Slow-Onset Hypoxia–Combatting a Silent Killer

The current training standards for rapid or explosive cabin depressurization have served the aviation industry well. Over the last 15 years there have been numerous high-altitude rapid decompression events that have all resulted in safe landings without associated fatalities. This success is attributed to the requirements of the high-altitude endorsement, which emphasizes the immediate donning of the mask in the event of a rapid or explosive decompression. Obviously, rapid or explosive decompression is easily identified, and therefore a trained pilot reacts appropriately.

However, there is a weak link when it comes to slow-onset hypoxia associated with the failure of pressurization on ascent, or a slow depressurization at cruise altitude. Since 1999 there have been more than 140 fatalities related to this scenario involving technically advanced commercial, corporate, and pressurized aircraft.

When looking at these fatal cases it can be determined that the cabin ascent rate was not explosive in nature, instead occurring over a 10-20-minute period. In some cases the pilots were task-oriented or possibly task-saturated while the insidious nature of slow-onset hypoxia ensued. This is extremely dangerous as there is a much greater degree of fixation resulting in loss of situational awareness.

As a result, there is a need to better understand the insidious nature of slow-onset hypoxia as it relates to the flight profiles and scenarios that would be seen in the Twin Commander fleet. Some may think that hypoxia is more of a phenomenon at altitudes of over 30,000 to 40,000 feet, and that it may not be as significant at altitudes of 15,000 to 25,000 feet—typical Twin Commander cruise altitudes. Nothing could be more wrong!

Upon review of the fatal accidents attributed to hypoxia, most pilot incapacitations occurred at these cabin altitudes, regardless of the type of aircraft that they were flying. When it comes to hypoxia, cabin altitude is critical and early identification is key to avoid incapacitation. There should never be a delay in the donning of the mask, because the longer you wait the more fixated you will become. When managing your avionics during slow cabin depressurization your symptoms will not be as obvious as you might think. Furthermore, if you are busy on the climbout or addressing another technical issue, this dangerous killer can be easily missed.

The innovative approach of our DeSat training program integrates a Garmin 1000 and Zodiac Aerospace EROS mask within the high-altitude chamber, along with ATC communications, in an attempt to create a realistic slow-onset training scenario. The DeSat experience allows the pilot to “explore” and better understand his or her hypoxia sensations in a safe and controlled environment.


Equally important is the need to learn and experience the post-donning procedures. These include the ability to communicate with ATC while wearing the mask, which is not as easy as one may think. Also, there is a need to learn how to equalize the ears during the emergency descent to avoid debilitating ear pain on final approach. The end result of the DeSat program is to achieve a skill set the pilot can draw on to effectively manage the emergency with confidence.

In comparison to other emergencies pilots are very well trained for, the phenomenon of a slow-onset hypoxia is the most difficult to identify. Hence, it is important that we remind ourselves on every takeoff to be vigilant with our cabin pressure settings and monitoring. It is also important to practice donning your mask, and ensure that you have proper oxygen pressure and flow before every flight. The one time that you need your mask is not the time to practice your mask skills or find out there is no flow.

Dr. Paul Buza is founder of the Southern AeroMedical Institute in Melbourne, Florida, a research-based institute that provides training to pilots on the dangers of slow-onset hypoxia. Dr. Buza is a board-certified Neurologist and graduate of Michigan State University, College of Medicine. In addition to his dedication to aviation safety, Dr. Buza is ardently dedicated to hyperbaric oxygen therapy and the power it has to heal. He was instrumental in the early development of hyperbaric medicine in the Melbourne area, which is now considered the standard of care.