Loss of Pressurization During Flight, Part 2
The vast majority of the sampled NASA Aviation Safety Reporting System (ASRS) reports occurred at altitudes where the “time of useful consciousness” can be rather small. The time of useful consciousness (TUC) is the time that an average individual has sufficient mental capacity to take the proper corrective and protective actions, such as donning an oxygen mask or completing a checklist.
The NTSB concludes that existing guidance and information on TUC is inconsistent and misleading because it does not accurately reflect the TUC for pilots trying to perform complex tasks in an emergency environment. The studies upon which the original TUC times were based were conducted using comfortably seated participants who were expecting a decompression and who were asked to perform simple repetitive tasks (such as counting backward from 1,000) until they could no longer accomplish them. Scientifically qualified experts at the NTSB note that these earlier studies did not accurately replicate the complex and changing environment of an aircraft that is losing cabin pressure, and the tasks performed did not accurately simulate the types of tasks involved in accurately identifying and responding to an emergency situation. This retroactive evaluation of the earlier studies is certainly a valid concern.
The NTSB cites other recent studies that suggest that impairment occurs much sooner for pilots required to perform complex tasks. For example, an altitude chamber study published in 1990 indicated that a delay of as little as 8 sec. in supplying oxygen to subjects who had been rapidly decompressed to a pressure altitude of 29,850 ft. resulted in a significant drop in oxygen saturation. Even a moderate drop in oxygen saturation has been shown to significantly impair cognitive functioning and increase the amount of time required to complete complex tasks. Further, the ability to learn new tasks measurably decreases at altitudes as low as 8,000 ft., and, at 15,000 ft., significant impairment is noted in the performance of even simple cognitive tasks. Simulator studies have shown that at 15,000 ft., the ability to manually maintain a given airspeed, heading or vertical velocity is reduced by 20-30%.
The NTSB has made several important safety recommendations regarding the training for oxygen system utilization after notable accidents. After the fatal accident of a UPS Boeing 747 in Dubai in 2010, the NTSB issued Safety Recommendation A-11-088 to the FAA to require operators of Part 121, 135 and 91 Subpart K flights to include, during initial and recurrent training, tactile, hands-on training on the use of operable oxygen mask/goggle sets, including the use of the regulator’s emergency selector and the venting of the smoke goggles.
In the aftermath of the Payne Stewart accident, the NTSB made recommendations requiring that operators of all pressurized cabin aircraft provide guidance to pilots on the importance of a thorough preflight of the oxygen system, including verification of supply pressure, regulator operation, oxygen flow, mask fit and communications using mask microphones. It also recommended requiring that all pressurized aircraft certificated to operate above 25,000 ft. have a clear emergency procedure associated with the onset of the cabin altitude warning. It should contain instructions for flight crews to don oxygen masks as a first and immediate action item, followed by instructions appropriate to diagnose, manage and resolve the condition indicated by the warning.
Some aircraft have the oxygen control valves mounted for external access only, which obviously can become problematic if the valve is accidentally left in the “closed” position during a preflight and the flight crew needs emergency oxygen during a loss-of-pressurization event in the subsequent flight. A thorough check of the oxygen masks prior to a day’s flights is pretty standard on most checklists and should not be skipped.
Some ground school instructors point out that holding down the oxygen flow button on the oxygen mask for at least 5 sec. will quickly alert a pilot to whether the emergency oxygen valve is open or closed. If the valve was mistakenly overlooked and left closed, the ground instructors advise me that the squirt of available air will quickly subside.
Newer Training Options
According to Dr. Quay Snyder, a retired U.S. Air Force command flight surgeon and president of Virtual Flight Surgeons, “Until recently, hypoxia training was primarily conducted in ‘altitude chambers.’ These chambers have provided valuable experiences to flight crewmembers in the recognition of personal hypoxia symptoms, the subtle incapacitation effects of hypoxia and the unpleasant physiological effects of rapid decompression. This training does come with some medical risk due to the effects of trapped gases and brief exposures to hypoxia. To mitigate these risks, careful medical screening, medical monitoring with emergency equipment and an emergency medical treatment plan is standard.”
For example, pilots are not allowed to participate in an altitude chamber training exercise if they have an acute respiratory and/or systemic infection, have a beard, have been scuba diving within 24 hr. (because of the risk of evolved gas disorder when going from the extreme pressure changes of being under water to high-altitude flight), have donated one unit (500 ml) of blood within 24 hr. or more than one unit of blood within 72 hr., have consumed any alcoholic beverage within 8 hr. or are under the influence of alcohol.
According to Snyder, “Recently, newer techniques of hypoxia training have become a popular alternative. It is economically advantageous because the equipment is portable and less expensive. Since it doesn’t involve the use of reduced pressure, it eliminates the risk of decompression sickness and trapped gas problems.” A recent method involves using a mixture of gases to reduce the percentage of oxygen inhaled in each breath. The equipment is portable and less expensive and eliminates the risk of decompression sickness and evolved gas problems that are possible with altitude chamber training.
Snyder says one of the best advantages of the new method is that “combining the demonstration with simulator training increases the value of this training by showing the subtle, yet significant, incapacitating effects of early hypoxia. These effects become apparent very soon after exposure because of the complex cognitive tasks required to operate an aircraft simulator.”
Snyder also warns, “If conducted without proper safeguards and participant education, however, hypoxia demonstrations can expose participants to career- and health-threatening risks without adequately training them in the prompt recognition and proper response to this insidious killer. When selecting training vendors, participants should exercise diligence in determining the safety and value of the training offered.”
Immediate Corrective Action
Nearly every training text on this topic prescribes the same general procedures. Upon first recognition of any symptom or detection of a loss of pressurization, pilots should immediately take the appropriate corrective action (don an oxygen mask, select 100% oxygen flow and, at higher altitudes, select the “emergency” volume of oxygen flow to ensure a positive flow of oxygen into the mask). Immediate descent should be considered to an altitude that will minimize the physiological effects of the pressure loss.
Fortunately, it appears that the combination of heightened awareness after the Payne Stewart accident, the repeated frequency of the “high dive” maneuver in the simulator during recurrent training, and the percentage of the pilot population that has undergone some form of hypoxia training has kept the consequences of a loss of pressurization down to “events” instead of accidents.
As we learned from the Stewart accident, this is one malfunction where we don’t get a second chance. In the words of Snyder, “Training all flight crewmembers to recognize the early symptoms of loss of cabin pressurization and take prompt corrective action is an essential component of any comprehensive aviation safety program. With the advent of very-light jets and increased single-pilot high-altitude operations, this training takes on an even greater significance.”