High-Altitude Flying: What You Need to Know | Flying Magazine

High-Altitude Flying: What You Need to Know

Learn the symptoms and dangers of hypoxia and how to prevent it.

High Altitude Flying

Venturing into the rarefied air in the flight levels can offer significant advantages for pilots, but dangers lurk at these altitudes as well.

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The pilots of a Learjet 36 cruising high over Cleveland Center’s airspace are fighting a desperate battle for survival — but they don’t realize it. Suffering the effects of extreme hypoxia in the thin air at 32,000 feet, the copilot is passed out. The captain in the left seat is barely hanging on. Most chilling about the ATC recording of the 2006 incident is that the crew in all probability survived only by a quirk of fate. The copilot, slumped unconscious with his arms flailing uncontrollably, continuously keys the microphone, alerting the Cleveland Center controller on duty that something is amiss.

When the controller queried the Learjet, the barely conscious captain sounded blind drunk. By then the copilot’s convulsions had nudged the yoke, disconnecting the autopilot — and initiating a climb. The captain, dimly aware of a problem but not cognizant of the urgent need to don his oxygen mask, declared an emergency. “Unable to maintain altitude. Unable to maintain airspeed. Unable to maintain heading,” the pilot radios in slow, stammered speech. “Other than that,” he wrongly adds, “everything A-OK.”

It was a second controller on duty who heard the exchange and recognized the symptoms of hypoxia. She instructed her colleague to tell the Learjet to descend immediately. Upon reaching a more oxygen-rich altitude of 11,000 feet, the pilots are suddenly both fully alert. Their nonchalant demeanor belies the fact that they have survived a brush with death at an altitude above that of Mount Everest. The controllers on duty that night would go on to receive the National Air Traffic Controllers Association medal of safety for their actions and thanks from a grateful crew.

No amount of hypoxia awareness training can ensure you won’t succumb to the effects of a lack of oxygen in the bloodstream.

Pilots who never venture into the flight levels tend to regard high-altitude flying in relatively simple terms. We all know what hypoxia is, though many of us have never experienced the symptoms ourselves in any meaningful way. We’re aware of the FAA regulations requiring supplemental oxygen when flying above 12,500 feet for more than 30 minutes or any time above 14,000 feet — and so we make sure we stay below those altitudes. We know that “jet routes” replace Victor airways starting at 18,000 feet and that pilots also switch over to a standard barometric pressure setting of 29.92 inches of mercury.

Beyond that, high-altitude flying is something of a mystery to a great many general aviation pilots who probably only get to experience this unique vantage point from the cabin of an airliner. But with high-performance piston singles and twins capable of cruising well up into the flight levels, and more aircraft buyers moving into the ranks of pressurized turboprops and light jets, high-altitude flight is common among GA pilots.

If you fly at high altitude, you must understand that this flight regime requires additional training and preparation. The amount of time a pilot has before the effects of hypoxia become incapacitating can be measured in minutes — or even seconds at very high altitudes.

Oxygen Mask

The FAA requires a high-altitude endorsement to act as pilot in command of a pressurized airplane with a service ceiling above 25,000 feet. FlightSafety and Simcom offer the training, and King Schools has an online course.

Lowell Sannes

Beyond basic considerations about oxygen use — admittedly of crucial importance when flying unpressurized airplanes in the flight levels — these operations also require knowledge of high-altitude aerodynamics, meteorology and physiology, special equipment requirements, flight planning considerations and more.

The FAA requires a high-altitude endorsement to act as pilot in command of a pressurized airplane with a service ceiling or maximum operating altitude above 25,000 feet. But pilots flying nonpressurized airplanes at high altitude aren’t required to receive additional training beyond an instrument rating necessary for flight at 18,000 feet and above. You don’t even need to be taught to use a supplemental oxygen system before taking off and climbing as high as your airplane’s performance will allow.

Prudence dictates that your own personal minimums include extra training and forethought. It behooves any pilot who will be flying above 10,000 feet to seek out additional high-altitude training. For night flying, you’ll want to cut that number to 6,000 feet, since diminished oxygen particularly affects vision. Pilots who use supplemental oxygen at night will tell you it’s like turning up the lights because visual acuity improves instantaneously when blood-oxygen saturation levels return to normal.

You’ve probably heard that the affects of hypoxia are insidious. In other words, they can sneak up on you. You may start to feel euphoric, lightheaded or dizzy as the saturation of the hemoglobin cells in your blood, which transport oxygen in the circulatory system, drops below about 87 percent. You may experience shortness of breath, rapid breathing or a fast heart rate. Your fingernails may start to turn blue. The trouble is, we all exhibit different symptoms of hypoxia. You won’t know how it affects you unless you’ve had physiological training.

If you’re interested in knowing how you’ll react to the effects of hypoxia, there are a handful of altitude chambers around the country you can visit, such as the one we tried out recently at Edwards Air Force Base in California. But there’s actually an easier way. A decade ago, FlightSafety International teamed up with the Mayo Clinic to create a device that changes the mix of nitrogen and oxygen in the air to exactly simulate the effects of high-altitude physiology. The training takes only a few minutes and can be a real eye-opener.

