodiac Aerospace of France Squeezing the nose piece of the crew mask will instantly inflate the webbing allowing the wearer to don the mask in only a few seconds PROFESSIONAL PILOT May 2018 65 Reasons for pressurization failure Loss of pressurization is usually due to technical reasons with the quality of maintenance being a contributing factor A slow rise in cabin altitude to critical values may be due to a lack of pressurized air input such as the failure of the bleed system or packs A rapid or explosive loss of cabin pressure is usually caused by a window or door failure In rare but well known cases its happened due to a cabin structural failure In 1988 Aloha Airlines Flight AQ243 a Boeing 737 200 lost large portions of the cabin roof enroute This caused a sudden and complete loss of cabin pressure and the death of a flight attendant who was sucked out of the aircraft Corrosion from exposure to salty ocean air was later found as the main reason for this terrible failure of a great aircraft While airliners usually operate between FLs 300 to 400 corporate jets may cruise at up to FL500 The speed and severity of the effects of a cabin depressurization depend on the pressure differential Since cabin pressurization technical schedules are designed based on type certification this usually means the cruise altitude and the cabin rate of climb A failure in the bleed system will allow a slow rise in cabin altitude if the outflow valves close properly and the cabin itself is well sealed In these cases the crew has enough time to descend and don oxygen masks On the other hand a failed window will cause a sudden loss of pressure with visibility in the cabin impaired by fog and dust Additionally pain in the ears and limbs of passengers and crew is a strong possibility The volume of the cabin relative to the cross section of the opening in the hull will determine the rate of depressurization This means that for the failure of a similarly sized window a wide body airliner at FL350 will have a slower rate of cabin climb than a corporate jet with smaller cabin volume at the same flight level Corporate jet pilots cruising at flight levels above FL350 or so should keep this in mind Time of Useful Consciousness Time of Useful Consciousness TUC is something we all learned about while studying to become professional pilots Its defined as the amount of time a crew member is still able to perform his duties when experiencing a lack of oxygen supply Although individual physical conditions such as age and smoking habits do make a difference its generally agreed that TUC varies between 15 to 20 minutes at FL200 1 to 3 minutes at FL300 30 seconds to 1 minute at FL350 15 seconds at FL400 and 5 to 10 seconds at FL450 and beyond Note the significant decrease in TUC at FL350 and above The bends a form of decompression sickness experienced by divers may also occur at altitude Its characterized by pain in one or more joints from dissolved gases forming bubbles as they come out of solution upon depressurization Sensations can range from a mild dull ache to incapacitating pain Decompression sickness DCS is caused by nitrogen in the blood flow that cant dissolve fast enough to stay ahead of ambient pressure that is decreasing too fast Nitrogen bubbles form in the blood vessels under the skin or even in the lungs or brain affecting the central nervous system Flying shortly after diving can make things worse The FAA recommends waiting 24 hours after 1 day of scuba diving and 48 hours after a week of scuba diving before flying at a cabin altitude of 8000 ft However should you be unlucky enough to experience a rapid decompression in your aircraft after scuba diving for a week all bets are off as serious DCS may occur Another limit that is nice to know but usually not of consequence in civil aviation is the Armstrong limit At about 58000 to 60000 ft the pressure of the atmosphere is so low that water in the body boils at body temperature a condition that can only be overcome with a military style pressure suit Effects of hypoxia Now on to lack of oxygen known as hypoxia Effects include fatigue inability to concentrate confusion or euphoria impaired decision making poor performance loss of consciousness and eventually death Caution Hypoxia doesnt cause discomfort or pain Its dangerous because it starts quietly sometimes unnoticed Individuals differ considerably in their ability to withstand hypoxia In the early stages one crewmember may be more seriously affected than the others A crewmember that yawns a lot may be experiencing the first stages of hypoxia Hypoxic hypoxia also known as altitude hypoxia occurs due to the reduced partial pressure of oxygen in surrounding air In a healthy individual oxygen saturation in the blood is initially little affected as oxygen partial pressure decreases in the surrounding air with increasing altitude Night vision may suffer first The heart is one of the most sensitive organs with respect to oxygen supply it extracts more oxygen from arterial blood than most other organs Photo by Jose Vasquez