The Air Up There!

  • Published
  • By Maj Brian "Moose" Musselman
Remember your last physio refresher? During the course of the discussion, there was probably mention of Class A aviation mishap data with an emphasis on the contributions of human factors. Due to increased emphasis on human error causation over the last decade, discussion of this topic is important; however, there is another component of military aviation still contributing to aviation hazards: human physiological responses to flight. Confused? You shouldn't be! I am talking about hypoxia, decompression sickness, trapped gases, smoke and fumes, etc. In FYs 03-07, there were 144 Class A mishaps with a rate of 1.31 per 100,000 flying hours. During this same time frame, the numbers for reported physiological incidents is somewhat surprising. There were 753 reported physiological incidents between FYs 03-07 at a rate of 6.85 per 100,000 flying hours. 

Feeling All Warm and Fuzzy. During FYs 03-07, hypoxia was reported on 139 separate occasions. Do you remember your cardinal signs of hypoxia? These 139 folks did! You may be thinking these events are limited to undergraduate pilot training (UPT) students; however, during an alert launch, an F-15E pilot experienced hypoxia symptoms. The crew was number two of an alert flight and scrambled in support of a troops-in-contact situation. They climbed to FL265 and the pilot noticed that no one was responding to his calls. When he checked the communication cord, he noticed the cord and the oxygen hose from the mask to the CRU-94 were disconnected. He reconnected both leads and continued the flight. About five minutes later, he felt like cabin air was entering the mask instead of oxygenated air. He looked down, noticed the oxygen hose was disconnected again, and reconnected the hose. Later, the pilot again noticed the same sensation of breathing cabin air instead of normal oxygen provided by the oxygen system. He had the weapons system officer (WSO) fly while he tried to get the oxygen hose to stay attached to the CRU-94. The pilot reported the cabin pressure was around 10,000 feet MSL and 15 minutes later started to experience hypoxia symptoms. He reported blurred vision, tingling of the hands, and slowed performance of aircraft duties. The pilot informed the WSO, gang-loaded the regulator, and started a rapid descent to 11,000 feet MSL. Realizing the oxygen mask was still disconnected, he reconnected the oxygen mask hose to the CRU-94 and held them together with one hand. He started to feel the oxygen coming through the mask and felt the bladder in the back of the helmet inflate, indicating good oxygen flow. The flight lead declared an emergency for the hypoxia symptoms. The pilot returned to base and attempted to land, but was told to go around for being too high and too fast. He landed successfully on the second approach. During the investigation, the CRU-94 was found to be dirty and difficult to use. It is required to be inspected every 30 days and was inspected 22 days prior to the incident. This example reiterates the importance of a proper P.R.I.C.E. check and hypoxia recognition training. 

The hazards of hypoxia are not unique to fighter/trainer aircraft either. An EC-130H crew departed for an OCONUS flight and as they climbed through 10,000 feet MSL, the pilot noticed a normal indication on the cabin altimeter. After receiving clearance, the pilot began his climb to a cruising altitude of FL210. As the aircraft passed through 17,000 feet MSL, the pilot felt what he described as "warm and uncomfortable" and noticed that the flight deck seemed "unusually quiet." The flight engineer also recalled losing his color vision. The pilot noticed that the cabin altimeter was indicating 17,000 feet MSL and directed the crew to go on 100% oxygen. Upon recognizing possible hypoxia symptoms, the pilot was able to correct what could have turned out to be a bad situation. The crew eventually discovered a stuck valve in the cargo compartment under the floor heating system which was preventing the aircraft from pressurizing. The pilot turned off the heat and was able to pressurize the aircraft. Every AF crew member is trained to recognize his or her hypoxia symptoms in a controlled environment to facilitate recognition and recovery during an actual flight. 

Get Bent. We do not purposefully induce decompression sickness (DCS) in physiology refresher students; however, this doesn't mean we haven't thought about using it as an incentive for students to remain coherent during the classroom instruction! Although some may think that DCS is no longer a hazard in modern military aviation, there were 36 reported DCS cases during FYs 03-07. Separating myth from reality, only 17 of these reported cases were in the U-2.

DCS is a known hazard for the U-2 pilots and they receive additional training on DCS and take preventative measures prior to flight. What about the other 19 cases, though? The majority of the cases occurred in fighter/trainer aircraft, but there were five reported DCS cases within the Tanker, Transport, Bomber category.  One reported incident involved a C-130H. The aircraft departed a deployed location and enroute to their destination at FL240, the crew noticed cabin pressure rising through 4,000 feet. The flight engineer (FE) was unable to maintain or control the rising cabin pressure. As the pressure rapidly passed through 7,000 feet, the co-pilot told the crew and passengers to go on oxygen, and the aircraft commander initiated an emergency descent. By FL230, the aircraft pressure equaled the outside air pressure. The crew diverted to a base with a Navy dive chamber to treat suspect passengers. The crew landed uneventfully, and all 72 personnel onboard the aircraft were seen by a Navy flight doctor. Of the 72 passengers and crew, 8 had experienced ear or sinus blocks or other symptoms. Three personnel had symptoms that warranted a precautionary Table VI hyperbaric dive. No delayed symptoms were observed. This event demonstrates the importance of crew members understanding the physiological factors of flight, and the crew should be commended on their response to this emergency situation. Another physiological concern for aircrew is trapped gases in the ears and sinuses. 

