In Flight Emergency – Cabin Pressure & Hypoxia
I was recently in Estonia for a NATO military training exercise. Flying in the back seat of an F-16D (two seater), I was ‘gently’ reminded of the importance of human factors in flight and the constant, inherent danger in combat aviation. Even in the training environment, significant risks and aeromedical stressors constantly lurk in the shadows. This is why military pilots train so extensively – to prepare for real-world threats.
The sortie (mission) was designated dissimilar BFM (basic fighter maneuvering, i.e. basic Air-to-Air tactics) against another NATO nation’s Eurofighters. Following takeoff, it became obvious that effective training would prove difficult to attain for our two-ship. Weather was not good. Aside from some broken layers, the cloud deck’s ceiling was from the bottom of the airspace to nearly 25,000 feet. Somewhere around FL 230, I began to experience mild discomfort in my right ear. I hadn’t had a recent upper respiratory infection. No allergies either. The cabin altitude should be somewhere between 8,000 and 9,000 feet; maybe enough to cause some discomfort I reasoned. As we broke out of the white and gray soup surrounding us into
brilliant blue skies, we continued a slow climb while driving towards the Eurofighters to initiate the fight. Following our G-checks, the discomfort in my ear increased. Taking my oxygen mask down, I pinched my nose with thumb and index finger, performing a Valsalva Maneuver to relieve the building pressure. As advertised, the maneuver worked. I replaced the MBU-20/P oxygen mask back over my nose and mouth, clicking the bayonet into place. As the flight doc in the back seat in bad weather, I was keeping quiet and not intimately involved with flight operations at this point. So, I began to read the F-16 emergency procedure checklists as I do from time to time during flight. Odd. I became aware that I found simple comprehension of what I read elusive. It was very subtle, but I felt slightly off. Why was I experiencing confusion?
The clinician in me quickly synthesized the history of ear discomfort (see previous post on ‘Trapped Gases‘ in flight) with subtle cognitive decline. Diagnosis – Hypoxia. I began to ask the pilot what the cabin altitude was. (The back seat of the F-16 does not have a cabin altitude indicator.) Halfway through my question, the MASTER CAUTION light activated and immediately the pilot bunted the nose over and we dove into the clouds. Yup, we had certainly lost cabin pressure. And the pilot automatically responded as trained by immediately descending to a safer altitude. It was a physiologic in-flight emergency (IFE).
All this happened simultaneously while I was in the middle of my question. It hung there for a moment unanswered as the pilot focused on flying the aircraft, called an IFE, and radioed to our number two about the situation. After all of this, he casually response to my yet-unanswered inquiry, “2-7-0” (Flight level 270 i.e. 27,000 feet). It was as if we had been immediately dropped onto the summit of Lhotse, the fourth highest mountain in the world. “Gang load?” I asked, knowing the answer to this obvious question. “Yeah, gang load”. Descending and now on 100% oxygen, my symptoms immediately resolved. We continued our descent until we were below 18,000 feet, ran the checklist for cabin depressurization, and RTB’ed without incident. In order to decrease any chance of decompression sickness, I recommended that we remain on 100% oxygen until shutdown. Returning to Amari Airbase in Northern Estonia, fire fighters and first responders met us in accordance with standard response to a physiologic IFE. “Do you need medical to respond to your jet?” asked the first firefighter to communicate with us? “Nope”, replied the pilot with a chuckle. Medical had been present and already provided an assessment and recommendation. Although not the most tactical sortie, I had learned a lot seeing a physiologic IFE from inside the jet!
The condition of hypoxia is simply defined as inadequate oxygen to meet the body’s metabolic needs. This physiologic state can be experienced by lowland dwellers due to a variety of medical conditions, or alternatively by anyone who ventures far from sea level and the earth’s protective atmosphere. Mountaineers and climbers often experience hypoxia on high ascents, but their bodies have time to acclimate and adapt to the lower oxygen density there. Aviators do not share this luxury and are at high risk of rapid, severe hypoxia that can occur within minutes or seconds if cabin pressurization is lost while flying at high altitudes. An entire post detailing the condition of Hypoxia in Aviation is covered elsewhere. There have been a number of high profile plane crashes that have been thought to be due to hypoxia, including the general aviation crash of an unresponsive pilot who ultimately crashed when the aircraft ran out of fuel in the Caribbean last year. Hypoxia kills.