Still, no amount of hypoxia awareness training can ensure you won’t succumb to the effects of a lack of oxygen in the bloodstream. Many readers will recall the tragic death of pro golfer Payne Stewart in 1999 after the suspected explosive decompression of the Learjet 35 on which he was a passenger. The captain had received high-altitude training in the Air Force yet still was overcome after the sudden loss of cabin pressure, which would have instantly sucked the air from both pilots’ lungs, further reducing their already severely limited time of useful consciousness. The jet traveled on autopilot for 1,500 miles, well off course and shadowed by military fighter jets, before running out of fuel and crashing in a field in South Dakota.

The speed of hypoxia onset varies by individual. At 45,000 feet in a rapid decompression, on average you will have less than 12 seconds to respond.

Luckily more airplanes are being equipped with safety systems designed to defeat the crippling effects of hypoxia. In the Cirrus I fly, for instance, the Garmin Perspective avionics will query the pilot at regular intervals if no keystrokes have been made when flying at high altitude. If the pilot fails to respond, the autopilot initiates an automatic descent to a safe altitude. Considering that my Cirrus is capable of cruising at 25,000 feet, where the time of useful consciousness without supplemental oxygen is about two minutes, I’m thankful for that safety net.

On the Garmin avionics system in the Cirrus, an annunciation periodically appears, asking the pilot “Are you alert?” Press any key and the message disappears. If you don’t respond, a yellow “Hypoxia Alert” message appears, accompanied by an aural chime. If the pilot still does nothing, a red “Auto Descent” warning message appears, followed by a flashing warning annunciation and continuous chime. The autopilot will then automatically initiate a descent to a preselected altitude.

The speed of the onset of hypoxia varies among individuals, and it depends on how weak the oxygen’s partial pressure is in the lungs compared with that in the blood. People who regularly spend time at higher altitudes can tolerate the lack of oxygen better. The higher you fly, the less the partial pressure of oxygen and the shorter the time of useful consciousness. At 45,000 feet in a rapid or explosive decompression event, the average person will have less than 12 seconds to respond. Things could get dicey as you try to sort out the confusion caused by the noise, cold, flying debris, fog and air rushing from your nose and mouth, not to mention the abdominal and ear pain that accompanies an explosive decompression.

The low density of the air, of course, also provides one of the great advantages of flying at high altitudes. Lower air density translates into less drag and, hence, higher airspeeds and reduced fuel consumption. Another benefit is the ability to take advantage of strong tailwinds, which can shorten en-route times considerably. Flying up high can also allow us to avoid the worst weather by climbing over icing conditions and turbulence. Traffic at higher altitudes is typically lighter as well.

The turbo Cirrus I fly has a built-in oxygen system that connects to ports in the ceiling. The system includes a cannula that can be used up to 18,000 feet with prongs that are placed in the nostrils to deliver a mixture of air and oxygen. For flights above 18,000 feet, a tight-fitting oxygen mask includes a built-in microphone, allowing communications with ATC.

Climbing above 10,000 feet during the day (or 6,000 feet at night), you’ll want to use a pulse oximeter to regularly check your blood-oxygen saturation levels. Check frequently for hypoxia symptoms, especially if you are flying alone. If you are flying with passengers, check on them often as well.

A final consideration if you’re flying in the flight levels is to make sure your preflight planning includes a weather check not just at your departure airport and destination but also along your flight route. It’s tempting to ignore en-route weather when you know you’ll be flying well above it, but this can be risky, since an emergency could necessitate a descent and landing anywhere along your route. Be especially cognizant of areas of icing, convective activity and low IFR conditions.

Hypoxia Symptoms

The following is a list of potential signs and symptoms associated with a lack of oxygen concentration in the blood. Specific symptoms will vary by individual, making high-altitude physiology training a good idea for pilots who will venture into the flight levels.

  • Increased breathing rate
  • Increased heart rate
  • Lightheadedness
  • Dizziness
  • Headache
  • Tingling or warm sensations
  • Sweating
  • Drowsiness
  • Nausea
  • Blue lips and fingernails
  • Poor coordination
  • Impaired judgment
  • Tunnel vision
  • Euphoria
  • Loss of consciousness

Time of Useful Consciousness at Altitude

This table lists the amount of time a pilot is able to perform flying duties efficiently in an environment of inadequate oxygen. Keep in mind that a rapid decompression event can reduce time of useful consciousness by up to 50 percent as air is suddenly forced out of the lungs.

  • FL 180 — 20 to 30 minutes
  • FL 220 — 5 to 10 minutes
  • FL 250 — 3 to 6 minutes
  • FL 280 — 2.5 to 3 minutes
  • FL 300 — 1 to 3 minutes
  • FL 350 — 30 to 60 seconds
  • FL 400 — 15 to 20 seconds
  • FL 450 — 9 to 15 seconds
  • FL 500 — 6 to 9 seconds

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