"Ears" What I'm Talking About. There were 143 Class E (Physiological) mishaps related to ear and sinus pressure issues reported during FYs 03-07. Once again, aircrew experience pressure change in the hyperbaric chamber during physiological training and are taught how to compensate for this change. We all know the number one way to prevent trapped gases in the ears and sinuses is to avoid flying with a cold or congestion, but much to my chagrin, 43 percent of the reported ear and sinus pressure issues said something along the lines of "Crew member felt sick prior to flight, but thought he could make it." When's the last time you were questionable and flew anyway? One particular incident involved a crew member on an AC-130 who presented sinus pain shortly after takeoff. During a descent from 6,000 feet, the instructor fire control officer requested a shallow descent for an ear block. Once at 1,500 feet, he used Afrin to clear his ears, and the crew landed the aircraft. The fire control officer reported having very mild cold symptoms prior to the flight, but had been able to easily valsalva on the ground. However, upon evaluation, his left ear had some clear fluid present and both eardrums did not move adequately. Despite flying with cold symptoms, this crew member and others involved in ear and sinus trapped gas incidents should be commended on their use of Afrin in-flight to clear symptoms reducing injury to the ears. Another flight involved a crew member in the backseat of an F-16. During a rapid descent from FL400 to 15,000 feet MSL, the crew member was unable to keep up with pressure changes. He felt pain and pressure in his left ear, but didn't inform the pilot of any ear problems, as he didn't want to interrupt the flow of the flight. If this guy had notified the pilot in a timely manner, it would have prevented or minimized his injury. After landing, the crew member still didn't report any ear problems! In fact, he waited approximately 24 hours before seeing a flight surgeon. During examination, the flight surgeons discovered blood behind an intact left eardrum. The crew member had no indication of pre-existing medical conditions (like a cold) that could have contributed to the barotrauma. The fluid was cleared over 5 days with systemic medication. A follow-up examination 5 days later showed a full recovery and the crew member were returned to flight status. This report also mentioned that due to a couple of barotraumas within this wing, awareness training for local aircrew would be worthwhile as they approached the flu season. Training should discuss communication of physiological issues between the aircraft commander, as well as crew members and passengers. Early recognition in any physiological incident is crucial to proper treatment. This individual could probably have avoided pain and DNIF by leveling the aircraft and providing himself time to adjust to the pressure change. Don't be afraid to mention a problem ... because let's face it, you're only human. 

The Smokin' Gun. The final category discussion is smoke and fumes. A smoke and fumes incident should be reported as a Class E (Physiological) mishap if there are "symptoms or health effects caused by toxins, noxious, or irritating materials such as smoke, fumes (including carbon monoxide) or liquids." If there are no symptoms or health effects, but "a member of the crew executed any portion of an emergency checklist in response to smoke and fumes," then the incident should be reported as a Class E (Miscellaneous) mishap. There were 135 reported Class E (Physiological) mishaps related to smoke in fumes during FYs 03-07. About 75 percent of the incidents involved heavies (non-fighter/attack/trainer aircraft) and about 25 percent of the total involved various C-130 models. Although a fair number of C-130 smoke and fumes incidents were caused by a dirty water separator sock, some resulted from other causes. In one incident, fuel began to leak from a generator located in the cargo compartment of a C-130 departing from the deployed location. The crew leveled off and the loadmaster contained the fuel after approximately two gallons had leaked. The aircraft commander directed the crew to don their oxygen masks, declared an emergency, and landed the aircraft. Initially there were no injuries; however later, the loadmaster reported to the flight surgeon complaining of headaches and other symptoms. He was subsequently brought to the base hospital and treated for symptoms resulting from inhaling fuel fumes. 

Another incident involved an F-16. As the pilot prepared for takeoff, he noticed a faint smell in the cockpit, lowered his oxygen mask briefly, and noticed the smell had gotten much stronger. He described the odor to be similar to "turpentine." The pilot put his mask on, selected 100% oxygen, and taxied clear of the runway. There were no caution/warning lights or abnormal indications from any instruments, and there was also no smoke present. As he taxied back into the arming area, he executed the "smoke/fumes in cockpit" emergency checklist. As soon as the pilot shut down and opened the canopy, the odor quickly dissipated. As the pilot egressed, he started to feel "light headed." Simultaneously, the fire department personnel noticed "significant" smoke coming from the engine intake, and the aircraft continued to smoke for several minutes after engine shutdown. Inspection of the Environmental Control System (ECS) revealed gun lubrication inside the turbine exhaust duct and throughout the immediate area. Further inspection of the duct revealed that the gasket was worn, allowing residual lubrication, which dripped down from the gun system components, to enter into the ECS and cause the fumes experienced by the pilot. Maintenance personnel followed all T.O. guidance correctly, but this problem is all but impossible to completely eliminate due to the design of the F-16 (the aircraft simply does not provide complete isolation of the two systems). 

Finally, the T-6A had three separate reported incidents where the electrical attitude direction indicators (EADI) failed causing smoke and fumes. The crew reacted appropriately in all three situations. Appropriate reaction to these incidents prevented further problems. The USAF will continue to experience smoke and fumes incidents, and crews should remain vigilant and study their smoke and fumes EPs. 

Aerospace Physiology original and refresher training provides valuable information to assist aircrew in preventing mishaps. This article covered several of the Class E (Physiological) mishap categories, but is not all inclusive. There are other physiological concerns in aviation that still require training and discussion. All physiological manifestations discussed during chamber training are ever-present in aviation. What are your hypoxia symptoms? Are you drinking enough fluids? What did you eat for lunch? How much sleep did you get last night? Do your passengers understand how to compensate for pressure change? What will you do if you recognize smoke and/or fumes in your aircraft? What flight conditions set you up for spatial disorientation? These are important questions you must ask yourself in continuing your career as an AF crew member. The answers may very well prevent pain, DNIF, or the next mishap. Fly safe!