Slow-Onset Hypoxia: Situation Unbreathable

hx-in-action4The recent TBM 900 accident and other similar events involving hypoxia-related pilot incapacitation are tragic reminders that it is important to look at this issue more closely so that all of us can better understand the dangers of slow-onset hypoxia.

When an emergency or technical issue arises in flight we automatically become fixated on the problem and focus all of our energy and concentration on resolving the situation. If we add a concomitant slow cabin depressurization, the reduced oxygen to the brain further compounds and amplifies this fixation, leading to incapacitation. Let’s take a closer look so that we can better understand how to manage such a dangerous scenario.


Slow-onset hypoxia can be very difficult to identify while task-oriented in the cockpit. Recently we conducted a study of 100 pilots who underwent a slow cabin depressurization model of 1500 feet/min while flying simulators with ATC active communication, and we found that fixation was very pronounced. More remarkable was the very low oxygen saturations noted, even though the pilots reported that they felt “OK” while flying their simulators in the high-altitude chamber.

In the study we found that when the pilot had experienced two or three sensations of hypoxia and mild impairment, the average oxygen saturation was just 63 percent! This saturation is critically low and explains how experienced and excellent pilots can be vulnerable to slow-onset hypoxia. They may not be aware of the sensations, or they think they have plenty of time to solve the technical problem before donning the supplemental oxygen mask. Either way, this is a very dangerous scenario and in part may explain the recent TBM 900 accident and other like events. The slower the onset of hypoxia the more dangerous the fixation becomes.


We also found that 90 percent of all pilots noted their first sensation of hypoxia at 18,000 feet, with an average oxygen saturation of 75 percent. These findings were so reliable that we decided to call this phenomenon, “The Poor Man’s Altimeter.”  It means that, even if aircraft alarms and message systems fail to warn of a cabin pressurization problem, the pilot can rely on one’s own body. However, this physiological alarm will be subtle and the pilot must be able recognize it for it to be effective.


A review of all fatal accidents associated with hypoxia since 1999 found that most occurred as a function of takeoff. In most cases the scenario was a failure of the pressurization system on the ascent. Cabin pressurization management is crucial during climb to altitude, and donning the mask must be accomplished early to avoid the insidious onset of hypoxia-induced fixation.

In our experience we see that a slow ascent from a cabin altitude of 10,000 feet to 20,000 feet is as dangerous as an explosive decompression at higher cruise altitudes in the flight levels.

Even though this scenario may be complex in nature, the solution is remarkably easy: don the mask! The better understanding we have of this dangerous trap the sooner we can establish situational awareness. It is within our reach to make hypoxia 100-percent preventable.

Paul-Buza-Uniform-240x300Dr. Paul Buza is founder and Medical Director of Southern AeroMedical Institute (SAMI; link) and the author of “Scenario Based Physiology.” He is also a faculty member at the University of Central Florida College of Medicine and the Florida Institute of Technology College of Aeronautics. Dr. Buza will be a featured speaker at the 2015 Twin Commander University.