Ascent and Scenario-Based Time of Useful Consciousness (TUC)
The Time of Useful Consciousness (TUC) is a standardized table that has been used by aviation regulatory, safety, and manufacturing organizations for over 50 years to better understand the potential danger of cabin depressurization. It emphasizes the importance of the immediate donning of oxygen masks at high altitudes in the event of a cabin depressurization which otherwise would result in the loss of consciousness. The current TUC table was developed from research in high-altitude (hypobaric) chambers by taking the pilots to various high altitudes while wearing oxygen masks at which point they removed their masks and engaged in various psychological tests until sufficient neurocognitive degradation was noted resulting in an approximate TUC determination. Very limited slow ascent research has concentrated on determining oxygen saturation and heart rate without TUC determination. Also, recent research has been concerned with rapid decompression but has been lacking scenario-based training closely related to flight deck environment. Rapid onset hypoxia scenario training has effectively reduced the number of fatalities related to explosive and rapid decompressions; over the past decade there has not been a commercial related fatality associated with a rapid or explosive decompression. Since 1999, 137 fatalities have been associated with slow onset hypoxia that is more insidious, slowly distorting pilot cognitive processes. The ability to multi-task is reduced resulting in loss of situational awareness and ultimately, loss of consciousness. Review of these recent fatal accidents found all to be related to a failure of pressurization on ascent; the estimated ascent rate determined by ATC transcript analysis was approximately 1,500 fpm. In view of these accidents, it appears that the existing rapid onset based TUC tables are insufficient and require modification to better understand this phenomenon. Hence, the current TUC table should be augmented to address issues of gradual decompression by introducing the concept of “ascent rate based” TUC. This study used Southern Aero-Medical Institute’s (SAMI) data of 100 subjects to lay the foundation for a new TUC table. The research design was modeled according to slow ascent decompression accidents reports in hypobaric chamber using SPO oximeter and scenario-based training simulating real cockpit duties. The new table is based on varied ascent rates with the first being 1,500 fpm that approximates a more realistic, slow-ascent scenario based model. The scenario was developed to more closely match pilot performance while interfacing with flight deck controls as opposed to prior psychological tests. This matches more closely to what is expected in real time commercial flight profiles. The subjects’ TUC (n=100) was determined from data taken upon oxygen pre-breath up to 5,000 feet and removal of the masks. Subject deteriorating performance and physiological symptoms created the endpoints for TUC determination. A continuous SpO2 and HR monitor worn by all subjects was used to collect data for TUC, HR and SpO2 determination. The research results (n=100) were: total mean TUC of 9.21 minutes (SD=1.79). The CPI Net Increase doubled from resting values (CPI= 48% (SD=0.17). The mean SpO2 was 63,54% (SD=8.03) and the mean of the negative slope of oxygen degradation was -3.71 (SD=1). The newly determined TUC at 1,500 fpm of 9.21 min is the first parameter established for the ascent based TUC table. Subsequent research is needed to identify the ascent based TUC at 500, 1,000 and 2,000 fpm to complete the table. One hundred subjects having nearly identical negative slope of oxygen degradation determined information that is expected to be useful for incorporation into algorithms for equipment design. Such consistent outcome leads to an implementation of cabin altitude alarm that would trigger crew response to check pressurization system working properly and the cabin altitude is not increasing beyond unsafe human conditions. These findings will result in a valuable addition to the existing TUC tables and raise awareness of this dangerous scenario thereby further improving aviation safety.