Slow Onset Hypoxia Represents the Highest Risk of Fatality

The current training standards for rapid or explosive cabin depressurization have served the commercial 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 extensive training resources available to the commercial pilot during initial and recurrent training. The fundamental premise of this hypoxia training is the study of the classic time of useful consciousness table (TUC) where at certain altitudes it is taught that there is a limited time in which the pilot can perform. Examples include that at 25,000 feet the TUC is 3-5 minutes; 30,000 feet is 1-2 minutes; and 40,000 feet is 15-20 seconds. This humbling message emphasizes the need for immediate donning of the mask and, again, has served the industry well.

The Danger of Slow Onset Hypoxia
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 while addressing a primary inflight threat. Since 1999, there have been over 140 fatalities related to this scenario involving commercial, corporate and pressurized technically advanced aircraft. When looking at these fatal cases more closely it can be determined that the cabin ascent rate was not rapid or explosive in nature, and actually were slower occurring over 10-15 minutes while the pilots were task oriented or possibly task saturated. This emphasizes the danger of slow onset hypoxia which is defined as a cabin ascent rate of 1500ft/min over a 10-15-minute period of time. In this slow onset model the signs and symptoms of hypoxia are subtle, and if the crew is attentive to ascent related procedures it is very easy to become fixated, resulting in loss of consciousness.

Recognizing the Subtle Signs and Symptoms
It is often assumed by most pilots that hypoxia will always be easily identified as the signs and symptoms will be quite strong. However, this is not the case with slow onset hypoxia as it is very insidious. This is further complicated by the fact that the crews are task oriented on the flight deck, especially during the ascent phase of the flight. Of special note is the increased level of fixation that occurs as pilots remain focused on their instruments, often missing the fact that they are having mild symptoms of hypoxia, which can be easily ignored. Recent studies have shown that neurocognitive impairment occurs even before the symptoms of hypoxia are recognized. Examples include: diminished scanning, memory read-back with ATC, heading and altitude mistakes, and difficulty with waypoint and frequency changes, to mention a few. Therefore it is imperative that flight crews be able to identify these changes of impairment and early subtle symptoms to avoid incapacitation.

Safety initiatives driven by SMS departments of flight operations will benefit significantly by reviewing this dangerous phenomenon and determine the appropriate steps to better educate the flight crew to this most dangerous scenario.