The Highs, and Lows, of Pressurization and Flight Levels Flying

Flying a pressurized Twin Commander offers all of the benefits of flight levels flying: cruising in typically smooth, clear air; avoiding much of the weather found at lower altitudes; occupying uncongested airspace; and enjoying high true airspeeds at reduced fuel flows. But those benefits come with the responsibility to recognize the potential risks associated with high-altitude flying, including hypoxia resulting from the sudden or gradual loss of cabin pressurization. The concern about hypoxia is especially critical in single-pilot operations. The September 5, 2014 fatal accident in which the pilot of a TBM 900 and his wife became incapacitated at altitude is a sober reminder of the potential consequences of hypoxia.

The accompanying sidebar details the pressurization systems and the associated emergency systems installed in various turboprop Twin Commander models. As is evident, pressurization systems improved over the years both in terms of cabin pressure differential and cabin altitude warning systems.

It goes without saying that it is important to understand the pressurization system on the Twin Commander you fly, and do the preflight checks of the system: proper supplemental oxygen bottle pressure and valve ON, cabin altitude annunciator test, passenger oxygen control selector on desired setting, and proper functioning and flow setting of the crew oxygen masks. It’s also recommended to occasionally check the emergency system in flight –– preferably with no passengers aboard –– by selecting a cabin altitude in excess of the value that activates the cabin altitude annunciator and, if applicable, an aural alert.

The Twin Commander pressurization system is designed to address the primary physiological concern when flying in the flight levels, which is not lack of oxygen content in the atmosphere but the low partial pressure of the oxygen component of the atmosphere. Without proper cabin pressurization or the wearing of a supplemental oxygen mask, the body is unable to absorb enough oxygen, which results in altitude hypoxia. Untreated, hypoxia can quickly lead to degradation of cognitive functions and, ultimately, unconsciousness.

FAA Advisory Circular 61-107B, “Aircraft Operations at Altitudes Above 25,000 Feet Mean Sea Level or Mach Numbers Greater Than .75,” explains in detail the physiological and aerodynamic concerns when operating in the flight levels. This statement in the AC illustrates the effects of operating at reduced ambient pressure at higher altitudes: “The human body functions normally in the atmospheric area extending from sea level to 12,000 ft MSL. In this range, brain oxygen saturation is at a level that allows for normal functioning. Optimal functioning is 96 percent saturation. At 12,000 ft MSL, brain oxygen saturation is approximately 87 percent, which begins to approach a level that could affect human performance.”

Sudden, rapid loss of cabin pressurization is impossible to ignore –– it likely will be signaled by a loud whooshing sound, various cabin altitude annunciators and alarms going off, and passenger oxygen masks dropping from the headliner. The challenge is to respond quickly by donning the crew oxygen mask, initiating an emergency descent, making sure passenger oxygen masks have deployed and are being used, and communicating with ATC.

A slow loss of pressurization or failure to pressurize at all and the corresponding rise in cabin altitude –– and the onset of hypoxia –– is much more difficult to recognize unless you are diligent in monitoring pressurization system controls and displays, and are familiar with the symptoms of hypoxia. “If you haven’t experienced the insidious onset of hypoxia, it can be very difficult to recognize,” said Lt. Col. Matt Albright, an aerospace physiologist and 19-year-veteran of the U.S. Air Force who currently is stationed at Fairchild Air Force Base in Washington. The Air Force requires all of its aviators, including paratroopers, to take hypoxia training in an altitude (hypobaric) chamber or with a mask-on device that replicates lower oxygen intake over a long period, according to Lt. Col. Albright. The training includes a recurrency session every five years.

The eyes are most sensitive to oxygen deprivation, and thus the first symptom of the onset of hypoxia is loss of visual acuity, especially at night. The brain requires about 20 percent of the body’s oxygen, so a reduction in oxygen intake will quickly affect brain functions, in particular the higher reasoning portions of the brain. Judgment and cognitive skills quickly diminish.

Age, physical condition, hydration, sleep hygiene, diet, and medical conditions affecting the lung’s capacity to transfer oxygen to the blood or the heart’s ability to efficiently pump oxygenated blood throughout the body can significantly contribute to a pilot’s susceptibility to hypoxia, Lt. Col. Albright noted.

Air Force installations used to make their hypobaric chambers available to general aviation pilots for altitude training, but in recent years the number of bases that conduct the training for civilians has diminished. The FAA’s Civil Aeromedical Institute offers a one-day course for pilots on “physiological and psychological stresses of flight.” The course, which includes a stint in a hypobaric chamber, is offered at the FAA’s Mike Monroney Aeronautical Center in Oklahoma City. To enroll, see

Formal altitude training provides pilots with knowledge of the symptoms they might experience that signal the imminent onset of hypoxia –– dizziness, headache, euphoria, hot and cold flashes, numbness or tingling in the extremities, and/or blurring vision, among others, according to Lt. Col. Albright. Recurrent training refreshes that knowledge, and accounts for changes in the pilot’s physiology that could affect his or her susceptibility to hypoxia, he added.

Along with altitude training, maintaining proficiency in the knowledge and operation of the pressurization system and emergency procedures, doing a thorough preflight of the system, and monitoring cabin altitude during flight-levels flying, pilots can add a simple and inexpensive form of protection against hypoxia by wearing a pulse oximeter when flying. The device, used by many doctors, fits on the end of a finger and measures the blood’s oxygen saturation level. An alarm threshold can be set at any desired oxygenation level.

And, one more hypoxia-prevention suggestion from Lt. Col. Albright, especially when flying at night: grab that crew mask every hour or two and take a few good gulps of 100 percent oxygen. You’ll be glad you did.