ATC CONVERSATION WITH TWO HYPOXIC PILOTS
All pilots receive extensive training on the condition of hypoxia. In the U.S. military, aircrew receive altitude (hypobaric) chamber training as they go through their initial pilot training. The word ‘hypobaric’ literally means ‘low pressure’. It follows that this type of chamber is able to decrease the ambient pressure allowing the trainees inside to experience conditions that simulate a variety of altitudes. Since hypoxia can present in a variety of ways to different people and in an insidious manner, it is critical that pilots experience this condition in a controlled environment. One of the first symptoms of hypoxia is loss of judgement. An in-flight emergency where cabin pressure is lost is not the ideal scenario for a person to first try to determine their hypoxic symptoms. Additionally, the altitude chamber allows other subtle changes such as gas expansion and decreased temperature to be observed, which also cue the aviator into a possible loss of pressure before it is too late.
When I trained in the hypobaric chamber at Brooks City Base, we were brought to a simulated 25,000 feet. A rubber glove hanging in the chamber with us demonstrated gas expansion as it slowly inflated like a balloon as we ‘ascended’ (i.e. ambient pressure lowered). This phenomenon could also be observed as gas expanded in our GI tract and we began to pass gas (seriously!). We were separated into groups of two and the first group was instructed to drop their masks. Let the hypoxic symptoms begin. Each person sans mask was given a small clipboard with a list of symptoms, simple math problems, a few riddles, and a maze a toddler could complete. Watching the other group perform these exercises was highly entertaining. Many giggled like school girls. Faces turned red. Arms and legs began to shake uncontrollably. The purpose of the training is twofold: First, identify your unique hypoxic symptoms and other subtle cues of the hypobaric environment. Second, take the appropriate emergency action. In the USAF, we call this action ‘GANG LOAD’. It means to push all of the switches on the regulator into the forward position, which initializes 100% oxygen and emergency pressure breathing. The goal is to perform this action prior to passing one’s time of useful consciousness (TUC). It was interesting to observe an Army aviator sitting across me exceed his TUC as he sat and laughed to himself refusing to follow simple commands and completely unable to engage in intelligible conversation. Ultimately, the physiologist safety observer in the chamber had to place his oxygen mask back on his face and GANG LOAD for him prior to him passing out.
When it became my group’s turn, I almost immediately began feeling euphoric upon dropping the mask. I looked around the chamber quickly, but became locked in on the math problems. I noticed some tingling in my toes that began to progress to my finger tips. On the worksheet, I circled the symptoms I was having with focused intensity. I felt very task-saturated. I noticed a subtle tremor of my hand. Soon my right thigh began to shake uncontrollably and I was unable to stop it. I had finished the math problems, riddles, and the maze. I can’t recall if they were answered correctly or not. I looked up to see it was only me and one other in my group not wearing our oxygen masks. It had become a competition. Neither of us wanted to GANG LOAD first, but neither did we want to repeat the antics of our Army colleague. I felt tremendous air hunger. My leg was pumping like a machine gun, but I could see my competitor was experiencing similar distress. Finally, the physiologist told him to GANG LOAD, waiting a split second after him, I also pushed all three switches forward and replaced my oxygen mask. Immediate relief and all symptoms resolved. The miracle of oxygen!
HYPOXIA OBSERVED IN THE ALTITUDE CHAMBER
PUT TO THE TEST
It was interesting in retrospect to validate how our hypoxia and emergency training helped us in-flight. The expanding gas in my ear and the subtle euphoria/confusion raised my suspicion to a hypobaric cockpit. The pilot’s extensive training and prior memorization of emergency checklists allowed the appropriate response to follow immediately from observation of the MASTER CAUTION light as we descended to a safe altitude, GANG LOADED, and limited our airspeed.
Hypoxia remains an inherent risk to all involved in the aviation and space environments. However, through effective training and vigilant operational procedures, the chance for true catastrophe in response to the condition has become increasingly rare. In the safety-centered world of aviation, no emergency or unprecedented event is allowed to pass without addressing lessons learned. At our subsequent pilot meeting, much like a medical ‘Morbidity & Mortality’ conference, the pilot and I were asked to brief our experience so that each pilot in the squadron could learn from it. And prevent future mishaps from hypoxia.